Belkacem Manser

Idir Belaidi

Abderrachid Hamrani

Sofiane Khelladi

Farid Bakir

B Manser Oe

Belaidi Oe

A Hamrani Oe

S Khelladi Oe

F Bakir Oe

Texture shape effects on hydrodynamic journal bearing

Keywords: Hydrodynamic journal bearing, surface texturing, bottom profile, contour geometry, wedge effect, dimple shapes

come

Introduction

The industrial world is currently facing a challenge where the economic conditions are restricted, due to the rise in energy prices, and environmental require ments. Consequently, many researches are often directed towards the optimization of existing systems, reducing energy consumption and increasing perform ance. It is well known that the performances of any machine are directly influenced by its effectiveness in terms of friction, which is the main cause of wear and energy lasses. For example, in the automobile industry, more than one-quarter of the consumed energy is used to overcome the friction between components [l]. This fact is similar in other industrial sectors, such as aero nautics field. The study of these tribological aspects, i.e. friction, lubrication, wear and contact mechanics, becomes fundamental. Moreover, the technological progress that has been achieved in recent years, allows for a precise control of the surface properties (rough ness, grooves, discrete textures/dimples). The technique of surface texturing is commonly used to control the tri bological performances of the considered system. The idea is originated from the roughness, but in the case of surface texturing, the operator intentionally incor porate a well defined dimple shape to obtain the desired tribological characteristics [START_REF] Qiu | The effect of tex ture shape on the friction coefficient and stiffness of gas-lubricated parallel slider bearings[END_REF]. Textured surfaces are characterized by various geometrical parameters such as shape, size, area density and orientation distribution of the pattern. For composing the specific types of texture patterns, there are various well-known shapes such as rectangular, cylindrical, spherical, sinusoïdal, triangular, elliptical, chevron, etc, which are commonly used in published literature [START_REF] Khatri | Influence oftextured surface on the performance of non-recessed hybrid journal bear ing operating with non-newtonian lubricant[END_REF]. There is a wide range of industrial techniques that can be used for manufactur ing this surface texture, such as laser surface texturing [START_REF] Etsion | State of the art in laser surface texturing[END_REF], reactive ion etching [START_REF] Wang | Improving the anti-seizure ability of sic seal in water with rie texturing[END_REF], LIGA process (lithography, moulding and electroplating) [START_REF] Stephens | Deterministic micro asperities on bearings and seals using a modified liga process[END_REF], vibro-rolling [START_REF] Schneider | Formation of surfaces with uniform micropatterns on precision machine and instruments parts[END_REF], vibro-mechanical texturing [START_REF] Greco | Surface textur ing of tribological interfaces using the vibromechanical texturing method[END_REF] and abrasive jet machining [START_REF] Wakuda | Effect of sur face texturing on friction reduction between ceramic and steel materials under lubricated sliding contact[END_REF].

Surface texturing have many functionalities, e.g. in the seals industry, this technology is applied to mini mize friction and leakage in mechanical [START_REF] Etsion | Improving tribological performance of mech anical components by laser surface texturing[END_REF]/gas seals [START_REF] Feldman | A hydrostatic laser surface textured gas seal[END_REF][START_REF] Kligerman | Analysis of the hydrod yn amic effects in a surface textured circumferential gas seal[END_REF], in the automotive industry, it is used to reduce friction of piston rings [START_REF] Ronen | Friction-reducing sur face-texturing in reciprocating automotive com ponents[END_REF]. Furthermore, surface texturing has proved to be very effective in all lubrication regimes, particularly in the case ofhydrod yn amic bear ing systems [START_REF] Wang | Improving the anti-seizure ability of sic seal in water with rie texturing[END_REF], where it is employed to improve the load carrying capacity by providing supplementary hydrod yn amic pressure [START_REF] Hamilton | A theory of lubrication by micro-irregularities[END_REF], acting as lubricant reser voirs to ensure supply in cases of lubricant starvation [START_REF] Wakuda | Effect of sur face texturing on friction reduction between ceramic and steel materials under lubricated sliding contact[END_REF][START_REF] Pettersson | Influence of surface texture on boundary lubricated sliding contacts[END_REF], entrapping wear debris and abrasive particles to prevent severe wear [START_REF] Varenberg | Different aspects of the role of wear debris in fretting wear[END_REF][START_REF] Yamakiri | Effects of laser surface texturing on friction behavior of silicon nitride under lubrication with water[END_REF][START_REF] Etsion | Analytical and experimental investigation of laser-textured mechan ical seal faces[END_REF][START_REF] Etsion | Experimental investigation of laser surface textured parallel thrust bearings[END_REF][START_REF] Kovalchenko | The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact[END_REF].

The study of textured journal bearing has been the abject of numerous theoretical and experimental studies. Lu and Khonsari [START_REF] Lu | An experimental investigation of dimple effect on the stribeck curve of journal bearings[END_REF] presented a series of experimental results to examine the effect of fully and partially textured journal bearings on the Stribeck curve. They explored the effect of external force, oil type and texture parameters on the friction character istics. lt was shown that the friction coefficient in fully textured bearing, can be reduced, with the appro priate selection of texture size, depth and density. One of the first works focusing on the numerical aspect of textured journal bearings was conducted by Tala Ighil et al. [START_REF] Tala-Ighil | A numerical investigation of both thermal and texturing surface effects on the jour nal bearings static characteristics[END_REF][START_REF] Tala-Ighil | Effects of surface texture on journal-bearing characteristics under steady-state operating conditions[END_REF], where they used a finite difference model to solve the two-dimensional Re yn olds equation under steady-state conditions. Their objective was to investigate the effect of the spherical and cylindrical shapes on the bearing performance, where it was reported that textured surface affected the main bear ing characteristics. Depending on the dimple par ameters (size, depth, density and distribution), texturing can exert positive or negative effects. Further, they reported, that the partial texturing configuration including the texturing of multiple zones has a positive influence on hydrod yn amic contact characteristics. Another study was presented by Ausas et al. [START_REF] Ausas | The impact of the cavitation model in the analysis of microtextured lubricated journal bearings[END_REF], where they numerically investigated the impact of the cavitation model on the performance of textured jour nal bearings. A comparison was made between results obtained from the classical Reynolds model and the mass conserving algorithm proposed by Elrod and Adams [START_REF] Elrod | A computer program for cavi tation and starvation problems[END_REF] for fully textured journal bearings with rectangular dimples. A slight increase in the minimum film thickness and friction torque was observed. Another, numerical study, performed by Kango et al. [START_REF] Kango | Thermal analysis of microtextured journal bearing using non-newtonian rheology of lubricant and jfo boundary conditions[END_REF][START_REF] Kango | Comparative analy sis of textured and grooved hydrodynamic journal bearing[END_REF], shows the combined influences of viscous heat dissipation and non-Newtonian rheology of lubri cant on the performance of textured journal bearings. Spherical dimples using mass conservation algorithm and oil mixing temperature concepts were used. The finite difference technique for solving the Re yn olds and energy equations was adopted. It was noticed a reduction in average temperature of the lubricant in the presence of surface texturing in comparison to smooth surface of the bearing. Zhang et al. [START_REF] Zhang | Parametric design of surface textures on journal bearing[END_REF] pre sented a parametric design of the surface texturing on the journal bearing, with the aim of achieving the optimal textures distribution for improving the load capacity.

To explain the behaviour of the lubricant inside the micro-textured surfaces, several mechanisms have been proposed. The most common mechanisms were: the 'micro-wedge action' [START_REF] Hamilton | A theory of lubrication by micro-irregularities[END_REF][START_REF] Anno | Microasperity lubri cation[END_REF], the 'collective dimple effect' [31], the 'inlet roughness' [32,33], the 'load sup port mechanism ' [34,35], and the 'balancing wedge action ' [36]. Based on these mechanisms, Nanbu et al. [37] reported that textures with bottom shapes containing a micro-wedge and/or a micro-step bearing tend to increase film thicknesses and pressure lift. Recently, Uddin et al. [38] investigated the effects of surface texture shape, geometry, bottom profile, orien tation, depth and density on the tribological perform ances of parallel rotating thrust bearings. It was mentioned that considerable improvement in the bear ing performance is achieved by texture shapes with micro-wedge and/or micro-step features.

The above-mentioned works show that, the tribologi cal performances of textured surfaces are very sensitive to the texture mechanism. Indeed, some dimple shapes with micro-wedge and/or micro-step features perform better by increasing the load lifting capacity and lowering the friction force. No systematic study dealing with the wedge effect and/or contour geometries of texture shapes on the particular case of journal bearing appears to exist in the open literature. Thus, the objective of this numerical study is to bring elements of a response to the questions related to the effect of texture contour geo metries and bottom profiles on the main journal bearing characteristics (i.e. film thicknesses, pressure distri bution, load carrying capacity, friction coefficient, side leakage, etc), using Elrod's efficient mass conservation algorithm for implementation the JFO boundary conditions.

The content of this paper is organized as follows : Section 2 provides the description of the modified Rey nolds equation, cavitation boundary condition and the computational procedure. Section 3 is devoted to the numerical results and discussions, which contains the preliminary validation and performance analysis. Finally, conclusions are given in the last section.

Governing equations for hydrodynamic journal bearing

Reynolds equation and Elrod cavitation algorithm

In the lubrication theory, Re yn olds equation is employed for the computation of the pressure distri bution of thin viscous fluid films. Based on the assump tions [I] that : the fluid is Newtonian; isoviscous and incompressible under laminar flow, the Re yn olds equation for a journal operating at steady state regime (in Cartesian coordinates), can be written as:

where p : lubricant pressure, h : film thickness, µ : d yn amic viscosity, p : lubricant density and U : shaft speed.

In the full film zone, the density is constant and Equation (1) becomes:

__?__ ( __!__ h 3 a p ) + __?__ ( 2-h 3 a p ) = ! µ u ah ax 12 ax az 12 az 2 ax < 2)
In rupture zone, the pressure remains constant at the cavitation pressure. Hence, Equation (I) is reduced to:

�(phU) = 0 ax 2 (3)
In order to take into account the effect of film rupture and reformation, Jakobsson, Floberg [39] and Olsson [40] proposed the so-called JFO boundary conditions. The JFO boundary condition have been added to be more realistic then Re yn olds boundary condition and sufficiently accurate model to account for lubri cant cavitation, especially in: misaligned, d yn amical loaded and textured bearings [START_REF] Ausas | The impact of the cavitation model in the analysis of microtextured lubricated journal bearings[END_REF][START_REF] Kango | Thermal analysis of microtextured journal bearing using non-newtonian rheology of lubricant and jfo boundary conditions[END_REF]41].

To use the JFO boundary conditions, Elrod [START_REF] Elrod | A computer program for cavi tation and starvation problems[END_REF] suggested to combination of the two Equations ( 2) and ( 3) into a single 'universal' equation that covers both the cavitated and the full film regions, by introdu cing a new parameter called fractional film content = p/ P c , where P c is the cavitated oil density.

The resulting mass conservative form of the Reynolds equation where the unknown is the factional film content instead of the pressure can be written as follows:

where f3 is the bulk modulus which represents the vari ation of the lubricant compressibility in the full film region and the pressure p is given by: [START_REF] Wang | Improving the anti-seizure ability of sic seal in water with rie texturing[END_REF] and g( <9) is the switch fonction ( cavitation index) which is zero in the cavitation zone and equal to unity in the full film zone. g= 1 if@::: 1 g=O if<9<1 [START_REF] Stephens | Deterministic micro asperities on bearings and seals using a modified liga process[END_REF] After solving Equation (4) for 0, the pressure in the full film region can be determined using the following relation [START_REF] Elrod | A computer program for cavi tation and starvation problems[END_REF]:

P = Pc + g {3 ln (<9) (7)
where, Pc is the cavitation pressure. By introducing the following dimensionless par ameters:

-13c 2 0=x/R Z=z/L h= h /C y=y/ h /3= UR where, R: bearing radius, L: bearing length and C: radial clearance.

The dimensionless form of the modified Re yn olds equation becomes:

1 a ( --3 ae ) ( R )
2 1 a ( --3 ae ) 12 ae f3g h ae + L 12 az f3g h az 1 a( @h)

ae

and the dimensionless pressure is described by:

P = 1\ + gf3ln(<9)
where dimensionless film thickness is described by: (8) (9) h c0, Z) = hsmooth(0) + dh c0, Z) [START_REF] Etsion | Improving tribological performance of mech anical components by laser surface texturing[END_REF] where, hsmooth(0) denotes the smooth part of the film geometry (hsmooth( 0) = 1 + e cos 0) and dh c e, Z) is the part due to the textured surface, it characterizes the depth of the fluid film in the texture (Figure 1).

Texture shapes, geometries and configurations

In general, textures can be characterized by their shape (contour geometry, bottom profile), size (con tour dimension, depth), number (along axial/circum ferential directions), density and distribution. Based on textured thrust pad bearing studies reported in the literature, Zhang et al. [42] demonstrated that, square texture shape performed the best hydrod yn amic enhancement, followed by the triangle, circle and rectangle. While, Uddin et al. [38] showed that texture shapes with convergent (positive) wedge bot tom profile offer the best performance, while, texture shapes with divergent (negative) wedge bottom profile perform the worst. According to that reasoning, in this study, eight different texture shapes are con sidered and grouped into two main categories. Four texture shapes with constant dimple depth or fiat bot tom (dh = est): square, cylindrical, triangular and chevron fall within the first category. The second cat egory concerns another four texture shapes with vari able dimple depth or wedge bottom (dh =I est): TI, T2, T3 and spherical. It should be emphasized that the first category is chosen to highlight the effect of texture contour geometries at the bearing surface and the second one to examine the effect of bottom shape profiles. Texture geometries, parameters and equations are summarized in Tables 1 and2 texturing with one or multiple separated zones of the bearing surface. They reported that, in one sicle, full texturing have a detrimental effect, and the partial texturing, in the other sicle, has a positive influence. Furthermore, they reported that, the optimal region where texturing can provide more efficiency is Table 1. Texture profiles and geometric equations four shapes with fiat bottom profiles (adapted from [START_REF] Qiu | The effect of tex ture shape on the friction coefficient and stiffness of gas-lubricated parallel slider bearings[END_REF]). [44] studied fully and partially tex tured parallel slider bearings by applying a two dimensional mass-conserving form of the Reynolds equation. They showed that, in one hand, the fully textured surface can only generate load car rying capacities when the inlet is textured, on the other hand, the partially textured sliders are capable of creating substantial hydrod yn amic lift, especially when the supply pressure is higher than the cavita tion pressure and textures are placed at the inlet. Fur thermore, they demonstrated that the texture density cannot be optimized for the partially textured sliders, because the load carrying capacity always increases with density, resulting in an optimum density of 100%. However, a density of 100% actually results in a stepped slider and the term texture becomes questionable. In practice, that means that the optimal

Geometric Equations if { -fx S X S fx flh _ -2r, I I -r, s z s r, => W.zl ----,;-x + ry if { -fx S X S fx => !lh(e.z) = -t, X+ fy -r, s z sr, if { -r, :'.': x :'.': r, => fl h w.zl = t, x -r, s z sr, { fx = r, = r if ey+(â/J-ycl' +�= 1 'x � 'z _,_ !lh _ r ✓ 1 _ (x-xcl ' _ (z-zcl ' _,. (0.z) -y ------,, ------,,
texture density is the maximum achievable, limited by the fact that high densities may lead to stress concentration, weakening the surface. In this paper, four densities (20%, 40%, 60% and 80%) are considered.

Numerical procedure

Based on the mass conservation algorithm (JFO boundary conditions), the approach proposed by Vijayaraghavan and Keith [41] is used herein. The finite differences method with Gauss-Seidel iterative process (successive over-relaxation 'SOR') is employed to compute the fractional film content (@). Usually, an iterative loop with a convergence criterion is required when JFO boundary conditions are imposed [START_REF] Ausas | The impact of the cavitation model in the analysis of microtextured lubricated journal bearings[END_REF]. Moreover, this method is very easy to implement which makes it the preferred solver for small and medium-sized problems [45]. In order to improve the convergence speed and avoid the problem in stabilizing the cavitation zone, the --------�------------ modified switch function algorithm developed by Fesanghary and Khonsari [45] is used in the compu tational code, where the 'gfactor' is chosen from the range of O to 0.9, and then, the dimensionless pressure field in the full film region can be deter mined using the Equation ( 9). The computational process consists of:

z .Q Z=0��---------�---------�-0 0=360 ° a) z -EB ---EB ---EB ---EB ---EB - -EB ---EB ---EB ---EB ---EB - -EB ---EB ---EB ---EB ---EB - Z=O �-
• Introduction of input data: geometrical parameters (L, D, C, and e), operating conditions (N, µ and (3), texturing parameters (nC0> nC v r X> r v r y , 0 1 , 0 2 , z 1 and z2) and grid size and convergence criteria; • Input initial values of: fractional film content 6/ t o ,z) and binary switch function g?o ,z) ; • Compute the film thickness using Equation ( 10); • Solve the modified Reynolds Equation (8); • Imposition of JFO boundary conditions on affected nodes (Equation ( 6) ); • The fractional film content is obtained by verifying the following convergence condition: lii:, ,jl :s tal e at each node (i, j) of the bearing surface 'J mesh; • Once the convergence condition is satisfied, the pressure profile can be determined using Equation ( 9) and the bearing characteristics can be deduced.

The process described above corresponds to the resolution of direct problem whose eccentricity ratio E: is known, for the inverse problem which consist of determining the position (the relative position t:) of the centre journal in the bearing for an imposed applied load F.

The procedure consists of:

• Input initial values of: the eccentricity ratio Bo;

• Solve the direct problem for the value Bo;

• The calculated load carrying capacity W c is com pared with the applied load F; • Calculation of a new eccentricity ratio E: by the method of Brent [ 46]; • The process stopping after the load convergence condition IW r F I :':: tol w is satisfied.

W e present, the global computational procedure in the following flowchart (Figure 3).

Static characteristics of hydrodynamic journal bearing

Performance characteristics of the journal bearing for smooth/textured cases are computed using the follow ing relations:

----" \ \ 1 ---------------------- E Cl) :ë 0 i... p.. Cl) Cil i... Cl) • = / r 1 1 E Cl) 1 :ë 1 8 1 p.. 1 t 1 t 1 -:a 1 \ \ ,------------+! '

No

Compute the fractional film content in ail nodes by solving the modified Reynolds equation (8

)
Compute of pressure distribution i'i ce.zJ using equation ( 9) -✓-2 -2 (W0 )

1 1 1 1 1 1 1 1 1 1 1 ....... ____________ _ ________ _,
-• -•-•-•-•-•-•-•-•-•-• -•-•-•
Wc =W 0 + W z <p = arctan Wz (12)
The dimensionless lubricant axial film flow discharging

Evaluation of journal bearing performance characteristics

End from the two ends of the bearing, is given by:

The dimensionless friction torque acting on the journal surface is given by:

1 0 ( - ) _ s 1 aP ( 0 ,Z) - 1 Ft = J J ---h<0.zJ +=---d0dZ o o 2 a0 h<0.z) 1 2 - + J J "'� � , , Z l d0dZ O 0s h((J,Z) (14)
Where, h ( o,,z) is the equivalent of oil film thickness in cavitation zone. Consequently, the frictional coefficient is obtained as follows:

!(!!_ ) = Pt C Wc ( 15
)
All numerical integrations was carried-out by means of Simpson's I/3rd ru.le.

Numerical results and discussions

Preliminary validation

Based on the analysis described in the present paper, a Matlab computer program was developed. This section is devoted to the validation of our code. T o this end, a comparison analysis is performed between our results and those from researchers works [START_REF] Tala-Ighil | Effects of surface texture on journal-bearing characteristics under steady-state operating conditions[END_REF]47], for smooth/textured surfaces of the bearings. obtained by the computational code are plotted in Figure 4 in order to further check its validity. Note that, in fractional film content distribution, the area where e :=:::: 1 represents the full film region, and the area where e ::: 1 represents the cavitation region; the variation in fractional film content in full film region is very small due to the large bulk modulus. However, a small variation in fractional film content in the full film region causes large change in the pressure distribution, as a results of the large value of the compressibility. Due to the atmospheric pressure imposed on the axial boundary, the frac tional film content at z = 0 and z = L is assumed to be e = 1, while on circumferential boundary, is due to the bearing line groove, situated at 0 = 0 ° and extending at the ends of the bearing. The comparison results are plotted in Figure 5. The results of the present comparison are in good agreement with one slight difference which may be due to the computational errors in the simulation, especially the choice made concerning the numerical integration method, where in [START_REF] Tala-Ighil | Effects of surface texture on journal-bearing characteristics under steady-state operating conditions[END_REF] they used the rectangular integration method and in our model we used a more precise method i.e. Simp son' s I/3rd ru.le.

In order to further show the effectiveness of our computational code, Figure 6 presents the prediction results for the film thickness and the pressure distri bution for smooth/textured cases. The results in i: .

Performance analysis of textured journal bearing

In this section, our objective is to point out the prepon derant effect (in terms of tribological properties) between texture contour surface and dimple bottom profile.

Bearing geometrical characteristics, as well as tex turing parameters are given in Table 4. Note that, the bearing is with a line groove of negligible thickness, situated at the maximum film thickness and extending at the ends of the bearing.

In the next subsections, the obtained results obtained from the full-textured/smooth bearing cases are compared in terms of: maximum pressure, load carrying capacity, attitude angle, axial film flow, friction coefficient, film rupture angle, film thickness, fractional film content and pressure distributions. Moreover, the choice of the best texture shape depends on the following criteria: the increase in pressure and

Maximum pressure

The curves of dimensionless maximum pressure P max for various eccentricity ratios; are shown in Figure 7 (a,b) (for flat/wedge bottom profile cases). It is found that the rate of increase in P max for all considered cases is pronounced at higher values of eccentricity ratio E:. Furthermore, the previous fi gu res also show the comparative graph for smooth/textured surfaces with all studied texture shapes. Maximum pressure values are clearly different from those recorded for the smooth surface. However, it can be seen from Fi gu re 7(a) that the main feature of all texture shapes with flat bottom profiles is to reduce the maximum pressure, which is due to the sudden change in the gap, causes a much large film thickness divergence. In the opposite way, texture shapes with wedge bottom profiles (Figure 7(b)) gradually reduce pressure via their diverging leading wedges. While, the reduction rate of T3 is more severe because it has a longer diver gent wedge, which causes the highest film thickness divergence and largest pressure drop. Triangular shape T2 shows a significant increase in the maximum pressure P max, this singular behaviour is due to the highest pressure recovery given by the long convergent wedge of this texture shape, which results in shallow pressure drop and highest additional pressure gener ation in the lubricated area. These results are coherent with the analysis described in [37,38].

Load carrying and attitude angle

Figure 8(a,b) show the variation of dimensionless load carrying capacity W c with eccentricity ratio t:; for smooth and textured profiles. Firstly, the increase in W c has the same behaviour as for the previous dimen sionless maximum pressure (Figure 7(a,b)) for smooth and textured cases. Secondly, texturing the whole bearing surface does not enhance the load carrying capacity except for the T2 texture shape. This enhancement is a consequence of the convergent wedge action provided by T2. This can be explained by the fact that, in full film region, dimple shapes with fiat bottoms have a micro-step bearing feature. While, the dimple shapes with micro-wedge has a divergent/convergent wedge (an arc in SP or linear in Tl, T2 and T3). Due to these mechanisms, the micro-step bearing causes a much large film thickness divergence, so a significant pressure drop (in full film region) and a smaller pressure recovery will be generated (in rupture film region), which results in a poor load carrying capacity. Note that, the texture con tour geometry of the fiat bottom profile, in the full film region, has an important effect, as the texture contour geometries increases, the micro-step bearing mechanism effects increases (higher film thickness divergence and larger pressure drop). In one hand, the comparison shown in Figure 8(b ), indicates that the same reduction rate in load carrying capacity, compared to the smooth surface, for textured bearing with triangular shape T 1 and spherical is observed. This can be justified by the fact that the two shapes are affected by the same mechan ism, the micro-wedge action, with a slight difference in geometrical bottom profile ( an arc in SP and a broken line in Tl). In the other hand, the two extreme trends cor responding to the load carrying capacities of T2 and T3 texture shapes are also due to the micro-wedge action mechanism, which is positive ( convergent) in the first case (T2) and negative (divergent) in the second case (T3), these results are consistent with those described in [34,35]. Here, an emphasis is placed on the rotation direction, which present a key parameter in the wedge profile orientation (convergent or divergent). Figure 9(a,b) present the variation of the load atti tude angle cf> against the eccentricity ratio t: for smooth/textured surface bearings. It can be discerned from the figures, that for any texture shapes, the values of load attitude angle are obviously different from those of smooth surfaces. For all studied cases, cf> decreases with the increase in c, this decrease is more consider able at higher values of eccentricity ratio to support the large bearing load. However, for all considered texture shapes with fiat bottom profiles (Figure 9(a)), l i;:: the load attitude angle slightly increases compared to the smooth surface bearing case. These results can be explained by the presence of micro-step bearing mech anism, which causes a pressure drop and makes its distribution more asymmetric on the circumferential direction. In Figure 9(b), the effect of texture bottom profiles on load attitude angle is more noticeable, where, it adopts two different behaviours: increases for spherical and Tl and even more for T3 profiles and decreases for T2, when compared to smooth sur face. This is due to the micro-wedge bearing effect, which causes a large pressure drop in the case of diver gent bottom profile: spherical, Tl and T3, and a high pressure recovery in the case of convergent bottom: T2, giving more symmetric pressure distribution on the circumferential direction, and leading towards nil load attitude angle.

Axial film flow

The evolution of the dimensionless axial film flow Oz for different values of eccentricity ratio t: in the cases of smooth and textured surface bearing are shown in Figure lO(a,b). In Figure lO(a), it is noticed that for all considered configurations (smooth and textured surface bearing with constant/fiat dimple depths), the axial film flow increases with the increase of t:, which can be justified by the large pressure gradient on the both ends, and it is also visible that textures with fiat bottom profiles reduce the axial film flow, as a result of micro-step bearing mechanism effects. In Figure lO(b), the same evolution is observed for all texture shapes with wedge bottom profiles except for T2 texture, which has an important axial flow when the eccentricity ratio is less than 0.75, and decreases for e > 0.75, compared to smooth bearing case.

This can be explained by the fact that, at low eccentricity ratios, the additional hydrod yn amic pressure generated by the long convergent wedges of the triangular T2 textures in the lubricated area is more important, which increases the bearing pressure gradient and so increases the axial film flow. At high eccentricity ratios (e > 0.75) the effect of T2 texture shape is negligible when compared with the effect of bearing convergent zone, in this case, the T2 texture shape adopt the same behaviour as the other textures. In order to clarify the previous behaviour of T2 tex ture, a variation analysis of the main components of axial film flow Equation ( 13) (pressure gradient -3 aP �r d) , film thickness h ( 0,end)' radial direction y(y-I))

will be presented in Figures 11 and12. makes it independent to the surface' s nature and the eccentricity ratios. The curves of dimensionless pressure gradient at bearing end is presented in Figure 12(a) for smooth and T2 texture shape, at e = 0.6 and e = 0.8, it is well observed that the areas under the curves of T2 are important compared to smooth case, which in the fact, confirms the previous finding of dimensionless maximum pressure (Figure 7(b)) and load capacity (Figure 8(b)).

The variation of the dimensionless axial film flow gradient, which is in fact resulting from the multipli cation of the previous components entities, is shown in Figure 12(b). It is noticed that, at e = 0.6; the area in the case of smooth surface is under

Friction coefficient

The dimensionless friction coefficients f (R/ C) for smooth/textured surface bearings are plotted in Figure 13(a) (for flat/wedge bottom profile cases, respectively). It can be noticed that, compared to the maximum pressure (Figure 7(a,b)) and load carrying capacity (Figure S(a,b)), the resulting friction coeffi cients values have an opposite trend. Moreover, it can be also observed that, the friction coefficient f (R/C) decreases with the increases of eccentricity ratio t:, for all studied cases. However, in the case of textured bear ings with fiat bottom profiles, the friction coefficients shallowly increases compared to the smooth surface case, presenting a bad performance. This is due to the fact that the fiat bottom profiles causes a pressure drop, which decreases the load carrying capacity, and so increases the friction values. In the other case of tex tured bearings with wedge bottom profiles, the triangu lar shape T2 with convergent (positive) wedge action shows the greatest friction reduction. This is due to the increase in load carrying capacity, caused by the highest pressure recovery. The triangular shape T3 with divergent (negative) wedge action performs the worst friction value.

In the following sections, the eccentricity ratio will be fixed at e = 0.6, in order to present properly the results of film thickness evolution, fractional film con tent distribution and pressure distributions.

Film thickness, fractional film content and pressure distributions

For the sake of clear representation, Figure 14 shows the three-dimensional distribution of lubricant film thickness computed at the contact interface for all studied cases. This figure shows that the film thick ness evolution for smooth (Figure 14 Figure 15 shows the variation of fractional film content in circumferential (at the mid-plane) direction, for smooth and textured surface bearing, with constant (fiat bottom) and variable (wedge bot tom) dimple depths. In the case of texture shapes with fiat bottom profiles (Figure 15(a)), it can be seen that ail curves of texture fractional film content are less than the smooth surface, as a results of micro-pressure drop eff ect of these textures, it is also very clear that as the texture contour geometry increases, the micro-pressure drop effect increases. U nlike the previous case, a considerable eff ect of texture shapes with wedge bottom profiles (Figure 15 (b)) on fractional film content is clearly noticed, especially in the case of triangular shape T2, which presents a significant improvement in the film fluid area (increasing the value of rupture angle).

The effect of texture shapes with flat/wedge bottom profiles on the dimensionless pressure profile is visu alized in Figure 16(a,b), respectively. Each figure is plotted in circumferential (at the mid-plane, case (c)) and axial (at the maximum pressure plane e;,, case ( d)) directions. The dimensionless film pressure for all texture shapes with fiat bottom profiles (Figure 16(a)) are less than the smooth surface film pressure, which confirm the influence of micro-step bearing mechanism. Moreover, it can be noticed that the maximum pressure plane for all textures profiles is constant at e<;, = 146.45 °.

The results shown in Figures 16(b) for all texture shapes with wedge bottom profiles are the same as in the previous figure, except for some minor differences:

• The dimensionless pressure profile and the rupture angle of T2 texture shape are sharply higher than other cases, • Sorne fluctuations appeared on the Tl and T2 pressure profiles,

• The maximum pressure planes e<;, increase in the case of triangular texture shape with convergent wedge T2 and decrease in T3 triangular texture shape with divergent wedge.

These differences are due to the convergent and/ or divergent wedge bottom profiles micro-wedge bearing mechanism, that directly affect the pressure distribution.

Effect of texture density and location

In the previous sections, the whole bearing surface was textured (from 0 ° to 360 °) and the texture density P tex • which is the surface occupied by dimples units per the total bearing surface, was set to 40%. Here, two texture confi gur ations (Fi gur e 2) and four texture densities are ------------, -------.------,---------- considered, including the previous one of 40% and 20%, 60% and 80%. Computed characteristics corresponding to each texture density are summarized in Table 5. In full textured cases, the result observed is that: except T2 texture shape, ail textures have a detrimental effect (redu cing the load carrying capacity and increasing the friction coefficient). Where, for fiat bottom profiles, the square texture performs the worst eff ect, due to the larger contour geometry followed by the circle, triangle and chevron. While, in wedge bottom profiles, T2 performs the best and T3 the worst, as a results of convergent/divergent wedge profiles, which can positively or negatively affects the tribological characteristics of the journal bearing. This was confirmed by simulation results shown in Wedge effect mechanism (micro-wedge action) has produced tangible results in the particular case of textures with variable dimple depths or wedge bottom profiles (SP, Tl, T2 and T3 shapes). A pressure reduction has been noticed for textures with divergent wedge profiles (SP, Tl and T3), while the opposite trend is observed for texture with convergent wedge shape (T2). The produced pressure in T2 texture shape enhances the load lifting capacity, while the divergent film in other textures causes a net load loss. Micro-step bearing mechanism, with the right arrangement of textures on the contact surface, can significantly enhance the bearing performances, by the effect of texture micro-pressure recovery mechanism.

Conclusion

In this paper, a numerical study was conducted to high light the potential of surface texturing on journal bear ing performances. Texture shapes with (1) fiat bottoms: square, cylinder, triangle (TR) and chevron; and (2) wedge bottoms: triangular (Tl, T2, T3) and sphere were considered. The computational model employed herein was solved by means of finite difference tech nique with taking into account the lubrication film rup ture and reformation by adopting the mass-conserving cavitation algorithm ('JFO' boundary conditions). The simulations made with our numerical model were compared to the existing theoretical data, available in the literature. The findings from the present numerical investi gation can be summarized in the following points:

( 1) The geometric shape of the texture has proved to be a k ey parameter in journal bearing performances. (2) Pully textured bearing with fiat/constant bottom dimples slightly affect the main performances as compared to the smooth bearing case, which can be explained by the effect of micro-step mechan ism (micro-pressure drop effect), Moreover, as the texture contour geometry increases, this mech anism effect increases. While, textures with wedge/ variable bottom profiles significantly influence ail bearing characteristics, which is directed by the micro-wedge mechanism effect. (3) Partial textured cavitation bearing area with con stant dimple depths enhance the load lifting capacity and lower the friction coefficient, due to the eff ect of micro-pressure recovery mechanism. These facts were more noticeable at high texture densities. While, textures with divergent bottom profiles causes a net load loss. (4) Among all considered cases of texture shape, T2 appears to be the most favourable to improve the bearing performances; e.g. for a texture density of 40%, it increases the pressure field by a factor of 5 to 6 over the value of the smooth one which in turn increases by 43% the load carrying capacity and decreases the friction coefficient by 26%. While T3 texture shape, presents the worst performances. (5) The previous observations concerning T2 texture shape are due to the convergent wedge effect, while, texturing the second angular part of the bearing (beyond 180 °) by textures with fiat bottom profiles, are due to the micro-pressure recovery effect, which generates additional hydrodynamic pressure and serve as fluid reservoirs that provides lubricant to the contact in starvation cases, and so, enhance the main bearing performances. [START_REF] Stephens | Deterministic micro asperities on bearings and seals using a modified liga process[END_REF] The square 'SQ' texture shape appears to be the most favourable to improve the bearing perform ances, followed by the triangular 'TR' then chevron 'CH' and cylindrical 'CY'.

Figure 1 .

 1 Figure 1. Representative scheme of: (a) straight section of the textured journal bearing; (b) detailed section for the texture scheme.

Table 2 .

 2 :': X :': fx =} lih ( 0,z) = fy -r, :s z :sr, 'f { fx = r, = r 1 (x -XcJ 2 + (z -Zef = f 2 iff,;x ::S z ::S -tx+c, if { -fx :': X :': fx -r, :s z :sr, if fx { --"-2 ' X + r, -Cr :': Z -f,;x+rz �z else Texture profiles and geometric equations for shapes with wedge bottom profiles (adapted from [37]). at the second half of bearing surface, where the area under the declining part of the pressure field is situated. In this study, two texture confi gu r ations, as shown in Figure 2, are considered. Dobrica et al.

Figure 2 .

 2 Figure 2. Illustration of textures distribution in the circumferential and axial directions on the bearing surface: (a) full textured case (0 ° to 360 °); (b) partial textured case (180 ° to 360 °).

  size and convergence criteriaInitializationFractional film content 0ca.z), Binary switch function g(9,z), Eccentricity ratio E 0 Compute of film thickness hce.z) using equation[START_REF] Etsion | Improving tribological performance of mech anical components by laser surface texturing[END_REF]

  carrying capacity W c by numerical integration of the pressure fields with Simpson's 1/3rd ru/e / / / •

Figure 3 .P

 3 Figure 3. Flowchart for resolution of hydrodynamic problem.The dimensionless load components in the circum ferential and the axial direction are given as:

Case 1 :

 1 Smooth surface Parameters adopted for this comparison are: • Bearing data: (a) amplitude of external load F = 20000 N; (b) journal diameter D = 40 mm; ( c) bear ing length L = 80 mm; ( d) radial clearance C = 30 µm; (e) Shaft speed N = 2500 rpm; (f) lubri cant viscosity = 0.02 Pa s; (g) bulk modulus f3 = 5 GPa. • Computational method parameters: (a) finite-differ ence method (FDM) with JFO boundary conditions (b) grid size (91, 41); (c) convergence criteria tal e = 10-6 , tol w = 10-4 _ Table 3 presents the comparison between our pre dictions and those of the already mentioned paper [47]. It can be noted that the obtained values are very close to the reference ones. The two dimensional results of film thickness, frac tional film content and pressure distributions (a) (b)

Case 2 :

 2 Textured surface Parameters adopted for this case are: • Bearing data: (a) amplitude of external load F = 12600N; (b) journal diameter D = 63 mm; (c) bear ing length L = 63 mm; (d) radial clearance C = 30 µm; (e) angular speed w 1 = 625.4 radis; (f) lubricant viscosity = 0.0035 Pa s. • Computational method parameters: (a) finite differ ence method (FDM) with Reynolds boundary con ditions (b) grid size (391, 142); (c) convergence criteria tol p = 10-4 and tol w = 10-5 _ • Bearing texture data: (a) spherical texture shape (b) texture radius r x = r z = 5 mm; (c) texture depth r y = 1 µm; (d) distribution along: circumferential nC0 = 1, 2, ... , 9 and axial nCz = 2 directions.

Figure 4 .

 4 Figure 4. Film thickness evolution, fractional film content and pressure distributions on the contact for the validation case.

Figure S .

 S Figure S. Variation of: (a) maximum pressure; (b) minimum film thickness for smooth and textured surfaces.

Figure 6 (

 6 Figure 6(b) show very clearly the effect of textures on film thickness evolution and pressure distribution, when compared to smooth case (Figure 6(a)).

  and the decrease in load attitude angle and friction coefficient.

Figure 6 .

 6 Figure 6. Film thickness evolution and pressure distribution on the contact for smooth/textured journal bearing for the validation case. (a) smooth journal bearing and (b) textured journal bearing.

Table 4 .

 4 Geometrical and Texturing parameters for the studied journal bearing.

  l ....... -�J î �i"""' ....... ("'";' '='=�����=-----"--

Figure 7 .

 7 Figure 7. Dimensionless maximum pressure versus eccentricity ratio for smooth/textured surface bearings. (a) fiat bottom cases and (b) wedge bottom cases.

Figure 8 .Figure 9 .

 89 Figure 8. Dimensionless load carrying capacity versus eccentricity ratio for different texture profiles. (a) fiat bottom cases and (b) wedge bottom cases.

Figure 10 .Figure 11 .

 1011 Figure 10. Dimensionless axial flow versus eccentricity ratio for different texture profiles. (a) Textures with a constant/fiat dimple depths and (b) Textures with a variable/wedge dimple depths.

  Figure ll(a) is concern the bearing end at z = L, which is the same in both cases ( smooth and T2 tex ture shape), while in Figure ll(b), results are about the variable y ( dimensionless distance between the shaft and the bearing ranging on [0, 11) which

Figure 12 .

 12 Figure 12. Variation of the dimensionless pressure gradient and axial film flow gradient component at the bearing end for smooth surface and T2 texture shape. (a) dimensionless pressure gradient and (b) dimensionless axial film flow gradient.

Figure 13 .

 13 Figure 13. Friction coefficient versus eccentricity ratio for studied texture profiles. (a) fiat bottom cases and (b) wedge bottom cases.

  (a)) and textured (with fiat, Figure 14(b), and wedge, Figure 14(c), bot tom profiles) surface bearings remain constant in the axial direction, and, its minimum value occurs near 0 = 181 °.

Figure 14 .

 14 Figure 14. Evolution of dimensionless film thickness versus axial and circumferential coordinates for smooth and different texture profiles. (a) smooth surface bearing, (b) fiat bottom profile cases and (c) wedge bottom profile cases.

1. 2

 2 .-

Figure 15 .

 15 Figure 15. Effect of various texture shapes with a fiat bottom profile on fractional film content distribution along the circumfer ential direction at bearing mid-plane. (a) fiat bottom cases and (b) wedge bottom cases.

Figures 7 --Figure 16 .

 716 Figure 16. Effect of various texture shapes with a flat/wedge bottom profiles on dimensionless pressure profile. (a) fiat bottom profile and (b) wedge bottom profile.

Table 3 .

 3 Comparison of performance characteristics of smooth journal bearing.

			Present study Jang and Khonsari [47]
	Eccentricity ratio	e	0.395	0.392
	Maximum pressure P m ,x (MPa)	5.799	5.90
	Leakage flow-rate	Q (cm 3 /s)	3.153	2.9
	Attitude angle	<po	67.618	69.1
	Friction coefficient	f	0.0037	0.0039

Table 5 .

 5 Variation of tribological characteristics for full/partial textured bearings with different texture densities.

	L/D = 1

This study is restricted to the selected texturing par ameters; fixed density, number, disposition, orientation and dimension; and the chosen problem assumptions; by applying Newtonian rheology and mass-conserving cavitation and neglecting changes in viscosity, density and temperature, etc. On one hand, breaking these con straints will open to us new possibilities of finding the opti mum texture shapes, on the other hand, difficulties on the computational aspect will be faced (CPU-time and cost). In this context, the application of model order reduction (MOR) [ 49] for the simulation of textured journal bearing is planned as the next step of this research.

Disclosure statement

No potential conflict of interest was reported by the authors.


Chantha Oeurng

Sabine Sauvage
email: sabine.sauvage@ensat.fr

Alexandra Coynel

Eric Maneux

Henri Etcheber

José-Miguel Sánchez-Pérez

Fluvial transport of suspended sediment and organic carbon during flood events in a large agricultural catchment in southwest France

Keywords: agricultural catchment, suspended sediment, dissolved organic carbon, particulate organic carbon, flood events, hysteresis

Water draining from a large agricultural catchment of 1 110 km 2 in southwest France was sampled over an 18-month period to determine the temporal variability in suspended sediment (SS) and dissolved (DOC) and particulate organic carbon (POC) transport during flood events, with quantification of fluxes and controlling factors, and to analyze the relationships between discharge and SS, DOC and POC. A total of 15 flood events were analyzed, providing extensive data on SS, POC and DOC during floods. There was high variability in SS, POC and DOC transport during different seasonal floods, with SS varying by event from 513 to 41 750 t; POC from 12 to 748 t and DOC from 9 to 218 t. Overall, 76 and 62% of total fluxes of POC and DOC occurred within 22% of the study period. POC and DOC export from the Save catchment amounted to 3090 t and 1240 t, equivalent to 1Ð8 t km 2 y 1 and 0Ð7 t km 2 y 1 , respectively. Statistical analyses showed that total precipitation, flood discharge and total water yield were the major factors controlling SS, POC and DOC transport from the catchment. The relationships between SS, POC and DOC and discharge over temporal flood events resulted in different hysteresis patterns, which were used to deduce dissolved and particulate origins. In both clockwise and anticlockwise hysteresis, POC mainly followed the same patterns as discharge and SS. The DOC-discharge relationship was mainly characterized by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment.

INTRODUCTION

Studies of fluvial suspended sediment and organic carbon transport through streams and rivers provide information on the rate of continental erosion, global carbon cycling and the contribution of terrestrial carbon to aquatic systems and oceans [START_REF] Meybeck | Carbon, nitrogen and phosphorus transport by World Rivers[END_REF][START_REF] Meybeck | Riverine transport of atmospheric carbon: sources, global typology and budget[END_REF][START_REF] Robertson | A scoping study into the role, importance, source, transportation and cycling of carbon in the riverine environment[END_REF][START_REF] Sarin | Significance of riverine carbon transport: A case study of a large tropical river. Godavari (India)[END_REF]. The transportation of organic carbon from terrestrial ecosystems by rivers and hydrological fluxes to the oceans plays an important role in regional budgets of organic carbon entering the continent-ocean interface [START_REF] Sarin | Significance of riverine carbon transport: A case study of a large tropical river. Godavari (India)[END_REF]. At the terrestrial scale, the previous estimations of global fluxes of organic carbon brought by the rivers are in the order of 400 ð 10 6 C per year in which 170-195 ð 10 6 C in particulate form [START_REF] Ludwig | Predicting the oceanic input of organic carbon by continental erosion[END_REF][START_REF] Meybeck | Global transfer of carbon rivers[END_REF] and 200-215 ð 10 6 C in dissolved form [START_REF] Meybeck | Global transfer of carbon rivers[END_REF].

Intensive agriculture has led to environmental degradation through soil erosion and carbon losses from agricultural land to stream networks [START_REF] Sharma | Streamflow, sediment and carbon transport from a Himalayan watershed[END_REF]. Suspended sediment (SS) transport from agricultural catchments to watercourses is responsible for aquatic habitat degradation, reservoir sedimentation and the transport of sediment-associated pollutants (pesticides, particulate nutrients, heavy metals and other toxic substances) [START_REF] Valero-Garcés | Sediment sources and siltation in mountain reservoirs: a case study from the Central Spanish Pyrenees[END_REF][START_REF] Heaney | Impacts of agriculture on aquatic systems: lessons learnt and new unknowns in Northern Ireland[END_REF][START_REF] Verstraeten | Regional scale variability in sediment and nutrient delivery from small agricultural catchments[END_REF]. Total organic carbon (TOC), comprising dissolved organic carbon (DOC) and particulate organic carbon (POC), is not only an important factor in stream water quality, but also an indicator of organic contamination [START_REF] Ni | Riverine inputs of total organic carbon and suspended particulate matter from the Pearl River Delta to the coastal ocean off South China[END_REF]. There is a general lack of studies determining organic carbon concentrations and fluxes in lowland agricultural catchments, particularly during flood events where there are many difficulties such as spatiotemporal variability in climatic conditions, different land uses and soil textures. Studies on river ecosystems have demonstrated that river discharge, primary production and litter pool sizes in catchments and the type and extent of agriculture in catchments are major processes influencing organic carbon fluxes in rivers [START_REF] Robertson | A scoping study into the role, importance, source, transportation and cycling of carbon in the riverine environment[END_REF]. Agriculture can significantly affect hydrological processes and organic carbon and nutrient transport in many ways. For instance, land use changes and tillage practices affect the hydrological response of a system, and thus, nutrient flux through changes in land cover, infiltration, evapotranspiration and soil characteristics (Roberstson et al., 1996). These changes are followed by feedback mechanisms for water, organic carbon and other chemical substances that bring further changes in these linked processes [START_REF] Alexander | Country-level estimation of nitrogen and phosphorus fertilizer use in the United States, 1945 to 1985[END_REF].

There is a wide range of existing literature investigating fluvial export of organic carbon from peatland environments [START_REF] Hope | Exports of organic carbon from two river systems in NE Scotland[END_REF][START_REF] Dawson | A comparison of particulate, dissolved and gaseous carbon in two contrasting upland streams in the UK[END_REF][START_REF] Worrall | Carbon budget for a British upland peat catchment[END_REF][START_REF] Pawson | Fluvial organic carbon flux from an eroding peatland catchment, southern Pennies, UK[END_REF]. Similar studies have been conducted in forest environments [START_REF] Meybeck | Riverine transport of atmospheric carbon: sources, global typology and budget[END_REF][START_REF] Molot | Storage of terrestrial carbon in boreal lake sediments and evasion to the atmosphere[END_REF][START_REF] Kao | Fluxes of dissolved and non fossil particulate organic carbon from an Oceania small river (Lanyang His) in Taiwan[END_REF][START_REF] Meybeck | Global transfer of carbon rivers[END_REF][START_REF] Shibata | Dissolved and particulate carbon dynamics in a cool-temperate forested basin in northern Japan[END_REF]. However, little attention has been paid to fluvial transport of organic carbon in large agricultural catchments, particularly during flood events when sediment transport can be significant.

The Gascogne area of southern Europe encompasses highly contrasting zones with various climatic influences (mountains, the Atlantic and the Mediterranean) and is dominated by anthropogenic activities, particularly intensive agriculture, causing severe erosion in recent decades. This is posing a major threat to surface water quality, since sediment transport within the catchment is the main factor mobilising aquatic contaminants and associated POC. For example, [START_REF] Oeurng | Dynamics of suspended sediment transport and yield in a large agricultural catchment, southwest France[END_REF] showed that sediment export during floods in the Save agricultural catchment in 2007 and 2008 represented 85 and 95% of annual loads (16 and 20% of annual duration), respectively. Within these floods, there was one extreme event which transported 63% of the total load. Moreover, [START_REF] Pawson | Fluvial organic carbon flux from an eroding peatland catchment, southern Pennies, UK[END_REF] found that POC export from a peatland catchment in southern Pennines, UK, accounted for 95% of flux in only 8% of the total study period. These results demonstrate the major role of floods in delivering sediment associated with POC transport from catchments. During flood events, hysteresis effect is often observed in sediment/nutrient concentrations and discharge relationships [START_REF] Asselman | Suspended sediment dynamics in a large basin: the River Rhine[END_REF]. When the concentration peak at the rising limb arrives before the discharge peak, it describes a clockwise hysteretic loop. When it arrives after the discharge peak, it describes an anticlockwise hysteretic loop [START_REF] Williams | Sediment concentration versus water discharge during single hydrologic events in rivers[END_REF]. However, when there are multiple peaks within a flood event, a complicated mix of clockwise and anticlockwise hysteretic loops occurs. Hysteresis patterns have been used in previous studies to indicate changing sources of sediment and nutrient supply to rivers during flood events [START_REF] Lefrançois | Suspended sediment and discharge relationship to identify bank degradation as a main sediment source on small agricultural catchments[END_REF][START_REF] Nadal-Romero | Relationships among rainfall, runoff, and suspended sediment in a small catchment with badlands[END_REF][START_REF] House | Hysteresis of the solute concentration/discharge relationship in rivers during storms[END_REF][START_REF] Bowes | Phosphorus discharge hysteresis during storm events along a river catchment: the River Swale[END_REF][START_REF] Stutter | Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events[END_REF].

The overall aim of the present study was to gain a deeper understanding of fluvial transport of SS and TOC from a large agricultural catchment during flood events. Specific objectives were to Study the temporal variability in suspended sediment, POC and DOC transport during flood events, including quantification of fluxes and controlling factors Analyze the relationship between discharge and SS, DOC and POC concentrations.

MATERIALS AND METHODS

Study area

The Save agricultural catchment is located in the area of Coteaux Gascogne, with an area of 1110 km 2 (Figure 1). The Save river has its source in the piedmont zone of the Pyrenees Mountains (southwest France) at an altitude of 600 m, joining the Garonne River after a 140 km course with a linear shape and an average slope of 3Ð6‰.

This catchment lies on detrital sediments from the Pyrenees Mountains. It is bordered on the east by the Garonne River, on the south by the Pyrenees and on the west by the Atlantic Ocean. Calcic luvisols (UN FAO soil units) have developed on the tertiary substratum, and local rendosols on the hard calcareous sandstone beds. The calcic cambisols that developed on hillsides with very gentle slopes have been subjected to moderate erosion. Calcic soils represent dominantly more than 90% in the whole catchment with a clay content ranging from 40 to 50%. Non-calcic silty soils, locally named boulbènes, represent less than 10% of the soil in this area (50-60% silt) [START_REF] Revel | Erosion due to cultivation of calcareous clay soils on the hillsides of south west France. I. Effect of former farming practices[END_REF]. The upstream part of the catchment is a hilly agricultural area mainly covered with pastures and little forest, while the lower part is flat and devoted to intensive agriculture, mostly sunflower and winter wheat in rotation (90% of the area used for agricultural purposes) (Figure 1).

The climatic conditions are oceanic, with annual precipitation of 700-900 mm and annual evaporation of 500-600 mm. The dry period runs from July to September (the month with maximum deficit) and the wet period from October to June. The mean temperature of the catchment is 13 °C, with a minimum in January (5 °C on average) and a maximum in August (20 °C on average). The hydrological regime of the catchment is mainly pluvial, i.e. regulated by rainfall, with maximum discharge in May and low discharge during summer (July to September). The catchment substratum is relatively impermeable due to its high clay content, and consequently, river discharge is mainly supplied by surface and subsurface runoff, while groundwater is limited to alluvial and colluvial phreatic aquifers [START_REF] Echanchu | Géochimie des eaux du basin de la Garonne[END_REF]. The maximum instantaneous discharge in the past 40 years was 620 m 3 s 1 (1 July 1977). During low flow periods, the Save River is sustained by about 1 m 3 s 1 from the Neste canal at the upstream area.

Instrumentation and sampling method

A Sonde YSI 6920 (YSI Incorporated, Ohio, USA) measuring probe and Automatic Water Sampler (ecoTech Umwelt-Meßsysteme GmbH. Bonn, Germany) with 24 1litre bottles has been installed at the Save catchment outlet (Larra bridge) since January 2007 for water quality monitoring. The Sonde was calibrated at the laboratory for turbidity with two points (0 and 1000 NTU) and recalibrated each three months in order to avoid sensor derivation. The Sonde is positioned near the bank of the river under the bridge, where homogeneity of water movement is considered appropriate for all hydrological conditions. The pump inlet is placed next to the Sonde pipe. The turbidity and water level are recorded at 10-min intervals. The turbidity values in water are detected by sensor on the Sonde YSI and the data are then transferred to the ecoTech memory. The Sonde is programmed to activate the automatic water sampler to pump water at water level variations ranging from 10 to 30 cm, depending on seasonal hydrological conditions for both the rising and falling stage [START_REF] Oeurng | Dynamics of suspended sediment transport and yield in a large agricultural catchment, southwest France[END_REF]. This sampling method provides high sampling frequency during storm events (3 min to 24 h per sample during floods). In the present study, manual sampling was also carried out using a 2-litre bottle lowered from the Larra bridge, near the Sonde position, at weekly intervals when water levels were not markedly varied. A total of 208 water samples were taken by automatic and manual sampling during the study period (January 2008 to June 2009).

Data sources and treatment

Hydro-meteorological data. Hourly rainfall data from five meteorological stations in the catchment (Figure 1) were obtained from Meteo France. Data on mean total rainfall depth and intensity in the whole catchment were derived using the Thiessen Polygon method [START_REF] Thiessen | Precipitation averages for large areas[END_REF]. Data on hourly discharge at Larra hydrometric station were obtained from CACG (Compagnie d'Aménagement des Coteaux de Gascogne), which is responsible for hydrological monitoring in the Gascogne region. The discharge was plotted by the rating curve in which water level was measured hourly by pressure with the form of a rectangular weir (length 12 m), then transferred by teletransmission.

Laboratory analysis. Water samples pumped by automatic sampling were generally collected from the field once a week, but during high flood periods they were collected twice a week. The water samples were filtered in the laboratory using pre-weighed glass microfibre filter paper (Whatman GF/F 0Ð7 µm). Volumes of water ranging from 150 to 1000 ml were filtered according to SS concentration (SSC). The sediment retained on the filter paper was dried for 48 h at 60 °C to ensure accurate sediment weight. The filters were then weighed to determine SSC.

-Sediment analysis for POC The dried filters containing SS (4 to 150 mg) were acidified with HCL 2N in order to remove carbonates and dried at 60 °C for 24 h. POC analyses were carried out using a LECO CS200 analyzer [START_REF] Etcheber | Particulate organic carbon in the estuarine turbidity maxima of the Gironde, Loire and Seine estuaries: origin and lability[END_REF]. POC content is expressed as a percentage of dry weight of sediment (abbreviated to POC%), and POC concentration as expressed in mg l 1 . -Water analysis for DOC The water samples filtered through 0Ð7 µm filter paper were acidified with HCL (12N; pH D 2) and kept cold at 4 °C until analyses were performed as soon as possible. The analyses were carried out with a Shimadzu TOC-5000 analyzer using the high-temperature catalytic oxidation method (HTCO).

Calculation of fluxes. Continuous data on SSC were generated from the relationship between SS and turbidity, with the interpolation method used for missing points [START_REF] Oeurng | Dynamics of suspended sediment transport and yield in a large agricultural catchment, southwest France[END_REF]. The SS load was calculated using high data resolution. The organic carbon flux for flood events and annual period was calculated using the [START_REF] Walling | Estimating the discharge of contaminants to coastal waters by rivers: Some cautionary comments[END_REF] method recommended by the Paris Commission for estimating river loads

Load D V ð n iD1 Ci ð Qi n iD1 Qi
Where Ci is the concentration for each instantaneous sample point (mg l 1 ), Qi is the discharge at each sampling point (m 3 s 1 ), V is the water volume over the period considered (m 3 ) and n is the number of samples. This is the preferred method for flux estimates given the available data [START_REF] Littlewood | Estimating constituent loads in rivers: a review[END_REF] and is common in the literature for estimates of organic carbon loads (e.g. [START_REF] Hope | Exports of organic carbon from two river systems in NE Scotland[END_REF][START_REF] Dawson | A comparison of particulate, dissolved and gaseous carbon in two contrasting upland streams in the UK[END_REF][START_REF] Worrall | Carbon budget for a British upland peat catchment[END_REF][START_REF] Worrall | Predicting the future DOC flux from upland peat catchments[END_REF].

Statistical analyses

Statistical analyses were performed using statistical techniques (Pearson correlation matrix) and Principal Component Analysis (PCA) by the STATISTICA package. The relationships between SS, POC, DOC and hydro-climatological variables were analyzed in order to determine the factors controlling SS, POC and DOC transport during flood events. A database was generated for each flood event and contained two main groups of variables: antecedent variables to the flood conditions and flood variables (precipitation, discharge, sediment and organic carbon) during the events (Table I). The antecedent variables used were accumulated precipitation one day before the flood (P1d, mm), five days before (P5d), and ten days before (P10d); initial baseflow (Qb) before the flood started; and the antecedent flood corresponding to the current flood (Qa).

A Pearson correlation matrix and factorial analysis that included all the above mentioned variables (Table I) were generated for 13 flood events (event 1 excluded due to lack of DOC and POC data). Event 4 (1 June 2008) was also excluded from the matrix because it was an extraordinary event making a high contribution to total variance. Flood variables were described by the precipitation that caused the flood, i.e. mean total precipitation (Pt) and hourly maximum intensity of the precipitation (Imax). Total water yield (Wt) during the flood was expressed by the total water depth of the event, total duration of the

RESULTS

Hydrometeorology during the study period

The term 'flood' is used here to represent a complete hydrological event with rising and receding limbs. Major rainfall events generally occurred in autumn (October-December) and particularly in spring (March-June) and minor rainfall events in summer (July-October). During the whole observation period, 15 flood events were recorded (3 in winter, 8 in spring and 4 in autumn) (Figure 2). The duration of these flood

SS, POC and DOC concentrations and relationship with discharge

Delivered SS characteristics increased with seasonal discharge and varied widely during the observation period. For all hydrological periods (flood and non-flood events), SSC ranged between 6 and 15 743 mg l 1 . Maximum SSC during flood events reached 15 743 mg l 1 (observed in event 4), while the minimum value was 391 mg l 1 , observed on 14 April 2009 (event 12). Mean discharge-weighted SSC for the whole period (estimated Maximum POC and DOC concentrations were recorded during flood events (Figure 3), whereas minimum concentrations occurred during base flow periods. POC concentration during all hydrological conditions at the catchment outlet ranged from 0Ð1 to 173Ð2 mg l 1 (discharge-weighted mean value of 14 mg l 1 ) and DOC concentration from 1Ð5 to 7Ð9 mg l 1 (discharge-weighted mean value of 4Ð1 mg l 1 ). There was a trend for decreasing POC% with increasing discharge and SSC during flood events, with POC% ranging from 0Ð9 to 8% (mean value 2Ð25%) (Figure 4). The Save catchment showed a good relationship between discharge and DOC concentration (R 2 D 0Ð50) during all hydrological conditions, but a weak relationship between discharge and POC concentration (R 2 D 0Ð18) (Figure 5).

In the present study, complex mixes of clockwise and anticlockwise loops were observed when there were multiple peaks of discharge together with multiple peaks of SSC during a flood event, coinciding with extreme rainfall intensity, e.g. in flood event 4. The relationship between POC/DOC and discharge showed clockwise, anticlockwise and mixed hysteresis due to temporal variability in concentrations during flood events in different seasons (Figure 6), as also observed for sediment concentration and discharge by [START_REF] Oeurng | Dynamics of suspended sediment transport and yield in a large agricultural catchment, southwest France[END_REF].

SS, POC and DOC fluxes

The results clearly demonstrated the temporal variability in SS, DOC and POC transport during seasonal flood events (Table III). The SS, DOC and POC loads transported during autumn were less than those in winter and spring due to lower flood magnitude. The transport rates during observed floods showed that SS load (per event) varied from 513 to 41 750 t; POC load from 12 to 748 t and DOC load from 9Ð3 to 218 t. The POC and DOC transported during flood events represented 76 and 62% of their total loads and occurred within 22% of the study period (January 2008-June 2009). The maximum SS and POC loads recorded in flood events occurred during spring flood (event 4), while the maximum DOC load was recorded during the flood of the longest duration (event 10). During the whole study period, POC from the Save catchment amounted to 3090 t and DOC export to 1240 t, representing 1Ð8 t km 2 y 1 and 0Ð7 t km 2 y 1 , respectively. The POC load ranged from 1Ð6 to 7Ð7% of sediment transport from the catchment during flood events and represented 2Ð5% of total sediment export during the whole study period. It is however noted that POC% by mass appears to underestimate the relative importance of POC in the sediment mix because of its low density.

Relationships between POC, DOC and hydro-climatological variables

Table IV shows the relationships between hydroclimatological, DOC and POC variables in the Save catchment. Total precipitation (Pt) showed a moderate correlation with mean discharge (Qm) (R D 0Ð56) and good correlations with maximum discharge (Qmax) (R D 0Ð73) and total water yield (Wt) (R D 0Ð79). Antecedent flood discharge (Qa) and baseflow (Qb) had weak correlations with total precipitation (Pt).

Organic carbon concentration (POCm, POCmax, DOCm, DOCmax) had weak relationships with total precipitation (Pt) and maximum rainfall intensity (Imax). DOCm was fairly well correlated with flood intensity (IF) (R D 0Ð57), while POCmax showed a moderate correlation with If (R D 0Ð62). DOCmax was slightly correlated with Qmax, while POCmax was more strongly correlated with this parameter (R D 0Ð71). SSt, DOCt and POCt showed significant correlations with flood duration (Fd), total precipitation (Pt), flood discharge (Qm; Qmax) and total water yield (Wt) (Table IV). SS, POC and DOC variables did not show any relationship with antecedent flow (Qa, Qb) or antecedent precipitation (P1d, P5d and P10d).

In Principal Component Analysis (PCA) taking samples and variables into account, two factors explained 59Ð1% of total variance, with factor 1 representing 44Ð2%. Factor 1 was characterized by high negative Eigen value for total rainfall (Pt), flood duration, flood discharge (Qm; Qmax) and total water yield (Wt), which indicates the response of SS, POC and DOC load transport during flood events. Four factors were retained for rotational analysis. A summary of varimax rotated factors for all variables is given in Table V. The first four axes absorbed 79Ð1% of the total variance.

DISCUSSION

Temporal variability in SS, POC and DOC transport and yield

SS, POC and DOC concentrations recorded during different seasonal flood events provide an insight into the temporal variability in these parameters in the Save agricultural catchment. Maximum SS, POC and DOC concentrations generally increased with increasing magnitude of flood events, particularly in spring, yielding SS, POC and DOC fluxes with strong variability. Basing on the statistical analyses, there were strong correlations between total precipitation (Pt), flood duration (Fd), flood discharge (Qm; Qmax), total water yield (Wt) and suspended sediment and organic carbon fluxes (SSt, POCt and DOCt). These variables could be the main factors controlling SS, POC and DOC yield from the Save catchment during the investigation period. Cooper et al. (2007) also attributed DOC transport to flood event magnitude. However, soil type, land use and the availability of SS and organic carbon sources are also major drivers of their temporal dynamics. The variability in sediment transport during successive peaks of similar magnitude is influenced by sediment exhaustion effects. After a period of relatively high sediment transport (supplyrich floods), sediment becomes less and less available (exhaustion phenomenon), and the sediment concentrations recorded during successive months are consequently lower [START_REF] Walling | Suspend sediment and solute response characteristics of river Exe, Devon, England[END_REF]. This was seen in successive floods (events 7, 8 and 9) during autumn 2008, recorded on 26 November 2008 (Qmax D 27Ð57 m 3 s 1 ; SSCmax D 1613 mg l 1 ), 6 December 2008 (Qmax D19Ð77 m 3 s 1 ; SSCmax D 569 mg l 1 ), and 14 December 2008 (Qmax D 26Ð74 m 3 s 1 ; SSCmax D 501 mg l 1 ). These exhaustion effects have been described by many previous studies [START_REF] Alexandrov | Suspended sediment transport in flash floods of the semiarid northern Negrev, Israel[END_REF][START_REF] Rovira | Temporal distribution of suspended sediment transport in a Mediterranean basin: the Lower Tordera (NE Spain)[END_REF]. The highest POC concentrations were measured in the flood event with the highest rainfall intensity (17Ð2 mm h 1 ). However, the maximum discharge during this flood event amounted to 44Ð02 m 3 s 1 , while the flood on 27 January 2009, with discharge of 112Ð60 m 3 s 1 , transported only 36Ð20 mg l 1 of POC. This shows that the level of peak discharge does not always control the peak of POC, as it can also be affected by other factors such as rainfall intensity and flood intensity that determine soil erosion within the catchment during rainfall events. The extreme POC concentration was linked to the highest SS associated with POC%. DOC also showed strong variability in concentrations during all hydrological conditions. However, it transpired that the level of increase in flood discharge did not solely control the increase in DOC concentration, as similar peaks in DOC were produced by different flood discharges (Table III). This is confirmed by the poor statistical relationship between maximum DOC and peak discharge (R D 0Ð31). The temporal dynamics of DOC are very complex [START_REF] Jones | A long-term perspective of dissolved organic carbon transport in Sycamore Creek, Arizona[END_REF] and may be controlled not only by microbial activity in sediments [START_REF] Bicudo | Fluxes of dissolved organic carbon within attached aquatic microbiota[END_REF] but also by variations in POC [START_REF] Vervier | Role of interaction zones between surface and ground waters in DOC transport and processing: considerations for river restoration[END_REF][START_REF] Jones | Vertical hydrologic exchange and ecosystem metabolism in a Sonoran Desert stream[END_REF]. However, during summer, the groundwater dilution of DOC is limited in the Save catchment, since the catchment substratum is relatively impermeable due to its high clay content, and therefore, DOC concentrations are not high (<8 mg L 1 ). Numerous authors have reported that groundwater may be high in DOC [START_REF] Wallis | The importance of groundwater in the transportation of allochthonous dissolved organic matter to the stream draining a small mountain basin[END_REF][START_REF] Mcdowell | Origin, composition and flux of dissolved organic carbon in the Hubbard Brook valley[END_REF][START_REF] Vervier | Role of interaction zones between surface and ground waters in DOC transport and processing: considerations for river restoration[END_REF][START_REF] Bernard | DOC cycling in surface and ground waters interaction zone in a fluvial ecosystem[END_REF] and have described groundwater as being a source of organic matter for surface water [START_REF] Fiebig | Inmobilization of dissolved organic matter from groundwater discharging through the stream bed[END_REF]. The mean DOC concentration in the Save catchment is similar to the DOC value of 4Ð1 mg l 1 reported for temperate zones [START_REF] Meybeck | How to establish and use world budgets of riverine materials[END_REF]. Compared with other rivers, the Save DOC range is close to the range (2-6 mg l 1 ) of the Niger River [START_REF] Martins | Geochemistry of the Niger River. In Transport of Carbon and Minerals in Major World Rivers[END_REF], slightly higher than the range (3-5 mg l 1 ) of the Amazon [START_REF] Richey | Biochemistry of the Amazon River: an update[END_REF] and the St. Lawrence River [START_REF] Pocklington | Organic carbon transport in the St. Lawrence River[END_REF] but much lower than the range (2-22 mg l 1 ) of the Indus River [START_REF] Arain | Persisting trends in carbon and mineral transport monitoring of the Indus River. In Transport of Carbon and Minerals in Major World Rivers[END_REF].

The specific POC yield (1Ð8 t km 2 y 1 ) of the Save catchment is comparable to the mean of the Garonne River (1Ð47 t km 2 y 1 ) [START_REF] Veyssy | Seasonal variation and origin of Particulate Organic Carbon in the lower Garonne River at La Re'ole (Southwestern France)[END_REF] and slightly higher than the mean of rivers in Europe (1Ð10 t km 2 y 1 ) [START_REF] Ludwig | Predicting the oceanic input of organic carbon by continental erosion[END_REF]. However, it is lower than the yield of the Amazon River (2Ð83 t km 2 y 1 ; [START_REF] Richey | Biogeochemistry of carbon in the Amazon River[END_REF], and much lower than that of the Nivelle River (5Ð3 t km 2 y 1 ) [START_REF] Coynel | Contribution of small mountainous rivers to particulate organic carbon input in the Bay of Biscay[END_REF], which drains a typical Pyrenean mountainous catchment into the Bay of Biscay (Atlantic Ocean). This could be attributed to lower soil erosion generating less POC yield, as POC is associated with sediment. The specific DOC yield of the Save catchment (0Ð7 t km 2 y 1 ) is

If 0Ð50 0Ð20 1Ð00 Pt 0Ð71 0Ð73 0Ð22 1Ð00 Imax 0Ð21 0Ð10 0Ð15 0Ð37 1Ð00 P1d 0Ð12 0Ð38 0Ð20 0Ð03 0Ð26 1Ð00 P5d 0Ð11 0Ð25 0Ð10 0Ð17 0Ð20 0Ð75 1Ð00 P10d 0Ð28 0Ð50 0Ð16 0Ð45 0Ð30 0Ð23 0Ð39 1Ð00 Qa 0Ð00 0Ð05 0Ð06 0Ð05 0Ð04 0Ð17 0Ð09 0Ð13 1Ð00 Qb 0Ð14 0Ð36 0Ð30 0Ð44 0Ð42 0Ð16 0Ð06 0Ð11 0Ð48 1Ð00 Qm 0Ð53 0Ð29 0Ð72 0Ð56 0Ð07 0Ð07 0Ð26 0Ð28 0Ð26 0Ð34 1.00 Qmax 0Ð72 0Ð43 0Ð74 0Ð73 0Ð11 0Ð02 0Ð29 0Ð34 0Ð10 0Ð12 0.93 1.00 Wt 0Ð76 0Ð44 0Ð66 0Ð79 0Ð13 0Ð08 0Ð22 0Ð37
- - - Tr - - 0Ð58 - If 0Ð72 - 0Ð51 - Pt -0Ð80 - - - I m a x - - - - P 1 d - - - 0 Ð75 P 5 d - - - - P 1 0 d - - - - Q a - - - - Qb - - 0Ð74 0Ð51 Qm -0Ð83 - - - Qmax -0Ð96 - - - Wt -0Ð94 - - - SSCm - 0Ð59 - - SSCmax 0Ð7 7 - - - SST -0Ð98 - - - DOCm 0Ð63 0Ð66 - - DOCmax - 0Ð67 - - DOCt -0Ð95 - - - POCm 0Ð6 3 - - - POCmax 0Ð7 8 - - - POCt -0Ð95 - - - Variance explained 44Ð3 14Ð8 10Ð9 9 Ð1 Cumulative variance 44Ð3 59Ð1 70Ð0 79Ð1
Bold numbers for value ½ 0Ð80.

2Ð5 times higher than that of a Himalayan catchment dominated by agriculture studied by [START_REF] Sharma | Streamflow, sediment and carbon transport from a Himalayan watershed[END_REF], a difference that can be attributed to land conservation preventing soil and carbon losses within the latter. However, peatland catchments, which are rich in organic carbon, have much higher specific DOC yields, e.g. 16Ð9 t km 2 y 1 for a catchment in northeast Scotland [START_REF] Dawson | A comparison of particulate, dissolved and gaseous carbon in two contrasting upland streams in the UK[END_REF]. This value is common in peatdominated headwater catchments in the UK, where soil carbon is the major source of organic carbon in stream water [START_REF] Aitkenhead | The relationship between dissolved organic carbon in streamwater and soil organic carbon pools at different spatial scales[END_REF][START_REF] Dawson | Is within stream processing an important control on spatial changes in headwater carbon fluxes?[END_REF].

Discharge, SS, POC and DOC relationships and probable origins

The relationship between sediment concentration and discharge revealed the existence of clockwise, anticlockwise and mixed-shape hysteretic loops (mixing of clockwise and anticlockwise patterns). Interpreting sediment and organic carbon delivery processes using hysteresis patterns could help understand the origins of dissolved and particulate matter in a catchment. Increasing SSC on the falling limb during floods may be related to sources of relatively more available sediment near the catchment outlet. Clockwise hysteresis occurs when the sediment source area is the channel itself or an adjacent area located close to the catchment outlet, with runoff triggering the movement of sediment accumulated in the channel during the previous seasons and with little or no contribution from the tributaries [START_REF] Klein | Anti-clockwise hysteresis in suspended sediment concentration during individual storms[END_REF]. [START_REF] López-Tarazon | Suspended sediment in a highly erodible catchment: The River Isábena (Southern Pyrenees)[END_REF] also reported that the clockwise phenomenon was found preferentially when rainfall was mostly located near the catchment outlet. In the Save catchment, this was the case for clockwise flood events in early autumn and late winter. Anticlockwise hysteretic loops occur when sediment sources are far from the catchment outlet, e.g. soil erosion from hillsides and upstream areas [START_REF] Braisington | Suspended sediment dynamics in small catchments in the Nepal Middle Hills[END_REF][START_REF] Goodwin | The temporal and spatial variability of sediment transport and yields within the Bradford Beck catchment, West Yorkshire[END_REF][START_REF] Orwin | The evidence for paraglacial sedimentation and its temporal scale in the deglacierizing basin of Small River Glacier, Canada[END_REF]. This type of hysteretic loop is mainly found in the Save catchment in spring and late autumn, when there are high flood magnitudes with sufficient capacity to transport sediments from distant areas of the upstream catchment to the outlet [START_REF] Oeurng | Dynamics of suspended sediment transport and yield in a large agricultural catchment, southwest France[END_REF]. However, it is noted that clear interpretation of sediment sources using hysteresis patterns is limited within this study because the Save catchment is long with only one sampling station at the catchment outlet. Some hysteresis studies from existing literature were used to identify the sediment sources which are close or far referring to the sampling station, mainly in small catchments [START_REF] Lefrançois | Suspended sediment and discharge relationship to identify bank degradation as a main sediment source on small agricultural catchments[END_REF][START_REF] Nadal-Romero | Relationships among rainfall, runoff, and suspended sediment in a small catchment with badlands[END_REF].

POC and DOC exhibited different hysteresis behaviour during flood events. This resulted from variability in concentrations during rising and falling limbs of floods. The relationship between discharge and POC for both clockwise and anticlockwise hysteresis followed the same patterns as discharge and SS hysteresis. Examples can be seen in flood events 4, 7, 10 and 15 (Figure 6). Although POC% decreased during flood events, POC concentrations remained high with high concentrations of SSC and therefore the hysteresis patterns were similar (Figure 6). Generally, POC% decreased as SS increased, following a hyperbolic relationship (Figure 4). This is a very typical trend as reported for other rivers [START_REF] Meybeck | Carbon, nitrogen and phosphorus transport by World Rivers[END_REF][START_REF] Ittekkot | Global trends in the nature of organic matter in river suspensions[END_REF][START_REF] Coynel | Contribution of small mountainous rivers to particulate organic carbon input in the Bay of Biscay[END_REF], and it is attributed to changes in organic matter sources during the hydrograph through declining organic carbon in eroded materials [START_REF] Ittekkot | Fate of riverine particulate organic matter[END_REF]. [START_REF] Probst | Géochimie et hydrologie de l'érosion continentale, Mécanismes, bilan global actuel et fluctuations au cours des 500 derniers millions d'années[END_REF] showed for the Garonne that high POC% corresponds to production of phytoplankton during low flood periods, while low POC content corresponds to POC from soil erosion during high flow periods. In the present study (SSC <20 mg l 1 , associated with low river discharge), the high POC content could be attributed to the phytoplankton and litter contribution. For the other classes, corresponding to medium or strong sediment mobilisation associated with high river discharge and turbid waters, organic carbon content is low and generally recognized as being of allochthonous origin [START_REF] Etcheber | Biogéochimie de la matière organique en milieu estuarien: comportement, bilan, propriétés[END_REF][START_REF] Lin | Etude du potentiel de dégradation de la matière organique particulaire au passage eau douce-eau salée: Cas de l'estuaire de la Gironde[END_REF][START_REF] Coynel | Contribution of small mountainous rivers to particulate organic carbon input in the Bay of Biscay[END_REF]. In this study, POC associated with SSC higher than the 2000 mg l 1 can be attributed to the terrigenous origins which mainly originated from the soil.

The relationship between DOC and discharge also showed clockwise, anticlockwise and mixed patterns during the study period, but the mixed patterns were mostly found when the SS peak arrived before peak discharge. An example can be seen in flood events 4 and 10 (Figure 6). This could be due to dilution effects between old water before the floods and new water during and after floods. For clockwise patterns, DOC before the flood events was low, but then it was diluted by new water containing higher DOC concentrations from soils which quickly released DOC during storm events before reaching the peak discharge. Many studies have examined the effect of storms on the ability of soils to release DOC and water fluxes are responsible for seasonal changes in DOC concentration in runoff [START_REF] Kalbitz | Controls on the dynamics of dissolved organic matter in soils: A review[END_REF]. The relationship between DOC and discharge showed anticlockwise hysteresis, with higher DOC concentrations on the falling limb of the high hydrograph than on the rising limb. This indicates that water entering the stream during the early part of the flood events had lower DOC concentrations than water entering the stream after peak discharge [START_REF] Morel | Sources of dissolved organic carbon during stormflow in a headwater agricultural catchment[END_REF], an effect associated with subsurface water from shallow soil horizons, which is rich in DOC.

CONCLUSIONS

Temporal characteristics of fluvial transport of suspended sediment and organic carbon during flood events were studied in a large agricultural catchment using an extensive dataset with high temporal resolution obtained by manual and automatic sampling. The results showed strong variability in SS and POC and DOC concentrations. Suspended sediment load during different seasonal flood events varied from 513 to 41 750 t; POC load from 12 to 748 t and DOC load from 9 to 218 t. Transport of POC and DOC during flood events amounted to 76 and 62% of their total fluxes and occurred within 22% of the study period (January 2008-June 2009). These results reveal the important role of floods in mobilising SS, POC and DOC transport from the Save agricultural catchment. Total POC export during the whole study period amounted to 3091 t and total DOC export to 1238 t, representing 1Ð8 t km 2 y 1 and 0Ð7 t km 2 y 1 , respectively.

Statistical analyses revealed strong correlations between total precipitation (Pt), flood discharge and total water yield and SS, POC and DOC, indicating that these variables are the main factors controlling sediment and organic carbon export from the Save catchment. Sediment and organic carbon sources are also important in yielding dissolved and particulate matter during flood events, as successive floods exhaust the amounts available. The relationships between SSC, POC and DOC loads and discharge over different temporal scales during flood events resulted in different hysteresis patterns, which were used to identify their origins. For POC, clockwise and anticlockwise hysteresis followed the same patterns as discharge and SS hysteresis. The relationship between DOC and discharge was mainly dominated by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment.

Figure 1 .

 1 Figure 1. Location, land use and topographical maps of the Save catchment

Figure 2 .

 2 Figure 2. Hourly discharge in the 15 flood events observed during the study period (January 2008 to June 2009) at Larra sampling station

Figure 3 .

 3 Figure 3. Temporal variability in particulate (POC) and dissolved (DOC) organic carbon during the whole study period (January 2008 to June 2009)

Figure 5 .

 5 Figure 5. Relationship between discharge and (A) dissolved organic carbon (DOC) and (B) particulate organic carbon (POC)

Flood)Figure 6 .Flood

 6 Figure 6. Relationship between discharge and suspended sediment (SS), particulate organic carbon (POC) and dissolved organic carbon (DOC), showing different hysteresis patterns

  is significant at P < 0.01 level for bold numbers and P < 0.05 for italics.

Table I .

 I Names, abbreviations and units for the variables used to characterize flood events and to perform Pearson correlation matrix and factorial analysis

	Antecedent conditions	Abbreviation	Unit
	Precipitation 1 day before the	P1d	mm
	event
	Precipitation 5 days before the	P5d	mm
	event
	Precipitation 10 days before the	P10d	mm
	event
	Baseflow before the event	Qb	m 3 s 1
	Antecedent peak discharge	Qa	m 3 s 1
	Flood event conditions
	Flood duration	Fd	h
	Time of rise	Tr	h
	Total precipitation during the	Pt	mm
	event
	Maximum rainfall intensity of	Imax	mm h 1
	the event
	Flood intensity ((Qmax	If	m 3 min 2
	Qb)/time of rise)
	Total water yield	Wt	Hm 3
	Mean discharge	Qm	m 3 s 1
	Maximum discharge	Qmax	m 3 s 1
	Mean suspended sediment	SSCm	mg l 1
	concentration
	Maximum suspended sediment	SSCmax	mg l 1
	concentration
	Total suspended sediment yield	SSt	t
	Mean dissolved organic carbon	DOCm	mg l 1
	Max.dissolved organic carbon	DOCmax	mg l 1
	Dissolved organic carbon yield	DOCt	t
	Mean particulate organic carbon	POCm	mg l 1
	Max.particulate organic carbon	POCmax	mg l 1
	Particulate organic carbon yield	POCt	t
	event (Td), and mean discharge (Qm) and maximum dis-
	charge (Qmax) corresponding to the time of rise to reach
	the peak discharge (Tr). The discharge speed to reach the
	peak flow during flood events was defined by flood inten-
	sity If (If D Q max Q b /Tr). Suspended sediment was
	expressed as the mean concentration (SSCm), the max-
	imum concentration (SSCmax) and the total suspended
	sediment yield during the flood event (SSt). Dissolved
	and POC loads during floods were expressed by mean
	values (DOCm, POCm), maximum values (DOCmax,
	POCmax) and their yield (DOCt; POCt).

Table II .

 II Summary of the main flood characteristics recorded during the study period in the Save catchment Mean daily discharge in the whole study period was 6Ð28 m 3 s 1 . Table II summarizes all flood characteristics during the observed flood events and their antecedent conditions.Total rainfall in the catchment for the whole study period (January 2008-June 2009) was 1152 mm (i.e. 768 mm y 1 ). The maximum rainfall intensity reached 17 mm h 1 in event 4 (1 June 2008). The mean total water yield of the whole study period (January 2008-June 2009) was 178 mm y 1 higher than the longterm mean value of 136 mm for the period1985-2008.

	N °Flood	Season	P1d	P5d	P10d	Qb	Qa	Fd	Tr	Pt	Imax	If	Wt	Qm	Qmax
		date		(mm)	(mm)	(mm)	(m 3 s 1 )	(m 3 s 1 )	(h)	(h)	(mm)	(m (m h 1 )	(m 3 min 2 )	(Hm 3 )	(m 3 s 1 )	(m 3 s 1 )
	1	19/01/2008 Winter	17Ð7 27Ð7 41Ð6	3Ð16	6Ð75 184 43 19Ð9	3Ð4	0Ð87	7Ð34 10Ð74 40Ð64
	2	28/03/2008 Spring	7Ð2 24Ð9 26Ð8	2Ð56 40Ð64 228 84 39Ð3	2Ð8	0Ð42	8Ð56 10Ð39 37Ð60
	3	21/04/2008 Spring	13Ð3 22Ð4 51Ð3	4Ð06 37Ð60 189 22 19Ð4	4Ð0	1Ð19	7Ð1	9Ð60 30Ð20
	4	01/06/2008 Spring	24Ð0 48Ð9 61Ð1	4Ð28 30Ð20 228 16 50Ð0	17Ð2	2Ð48	12Ð75 15Ð70 44Ð02
	5	12/06/2008 Spring	7Ð5 14Ð6 54Ð5	4Ð28 44Ð02 259 29 28Ð5	8Ð5	1Ð40	12Ð61 15Ð01 44Ð80
	6	08/11/2008 Autumn	3Ð1 14Ð5 47Ð3	2Ð96 44Ð80 105 46 23Ð8	4Ð6	0Ð22	2Ð4	6Ð18 12Ð97
	7	26/11/2008 Autumn	3Ð3 13Ð1 14Ð7	4Ð90 12Ð97 191 43 35Ð9	4Ð4	0Ð53	3Ð42	9Ð08 27Ð57
	8	06/12/2008 Autumn	4Ð2	9Ð6 32Ð7	4Ð90 27Ð57 126 54 27Ð7	5Ð3	0Ð28	3Ð21 10Ð12 19Ð77
	9	14/12/2008 Autumn 11Ð7 22Ð6 41Ð0	6Ð95 19Ð77 256 27 13Ð3	1Ð6	0Ð73	6Ð01 11Ð63 26Ð74
	10 27/01/2009 Winter	11Ð5 11Ð7 13Ð0	4Ð06 26Ð74 351 69 74Ð5	4Ð1	1Ð57	43Ð71 34Ð50 112Ð60
	11 11/02/2009 Winter	0Ð2	7Ð7 12Ð6	9Ð99 112Ð60 233 54 32Ð9	4Ð2	0Ð94	19Ð71 25Ð94 60Ð66
	12 14/04/2009 Spring	17Ð6 48Ð3 49Ð1	5Ð10 60Ð66 141 29 29Ð5	4Ð5	0Ð64	7Ð15 14Ð08 23Ð80
	13 22/04/2009 Spring	3Ð1	9Ð2 51Ð5	6Ð75 23Ð80 112 36 19Ð3	4Ð2	1Ð26	9Ð80 24Ð31 52Ð24
	14 02/05/2009 Spring	9Ð6 25Ð1 38Ð9 11Ð00 52Ð80 116 22	1Ð1	0Ð7	1Ð20	7Ð18 15Ð90 37Ð47
	15 15/05/2009 Spring	11Ð3 12Ð7 13Ð2	5Ð10 37Ð47 95 26 13Ð0	1Ð9	0Ð48	3Ð31	9Ð68 17Ð62
	Maximum values in bold, minimum values in bold italics.
	events ranged from 95 h to 351 h, with a mean value
	of 188 h. The longest event (event 10; 351h) occurred
	on 27 January 2009, with total precipitation of 74Ð5 mm
	in the whole catchment. This event was unusual since
	it had a 10-year return period and it represented the
	biggest flood during the whole study period. Maximum
	hourly discharge during observed flood events varied
	from 12Ð97 m 3 s 1 (8 November 2008) to 112Ð60 m 3 s 1
	(27 January 2009).

Table III .

 III Concentrations and transport rates of total suspended solids (SS), dissolved organic carbon (DOC) and particulate organic carbon (POC) during the 15 flood events observed

	N °Flood date	Season	SSCm	SSCmax	SSt	DOCm	DOCmax	DOCt	POCm	POCmax	POCt
				(mg l 1 )	(mg l 1 )	(t)	(mgl 1 )	(mgl 1 )	(t)	(mgl 1 )	(mgl 1 )	(t)
	1	19/01/2008	Winter	652	1380	4801	NA	NA	NA	NA	NA	NA
	2	28/03/2008	Spring	562	1160	4820	4.0	6.1	34	11Ð5	2 4 Ð1	9 8
	3	21/04/2008	Spring	650	1536	4385	3.8	5.1	25	13Ð0	2 3 Ð8	8 5
	4	01/06/2008	Spring	1597	15743	41750	4.5	7.9	58	58Ð0	173Ð2	748
	5	12/06/2008	Spring	850	1322	9077	5.0	6.1	70	12Ð5	1 7 Ð6	176
	6	08/11/2008	Autumn	159	466	513	4.3	4.8	10	16Ð8	2 1 Ð9	3 9
	7	26/11/2008	Autumn	494	1618	2959	3.6	5.2	22	7Ð4	1 0	4 6
	8	06/12/2008	Autumn	278	569	1018	3.3	4.3	15	4Ð4	5Ð6	20
	9	14/12/2008	Autumn	128	501	1085	3.6	4.1	38	4Ð9	6 Ð9	5 2
	10	27/01/2009	Winter	337	2003	23374	5.0	5.7	218	16Ð2	3 6 Ð2	706
	11	11/02/2009	Winter	396	1030	6867	3.4	4.8	75	7Ð2	1 6 Ð8	157
	12	14/04/2009	Spring	268	391	1690	4.5	6.7	32	5Ð5	8 Ð6	3 9
	13	22/04/2009	Spring	678	1055	5029	5.2	6.3	51	12Ð6	2 4 Ð8	123
	14	02/05/2009	Spring	344	1246	3113	3.8	5.3	25	8Ð8	2 4 Ð2	5 8
	15	15/05/2009	Spring	204	434	666	2.8	4.6	9	3 Ð6	6Ð1	12

Maximum values in bold and minimum values in bold italic.

Table IV .

 IV Pearson correlation matrix among all variables (n D

	13)	Qm Qmax Wt SSCm SSCmax SSCT DOCm DOCmax DOCt POCm POCmax POCt
		Qb
		Qa
		P5d P10d
		Imax P1d
		Pt
		If
		Fd Tr	1Ð00	0Ð42 1Ð00
			Fd	Tr

Table V .

 V Summary of varimax rotated factors for all variables presented in Table I (Eigen values <0.50 excluded)

	Variables	Factor 1 Factor 2 Factor 3 Factor 4
	Fd	0Ð7 6

ACKNOWLEDGEMENTS

This research was financially supported by a doctoral research scholarship from the French government in cooperation with Cambodia. The work was performed within the framework of GIS-ECOBAG, Programme P2 Garonne Moyenne and IMAQUES, and supported by funds from CPER and FEDER (Grants nos OPI2003-768) of the Midi-Pyrenees Region, Zone Atelier Adour Garonne (ZAAG) of PEVS/CNRS347 INSUE. This work was also performed within the framework of the EU Interreg SUDOE IVB program (SOE1/P2/F146 AguaFlash project, http://www.aguaflash-sudoe.eu) and funded by ERDF and Midi-Pyrénées Region. We sincerely thank the CACG for discharge data and Meteo France for meteorological data. The authors would like to thank Ecolab staff for access to the site and assistance with monitoring instruments and in laboratory. Much thanks are particularly offered to the two anonymous referees whose comments improved this manuscript.