Jordi Seuba

Jerome Leloup

Stephane Richaud

Sylvain Deville

Christian Guizard

Adam J Stevenson

Fabrication of ice-templated tubes by rotational freezing: microstructure, strength, and permeability

Keywords: Ice-templating, Ceramics, Tubular membranes, Fracture, Air flow

We demonstrate a facile and scalable technique, rotational freezing, to produce porous tubular ceramic supports with radially aligned porosity. The method is based on a conventional ice-templating process in a rotatory mold and demonstrated here with yttria-stabilized zirconia (YSZ). We investigated the effects of solid loading, freezing temperature, and volume of the slurry on the microstructure, strength (o-ring test and four-point bending), and air permeability. The results show that pore volume and pore size can be controlled by the solid loading and freezing temperature respectively, and overall tube thickness can be adjusted by the volume of slurry initially poured into the mold. Decreasing pore size and pore volume increases the mechanical properties but decreases the air permeability. These tubes could be particularly interesting as tubular membrane supports such as oxygen transport membranes.

Introduction

Porous ceramic membranes are increasingly used in technologically important applications such as energy production (solid oxide fuel cells, oxygen and proton transport membranes) [START_REF] Pan | Processing of YSZ thin films on dense and porous substrates[END_REF] [2] [3] [START_REF] Weirich | Preparation of lanthanum tungstate membranes by tape casting technique[END_REF], solid-liquid filtration [START_REF] Barg | Physical and high-temperature permeation features of double-layered cellular filtering membranes prepared via freeze casting of emulsified powder suspensions[END_REF] [START_REF] Ambrosi | Membrane Separation Processes for the Beer Industry : a Review and State of the Art[END_REF], and gas separation [START_REF] Zivkovic | Gas transport efficiency of ceramic membranes: comparison of different geometries[END_REF]. These membranes usually have an asymmetric configuration where a membrane layer is deposited on top of a porous substrate. The membrane layer provides the selectivity required for the process, while the porous support provides the mechanical stability. Besides strength, porous supports must also exhibit a high permeability to facilitate the evacuation of the fluid and thus minimize detrimental effects such as concentration polarization or exaggerated pressure gradients that compromise the overall performance of the membrane [START_REF] Baumann | Manufacturing strategies for asymmetric ceramic membranes for efficient separation of oxygen from air[END_REF].

Typically, tubular porous substrates are prepared by extrusion [START_REF] Zhou | Preparation and characterization of tubular porous silicon carbide membrane supports[END_REF] [10], slip casting [START_REF] Choi | Oxygen permeation properties of bscf5582 tubular membrane fabricated by the slip casting method[END_REF], centrifugal casting [START_REF] Kim | Centrifugal casting of alumina tube for membrane application[END_REF], or isostatic pressing [START_REF] Dong | Reaction-sintered porous mineral-based mullite ceramic membrane supports made from recycled materials[END_REF]. However, these techniques rely on increasing the total pore volume and pore size to achieve the required permeability at the expense of mechanical properties. Therefore, it is necessary to explore other strategies such as an optimized pore shape or reduced tortuosity to simultaneously improve permeability and strength.

Nowadays, the method to obtain unidirectional macroporous materials most conventionally used is extrusion. However due to technical limitations is not possible to produce materials with a high pore volume content (>50%) and reduce the pore size below 100 µm [START_REF] Isobe | Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores[END_REF]. On the other hand, direct replication of wood has successfully been demonstrated as a feasible technique, although the large number of steps and the microstructural variability among samples remain as important drawbacks to overcome. Alternatively, in the last decade ice-templating (or freeze-casting) has emerged as a popular shaping route to produce macroporous materials with a tailored pore morphology and aligned porosity. The technique is based on the freezing of a colloidal suspension and the segregation of ceramic particles between the crystals. Afterwards, the solvent crystals are removed by sublimation and the green body is sintered to consolidate the structure. The final porosity is thus a direct replica of the solvent crystals [START_REF] Deville | Freeze-Casting of Porous Ceramics: A Review of Current Achievements and Issues[END_REF].

The mechanical and gas flow properties of ice-templated monoliths have been extensively evaluated [START_REF] Deville | A meta-analysis of the mechanical properties of ice-templated ceramics and metals[END_REF] [17] [START_REF] Porter | Bioinspired Scaffolds with Varying Pore Architectures and Mechanical Properties[END_REF] [19] [START_REF] Han | Highly porous ZrO2 ceramics fabricated by a camphene-based freeze-casting route: Microstructure and properties[END_REF] [21] [START_REF] Seuba | Gas Permeability of icetemplated, unidirectional porous ceramics[END_REF] [23] [START_REF] Pekor | The Effect of Polyvinyl Alcohol on the Microstructure and Permeability of Freeze-Cast Alumina[END_REF]. However the literature on ice-templated tubes is scarce. Moon et al. [START_REF] Moon | Preparation of NiO-YSZ tubular support with radially aligned pore channels[END_REF] used an external metallic mold with a central PTFE rod to induce the thermal gradient required to align the porosity. They studied the effect of solids loading and freezing temperature on the microstructure and the feasibility of depositing an external dense layer. Liu et al. [START_REF] Liu | A novel way to fabricate tubular porous mullite membrane supports by TBA-based freezing casting method[END_REF] used a similar set-up to evaluate the impact of solids loading, pore size, and particle size on compressive strength, nitrogen flux, and water flux. However, the difficulties demolding the tubes after solidification is drawback of this set-up that limits further industrial development. Moon et al. [START_REF] Moon | Novel ceramic/camphene-based co-extrusion for highly aligned porous alumina ceramic tubes[END_REF] overcomes this issue combining ice-templating with co-extrusion, however the resultant porosity is not radially aligned.

In this work, we propose a novel method based on a rotational freezing to produce ice-templated tubes. We investigate the impact of pore volume, size, and tube thickness on mechanical properties using an o-ring test and four-point bending. Furthermore, we studied the effect of the same morphological parameters on permeability to assess the applicability of these materials as a membrane supports.

Experimental procedure

Sample preparation

Ceramic suspensions were prepared by mixing distilled water with 0,75 wt% of dispersant (Prox B03, Synthron, Levallois-Paris, France), 3 wt% of organic binder PVA (PVA2810, Wacker, Burghausen, Germany), and 3 mol% yttria-stabilized zirconia (TZ-3YS, Tosoh, Tokyo, Japan). All the percentages are referred to the total solids loading of the slurry. After a first mixing step with a magnetic stirrer, the slurry was ball milled at 800 rpm for a minimum of 18 h to ensure a good homogenization. Afterwards, it was deaired during 10 minutes and poured in a tubular cooper mold.

The set-up used to ice-template tubes consists of a cryothermostat (Model CC 905, Hubert, Offenburg, Germany), freezing liquid (SilOil M90.055.03, Hubert, Offenburg, Germany), metallic container, tube-shape mold, and a rotating system. The tube filled with slurry was attached to the rotational clamp and the speed set at 70 rpm. The silicon oil was pumped in the container with a preset temperature (-80 • C or -30 • C) until it achieved a steady flow. The system was left under rotational freezing for a minimum of 5 minutes. The next steps were similar to the regular ice-templating process and detailed elsewhere [START_REF] Seuba | Mechanical properties and failure behavior of unidirectional porous ceramics[END_REF]: demoulding, freeze-drying (Free Zone 2.5 Plus, Labconco, Kansas City, Missouri, USA), and sintering. Tubular specimens were sintered in air at 1400 • C for 3 h with a heating and cooling rate of 5 • C/min. A previous step at 500 • C for 5 h with the same heating rate was used to ensure a proper organic burn-out.

The process was repeated with variations of the solid loading of the initial slurry (50 wt.%, 55 wt. %, and 65 wt.%), temperature of the freezing liquid (-30 • C and -80 • C), and volume of slurry initially poured into the mold (16, 18, and 20 ml). The final dimensions of the tubes in all tested conditions were: 150 mm length (L) and 10 mm of external diameter (D ext ).

Sample characterization

Micrographs of the ice-templated tubes were taken with a scanning electron microscope (Nova NanoSEM 230, FEI, Hillsboro, USA) at 10-15 kV. Total pore volume P(%) was calculated based on the relative density measured geometrically with respect to that of fully dense TZ-3YS (ρ ysz = 5.8 gcm -3 ). P(%) measure-ments were performed in a minimum of five rings per tube, and two tubes per condition. The average pore size was measured by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics, Norcross, USA) with an applied pressure up to 0,31 bar.

Mechanical properties of ice-templated tubes were characterized by two different tests: 1) four-point bending on half tubes, and 2) o-ring test. All the experiments were conducted with a universal testing machine (Shimadzu model AGSX, Kyoto, Japan), equipped with a 10 kN load cell at a crosshead speed of 0.2 mm/min. 1. Samples were cut with a slow-speed saw first in the radial direction and then in the longitudinal direction with a final length (L) ≥ 55 mm, and external diameter (D ext ) = 10 mm. The spacing between the outer supports was adjusted to S 1 = 40 mm whereas the span between the inner supports was S 2 = 20 mm. A minimum of three samples per condition were tested.

The maximum stress (σ max ) at the middle point between the two inner supports was calculated based on the approximation proposed by Kwok et al. [START_REF] Kwok | Strength characterization of tubular ceramic materials by flexure of semi-cylindrical specimens[END_REF]. Tests were considered invalid when the crack propagated outside the inner supports. When the test crack pattern resulted in a successful test, the maximum stress was calculated by:

σ max = P ay c I ( 1
)
where P is the maximum load, a is the distance between the load and the support, y c is the distance between the middle point and the centroid of the specimen cross section along the radial direction, and I is the second moment of inertia of the cross section. For a semi-circular annulus, y c and I are expressed as:

y c = R - 4 3π R 3 -R i 3 R 2 -R i 2 (2)
and

I = π 8 (R 4 -R i 4 ) - 8 9π (R 3 -R i 3 ) 2 R 2 -R i 2 (3)
where R and R int are the external and internal radius respectively.

2. Eight tubes were sliced in the radial direction with a slow-speed saw leaving specimens with an external diameter (D ext ) = 10 mm, height (h) = 4 mm, and ring thickness (t) ≈ 2 mm. Tests were performed with a piece of alumina supporting the samples to avoid the rotation and distribute evenly the load through the surface.

The maximum stress (σ max ) is located on the inner diameter of the ring and was determined by:

σ max = 1, 8 P r a ht 1 + t 3r a ( 4
)
where P is the maximum load, t the tube thickness, h the width of the ring, and r a the average radius (r ext -r int )/2.

Gas permeability was evaluated using custom built equipment to measure the air pressure drop across the tubes. A preliminary test was performed to assess the tightness of the system. Synthetic air was passed through the tubular samples which were held by a silicon ring. The specimens exhibited one closed side, 120 mm length (L), and 10 mm external diameter (D ext ). The air flow studied ranged between 0-25000 ml/min (0-0.25 m/s) and was controlled by an electrovalve.

Two sensors were placed before and after the tube to record the inlet (P i ) and outlet pressure (P o ).

Results and discussion

Microstructure control

Fig. 1 schematically depicts the process that occurs during the rotational freezing. First, the tube rotation spreads the slurry evenly across the inner surface of the mold. Then, the freezing oil is pumped into the container and the slurry in contact with the mold solidifies almost instantly. The freezing rate at this initial stage is very high due to the high heat transfer between the copper mold and the slurry. Afterwards, the velocity of the solidification front undergoes an abrupt decrease, until it reaches a steady rate.

The ice-templated tubes exhibited two different pore morphologies, random (area in contact with the copper mold) and lamellar porosity, similarly to that reported in unidirectional frozen ice-templated monoliths [START_REF] Deville | Influence of particle size on ice nucleation and growth during the ice-templating process[END_REF]. The external layer with random porosity is generated when the velocity of the solidification front is too high for the ice crystals to grow with a preferential orientation and thus they are not able to rearrange the ceramic particles (Fig. 2-A). When steady state is reached, the ice crystals grow continuously along the thermal gradient (i.e. in this case the radial direction) and the pore morphology becomes lamellar (Fig. 2-B). Due to the constant rotation of the system, the remaining liquid remains in contact with the frozen part and provides the slurry required to continue with the ice-templating process until the tube is entirely solidified. Fig. 3 shows the effect of solid loading on total pore volume. Increasing the solid loading causes a variation in the ceramic particles-water ratio and therefore an increase in the relative density. The total pore volume of the ice-templated tubes ranged between 53% and 73% for a variation in solid loading from 65% to 50%.

The dashed line in Fig. 3 represents the relationship between solid loading and porosity obtained in a previous work for ice-templated YSZ monoliths [START_REF] Seuba | Mechanical properties and failure behavior of unidirectional porous ceramics[END_REF]. The good match between the monolithic samples and the tubular samples shows that total pore volume is independent of the mold used and mainly determined by solid loading. Since the pore volume measurements were taken at different locations along the tube, it also indicates that the pore volume is homogeneously distributed along the length of the tube. During ice-templating, pore size can be controlled by the freezing temperature and solids loading. Table 1 shows how, for a given solids loading, samples frozen at -80 • C systematically exhibited a smaller pore size compared to their counterparts frozen at -30 • . At lower freezing temperatures, the temperature gradient and the velocity of the freezing front increase, which result in smaller ice crystals and creates thus a finer microstructure [START_REF] Farhangdoust | The effect of particle size on the mechanical and microstructural properties of freeze-casted macroporous hydroxyapatite scaffolds[END_REF]. Furthermore, solids loading also affects the pore size. This phenomena is caused by the thicker walls created when the solid loading increases hindering the ice crystal growth.

Mechanical properties 3.2.1. O-Ring test

Fig. 5-A shows two representative load-displacement curves for samples frozen at -80 • C and -30 • C. In both cases, the fracture behavior is described by an initial linear stage followed by two consecutive load drops. When the ring is subjected to a diametrical compression i.e. like in the o-ring test, the maximum tensile stresses are located on the intersection between the inner diameter and the loading plane [START_REF] Hudson | Tensile strength and the ring test[END_REF]. This load corresponds to the first peak in the load-displacement curve and was used to describe the strength of the rings. The second peak corresponds to the fracture of the two remaining "half rings", originated by the tension developed on the outer periphery. Both types of cracks can be observed in Fig. Fig. 6 and Table 1 show the effect of pore volume on strength for samples frozen at -80 • C and -30 • C. As expected, rings with higher pore volume exhibited a lower strength. Moreover, samples frozen at a lower temperature (-80 • C) have a smaller pore size and therefore should exhibit an increase in strength [32] [17].

5-B.

However, the trend obtained in this study seemingly contradicts this observation. The explanation for this behavior could rely on the large scattering of the strength data that makes it difficult to conclude whether this effect is seen here or not.

There are two contributions to the scattering of strength values: the probabilistic nature of the strength in ceramics [START_REF] Danzer | A general strength distribution function for brittle materials[END_REF], and small, local variations of the tube thickness, as observed in Fig. 7-B. Such geometrical imperfections induce an asymmetry in the stress field that have a deleterious effect on the strength [START_REF] Kwok | Mechanical reliability of geometrically imperfect tubular oxygen transport membranes[END_REF],

and in this case may hide the pore size effect.

Four-point bending

We modified the solid loading, freezing temperature, and volume of the slurry initially poured into the mold to obtain ice-templated tubes with different pore volume, pore size, and thickness respectively. Fig. 8 shows the effect of each parameter on the flexural strength. In all conditions, when the solid loading increases the total pore volume of the sample decreases, causing an increase in strength. Unlike in the diametrical compression, in four-point bending we did observe an increase of strength with pore size reduction (Fig. 8-A). We attributed this effect to the increasing amount of ceramic struts between the adjacent walls observed at lower freezing temperatures [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics Through Additives and Templating[END_REF] that reinforce the structure. When the half ice-templated tube is bent, the external midpoint is subjected to tensile forces. Since the porosity is perpendicular to the tensile forces, the strength of the tube is mainly determined by the resistance of the structure to opening the pores.

On the other hand, it is more difficult to discern the effect of tube thickness on flexural strength (Fig. 8-Bb). As discussed above, the strength data are too scattered to drawn any conclusion due to local imperfections of the tubes geometry.

Permeability

Tubes obtained with different solids loading, freezing temperature, and volume of slurry poured into the mold were tested to evaluate the independent impact of pore volume, pore size, and tube thickness respectively on permeability, Fig.

9.

In all conditions, pressure drop (∆P ) exhibits a linear dependence with air velocity (ν s ). A similar behavior was observed in a previous work [START_REF] Seuba | Gas Permeability of icetemplated, unidirectional porous ceramics[END_REF] with icetemplated YSZ monoliths in the same pore volume and pore size range. Therefore, permeability can be calculated from their slopes according to the Darcy's law (Eq. 5) instead of the Forchheimer equation [START_REF] Innocentini | Assessment of Forchheimers Equation to Predict the Permeability of Ceramic Foams[END_REF]. 2.

∆P L = P i 2 -P o 2 2P o L = µ k 1 ν s (5
Permeability values (k 1 ), experimental conditions, and the most relevant pore descriptors are specified in Table 2. When the pore volume increases due to the solid loading reduction, there is less resistance to the airflow and permeability increases accordingly, Fig. 9-A. Additionally, increasing pore volume increases the mean pore size, enhancing further this effect. µm) and lower permeability (2.80×10 -14 m 2 ). When the tubes are frozen using a constant solid loading and lower temperature, the total pore volume remains invariant and the pore size reduces, affecting the permeability. Interestingly, the sample frozen at higher temperature (-30 • C), pore size of 6.2 µm, and 52% porosity in Fig. 9-B has a comparable permeability (and hence pressure drop) to the sample frozen at -80 • C, pore size 7.3 µm, and porosity 68% in Fig. 9-A. This result emphasizes the importance of pore size over pore volume as the critical parameter to tailor permeability.

This observation is in agreement with the Hagen-Poiseuille equation that predicts a gas flow dependence (i.e. in a viscous flow) proportional to the pore volume compared with the square exhibited by pore size [START_REF] Ohji | Macro-porous ceramics: processing and properties[END_REF]. Finally, permeability can also be controlled by the volume of slurry initially poured into the mold (Fig. 9-C). When the volume of slurry decreases, the overall thickness of the tube decreases as well and the tube becomes more permeable due to the reduction of the pathway to evacuate the air.

Solids loading (wt.%) Porosity (%) Freezing Temperature ( Although, the external layer with finer and non-oriented porosity (referred here as transition zone) can facilitate the subsequent deposit of a dense coating [START_REF] Moon | Preparation of NiO-YSZ tubular support with radially aligned pore channels[END_REF],

it might also act as a bottle-neck and consequently enhance the pressure drop.

This effect can be observed in Fig. 10 where a permeability test was performed in a tube before and after sanding off the external surface. The tube had a P(%) of 45%, a mean pore size of 3 µm, and an overall thickness around 2 mm.

The specimen before the superficial treatment, and thus still with the transition zone, exhibited a higher pressure drop (i.e. lower k 1 ) compared with the sample after removing the superficial layer. The increase of permeability by a factor 6 highlights the role of this transition zone on permeability and draws the attention towards new hierarchical bilayared microstructures as the path to enhance the gas or liquid flow without an exaggerated increase in pressure drop.

Conclusions

We developed a set-up to produce ice-templated tubes with accurate control of pore volume, pore size, and average thickness. Mechanical testing results

showed a clear impact of pore volume and size on flexural strength, but results from radial crushing were inconclusive. The permeability of tubes was successfully characterized, and we observed a high dependence on pore size.

The formation of a hierarchical microstructure obtained with this set-up could be beneficial in applications that require an asymmetric configuration such as solid oxide fuel cells, oxygen transport membranes, or water filtration. The external layer with finer and non-oriented porosity can facilitate the deposit of a dense coating and also increase the electrochemical reaction sites, while the unidirectional lamellar porosity minimizes the energetic losses reducing the effective transport pathway.

Figure 1 :

 1 Figure 1: Schematic of the rotational freezing process.

Figure 2 :

 2 Figure 2: SEM micrographs at different locations of an ice-templated tube with a 65 wt.% solid loading, frozen at -30 • C, and 20 ml. A) Top view of the external layer B) Cross section of the tube and C) Top view of the inner surface.

Figure 3 :

 3 Figure 3: Variation of porosity as a function of the solid loading of the slurry. Dashed line represents the the relationship between solid loading and porosity obtained in a previous work for ice-templated YSZ monoliths [17].

Fig. 4

 4 Fig.4shows a top view of three ice-templated tubes with a different amount of slurry initially poured into the mold (20, 18, and 16 ml). Decreasing the volume of slurry reduces the overall thickness of the tube because essentially there is less material to be ice-templated. The overall thickness of the tubes after sintering was 2.0, 1.8, and 1.6 mm for slurry volumes of 20, 18, and 16 ml.

Figure 4 :

 4 Figure 4: Detail of the thickness variation by changing the initial amount of slurry.

Figure 5 :

 5 Figure 5: A) Representative load-displacement curves of two rings obtained from tubes with a 65% solids loading and frozen at -80 • C (red) and -30 • C (blue). B) Detail of the characteristic fracture in a O-ring test with the two types of cracks highlighted.

Figure 6 :

 6 Figure 6: Effect of porosity and freezing temperature on radial crushing strength obtained by O-ring test. Data summarized in Table1.

Figure 7 :

 7 Figure 7: Examples of tubes made by rotational freezing. A) Overview B) Cross section of a tube.

Figure 8 :

 8 Figure 8: Effect of freezing temperature A), and volume of slurry (which determines the tube thickness) B) on flexural strength obtained by four-point bending.

  ) L is defined as the thickness of the tube, µ the viscosity of the fluid (in this case air at 25 • C), ∆P is the pressure drop, ν s the fluid velocity (air flow divided by the internal area of the tube), and k 1 the Darcian permeability.

Figure 9 :

 9 Figure 9: Effect of experimental conditions on pressure drop. A) 65% and 50% solid loading, B) -80 • C and -30 • C, and C) 20, 18, and 16 ml poured initially in the mold. Values specified in Table2.

Fig. 9 -

 9 Fig.9-B shows how the specimen frozen at -80 • C exhibits a smaller pore size(3.3

Figure 10 :

 10 Figure 10: Effect of transition zone on pressure drop.

Table 1 :

 1

	65	53.96 ± 2.54	-80	3.3	20.81 ± 6.6
	55	65.70 ± 0.83	-80	6.8	14.01 ± 3.90
	50	70.89 ± 1.55	-80	7.3	8.64 ± 3.36
	65	52.64 ± 1.85	-30	6.2	28.31 ± 4.86
	55	67.64 ± 1.18	-30	10.6	15.98 ± 1.39
	50	72.50 ± 0.96	-30	14.6	7.63 ± 1.31

Solids loading (wt.%) Porosity (%) Freezing Temperature ( • C) Mean d p (µm) σ ORing (MPa) Summary of the effect of solids loading and freezing temperature on porosity, mean pore size (dp), and radial crushing strength (σ ORing ). dp was obtained by mercury intrusion porosimetry.

Table 2 :

 2 Summary of the main experimental conditions and the effects on porosity, mean pore size (dp), and permeability (m 2 ).** mean pore size and * thickness of the tube not measured but considered equivalent to the sample frozen at -80 • C, 65% solids loading, and 20 ml of volume of slurry (V Slurry ).

Preprint submitted to Journal of the European Ceramic SocietyDecember 6, 2016

Acknowledgements

The research leading to these results has received funding from the CNRS and Saint-Gobain under BDI grant agreement 084877 for the Institut National de Chimie (INC), and from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement 278004 (project FreeCo).


Chantal Aspe
email: chantal.aspe@univ-amu.fr

Andre ´gilles •
email: andre.gilles@imbe.fr

Marie Jacque Ŕeceived

Á M Jacque

Á A Gilles

Irrigation canals as tools for climate change adaptation and fish biodiversity management in Southern France

Keywords: Irrigation canals, Ecosystem services, Climate change adaptation, Interdisciplinary research

This paper is based on the interdisciplinary research conducted in the south of France that analyses the different economic, social and environmental roles played by agricultural irrigation canals. We argue that beyond their productive role, which is to supply farmers with water, they fulfil other environmental services and play an important role in the context of future climate change to face challenges of adaptation. We point up several ecosystem services provided by such canals, e.g. replenishing the groundwater table, the development of riparian vegetation and wet areas in the Mediterranean zone, tools for regulating flooding and drought, the bases for new cultural approaches to nature. Moreover, they play an important role in the maintenance of an ichthyological biodiversity that is indispensable for the persistence of natural ecosystem. Functioning as an ecological corridor, they display interesting capacities as refuges for certain fish species under stress. Indeed, they can potentially connect upstream and downstream zones over a continuum of more than 300 km and thus covering very contrasted climatic zones (alpine versus Mediterranean). For now, most of these services remain largely unknown and underestimated. However, they serve as assets for territorial development since they combine economic, ecological and social factors whose remodelling is becoming increasingly necessary in the face of climate change.

Introduction

The numerous issues surrounding climate change have spurred a great deal of research worldwide that aims to renew thinking about water management [START_REF] Milano | Facing climatic and anthropogenic changes in the Mediterranean basin : what will be the medium-term impact on water stress ?[END_REF]Garcia Ruı ´z et al. 2011). Most of this research advocates for water-saving measures and encourages the agricultural sector-the largest water consumer-to change its watering practices, notably by moving away from surface irrigation and instead developing overhead irrigation and drip irrigation techniques [START_REF] Diaz | Climate change impacts on irrigation wa ´ter requirements in the Guadalquivir river basin in Spain[END_REF]Iglesias et al. 2011a). In the south-east of France, agricultural irrigation canals, whose main productive role is to supply farmers with water, have indirect environmental effects that could be recognized as services: e.g. replenishing the water table, rainwater disposal and evacuation of torrential rains, landscape enhancement [START_REF] Aspe | The role of traditional irrigation canals in a long term environmental perspective-a case study in Southern France: the Durance basin[END_REF] and the maintenance of biodiversity. These services remain largely unknown and are under-appreciated, but they could be major assets for the development of irrigated territories, particularly in the context of climate change. The very dense hydraulic network of irrigation canals that exists across the Provencal Territory means that such canals can play a role in adapting to the effects of climate change. To do so, it seems necessary to move beyond a quantitative approach to water management (Iglesias et al. 2011b;[START_REF] Arnell | The implications of climate policy for the impacts of climate change on global water resources[END_REF]) and instead engage with thinking that examines the qualitative effects of its use.

In a first section, we shall begin by looking at how the indirect environmental effects of irrigation canals have gradually begun to be recognized thanks to the development of ecosystem services [START_REF] Wolter | Functional vs scenic restoration-challenges to improve fish and fisheries in urban waters[END_REF][START_REF] Braconnot | Analyse et mode ´lisation du changement climatique, 2 e `me e ´dition du Livre blanc Escrime. Socie ´te ´Me ´te ´orologique de France, Me[END_REF]. Their perpetuation is as such an important aspect of water resource sharing in the context of climate change. Over time, this approach may allow for better remuneration, via payments for environmental services, made to the facilities that manage them. Our current interdisciplinary research also aims to show that beyond their economic and social utility, irrigation canals are part of a complex hydro-social system that appears to play an important role in maintaining the ichthyological biodiversity of rivers [START_REF] Katano | Species diversity and abundance of freshwater fishes in irrigation ditches around rice fields[END_REF][START_REF] Rosenvald | Fish assemblages in forest drainage ditches: degraded small streams or novel habitats?[END_REF][START_REF] Colvin | Fish Use of Intermittent Watercourses Draining Agricultural Lands in the Upper Willamette River Valley, Oregon[END_REF]). The second section will show how irrigation canals help maintain biodiversity at the scale of the Durance basin and how they could constitute tools for managing biodiversity to address future climatic trends. For them to be considered as such, changes are likely needed in how they are managed; they also need to be permanently integrated into territorial development policies.

From productive water to environmental water: towards recognizing the ecosystem services of irrigation canals in the context of climate change

In the south-east of France, agricultural irrigation canals are a key infrastructure for the sharing and management of water resources. The first canals were built in the Durance River basin between the Middle Ages (St Julien canal, 1171) and the Renaissance (Craponne canal, 1554) to channel water for its driving force. The Durance region's hydraulic infrastructure grew in the late eighteenth century and really flourished in the nineteenth century, with the construction of numerous structures and the development of a network of secondary canals and distributaries (small canals made of earth that supply and divide plots) for farming purposes. Their construction was the product of a combination of the economic interests of large farmers and the planning ambitions of the state which sought to increase the agricultural yield from the plains of Southern France to secure food supplies for cities. From the late nineteenth century onwards, the Durance canals allowed irrigated agriculture to develop, an intensification of production, an increase in crop area and the introduction of new crops. There are currently fifteen canals which contain 200 million cubic metres of the 1.2 billion stored in the Serre-Ponc ¸on dam built in 1955. They distribute water across the entire Provencal Territory via a network of gravity-fed canals that represent 540 km of primary structures and 4,000 km of secondary networks, and cover a surface area of 150,000 hectares [START_REF] Balland | Rapport sur la Durance. Propositions de simplification et de modernisation du dispositif d'intervention de l'Etat sur la gestion des eaux et du lit de la Durance. Contribution a `un plan Durance[END_REF]. Figure 1 shows the main canals of the lower Durance and the expanse of territories-the so-called irrigated areasthey supply (Fig. 2).

Since the law of 21 June 1865, irrigation canals have been managed by Authorized User Associations (ASAs); landowners automatically have member status in the associations for as long as they own their property. Each landowner whose property contains a section of canal is obliged to participate financially (i.e. via the payment of a fee) and technically (i.e. via a servitude) in running the hydraulic infrastructure. In exchange, the person has a right to use the resource based on conditions (e.g. rates, irrigation turn) set by the ASA. The entire surface area of irrigated land comprises the canal's perimeter and corresponds to the ASA's area of intervention. Until fairly recently, the management of irrigation canals occurred in a predominantly agricultural context and the upkeep of the hydraulic network of each perimeter (cleaning distributaries, clearing, etc.) was an integral part of each farm's work. While this remains the case in most areas supplied by the Durance canals, newer property owners are less and less likely to be farmers, but they continue to benefit from the rights associated with the canal's presence on their property; housing developments, golf courses and business parks are increasingly common and occupy land previously used for farming.

These new landowners are often much less assiduous about maintaining the secondary canal networks, when their distributaries are not simply covered over or filled into save space. Weakened by agricultural abandonment (changes in land use), urbanization and the decreasing number of farmers, the irrigators' associations are now seeking to accommodate new uses of the resource in order to preserve the hydraulic structures. Agreements have been signed, for example, with municipalities for the watering of municipal ''green space'' and with private sector actors from golf courses.

The use of water for farming purposes, which represents 70 % of the resource's global consumption, is presented in international reports as the sector that needs to make the greatest water-saving efforts (FAO 2007;OECD 2013). It is as such that European and international policy support measures to modernize irrigation networks and reduce the use of surface irrigation techniques (EEA 2012) in favour of systems that consume less. While we do not wish to challenge the utility of overhead and drip irrigation, we feel that the maintenance of irrigation canals-which mobilize large volumes of water-could become essential for local societies to adapt to changes in climatic conditions. Indeed, the specific services provided by gravity-fed networks address two major challenges that Mediterranean societies will have to face: the scarcity of the resource and protection during increased periods of intense rainfall. The Mediterranean region will likely experience a rise in temperatures and an increase in droughts (GIEC 2007;[START_REF] Deque | Frequency of precipitation and temperature extremes over France in an anthropogenic scenario: model results and statistical correction according to observed values[END_REF]Kuglitsch et al. 2010;[START_REF] Hertig | Temperature extremes in the Mediterranean area: trends in the past and assessments for the future[END_REF][START_REF] Hoerling | On the increased frequency of Mediterranean drought[END_REF]) that will in turn likely affect the availability of water resources (Douville and Terray 2007; Climate change adaptation and fish biodiversity management in Southern France [START_REF] Braconnot | Analyse et mode ´lisation du changement climatique, 2 e `me e ´dition du Livre blanc Escrime. Socie ´te ´Me ´te ´orologique de France, Me[END_REF][START_REF] Lespinas | Impact of recent climate change on the hydrology of coastal Mediterranean rivers in Southern France[END_REF][START_REF] Milano | Facing climatic and anthropogenic changes in the Mediterranean basin : what will be the medium-term impact on water stress ?[END_REF]. According to IPCC experts, the estimated water deficit for the Mediterranean basin varies between 45 and 60 mm per decade, with greater variability in precipitation and, especially, an intensification of rainfall episodes and extreme hydrological phenomena that will cause an increase in water levels, run-off water, flooding, etc. (GIEC 2007;[START_REF] Durand | Reanalysis of 44 years of climate in the French Alps (1958-2002): methodology, model validation, climatology and trends for air temperature and precipitation[END_REF][START_REF] Bigot | Contraintes climatiques dans les Pre ´alpes franc ¸aises: e ´volution re ´cente et conse ´quences potentielles futures[END_REF].

The water in irrigation canals represents a useable supply for the needs of irrigated Mediterranean agriculture, as well as for new urban and peri-urban uses in the zones they transect. Use of water from the canals as such limits the amount of water drawn from the water table; further, the hydraulic infrastructure they form is a structural tool for sharing the resource that exists and can continue to play an essential role in a context of heightened water stress. Given the way irrigation canals works, they also help renew groundwater resources. Indeed, the circulation of water through the entire network of distributaries and irrigation ensure that the region's main water tables are replenished. The most emblematic example is the Crau1 which is practically devoid of surface water but has a water table containing 550 million cubic metres that supplies 270,000 inhabitants with water in the Bouches-du-Rho ˆne department and Fos industrial zone. Two-third of this aquifer's replenishment comes from irrigation water diverted from the Durance via the Craponne canal (CED 2007;[START_REF] Berard | Actualisation du mode `le de la nappe de la Crau[END_REF][START_REF] Bourrie | Ro ˆle de la composition de l'eau d'irrigation, des interactions avec les constituants des sols et de l'absorption de nutriments dans la protection de la qualite ´de la nappe de la Crau et des sols-un exemple ancien d'agriculture durable. Le sol face aux changements globaux[END_REF]. At present, the perpetuation of this water table is ''totally dependent on intake from leaks in canals and percolation from surface irrigation, estimated at 160 Mm3/year'' [START_REF] Balland | Rapport sur la Durance. Propositions de simplification et de modernisation du dispositif d'intervention de l'Etat sur la gestion des eaux et du lit de la Durance. Contribution a `un plan Durance[END_REF] [translated here]. In the Durance sector, irrigation water also plays an important role in the maintenance of several groundwater resources. According to a study commissioned by the St Julien canal ASA in 1999, the water table of the lower Durance region is supplied for 75 % with irrigation water versus only 25 % by natural means (watercourses and precipitation) [START_REF] Blavoux | La re ´alimentation de l'aquife `re alluvial par l'irrigation gravitaire en Basse Durance[END_REF]. Recent research into the water table that supplies the city of Avignon further confirmed the importance of this proportion and the major role played by irrigation water in replenishing the aquifer [START_REF] Nofal | Impact de la gestion des eaux d'irrigation sur la recharge des nappes en Basse Provence. Exemple de la plaine d'Avignon. In: Aspe C (ed) De l'eau agricole a `l'eau environnementale. Re ´sistance et adaptation aux nouveaux enjeux de partage de l'eau en Me ´diterrane[END_REF].

Irrigation canals are also water draining facilities. They as such supply another indirect service-rainwater disposal-and play a significant role in protecting against flooding during periods of intense rainfall. For example, in the Salon-de-Provence sector, irrigation canals drain off 35-100 % of rainwater depending on the municipality (FDSH 13 2008). An economic evaluation of the canal's replacement cost was conducted by the city of Sisteron.

Were the Saint-Tropez canal hydraulic structure to disappear, the municipality would have to invest a million euros, in addition to the cost of maintaining a drainage system, whereas the annual maintenance cost of the main agricultural canal and its secondary network is estimated at 30,000 euros/year (Chambre d'Agriculture Alpes de Haute-Provence 2005). The Carpentras canal ASA has signed an agreement with the city of Carpentras for the regulation of rainwater. As a result, the municipality invested in the construction of a balancing tank to relieve the main canal when necessary. The modernization and maintenance of hydraulic networks by ASAs increasingly include such new services, although they are still for the most part funded by the ASAs; their main challenge resides in ensuring that the entire hydraulic network covered is in good working condition. Some ASAs have acquired remote operated valves that allow them to react more quickly during periods of intense rainfall. Others ensure that distributary networks no longer used for farming are up-kept to ensure rainwater disposal. In the event of an increase in extreme rainfall events, the hydraulic network of irrigation canals could as such become an efficient monitoring and evacuation tool to prevent flooding. The decree of 1 July 2004, which modified the status of ASAs and repealed the law of 21 June 1865 that had governed them up until then, broadened the role and scope of user associations to include ''the creation and management of facilities that help in risk prevention, the management of natural resources, and the planning of watercourses, roadways and networks'' [translated here] ( [START_REF] Balsan | Innovations sociales et Institutionnelles dans la Gestion de l'Irrigation en Me ´diterrane ´e (ISIIMM)[END_REF]). The recognition of these services is a sign that irrigation canals are no longer valued solely for their productive merit but also for their environmental value [START_REF] Aspe | De l'eau agricole a `l'eau environnementale. Re ´sistance et adaptation aux nouveaux enjeux de partage de l'eau en Me ´diterrane[END_REF].

That is why it is particularly through new types of public involvement in the environment field that a new assessment and remuneration of these services may be envisaged. Since the recommendations of the 2005 Millennium Ecosystem Assessment, promoting ecosystem services is one important angle of contemporary environmental policy. These services are defined as ''the favoured means to understand how certain ecosystems have a positive, often unknown, impact on economic activity and 'human wellbeing''' [translated here] (MEA 2005). Such services are divided into four categories: provisioning (food, energy), regulating (water quantity and quality), supporting (soil creation) and cultural services. The report estimates that 60 % of these services are in decline due to anthropic pressure on ecosystems [START_REF] Mea) | Ecosystems and Human Well-Being. Synthesis. Island Press, Washington, DC Ministe `re de l'e ´cologie, du de ´veloppement durable, des transports et du logement (MEDDTL)[END_REF]. It therefore appears that one of the current challenges in water management is limiting its agricultural use while maintaining the ecosystem services provided by irrigation practices. A recent report entitled Releasing the Pressure: Water Resource Efficiencies and Gains for Ecosystem Services by researchers from the Stockholm Environment Institute (SEI) for the UNEP invites decision-makers and resource managers to shift from thinking in terms of traditional water productivity per unit of agricultural yield to a broader understanding of the concept that would encompass ecosystem services. ''This is a challenge, because all too often the benefits from ecosystem services are neither immediately recognized, nor easily valued… Recognizing these wider benefits generated by water in respect of, for example, nutrient flows, cooling, providing habitats, and other supporting and regulating ecosystem services is the aim of our work'' [START_REF] Keys | Releasing the Pressure: Water Resource Efficiencies and Gains for Ecosystem Services[END_REF]). Among other things, the report proposes ecosystem management tools in order to identify the different types of benefits of ecosystem services in their local context. These different types of benefits, as we have underscored, are numerous when it comes to agricultural irrigation canals and focusing solely on saving water does not seem appropriate in the Mediterranean context.

The maintenance of hydraulic irrigation infrastructure is particularly contingent on recognition of the specificities of Mediterranean agriculture and the types of water resource management that have allowed it to develop; rather than seeing the water used as ''lost'', this allows it to be understood as working to produce ecosystem services. For now, the monetary assessment and remuneration of these services remain fairly uncharted territory. Certain ASAs have signed local agreements with municipalities, which in most cases involve the participation of municipal technical staff in the network's maintenance. The monetary assessment of services provided ''unintentionally'' by the canals-like replenishing the aquifer-requires an assessment of what the services cost the irrigation activity on the one hand, and a definition of who are the direct beneficiaries and their remuneration methods (taxes, public funding, etc.) on the other hand [START_REF] Kuhfuss | Comment e ´valuer les services rendus par les canaux d'irrigation? Application au cas de la recharge de nappe[END_REF]. Since the implementation of agro-environment measures into the Common Agricultural Policy in 1992, farmers have slowly begun to include ''services provided'' to the environment in their farming practices. While they were initially funded in the context of public grants, environment services now tend to be negotiated in the context of market regulations that require the definition of beneficiaries deprived of these services [START_REF] Pagiola | Payments for Environmental Services Environment Strategy Notes 3[END_REF][START_REF] Ferraro | The cost-effectiveness of conservation payments[END_REF][START_REF] Van De Sand | Payments for ecosystem services in the context of adaptation to climate change[END_REF]. Moreover, the implementation of payments for ecosystem services (PES) requires the emergence of new intermediary actors who see an interest in the monetary exchange of services (e.g. municipalities, private companies) [START_REF] Ph | Les e ´volutions inverse ´es de la multifonctionnalite ´de l'agriculture et des services environnementaux[END_REF].

Can the remuneration of environment services by the indirect users of canals truly provide a long-term management solution for hydraulic infrastructure facilities?

How can such funding be guaranteed in contexts in which farming plays an increasingly small part in the management of these facilities, evermore dependent on new uses? For now, the financial support garnered by ASAs for these new services remains marginal and is predominantly government subsidized via the Agence de l'eau (water agency). At the scale of the Mediterranean basin, the water agency has taken a governance approach via canal contracts whose goal is to get new actors involved in the canals' management (e.g. municipalities, non-farming users, nature conservation associations, fishing federations and river unions) through the signing of a charter of goals to help define the medium and long-term actions needed to ensure the perpetuation of hydraulic networks (Jacque ´2012). These charters are not a legal obligation and are not legally binding for the new actors associated with the management of canals.

The long-term viability of the Durance region's hydraulic infrastructure is now contingent on recognition of the indirect services it provides and on the funding it can secure from the facilities that manage them. As we have seen, these services may be advantages in future to help local societies adapt to climate change, whether in terms of managing water shortages or its abundance. If they are not rethought in a local context, water-saving policies currently conducted in the name of strategies to adapt to climate change could have a certain number of unwitting effects on the availability of water resources, as well as on the ecological quality of aquatic and land environments, both of which are main priorities in contemporary environmental policies.

Ecological assessment versus economic evaluation: canals as biodiversity-management tools for the integrated development of territories

The biodiversity of aquatic ecosystems will likely be particularly affected by hydrological changes connected to climate change (The Convention on Biological Diversity held in Nagoya in October 2010) adopted a strategic plan for biological diversity-the ''Aichi Targets''-which set 20 quantified targets for the conservation and restoration of biodiversity to ''halt the loss of biodiversity in order to ensure that by 2020 ecosystems are resilient and continue to provide essential services, thereby securing the planet's variety of life, and contributing to human well-being and poverty eradication'' (CDB 2010). In the field of aquatic environment management, controlling and limiting sampling is a key factor for the preservation of ichthyological biodiversity. ''It is almost certain that disturbances to freshwater ecosystems, such as dams, reservoirs and diversions for irrigation and industry, will endanger or extinguish many freshwater fish species in future, by creating physical barriers to normal movements and migration of the biota and by decreasing habitat availability'' [START_REF] Leadley | Sce ´narios de biodiversite ´: projections des changements de la biodiversite ét des services e ´cosyste ´miques pour le 21 e `me sie `cle[END_REF].

And yet, our initial findings on the ichthyological biodiversity of the Durance canals point up that the impact of these diversions on biodiversity is not necessarily negative. The hydrological network comprised of all of the main canals and their distributaries have produced an aquatic environment with ecological dynamics that are distinct from those in the regulated river but which are also complementary for the ichthyological population of the Durance. Indeed, the canals could not be considered as a ''habitat analogous'' (i.e. an environment that support indigenous biodiversity due to their structural or functional resemblance to natural ecosystem, Lundholm and Richardson 2010), because these artificial habitats displayed ''a priori'' none similar structures. The canals corresponded more to a functional novelty in which the different species could survive and spread through this aquatic network inducing a new area of biodiversity. Biodiversity here is measured by the abundance of fish and number of species, which is a particularly interesting indicator since fish species are central to the aquatic ecosystem (i.e. keystone species). Indeed, they play an important role in regulating the phytoplankton and zooplankton populations. As a result, fish are a bio-indicator of the state of the river as shown by the size of the populations present in an environment. The samples taken in irrigation canals (Fig. 3) show the presence of numerous fish species, proof of the broad ecological diversity, including of some protected species: vairone (Cyprinidae, Leuciscus souffia) and apron (Percidae, Zingel asper), the former of which is listed in Annex II of the Habitat-Fauna-Flora directive and Annex III of the Bern Convention and the latter of which is considered to be ''in critical danger of extinction''. Within the canals, we find a food web characteristic of the Durance region ranging from herbivore-diatomivorous cyprinid species like Chondrostoma (Parachondrostoma toxostoma and Chondrostoma nasus) to omnivores like chub (Squalius cephalus), spirlin (Alburnoides bipunctatus) and vairone, to insectivores like percid apron.

Our findings point up a close relationship between the hydraulic infrastructure of the canals and the circulation of fish upstream and downstream in the river. The canals play a role in maintaining freshwater fish biodiversity in the river via a return to the natural environment through tailwater and drainage water, as well as being a refuge zone for reproduction. One significant observation was that different species have even colonized infrastructure whose size does not exceed 50 cm in width and a water depth of 12-15 cm. There is a large proportion young-of-the-year fish (20-30 mm TL) in these distributaries, which allows us to deduce that they are reproduction zones or capture zones stocking young specimens. We can as such assert that, during the impoundment period (which can last several years in the EDF power canal in which we also took samples), canals have great ichthyological diversity combined with a diverse population (i.e. individuals of varying age groups). Individuals present in the canals that are released in the Durance River during closure periods (or during a 10-year draining) may form an important source of diversity that allows for a restocking of the river, whose stocks can decline drastically after flooding (flow and clogging) or during the warmest periods (summer in France) inducing an increase in water temperature specially when heatwave occurs. However, it could be a pitfall for species living \10 years (as the highly protected Z. asper) especially if no reproduction was possible in the canal; thus, future studies will focus on this last point. Considering the other species, the canals are actually a receptacle of the Durance's diversity, which can be restored during canal draining operations, as well as throughout their impoundment via the return of tailwater and drainage water to the river. They can as such be seen as temporary refuges. And yet, is this the only role played by these commensals of the Durance River? Or rather, could these newly formed environments stocked by the Durance River not also be the source of a reconstructed biodiversity?

Moreover, the existence of genetic polymorphism in the Durance River for different species (specially for the ''chondrostoma'' species complex) can be explained by two complementary phenomena: a long-established local population (the populations belonging to the Rhone basin are more polymorphic than those belonging to the Garonne basin) and introgressive hybridization (a supply of new genic variants from the common nase (C. nasus) compatible with the South-west European nase P. toxostoma) which increase genetic diversity both in individuals and the population (for more information, see [START_REF] Costedoat | Etude de l'hybridation introgressive entre Chondrostoma t. toxostoma et Chondrostoma n. nasus (Te ´le ´oste ´en, Cyprinidae) en utilisant une approche multiple[END_REF] for allozyme markers; [START_REF] Costedoat | Novelties in hybrid zones: crossroads between population genomic and ecological approaches[END_REF] for intron markers [START_REF] Sinama | Non-homogeneous combination of two porous genomes induces complex body shape trajectories in cyprinid hybrids[END_REF] for microsatellite markers). However, molecular marker involved in the immune response as the major histocompatibility complex (MHC) presented a similar level of MHC polymorphism between both species and the hybrid specimens. Taking into account that the MHC plays an important role in pathogen and parasite resistance, it seems obvious that Durance environment selected specimens with ''local'' alleles (P. toxostoma). This is particularly true for the C. nasus specimens (the non-native species) that present pure genetic pedigree on neutral markers (based on 41 microsatellites), but adaptive alleles from P. toxostoma based on MHC.

These two phenomena (ancestral polymorphism and introgressive hybridization), which are at the corner stone of such strong genetic diversity, confer a new plasticity in the life history traits [START_REF] Corse | Trade-off between morphological convergence and opportunistic diet behavior in fish hybrid zone[END_REF][START_REF] Corse | Plasticity of ontogenetic trajectories in cyprinids: a source of evolutionary novelties[END_REF] of the endemic species, allowing it to survive in a modified environment.

The biodiversity generated by the hybridization phenomenon has, however, traditionally been underestimated either out of a lack of interest or due to a poor understanding of the phenomenon. Indeed, hybrids are often viewed as an aberration between two ''pure'' bloodlines rather than as an adaptive potentiality, an evolving novelty from which selection can occur and which could allow for the establishment of new lines [START_REF] Crispo | Broken barriers: human-indiced changes to gene flow and introgression in animals[END_REF]. We now know that 25 % of embryophyte species (plants) are currently the product of hybrid lineages, as are 10 % of animals [START_REF] Mallet | Hybridization as an invasion of the genome[END_REF]. This research makes it clearly apparent that hybrids are an indicator of an ecosystem's biological diversity, living proof of the existence of unexploited or even new ecological niches [START_REF] Seehaussen | Speciation reversal and biodiversity with hybridization in changing environments[END_REF]. Indeed, eutrophication can cause speciation reversal in whitefish (Salmonids fish) due to demographic decline and speciation reversal through introgressive hybridization [START_REF] Vonlanthen | Eutrophication causes speciation reversal in whitefish adaptive radiations[END_REF]. However, [START_REF] Nolte | Understanding the onset of hybrid speciation[END_REF] have proposed an alternative model in which hybridization acts as a creative force, giving rise to novel traits imparting a new ecological potential absent from the parental lineages, which may be seen as a non-negligible source of adaptive variation in animals. It thus remains unclear whether these two outcomes (speciation reversal versus species formation) are mutually exclusive in a particular biological model (regardless of the environment) or whether the model responds differently in different environments (a mosaic of patterns).

The existence in the canals of individuals belonging to both ''Chondrostoma'' species and of a good number of hybrids allows us to assert that the canals have an impact on the evolutionary history of populations in the Durance River. Indeed, we found in Carpentras canal 17.39 % of hybrid specimens, 20.29 % of invasive species (C. nasus) and 62.32 % of the endemic species (P. toxostoma). Irrigation canals form a mosaic of pockets that contain the genetic resources of the Durance River and which can restock the river according to the variability of climatic conditions. The introduced common nase was meant to dominate and make the endemic South-west European nase disappear. As predicted by some author hybridization and introgression with non-indigenous species may lead to a form of extinction of endemic fish [START_REF] Rhymer | Extinction by hybridization and introgression[END_REF]. And yet this did not occur since both species coexist and are distinctly divided in the Durance ecosystem. The canals played a positive role in this context in the sense that they allowed for the maintenance of hybrid specimens [START_REF] Aspe | The role of traditional irrigation canals in a long term environmental perspective-a case study in Southern France: the Durance basin[END_REF]) and now function as reservoirs of biodiversity for the river, reservoirs whose evolution is governed by the genetic crossover they allow between the introduced species and the endemic species. In the context of climate change, these reservoirs could allow for the maintenance of genetic biodiversity and provide refuge conditions for these species.

The preservation of hydraulic irrigation infrastructure is currently justified more for its environmental value than for its productive value, even if the latter remains an essential facet of its role. And yet, the economic evaluation of ecosystem services for maintaining biodiversity appears more complex and also raises a certain number of ethical issues. The Stern Review on the Economics of Climate Change [START_REF] Stern | Review on the economics of climate change[END_REF]) was the first to calculate not the cost of environmental damage but rather the cost of inaction, advancing the idea that preventive measures are economically more profitable than those that seek to repair. Inspired by this approach, the UNEP created the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) to provide governments with a tool to evaluate the loss of biodiversity and the subsequent cost for national economies. In France, this evaluation was conducted by the Centre d'Analyse Strate ´gique (CAS), which published a report in 2009 (the ''Rapport Chevassus'') that provided an economic evaluation of the value of biodiversity at the national scale. The type of evaluation chosen was similar to the one used in the Millennium Assessment and adopted the same main categories of services. Although the authors specified several times that a monetary assessment was primarily meant to guide public decision-making for planning choices, ecosystems were nonetheless monetarily assessed and given a price that took into account their contribution to biodiversity (Chevassusau-Louis et al. 2009). The decision to attach a price meant that the assessment of the uses and territories managed was primarily market driven and used mainly economic tools rather than taking into account measures based on an ecological approach to conservation [START_REF] Aubertin | Compte rendu de document. Approche e ´conomique de la biodiversite ´et des services lie ´s aux e ´cosyste `mes[END_REF]. The principle of economic compensation for the loss of biodiversity is the backbone of such marketdriven reasoning. A compensatory measure is an ''ecological action that aims to restore or recreate a natural environment to offset the damage to biodiversity caused by a planning document or project. It addresses only the residual impact following prevention and impact mitigation measures, which are given priority'' [translated here] (Commissariat Ge ´ne ´ral au De ´veloppement Durable (CGDD) 2012). This approach was especially common in the so-called Grenelle 1 planning law of 2009, supplemented in 2010.2 Compensatory measures are usually considered when avoidance measures have been examined but not chosen and when notable residual impact remains despite impact mitigation measures. It is what is expressed by the triptych: ''avoid, reduce, compensate'' (Ministe `re de l'e ´cologie, du de ´veloppement durable, des transports et du logement (MEDDTL) 2012). Too often, however, the ''avoid'' and ''reduce'' steps are skipped and focus is placed directly on the act of ''compensating'' and estimating how great the impact that needs to be compensated. The proposed compensatory measures are often devoid of any real stakes in terms of ecological functionality. The purchase of land in good ecological condition or putting land aside as a compensatory measure does not do any good for the environment, although that is the stated goal of compensatory measures. Ecosystems are not interchangeable and the disappearance of certain areas raises questions about the ecological functions that disappear with them.

The services provided by irrigation canals cannot be disconnected from the overall working of hydraulic infrastructure (at once ecological, economic and social) and their preservation cannot be bound up in a compensatory logic. Rather, the contemporary challenge resides in their perpetuation and avoiding even their partial disappearance. We believe that when these facilities are seen as planning tools for environmentally sustainable land, political and economic solutions may be envisaged. In France, municipalities need to play a greater role in their management, even if their involvement remains limited for now. Different regulations and policy approaches in the field of territorial planning should encourage this involvement. Municipalities are now asked to include all canals, even those that are only distributaries, in their urban planning documents and land use plans. 3 Undertaken by a few municipalities in the Durance region, this approach is far from complete; one of the main challenges is to fully understand the entire hydraulic network that underpins each territory. The environmental value of irrigation canals as ecological corridors could also provide an argument for their perpetuation, and they could be included in new territorial planning tools such as green and blue corridors. 4 Here too, it is up to municipalities, in agreement with government authorities, to draw up development plans that include green and blue corridors. The contemporary challenge will surely be to find a new economic balance and social compromise in the technical handling and management of these facilities.

Given the global environmental problem posed by climate change, we need to imagine practical and local solutions to adapt. The maintenance-and even the protection-of gravity-fed irrigation canals could be an efficient preventive measure to address the anticipated effects of warming in Mediterranean regions. While they may not ''save'' water, these canals provide other environmental services that can play an important role in the balance of recognized advantages.

Fig. 1

 1 Fig. 1 Main agricultural irrigation canals and their irrigated areas in the Val de Durance (south-east of France)

Fig. 3

 3 Fig. 3 Electric fishing in an irrigation canal (Vaucluse-France)

The Crau or the Crau plain is a paleo-delta of the Durance near the Camargue in the Bouches-du-Rho ˆne department.

Planning law n°2009-967 of

August 2009 on the implementation of the Grenelle Environnement, JORF n°0179, supplemented in 2010 by law n°2010-788 of 12 July 2010 on the national commitment to the environment, completed on 1 January 2014.3 Law on risks n°2003-699 of 30 July 2003, Decree n°2005-115 of 7 February 2005 implementing articles L. 211-7 and L. 213-10 of the Code de l'environnement and article L. 151-37-1 of the Code rural.

Art. R. 371-16 of decree n°2012-1492 of 27 December 2012.

Acknowledgments Our research project HYODA (''Hydro-sociosyste `me-Durance-Acteurs''), Practices and social representations of environmental water. The Durance canals as the backdrop for new ecological, economic and social issues is financially supported by the Agence de l'Eau Rho ˆne Me ´diterrane ´e Corse and Electricite ´de France (EDF).