Kevin Samyn
email: k.samyn@brgm.fr

Olivier Cerdan

Gilles Grandjean

Romain Cochery

Séverine Bernardie

Adnand Bitri

Assessment of vulnerability to erosion: digital mapping of a loess cover thickness and stiffness using spectral analysis of seismic surface-waves

Keywords: Surface wave, Erosion, Digital mapping, Shear wave velocity, Cone resistance

soundings were also performed using a Dynamic PANDA penetrometer.

Concurrently, boreholes were used to sample the same horizon for bulk soil density (ρ b ) measurements. Based on these measurements, large variations in ThickLM were observed. The distribution of Vs values along a 130 m transect allowed for the distinction between two layers corresponding to different mechanical properties. The Vs value of 240 m/s was then used as a limit between the loamy material and the underlying clays. This limit was validated using drilling observations performed on the same transect. Therefore, it was possible to map the ThickLM, which varied between 0.2 and 6.5 m over the entire field. The comparison between the averaged values of Vs and Qd in the loamy material layer showed a significant correlation (R 2 =0.4) such that the mapping of StiffLM was realised from the Vs map and the Vs-Qd relationship.

Density comparison between the ρ b measured on drill samples and the ρ b calculated from Vs were also performed using previously published relationships; however, significant correlations were not observed. The obtained maps of ThickLM and StiffLM were consistent with the expected effects of erosion at the catchment scale and provide indications of historical erosion events. This methodology, which provides a structural and mechanical characterisation of subsurface materials, should help to focus conservation measures to the most threatened areas (i.e., the identification of areas that show a reduced ThickLM and increased StiffLM, which are associated with high soil erosion vulnerability and/or high compaction state).

Introduction

The current growing awareness of ecological issues has led to an increasing demand for environmental datasets that are necessary for adequate monitoring and management of various threatened ecosystems. There is also a growing concern regarding the sustainability of biomass production, not only in developing or semi-arid areas [START_REF] Hadgu | Spatial variation in biodiversity, soil degradation and productivity in agricultural landscapes in the highlands of Tigray, northern Ethiopia[END_REF][START_REF] Ye | Production scenarios and the effect of soil degradation on long-term food security in China[END_REF] but also in industrialised countries. The increasing demand for cereals and biofuels will further accentuate this trend. Strong relationships between soil erosion, soil depth and soil productivity have been reported in various environments (e.g., [START_REF] Biot | Loss of yield caused by soil erosion on sandy soils in the UK[END_REF]Tendberg et al., 1997;[START_REF] Heckrath | Tillage erosion and its effect on soil properties and crop yield in Denmark[END_REF][START_REF] Rejman | Topsoil reduction and cereal Yields on loess soils of Southeast Poland Land Degrad[END_REF]. The regolith and, more generally, the soil surface material are particularly important, as they support human activities and fulfil numerous ecosystem services (European Commission., 2002-2006). However, our knowledge of the nature and spatial extent of surficial materials is far from complete, and therefore further research is necessary to fill this gap. The ability to accurately and rapidly produce soil depth maps or to delineate areas with limited root penetration depths will, therefore, become crucial to address these issues. Conventional soil surveys are generally based on manual sampling and visual observations of soil pits or auger holes. Such observations are met with both methodological and economic constraints when used for the investigation of large areas. Not only are these observations extremely time consumingand thus costlybecause of the highly repetitive fieldwork that is needed, they can also be destructive to the soil. Moreover, this type of local observation may not represent larger-scale trends in soil properties.

Geophysical techniques offer an interesting alternative to traditional soil sampling methods, especially for estimating the spatial variability of physical soil parameters of large areas. These techniques are particularly relevant because most physical soil characteristics are closely related to soil properties (e.g., geoelectrical properties) [START_REF] Rhoades | An electrical conductivity probe for determining soil salinity[END_REF][START_REF] Robain | A multiscale electrical survey of a lateritic soil system in the rain forest of Cameroon[END_REF][START_REF] Samouëlian | Electrical resistivity survey in soil science: a review[END_REF]. For example, a significant correlation has been demonstrated between the apparent electrical resistivity () or electrical conductivity () and the soil texture [START_REF] Williams | The use of electromagnetic induction to detect the spatial variability in the salt and clay content of soils[END_REF], soil water content [START_REF] Kachanoski | Estimating spatial variations of soil water content using noncontacting electromagnetic inductive methods[END_REF][START_REF] Kalinski | Estimating water content of soils from electrical resistivity[END_REF][START_REF] Michot | Spatial and temporal monitoring of soil water content with an irrigated corn crop cover using electrical resistivity tomography[END_REF], soil salt or nutrient content [START_REF] Rhoades | Determining soil electrical conductivitydepth relations using inductive electromagnetic soil conductivity meter[END_REF][START_REF] Eigenberg | Geophysical electromagnetic survey methods applied to agricultural waste sites[END_REF] or soil depth [START_REF] Thompson | Color index for identifying hydric conditions for seasonally saturated mollisols in Minnesota[END_REF][START_REF] Chaplot | Mapping fieldscale hydromorphic horizons using Radio-MT electrical resistivity[END_REF]. In contrast to these electrical methods, seismic techniques are not well established in soil sciences but could be particularly promising. Due to the development of subsurface characterisation studies for environmental or geotechnical purposes, the efficiency of seismic methods for estimating ground velocity structures and mechanical properties has seen considerable progress in the recent decades and has been used in various applications in several fields: waste disposal [START_REF] Lanz | Refraction tomography over a buried waste disposal site[END_REF], landslides [START_REF] Grandjean | Geophysical data fusion by fuzzy logic for imaging mechanical behaviour of mudslides[END_REF], or hydrogeophysics [START_REF] Sturtevant | Hydrogeophysical characterization of bedrock fracture orientations using azimuthal seismic refraction tomography[END_REF]. Modern equipment, which generally features 48 or 72 recording channels and PC-piloted acquisition software, has made this method user-friendly and has contributed to its popularity. Recently, an adaptation of the sensor line, which is based on unplugged gambled geophones, was proposed to drastically reduce the acquisition times (Grandjean, 2006a;[START_REF] Debeglia | Karst investigations using microgravity and MASW; Application to Orléans, France[END_REF]. This improvement was supported by the development of new data processing protocols, such as acoustical tomography [START_REF] Azaria | High-resolution seismic mapping at a groundwater contamination site: 3-D traveltime tomography of refraction data[END_REF]Grandjean, 2006b) or spectral analysis of surface-waves (SASW) [START_REF] Nazarian | Use of spectral analysis of surface waves method for determination of moduli and thicknesses of pavement systems: Transport[END_REF][START_REF] Park | Multichannel seismic surfacewave methods for geotechnical applications[END_REF][START_REF] Grandjean | 2M-SASW: inversion of local Rayleigh wave dispersion in laterally heterogeneous subsurfaces: application to Super-Sauze landslide (France)[END_REF] and related multichannel MASW applications [START_REF] Foti | Multistation Methods for Geotechnical Characterization using Surface-waves[END_REF]Miller, 1999;Park et al., 1999aPark et al., , 1999b;;Xia et al., 1999), which allowed for the construction of shear waves velocity (Vs) profiles. For example, the analysis of fundamental-mode Rayleigh waves is one of the most common methods for using the dispersive properties of surface waves [START_REF] Bullen | An introduction to the theory of seismology[END_REF]. This type of analysis provides essential parameters that are commonly used to evaluate near-surface stiffness, which is a critical property for many geotechnical studies [START_REF] Stokoe | Characterization of geotechnical sites by SASW method[END_REF]. SASW uses the spectral analysis of the ground roll that is generated by an impulsive source and is recorded by a pair of receivers. This method has been widely and effectively used in many geotechnical engineering projects [START_REF] Stokoe | Characterization of geotechnical sites by SASW method[END_REF]. A single pair of receivers is configured and reconfigured (based on wavelength calculations made during the acquisition) as many times as necessary to sample the desired frequency range. Unfortunately, the necessity of recording repeated shots during multiple field deployments for a given site increases the time and labour requirements relative to a multichannel procedure. Multichannel analysis of surface waves (MASW) is designed to overcome the few weaknesses of the SASW method. The purpose of this study is to test the MASW method as a new tool for characterising soil mechanical properties (i.e., soil thickness and stiffness) with respect to erosion processes.

Instead of sounding the area by systematic drilling, seismic methods were tested to efficiently produce soil property maps. Specifically, we evaluated the feasibility of using mechanical contrasts that exist between the lithologies to map the thickness and stiffness (ThickLM and StiffLM, respectively) of the surface loamy material horizon using Vs data. Based on this process, we should be able to identify the most threatened areas at the catchment scale, i.e., areas that show a reduced ThickLM associated with high StiffLM, using Vs data coupled to cone resistance (Qd) data because StiffLM can be used as a surrogate for the soil erodibility.

Materials and methods

1. Description of the study field

The study area was located within the Bourville catchment in Normandy, a region of Northern France where erosion that removes the upper soil horizons is a major threat to the main soil functions (i.e., "food and other biomass production, storage, filtering, and transformation of elements among which water and nutrients, biological habitat, and source of raw materials", European Commission, 2002;[START_REF] Van-Camp | Reports of the Technical Working Groups Established under the Thematic Strategy for Soil Protection[END_REF]. The studied area includes a catchment of the European loess belt in Normandy; this region is severely affected by water and wind erosion with rates often exceeding soil production (between 5 and 10 ton.ha -1 .yr -1 ; [START_REF] Cerdan | The rate and spatial distribution of soil erosion in Europe[END_REF] (Figure 1). Normandy has a humid-temperate climate with few days of frost. The average temperature ranges between 10 and 12 °C throughout the year, and in August, which is the hottest month of the year, the temperature fluctuates approximately 18 °C. The hilly areas, characterised by a smooth relief (0-7%) and deep superficial layer (>5 m), are used for intensive production of alternating winter and spring crops (e.g., wheat, beets, and maize). An area of approximately 1.5 ha was selected in this catchment to test our approach. Not only was this area highly degraded with a quasi-complete removal of the soil cover in certain areas, its surrounding areas also showed a gradation in the thickness of the loamy material horizon from thick and well conserved in the low elevation locales to thin and eroded in the high elevation areas. The research plot was delineated within the transition zone. This plot, which extended along the steepest slope (Figure 1), shows clear evidence of soil degradation in its north-west and south-east areas with the presence of outcropping clays with flints, both of which are characteristics of on-going loamy material horizon erosion. The entire soil surface was covered by grass vegetation during the field study.

Geological setting and geomorphologic conditions

The study area was located in north-western France (Normandy), which is characterised by a humid-temperate climate. The topography was relatively smooth with slope gradients ranging between 1% and 4% on the plateau and 4% to 10% on valley sides. The area is covered by silt loam soils, which developed on the loess Quaternary deposit, and contains at least 60% silt in the surface horizons. These soils are classified as Neoluvisol in the French Classification system and are described as "excessively drained" according to the USDA (2003) soil drainage classification (Orthic Luvisol, World Reference Base, 1998). Such soils are very sensitive to soil sealing because of their low clay content (130 to 170 g.kg -1 ) and low organic matter content (10 to 20 g.kg -1 ) relative to more competent underlying clays that are enriched with flints. When in arable use, large areas are left bare and open to rainfall during most of the cultural season, which, combined with the sensitivity to sealing, renders them vulnerable to runoff and water erosion (Figure 2). In contrast, sediments accumulate due to erosion flows in sheltered areas. Based on the geomorphologic conditions, the thickness of the uppermost loamy sediments presents a high variability at the catchment scale.

3. Theory and basic principles of the MASW method

In the majority of surface seismic surveys, when a compressional wave source is used, more than two-thirds of the total seismic energy generated is imparted into Rayleigh waves [START_REF] Richart | Vibrations of soils and foundations[END_REF], which is the principal component of surface waves. Assuming vertical velocity variation, each frequency component of a surface wave has a different propagation velocity (called the phase velocity, Cf) at each unique frequency (f) component. This unique characteristic results in a different wavelength (λf) for each frequency that is propagated. This property is called dispersion. Although surface waves are considered noise in body-wave surveys (i.e., reflection or refraction profiling), their dispersive properties can be used to infer near-surface elastic properties via Vs evaluation [START_REF] Nazarian | Use of spectral analysis of surface waves method for determination of moduli and thicknesses of pavement systems: Transport[END_REF][START_REF] Stokoe | Characterization of geotechnical sites by SASW method[END_REF]Park et al., 1998a). The entire process typically used to produce reliable Vs profiles via the spectral analysis of surface waves involves three steps: (1) the acquisition of surface waves (Figure 3,5a), ( 2) the construction of dispersion curves (a plot of phase velocity as a function of f) (Figure 5b), and (3) the back calculation (inversion) of the Vs profile from the calculated dispersion curve (Figure 5c,d). A workflow diagram illustrating the processing method used to obtain reliable Vs profiles with depth is presented in Figure . 4.

For step (1), to obtain a good estimation of dispersion curves, we used a multistation (MASW) configuration, in which receivers are set at several locations and are regularly spaced along a straight line. A seismic source signal was generated via the impact of a hand-held hammer hitting a small iron anvil located on the ground. During the recording, the wavefield was discretised and truncated in both the time and space domain (Figure 5a). The sampling periods in the time domain were Dt=0.5 ms; the numbers of samples was M=1000. The near offset (also called origin offset; i.e., the distance between the source point and the first recording point along the line) of x 0 =50 cm, the geophone spacing of Dx=50 cm and the offset range of L=11.5 m (Figure 3) are the three important acquisition parameters that require proper selection to prevent aliasing, near field, and far field effects [START_REF] Xia | Configuration of near surface shear wave velocity by inverting surface wave[END_REF][START_REF] Miller | Multichannel analysis of surface waves to map bedrock[END_REF]. These effects determine the minimum and maximum depth in which Vs can be accurately measured using the MASW method. Due to certain undesirable effects, Rayleigh waves must be studied beyond the near field offset. Far from this distance, they can be considered as horizontally travelling plane waves and processed accordingly. The adaptation of MASW to soil investigations is first conditioned by the possibility of reducing the seismic array (originally consisting of several tens of meters) to approximately several meters, provided that nearfield effects are avoided. Second, the frequency range of the seismic signal is increased to obtain a depth of interest up to a maximum of 10 m below the ground surface. Finally, the selected seismic system involved a hammer source that was capable of generating signals in the 1 to few tens of Hz frequency range; and a seismic antenna composed of 24 geophones capable of recording signals from 10 to 200 Hz was used. The entire system was towed behind a vehicle (Figure 3a) to ensure a rapid acquisition. A total of 157 seismic observations were performed along 13 transects with regular 12 m spaces between data points, as shown in Figure 1.

For step [START_REF]Max: maximum; Av: average; Median; Stdev, standard deviation; Var: variance; CV, coefficient of variation; Skwe, skewness; Kurt, kurtosis for ThickLM[END_REF], the generation of a dispersion curve is one of the most critical steps for generating an accurate Vs profile. Dispersion curves are generally displayed as Cf as a function of f (Figure 5b). For impulsive data, a frequencydomain approach (Park et al., 1998b) is used to calculate the dispersion curve. This approach involves a 2D wavefield operation that transforms seismic data from the space-time domain into the Cf-f domain, which is more convenient for highlighting dispersion features.

For step (3), the Vs profiles were calculated using an iterative inversion process [START_REF] Tarantola | Inverse problem theory[END_REF] that requires dispersion data (Figure 5c). A least-squares approach allows for the automation of the process [START_REF] Xia | Configuration of near surface shear wave velocity by inverting surface wave[END_REF]. For the method used here, only the Vs and model thickness are updated after each iteration; Poisson"s ratio remains unchanged throughout the inversion. An initial earth model needed to be specified as a starting point for the iterative inversion process. The earth model consists of velocity (P-wave and S-wave velocity), density, and thickness parameters. Among these four parameters, Vs has the most significant effect on the reliable convergence of the algorithm. For each iteration of the inversion process, an update of Vs is calculated, and synthetic dispersion curves are back calculated from this new Vs model. The synthetic dispersion curve is then compared to the observed dispersion curve based on the least-squares method (Figure . 5d). A reliable Vs model is obtained when the misfit between synthetic and observed dispersion curves is minimised. The inversion algorithm used in our study implements all of these aspects and is based on [START_REF] Hermann | Computer programs in seismology[END_REF]. The stop criterion for the Cf residuals between synthetic and observed dispersion curves was defined as less than 5 m.s -1 .

Finally, the interpolation of contiguous Vs models resulting from the inversion process is realised using a natural neighbour method to obtain a 2D Vs section along the transect.

4. Penetrometer soundings

A dynamic penetrometer with variable energy (Afnor, French norm NF XP P 94-105) can be used to record the mechanical resistance (Qd in MPa) variation with depth [START_REF] Sanglerat | The penetrometer and soil exploration[END_REF][START_REF] Burns | Small and high-strain measurements of in-situ soil properties using the seismic cone penetrometers[END_REF] by manually driving a rod into the soil using a standardised hammer. Therefore, a penetrometer can be used to identify variations in soil profiles according to the penetration modulations observed on the rod at each blow of the hammer. Because this method is easy to use and relatively quick, this portable automatic penetrometer is well adapted for detailed prospecting and mapping [START_REF] Zhou | Caractérisation des sols de surface à l"aide du pénétromètre dynamique léger à énergie variable type Panda, thèse[END_REF]. Each penetrometric sounding (CPT) products a vertical cone resistance profile called a penetrogram; the interpretation of the penetrogram allows for the identification of different layers and that estimation of their thickness using two main criteria: well-defined cone resistance thresholds and the shape of the penetrogram. Qd is calculated using the following relationship [START_REF] Langton | The PANDA lightweight penetrometer for soil investigation and monitoring material compaction[END_REF]:

M P e MV A Qd     1 1 2 1 1 2 (1)
where A is the cone section (2 cm 2 ), M is the striking mass (kg), V is the impact velocity (ratio between the cell spacing and travel time between cells), P is the struck mass (kg) and e is the drill string progress (m). Qd soundings were performed using a Dynamic PANDA penetrometer [START_REF] Gourvès | Le pénétromètre dynamique léger Panda[END_REF] along transect n°5 of the field plot with a regular spacing of 12 m; this survey was conducted at the same location as the collection of seismic data points (Figure 1). In this study, we assimilated the cone mechanical resistance Qd into the soil stiffness.

5. Drilling observations

In addition to the abovementioned surveys, 4 drilling observations were performed on the same transect with a spacing of 40 m in the midslope and 20 m in the footslope regions (Figure 1). Each borehole was excavated in the north-west to south-east direction to depths of 1.85 m, 3.5 m, 6 m and 1.7 m.

Drilling cores were used to estimate ThickLM and for laboratory measurements of the water content (θ), the real density of the solid (ρ r ) and the bulk density (i.e., the density of the sample as a whole) (ρ b ) in the loamy layer; these results are shown in Table 1.

6. Sensitivity of Vs to soil mechanical properties

Numerical relationships between soil mechanical properties and Vs have been previously published. One of these relationships, which is given by elastic theory and is an essential property for evaluating dynamic responses and the stiffness of soil, is the small-strain shear modulus, G (i.e., a measure of solidity).

Values of G are typically determined indirectly by measuring the shear wave velocity, Vs, and the mass density of the soil, ρ, and computing G= ρVs 2 . Other commonly used relationships include the correlation between cone penetration resistance or SPT blow count (N) and Vs and functional forms; this relationship is reported to be Vs=A.N S , where the constants A and B are determined by a statistical regression of a data set. A significant number of correlations have been published for various soil types. [START_REF] Imai | The relation of mechanical properties of soils to P and S-wave velocities for ground in Japan[END_REF] studied the relationship between seismic velocities and certain index properties for 192 samples and developed empirical relationships for all soil types. [START_REF] Sykora | Correlations of in-situ measurements in sands of shear wave velocity[END_REF] reported that geological age and type of soil are not predictive of Vs, whereas the uncorrected SPT-N value is the most important term. [START_REF] Sykora | Correlations between dynamic shear resistance and standard penetration resistance in soils[END_REF] demonstrated a strong statistical correlation between the dynamic shear resistance and standard penetration resistance of soils. Iyisan (1996) examined the influence of the soil type on the correlation between SPT-N and Vs using data collected from an earthquake-prone area in the eastern part of Turkey. The results showed that with the exception of gravels, the correlation equations developed for all soils, sand and clay yield approximately similar Vs values. [START_REF] Jafari | Dynamic properties of the fine grained soils in south of Tehran[END_REF] presented a detailed historical review of the statistical correlation between SPT-N and Vs. [START_REF] Hasancebi | Empirical correlations between shear wave velocity and penetration resistance for ground shaking assessments[END_REF] studied similar statistical correlations using 97 data pairs collected from an area in the north-western part of Turkey and developed empirical relationships for sands, clays, and all soils irrespective of soil type. Ulugergerli and Uyanık (2007) investigated statistical correlations using 327 samples collected from different areas of Turkey and defined the upper and lower bounds of an empirical relationship instead of a single average curve for estimating seismic velocities and relative densities.

These previous studies, as well as a qualitative comparison of data available on our study site, demonstrate the good correlation of Vs to soil mechanical properties and allow for the expectation of a relatively good characterisation of StiffLM using Vs and an accurate estimation of ThickLM (Figure 6) in this context.

Evaluation of ThickLM and StiffLM of the loamy material horizon

1. Relationships between Vs and CPTs

The distribution of Vs values inverted along the transect n°5 allowed for the discrimination between two populations that corresponded to different mechanical properties. Each population was characterised by Gaussian laws of mean 1st layer = 171 m.s -1 , σ 1st layer = 80 m.s -1 and of mean 2nd layer = 347 m.s -1 , σ 2nd layer = 76 m.s -1 , where σ is the standard deviation (Figure 7). Moreover, the isovalue of Vs=240 m.s -1 was highly consistent with the isovalue Qd=20 MPa, which marks the boundary between a soft superficial unit and a competent underlying formation (Figure 8). This observation was then used for the calibration of a Vs threshold value (VsLim = 240 m.s -1 ) to characterise a mechanical limit used as a criteria to map ThickLM over the entire field based on the entire seismic dataset. Finally, the mean squared error (MSE) between ThickLM observations and estimations were computed using a validation set of 4 drilling points that were positioned to be representative of the observed main variability of ThickLM.

To study the stiffness of the loamy material-horizon, we compared Vs and Qd, obtained by CPTs, at a same depth. The two parameters can be interrelated because they are both influenced by effective level of confining stress, the anisotropic K 0 -stress state, mineralogy, aging, bonding, and other factors [START_REF] Mayne | Correlations between shear wave velocity and cone tip resistance in natural clays[END_REF][START_REF] Stuedlein | Shear-wave velocity correlations for Puyallup River alluvium[END_REF][START_REF] Dikmen | Statistical correlations of shear wave velocity and penetration resistance for soils[END_REF]. We first applied an exclusion filter to the Vs data with the condition Vs < VsLim to restrict the analysis to the loamy layer. Then, we computed the average Qd values in each interval of Vs 1D models. Nugget effects on the Qd data, which occur due to the punctual presence of various defects (e.g., pebbles) in the medium, were previously removed from the dataset using an interpolating operator. Figure 9 shows the linear regression between Vs and Qd where a correlation between Qd and e Vs is observed whith R 2 =0.4 and a two-tailed P value less than 0.0001. By conventional criteria, this difference is considered to be extremely statistically significant even if samples are highly scattered. Therefore, we predicted Qd according to Vs using the following equation:

Vs e Qd . 0057 . 0 2965 . 1  4 . 0 2  R 0001 . 0  P (2)
Mapping of the spatial variation of StiffLM as a function of the average Qd calculated over ThickLM using Eq. ( 2) then becomes possible over the entire field. Density comparisons between the ρ b measured for the drill samples and the ρ b calculated from Vs using published relationships [START_REF] Mayne | Stress-strain-strength-flow parameters from enhanced insitu tests[END_REF] were also performed, and no significant correlations were determined. This absence of correlations was probably due to the lack of ρ b values for the clay formation.

2. Spatial structure and interpolation of the data

The spatial structures of ThickLM and StiffLM were assessed using variograms that were estimated in four directions at 20°, 55°, 110° and 155° from geographic north. The variograms were generated using all possible sample pairs in a given direction and by grouping these into classes (lags) of approximately equal distance [START_REF] Matheron | Les variables régionalisées et leur estimation: une application de la théorie de fonctions aléatoires aux sciences de la nature[END_REF]. The variances (one-half of the mean squared difference) of these paired sample measurements were then plotted as a function of the distance between samples to provide a means of quantifying the spatial structure of the data. ThickLM and StiffLM obtained using MASW and Eq. ( 2), respectively, were then interpolated using ordinary kriging.

Ordinary kriging is a geo-statistical method that takes into account both the distance and the degree of variation between known data points and relies on the spatial correlation structure of the data to determine the weighting values. This type of kriging has been shown to provide better performance for soil parameters than other available methods (e.g., [START_REF] Burgess | Optimal interpolation and isarithmic mapping of soil properties. IV. Sampling strategy[END_REF][START_REF] Myers | Spatial interpolation: an overview[END_REF]. The interpolations were accomplished by fitting each of several theoretical variogram models (i.e., linear, Gaussian, spherical, and exponential models) to the empirical isotropic variogram using the least-squares method.

The best fit model was then used for the interpolation. Data points were subsequently interpolated to a regular 2×2 m grid using a full second-order polynomial drift function, which is the common practice.

Results

ThickLM ranged between 0.2 and 6.5 m with an average of 2.6 m and a median of 2.7 m (Table 2; Figure 10a). As shown by the variogram analysis (Figure 11a), ThickLM exhibited a moderate anisotropy and spatial structure.

Nevertheless, a slight azimuth dependence of less and greater variability was observed at the the 20° and 110° directions, respectively, relative to the other directions of less and greater variability than the other directions. The observations were interpolated over the 120×130 m plot and using an exponential model (sill = 0.75; range = 4.5 m) coupled with a Gaussian model (sill = 9; range = 70 m). The interpolated ThickLM map showed a gradual increase in thickness from the north-west limit of the plot, where values were approximately 0.20 m, to the mid-plot position; a gradual decrease was then observed from the mid-plot position to the south-east limit, where ThickLM was approximately the same as on the north-west side (Figure 10a). An area of greater ThickLM (between 3 and 6.5 m) was observed at the mid-plot position.

A local area of greater ThickLM (between 1.5 and 3 m) was observed on the north-west side of the plot. The differences between the observed and estimated ThickLM values are shown in Figure 12. The MSE of 0.043 m and R 2 =0.956 between these two variables over the 4 validation points shows that the estimated ThickLM could be considered to be accurate based on the consideration that the drilling observations are determined to be representative of the overall observed variability of ThickLM over the entire plot.

StiffLM, which is related to the mean Qd calculated over ThickLM, ranged between 2 and 6 MPa with an average of 3.78 MPa and a median of 3.68 MPa (Table 2; Figure 10b). As demonstrated by the variogram analysis (Figure 11b), StiffLM exhibited a moderate anisotropy and spatial structure. Anisotropy was observed between the 20 and 110° directions with less and greater variability than the other directions. The observations were interpolated over the 120×130 m plot using a Gaussian model (nugget = 0.1; sill = 0.7; range = 50 m). The interpolated StiffLM map showed a gradual decrease in stiffness from the northwest limit of the plot, where the values range between 5 and 6 MPa, to the midplot position; a gradual increase from mid-plot position to the south-east limit, where StiffLM presented an lower stiffness than on the north-west side, was also observed (Figure 10b). An area of lower StiffLM (between 2 and 4 MPa) was observed at the mid-plot position. A local zone of lower StiffLM (approximately 4 MPa) was observed in the north-west side of the plot.

Discussion

The maps of ThickLM and StiffLM were in good agreement with the expected consequences of the on-going erosion of the loamy material horizon: a gradation in the thickness of the loamy material-horizon from thick and well conserved in the lower parts to thin and eroded in the upper parts, was observed (Figure 10a). The increase of StiffLM at the limits of the study plot shows clear evidence of soil degradation in the north-western and southeastern zones with the presence of bare soils and outcropping clays and flints, which have a mechanical resistance. The local event of greater ThickLM and lower StiffLM indicated on Figures 10a and10b with a black dotted line should correspond to a buried former gully that was photographed at this location 2 years prior to the study and was caused by strong runoff activity (Figure 10c). This structural and mechanical characterisation of the loamy material horizon should not only help to focus conservation measures in the most threatened areas (i.e., areas that show a reduced ThickLM and increased StiffLM associated with high soil erosion vulnerability and/or a high compaction state), but will also help identify historical erosion events. The combination of maps of soil stiffness and surficial sediment depths will allow the development and implementation of soil conservation measures to target high-priority areas.

Where climatic scenarios are available, these maps can also be used to calculate the potential productivity loss using simulators, such as the SimPLE.ca model [START_REF] Bremer | SimPLE.ca: Simulator of productivity loss due to erosion for Canada[END_REF]. This method will also contribute to the broad discussion of reducing the inherent uncertainty in current soil mapping or attribute determinations. Soil maps show that there can be considerable uncertainty in map unit composition with resulting spatial variability in soil properties within map units (Webb and Lillburne, 2005). Actual soil maps define discrete soil classes, which represent the interpolation of only a limited number of modal soil profiles without capturing the full extent of soil variability [START_REF] Campbell | The missing geographic dimension to soil taxonomy[END_REF][START_REF] Qi | Comparing three methods for modeling the uncertainty in knowledge discovery from area-class soil maps[END_REF]. In the study presented here, the nondestructive mapping of continuous soil properties in space will help to improve the data frequency and allow for the derivation of probability distributions to parameterise the fuzzy nature of the geographical objects that comprise the soil maps [START_REF] Martin-Clouaire | Representing and processing uncertain soil information for mapping soil hydrological properties Computers and Electronics in[END_REF].

Conclusions

The objective of this study was to test and validate a new geophysical technique for mapping soil properties that are related to soil erosion processes. A seismic experiment coupled with penetrometric measurements form the basis of the proposed methodology. The MASW method was efficient for producing Vs models over large areas. When coupled with penetrometric measurements of Qd variations with depth, high-resolution maps of soil thickness and stiffness can be produced, provided that a correlation is found to occur between Vs and Qd.

We demonstrated that accurate mapping of variations in the thickness and stiffness of the loamy material horizon can be obtained by integrating information on the relationship between seismic Vs, soil mechanical resistance and drilling observations into the mapping process. An analysis of the correlation between seismic Vs and soil mechanical behaviours provided an effective basis for the accurate delineation of a specific soil attribute. According to the discussion, the analysis of the correlation between the seismic shear wave velocity and the soil bulk density at an interface of strongly contrasted mechanical properties should be studied further.

The future of precise mapping of selected soil properties using geophysical seismic techniques in dependent on the understanding of relationships between geophysical signals obtained from this technology and the overall spatial and temporal variability of soil patterns. As the value of the soil resources and associated ecological services receive greater recognition, digital soil mapping based on seismic shear wave velocity can provide spatial data regarding soil degradation that will serve as an essential tool for soil conservation and/or soil rehabilitation.

Acknowledgements

Figure captions

Figure 1 .

 1 Figure 1. Location of the study plot within the Bourville catchment in Normandy,

Figure 2 .

 2 Figure 2. Loamy sediments showing the critical thickness are rendered more

Figure 3

 3 Figure 3. a) Photograph of the towing system with the seismic acquisition

Figure. 4 .

 4 Figure. 4. Workflow diagram illustrating the methodology using to obtained

Figure 5

 5 Figure 5. a) Example of recorded surface waves. b) Observed dispersion curve

Figure 6 .

 6 Figure 6. Qualitative comparison between a drilling observation, a penetrometer

Figure 7 .

 7 Figure 7. Representation of the Vs distribution. Two populations characterised by Gaussian laws of mean 1st layer = 171 m.s -1 , σ 1st layer = 80 m.s -1 and of mean 2nd layer = 347 m.s -1 , σ 2nd layer = 76 m.s -1 are distinguished.

Figure 8 .

 8 Figure 8. Seismic and penetrometric sections along transect n°5. Threshold

Figure. 9 .

 9 Figure. 9. Observed Vs after exclusion filtering (Vs>240 m/s) versus averaged

Figure 10

 10 Figure 10. a) Spatial variations in the thickness of the loamy material horizon

Figure 11

 11 Figure 11. a) Directional variogram of ThickLM. b) Directional variogram of

Figure 12 .

 12 Figure 12. Plot of estimated ThickLM values obtained using the MASW

  Figure 1

Figure 6

 6 Figure 6

This study was funded by the FP7-DIGISOIL project. The DIGISOIL project (FP7-ENV-2007-1 N°211523) is financed by the European Commission under the 7th Framework Programme for Research and Technological Development, Area "Environment", Activity 6.3 "Environmental Technologies". The authors thank J. F. Ouvry from AREAS for his help in focusing the study field and Alexis Rochat for his help in acquiring field seismic data. Finally, the authors thank the editors of Geoderma and two anonymous reviewers for their comments and suggestions that led to an improved version of the manuscript.


J-D Rinaudo
email: jd.rinaudo@brgm.fr

M Montginoul

M Varanda

S Bento

Envisioning innovative groundwater regulation policies through scenario workshops in France and Portugal

Keywords: Climate change, groundwater management, water markets, self-regulation, France, Portugal, scenario workshops

Groundwater management policies will need to be revised in many Mediterranean countries, in light of the impact of climate change and the increasing demand for water. In this paper, we analyze stakeholder perceptions of three groundwater policy scenarios which respectively assume a strengthening of state intervention, the introduction of market regulation mechanisms, and the transfer of regulation responsibility to farmers. The method consists of organizing scenario workshops with experts, institutional representatives, and farmers. It is applied in two case studies in France and Portugal. From a methodological viewpoint, the research demonstrates the farmers' ability to contribute to an exploratory assessment of possible future water management scenarios. From a policy viewpoint it clarifies expectations concerning State intervention and self-regulation by farmers.

It also provides some insights regarding the acceptability of tradable groundwater permits in two European contexts.

Envisioning innovative groundwater regulation policies through scenario workshops in France and Portugal INTRODUCTION Over the last several decades, groundwater abstraction has increased considerably in northern Mediterranean countries [START_REF] Easac | Groundwater in the Southern Member States of the European Union[END_REF] and even more in southern Mediterranean countries.

Government agencies have not paid much attention to this 'silent revolution' [START_REF] Llamas | Lessons from intensive groundwater use in Spain : Economic and social benefits and conflicts[END_REF], and groundwater development has thus taken place in an institutional setting that placed no or few limits on groundwater use [START_REF] Schlager | Community management of groundwater[END_REF]. As a result, it is only after several decades, and investments made by thousands of farmers, that the consequences of these overdrafts, e.g., sea water intrusion, have started to appear, conflicts are escalating, and that establishing new rules for governing access to groundwater is seen as an absolute necessity.

In France and Portugal, the two countries considered in this paper, controlling groundwater abstraction is becoming an significant water policy issue. To comply with the requirements of the European Union Water Framework Directive, water authorities now have to develop a program of actions leading to the "good quantitative status" of all groundwater bodies. One of the first actions implemented has consisted of estimating the maximum inter-annual average volume that can be pumped from a number of critical groundwater basins characterized by a risk of over-exploitation. In parallel, specific efforts are made to complete the registration of all existing boreholes, including illegal ones. The next step for groundwater policy development will consist of developing and implementing new rules for sharing a limited quantity of water among economic sectors, and between users within each sector. Long-term climate change prospects will need to be considered while crafting such new rules for governing groundwater allocation, as they will probably have to be enforced in a context of increased water scarcity [START_REF] García-Ruiz | Mediterranean water resources in a global change scenario[END_REF].

For agricultural water users and their institutional representatives, the development and implementation of rules regulating access to and use of groundwater will be perceived as a paradigm shift, since most of them still consider groundwater to be an open-access good. To enhance the adequacy of regulations, as regards equity but also acceptability, and to reduce future implementation and enforcement problems, the stakeholders and users should be involved in the development of such regulations [START_REF] Ostrom | Governing the commons: the evolution of institutions for collective action[END_REF]). Their participation is essential because the policy options considered represent drastic changes that will call into question established institutions, societal values, and dominant ideologies and beliefs. Moreover, the practical (lay) knowledge of the stakeholders is essential for crafting rules that can work effectively in the context of local, technical, environmental, institutional, socio-economic, and. more generally cultural constraints [START_REF] Rinaudo | The benefits of combining lay and expert input for watermanagement planning at the watershed level[END_REF]. There are very few 'off-the-shelf' groundwater regulation models that could be imported from other countries without significant adaptation to the local context. This adaptation may require a lot of creativity which is likely to be enhanced by the confrontation of multiple visions of existing constraints and possible futures.

This paper aims to contribute to the scientific literature which explores how to craft rules for regulating groundwater usage. It is based on two concrete case studies conducted in France and Portugal, where a similar methodology was used. This consisted of organizing workshops with a group of institutional stakeholders, i.e., local administration representatives, economic actors, NGO's staff, and scientific experts, and several groups of farmers, during which three contrasted groundwater regulation scenarios were scrutinized and debated. The paper describes both the visions that emerged from the workshops and the social learning process that took place during the workshops.

The remainder of this paper is organized as follows. The next section presents the participatory foresight methodology used in this study, and the two case studies -France and Portugal -where it was applied. The second section describes the three groundwater regulation policy scenarios which were constructed and debated in the workshops by the French and Portuguese farmers and institutional stakeholders. The third section shows how these scenarios were perceived by the workshops' participants, and the lessons learnt for policy making. The last section evaluates the methodology and provides some policy recommendations.

METHODOLOGY AND CASE STUDIES

Unfolding the future in three steps

The common methodology implemented in the French and Portuguese case studies was inspired by the emerging literature on scenario planning and participatory foresight for water resources management (see for instance van der Helm 2003, and[START_REF] Hatzilacou | Scenario workshops : a useful for participatory water resources planning ?[END_REF]. Foresight is defined here as a multi-disciplinary approach whose goal is to help decision-makers explore the uncertainty associated with future developments, and to assess the consequences of the various strategic choices available. The approach here consisted of exploring a limited number of scenarios, each of which describes a possible future institutional set-up for groundwater regulation. The aim here is "to illuminate the choices of the present in the light of possible futures" [START_REF] Godet | Creating the future: the use and misuse of scenarios[END_REF], rather than to make predictions.

A group of institutional stakeholders and several groups of farmers were invited to participate in three series of workshops. In order to enhance the envisioning capacity of the participants, we adopted a sequential approach to future exploration. The first sequence was devoted to the analysis of past trends and the exploration of the future evolution of agricultural production in the case study area, at the 2030 time horizon 1 . Discussions focussed on agricultural system dynamics, water being considered only as a factor of production. This sequence aimed at developing temporal awareness. It also helped to reveal the common goals and shared assumptions of the participants, contributed to forging a team spirit, and generated confidence (among the participants, and with the researchers).

Water-management-related issues were not specifically debated and nothing in the scenarios discussed in this first phase challenged the established water-management paradigm.

The second sequence introduced a major uncertainty with the prospect of climate change. The impacts of climate change on the hydrological cycle were described in detail, using the results of research conducted in the case-study areas. Climate change forecasts introduce an anomaly that challenges the prevailing mind-set and forces participants to imagine a different future in terms of farming practices and water management. In this second sequence, we asked the participants to focus on changes that might occur at the farm level (cropping and irrigation practices) while issues related to water resource management in the river basins were postponed to the third sequence.

The third and last sequence was devoted to envisioning possible future groundwater regulation mechanisms, under three alternative models based on (i) strengthened administrative regulation, (ii) market-based regulation, and (iii) self-regulation by the farming community. In this sequence, participants were asked to build on the results of the first two sequences. The objective was to mobilize their analytical and creative capacities and to overcome the limitations imposed by the currently established institutional framework, and also by received values, beliefs, and expectations, when thinking about the future. The present paper focuses on this last sequence; the results of the two others are described elsewhere (Maton et al. 2010, Rinaudo et al. forthcoming). The following paragraphs describe how the groups were established and the workshops organized. 1 The analyses of past trends and of the current situation were carried out through interviews in France and through interviews and a preliminary workshop in Portugal. The exploration of the future development of agriculture was undertaken in a specific workshop during the two case studies.

Constitution of the group, and workshop organization

Although the scenario-workshop method usually involves the mixing of policy makers, business representatives, experts, and citizens [START_REF] Street | Scenario workshops: A participatory approach to sustainable urban living?[END_REF][START_REF] Andersen | Scenario workshops and consensus conferences: towards more democratic decision-making[END_REF][START_REF] Lienert | Future Scenarios for a Sustainable Water Sector: A Case Study from Switzerland[END_REF][START_REF] Hatzilacou | Scenario workshops : a useful for participatory water resources planning ?[END_REF]), we decided to set up separate groups for various public -institutional representatives and experts on the one hand, and farmers on the other hand. We also opted for smaller groups (five to fifteen persons) in contrast to the scenario groups described in the literature, where each workshop comprises between 25 and 40 persons [START_REF] Andersen | Scenario workshops and consensus conferences: towards more democratic decision-making[END_REF][START_REF] Hatzilacou | Scenario workshops : a useful for participatory water resources planning ?[END_REF][START_REF] Patel | Participatory scenario construction in land use analysis: An insight into the experiences created by stakeholder involvement in the Northern Mediterranean[END_REF]). This methodological choice was adopted to overcome the mistrust which characterizes relationships between farmers and public-sector experts.

To facilitate the debates, three contrasted, predefined future water management scenarios were used as material to initiate the debates. Scenario narratives were written by researchers in advance of the workshops, because we assumed that we would not have sufficient time to make trendbreaking scenarios emerge during the 4-5 hour scenario workshops. Each scenario consisted of a narrative describing groundwater management rules in 2030-2040 (see next section for a detailed presentation of the scenarios). The narratives used in Portugal and France were consistent, although adapted to the specific regional contexts. The three scenarios were clearly identified as starting points for discussion, which participants should deconstruct, criticize, and adapt according to their views. The workshops were tape-recorded to allow detailed analysis of the views expressed by participants.

In France, discussions of the scenarios took place during workshops held over a half-day period in the evening (6 to 11 PM), including a meal for the farmers. The workshops were organized as follows.

After a brief presentation of the workshop's objectives, the groundwater-management scenarios were successively presented, using posters as support. After each presentation, the participants were invited to comment individually during a round-table session before discussing any conflicting points of view. After all the scenarios had been discussed, the participants were invited to prepare individual rankings of the three scenarios according to their probability and desirability. Comparison of these pictures of the probable and desirable futures gave rise to a final group discussion.

A similar approach was implemented in the Portuguese case study. Prior to debating the scenarios, the research team presented the current administrative framework of water management. Then a reference scenario was presented followed by three separate presentations of the alternative groundwater-regulation scenarios, and each was discussed first in small groups and then in a roundtable format. The issues of probability and desirability for each of these three scenarios were also discussed. Whereas the farmers were invited to a workshop, the experts and institutional stakeholders were met individually in face-to-face interviews. In both case studies, a final meeting was then organized with farmers and institutional stakeholders together.

A key feature of our approach is that, since the workshops were organized in a research context, the aim was to encourage the expression of various pictures of the future, and to facilitate the mutual discovery of differing points of view concerning the future development of agriculture, and the associated irrigation-water demand and water-management frameworks. The posture adopted by the team was thus exploratory and not normative.

The key features of the two case studies are presented in the following table and in subsequent sections.

French case studies

In France, the Roussillon groundwater basin was selected for implementing the methodology described above. Located in southern France, and covering an area of 700 km², the Roussillon is characterized by one of the driest climates in France (average precipitation of 570 mm). The presence of relatively abundant surface and underground water resources has enabled the development of irrigation farming (about 15,000 ha irrigated), which produces mainly vegetables (tomatoes, cucumbers, potatoes, and artichokes) and fruit (peaches, nectarines, and apricots). About 11,000 ha are irrigated by open gravity channels which draw their water from three rivers, two of 2 Source : Montginoul and Rinaudo, 2009.

3 Source : adapted from [START_REF] Stigter | Screening of sustainable groundwater sources for integration into a regional drought-prone water supply system[END_REF] and Relatório PGBH Informação de suporte (2011). This evolution is taking place in a context of changing groundwater governance. Historically, access to groundwater use has been loosely regulated in France, which has resulted in a widespread perception that groundwater is an open-access good [START_REF] Barraque | Groundwater management in France: from private to common property ? in Managing common pool groundwater resources : an international perspective[END_REF]). This situation is currently changing, with the progressive implementation the Water Framework Directive and the subsequent 2006 Water Law. This law requires that a Maximum Permissible Annual Volume (MPAV) be defined

for each aquifer at risk of overexploitation, and that pumping authorizations be allocated in light of this constraint. Although the law and its implementing regulations do not specify how this overall quota has to be shared among users, it certainly represents a first step towards a more regulated water-allocation mechanism. In this context, local stakeholders and government agencies have recently initiated a Groundwater Management Plan for the Roussillon aquifers. Hydrogeological studies are currently been conducted to determine the maximum annual permissible volume (MAPV)

that can be pumped from the aquifer. Stakeholders and government agencies are also beginning to discuss arrangements that could be put in place to share the MAPV among farmers. This discussion is taking place in a very unfavourable economic situation for agriculture (a succession of several bad economic and climatic years).

Portuguese case study

Located in the South of Portugal, in the heart of the Algarve Region, the 318 km2 Querença-Silves aquifer is the largest and the most important aquifer in the region. The main water uses are for farming and drinking water, with more limited use by golf courses and industries. This aquifer is located in one of the most popular seaside tourism areas of Portugal, in which there is a tenfold population increase during the summer months. Farmers grow mainly citrus fruit, and overall their farms face marketing difficulties. In the summer of 2005, the extreme drought Portugal was going through brought this aquifer to record low levels, and pumping was restricted because of fears of saltwater intrusion. While there is as yet no evidence of groundwater overdraft, the Regional Water Administration of Algarve is keen to ensure that it does not occur in the future. In a 2009 report, the administration considered the Querença -Silves aquifer to be at risk of overexploitation, with water extraction exceeding 90% of the recharge [START_REF] Arha | Questõessignificativas da gestão da água, RegiãoHidrográfica das Ribeiras do Algarve, Participaçãopública, Informação de Suporte[END_REF]. Moreover, recent research has produced simulations showing that climate change could exacerbate the problem, resulting in seawater intrusion [START_REF] Stigter | Screening of sustainable groundwater sources for integration into a regional drought-prone water supply system[END_REF].

In Portugal as in France, groundwater is privately owned, while most surface water has been placed in the public domain. However, overexploitation reported in the south has led to changes. In 1966, a new civil code gave Government the ability to subject groundwater use to regulations, but this was not enforced until 1977, when a decree-law (No. 376/77) created a licensing system for water abstraction in the southern areas where aquifers are the only significant resources. The licensing system was expanded to cover the whole country in 1995, with varying levels of restrictions. Thus the trend is towards greater administrative control of private water rights, rather than towards redefinition of groundwater as a common property subject to user rights, which would lead to user boards like those proposed in Spain [START_REF] Barraque | Groundwater management in France: from private to common property ? in Managing common pool groundwater resources : an international perspective[END_REF].

The Portuguese case study was carried out while the just established Regional Water Administration of Algarve (RWAA) was developing its Regional Water Management Plan, in compliance with the Water Framework Directive. A specific groundwater management plan was not planned at the time.

The RWAA is the agency that issues licenses for the use of water and determines the maximum permissible volume that can be pumped per borehole. These regulations work well on paper but not in practice. Even though the RWAA has a Geographic Information System that supports the issuing of groundwater licenses, there are still a significant number of unregistered boreholes [START_REF] Arha | Questõessignificativas da gestão da água, RegiãoHidrográfica das Ribeiras do Algarve, Participaçãopública, Informação de Suporte[END_REF].

Furthermore, many water users do not have metering devices or do not use them, in spite of an obligation to send this information to the administration every three months. All of these are considered serious or very serious infractions, which can incur penalties ranging from €25,000 to €2,500,000 (the maximum penalty for not registering the borehole). Since control by the administration is very weak, the farmers are unconcerned about complying (as they say: "no one does it"). A recent campaign of compulsory well registration was implemented in an effort to change this state of affairs, resulting in an almost systematic declaration of wells by farmers (currently 19 626 declared wells in the Algarve region). Farmers harshly criticize the current system of water use regulation, as they find it ignores their needs, e.g., the volumes authorized are considered to be insufficient in the summer months. Another issue worrying farmers is the establishment (in 2009) of a new tax on water abstraction. Although the tax level is very low, farmers see it as a first step towards the enforcement of groundwater payments, something they deem unfair considering the already high electricity costs and their significant investments for well construction.

GROUNDWATER REGULATION SCENARIOS

In both France and Portugal, groundwater policy is currently at a crossroads where three different models may be chosen to develop national groundwater regulation policies. The first model consists of developing direct administrative regulation, with systematic registration of abstraction points, the issuance of permits, the definition of groundwater rights, and the awarding of concessions. The second model consists of relying on economic instruments, such as water pricing or tradable water licenses, to regulate the use and allocation of groundwater. The third model is self-regulation, consisting of establishing semi-autonomous groundwater users' associations (GWUAs) with the internal capacity of organizing groundwater regulations, supported by higher-level authorities (Lopez-Gunn and Cortina 2006). Three narrative storylines were developed to introduce and to arouse debate on these three models with institutional representatives, experts, and farmers. These storylines are briefly presented below.

Scenario A: the Regulatory Approach

This scenario assumes that groundwater becomes entirely regulated by the State. Drawing on approaches implemented in Spain with the Regimen de Explotacion (Lopez-Gunn 2003) and in some groundwater basins in western states of the U.S.A. [START_REF] Schlager | Challenges of governing groundwater in U.S. western states[END_REF], this scenario assumes that a government agency registers all groundwater abstraction points, issues permits, defines groundwater rights (volume per year), and awards concessions. It further assumes that farmers provide detailed information (at the plot level) on where they use water and for which crop, in a framework similar to what is currently done for obtaining Common Agriculture Policy subsidies. The government agency then checks that the declared irrigated areas and cropping patterns are consistent with the authorized water use. The enforcement problem is dealt with by the use of sophisticated remote-sensing technology coupled to a Geographic Information System. Satellite images are regularly acquired and analyzed to identify and map all irrigated areas. The volumes used in each field are estimated using the same images. This information is integrated into a numerical land registry in order to compare actual water use with legal water entitlement. In the event of inconsistency between declared and estimated water use, a field inspection is carried out. Severe sanctions apply in cases of non-compliance, the sanction consisting of a fine proportional to the excess water used. One of the problems associated with this scenario is the lack of flexibility: newcomers (young farmers) are unable to obtain a concession until another farmer relinquishes a license -possibly providing incentives for farmers to drill illegal wells or to engage in informal water trading.

Scenario B: Tradable Water Permits

The second scenario reatins all the assumptions of the regulatory scenario and further assumes that water concessions can be traded among farmers. Both short-and long-term exchanges can take place. Transactions are approved and registered by a regulatory authority which checks that the transaction has no third-party impact. Details of all transactions are published by the regulatory authority (price and volumes exchanged) to ensure transparency. Specific measures are implemented to prevent speculation, mainly consisting of recalling dormant licenses from farmers who have not directly used them for several consecutive years. The main questions raised by this scenario are related to the functioning of water markets (type of farms likely to actively engage in trading, type of exchanges that would take place, namely short-or long-term sales), and to the social acceptability of considering trading water concessions in a legal framework in which water is considered to be a common good (France), and the risks of market failure in both the French and Portuguese contexts.

Scenario C: Self-Regulation

The third scenario is inspired by various examples around the world where water users have successfully organized themselves into associations to safeguard groundwater resources [START_REF] Lopez-Gunn | The role of collective action in water governance: a comparative study of groundwater user associations in La Mancha Aquifer in Spain[END_REF][START_REF] Lopez-Gunn | Is self-regulation a myth? Case study on Spanish groundwater user associations and the role of higher-level authorities[END_REF][START_REF] Van Steenbergen | Promoting local management in groundwater[END_REF][START_REF] Schlager | Community management of groundwater[END_REF] and to prevent the tragedy of the commons from happening. The example of the Ground-Water User Association (GWUA) established in Eastern Mancha (Junta General de Regantes de la Mancha Oriental) was specifically studied when constructing this scenario [START_REF] Calera Belmonte | GIS tools applied to the sustainable management of water resources: application to the aquifer system 08-29[END_REF][START_REF] Lopez-Gunn | The role of collective action in water governance: a comparative study of groundwater user associations in La Mancha Aquifer in Spain[END_REF][START_REF] De | Monitoring irrigation water use by combining Irrigation Advisory Service, and remotely sensed data with a geographic information system[END_REF].

This scenario assumes that GWUAs are established and made responsible for the allocation of the Maximum Permissible Annual Volume (MPAV) among farmers. The GWUA develops the internal capacity to register boreholes and to monitor the volumes of water used by its members, without State interference. Membership in a GWUA is mandatory for any farmer wishing to exploit groundwater (the State ensures compliance with this). The GWUA, which is responsible for enforcing the overall quota (MPAV) controls the individual behavior of its members. A Groundwater Tribunal, composed of elected farmers, is established to arbitrate conflicts, and to impose penalties on offending farmers (illegal wells or over-pumping). Government Agencies are asked to intervene only if the Tribunal's order is not carried out. The main questions raised by this scenario are related to the assumption that GWUA can effectively monitor groundwater access and use by its members, and to the assumption that self-adjudication and enforcement of sanctions can work.

RESULTS: WHAT DID WE LEARN FROM THE SCENARIO WORKSHOPS FOR POLICY FORMULATION?

Lessons learnt concerning the definition and enforcement of individual water rights

The regulatory scenario is perceived as a rigid and authoritative solution that may escalate the conflicts between the authorities and the water users. In both case studies, farmers and stakeholders stressed that a major constraint that limits the scope for this scenario is that there is insufficient shared knowledge of the aquifer boundaries, the volume that can be extracted, and the risks of deterioration. Owing to this lack of scientifically sound information, water rights defined by the State are likely to be contested. In the Roussillon, this problem had already been identified in interviews conducted several years ago [START_REF] Montginoul | Quels instruments pour gérer les prélèvements en eau souterraine? Le cas du Roussillon[END_REF]. Surprisingly, farmers are not opposed to strict water-use monitoring carried out via a combination of meters and the use of remote-sensing imagery. In France, farmers consider this as being an unavoidable development in the short term (five years) which is totally consistent with the practices of Government agencies responsible for implementing the European Common Agricultural Policy. No conflict is expected concerning the use of remote sensing as a monitoring tool, in contrast with what Lopez-Gun (2003) reports from Western Mancha. Portuguese farmers would also accept stricter monitoring systems in a context of scarcity but they want to be better informed about such a system and also require greater transparency and equity in its implementation (mix of scenarios A and B). The necessity of penalties is not contested; however, in the case of over-pumping, French farmers suggest replacing financial sanctions by a reduction of the quota authorized for the following year (as is already happening in the Vendée county in western France for instance, and in the Eastern Mancha aquifer in Spain). For

Portuguese farmers also, penalties are not contested, but it is definitely a sensitive topic. For farmers, penalties should be avoided given the difficult economic situation of agriculture, but the State could instead withhold all public subsidies (CAP in particular) from offending farmers. Penalties for infraction should be the very last resort.

Lessons learnt concerning trading of rights

Overall, French and Portuguese farmers all express major concerns related to the ethical implications of establishing a market for water rights. They firmly oppose considering water as a commodity that can be traded. As one farmer puts it, "we can sell everything but not water". Also, they are particularly concerned by the assumed separation of land and water property, fearing that this would lead to the concentration of water rights in the hands of a few better-off farmers, resulting in the abandonment of large tracts of land. Concerning small farmers (who refer to themselves as "peasants"), the scenario somehow threatens their own existence, whereas it opens new prospects for large farmers (who were not well represented in our groups).

After having made clear that this scenario was not a desirable outcome, the French farmers, who agreed to discuss this scenario in detail, acknowledged that it was not totally improbable. To support this judgement, they spontaneously quoted the existence of similar cap and trade mechanisms used for the regulation of greenhouse gas emissions. They also mentioned that similar markets already exist in agriculture for regulating the allocation of certain production rights, e.g., vine plantation rights, milk quotas, shellfish concessions, etc. Finally they agreed that tradable water rights should be considered and debated, given that these could appear on the policy maker's agenda in the future.

From that point in the discussion, some of the workshop participants clearly shifted to an exploratory mode, making an effort to envision how the mechanism would work. They identified a number of situations where trade could offer flexibility (for instance during the first 4-5 years of orchard plantation farmers could lease their water rights since they would not fully use them). They agreed that trade could be beneficial to farmers in specific situations. Trade would most probably remain limited and consist of long-term (or definitive) transfers (sale of water right after retirement), although short-term transactions (from a few weeks to a 1-2 years) might also take place at the margin. Participants were quite imaginative in identifying potential problems and drawbacks, including prohibitive transaction costs and third-party effects generated by the concentration of water rights in specific geographic areas. Tradable water rights were also expected to increase the complexity of land markets and to increase the resources needed by young farmers entering the sector (purchase of water rights in addition to land, machinery, etc.). A clear risk of speculation was pointed out, assuming that some farmers (market gardeners) might purchase water rights, use them during wet years, and speculate during drought years. Last but not least, farmers were concerned that water rights could be transferred to cities, for instance by farmers retiring, resulting in the progressive disappearance of agriculture. In conclusion, some of the farmers agreed that a system of tradable water permits could be established, provided that water permits would remain linked to the land and exchanges were limited to short-term leases of permits.

Lessons learnt concerning self-regulation

In the two case studies, farmers clearly expressed the desire to be more closely involved in decisions defining groundwater regulation rules. However, some differences emerged between the French and Portuguese farmers.

In Portugal, farmers did not believe that Groundwater Users' Associations could take over the groundwater regulation responsibility, citing the lack of trust among them. Bonding social capital seems to be fairly weak within the community, given past experiences of failed collective action in associations and cooperatives, whose leaders are often accused of favoring some associates and even of corruption. For these farmers, collective action for the self-regulation of the aquifer was viewed as possible only in a critical situation, such as a serious lack of water or in reaction to a more threatening alternative that might be imposed by the State, such as a higher water abstraction tax, for instance. The Groundwater Tribunal was also criticized. It should not be made up of farmers only but should rather include a wide range of representatives, including some external to agriculture to insure its independence and impartiality. It should also be led by a professional judge, not by a farmer. They nonetheless pointed out some advantages of self-regulation such as the greater proximity to the local reality of those managing the aquifer, which they contrast with the "State's technocrats", and their greater participation in decisions that are important to them, e.g. the rules for allocating quotas. They emphasize that these associations need to be a bottom-up creation, as associations created by the State are doomed to fail. Concerning the representatives of government agencies, these are strongly dubious of the capacity of farmers to get organized and to devise and implement groundwater-governing institutions, although they are in favor of such a system, within a wider governance framework defined by the State. One representative went so far as to say that the State should test self-regulation in a site such as that of the Querença-Silves aquifer.

In the French case study, workshop participants were much more supportive of the self-regulation principle. This is probably due to the positive experience with the irrigation associations which have been successfully managing gravity-irrigation canals. Representatives from government agencies adopted a very positive attitude, agreeing with the idea of giving up part of their powers and recognizing the right of users to devise their own institutions. Farmers also supported the scenario, although they were fully aware that a large number of the social problems arising from diverging interests and the effective implementation of the rules would shift from the authorities to the farmers' organization. Beyond their general support, farmers expressed a number of concerns on the implementation of the proposed scenario. First, they said that it was unlikely to become a reality until a serious crisis occured, triggering collective action. Crisis was understood as a situation where agriculture would be designated as responsible for (or even the culprit in) the emerging water crisis, forcing farmers to engage in collective action to defend their interests; or a situation where water shortage might reach such an intensity that sharing water would become a survival issue, forcing farmers to act collectively.

French farmers also pointed out that the scenario breached the fundamental principle of separation of powers, as the farming community itself would be in charge of formulating the regulating rules, interpreting those rules, adjudicating conflicts, and implementing penalties. They called for the State to remain involved in the adjudication and enforcement of sanctions, suggesting that the Tribunal could, for instance, be composed of State representatives and farmers in equal proportions. This position was reinforced by the relative lack of trust, reflecting fears that a group of farmers would highjack the collective organization for their personal benefit, by influencing the decisions of the tribunal or the enforcement of sanctions. They also believe that self-monitoring is unlikely to be effective, since "neighbors do not denounce neighbors". Last but not least, the farmers claimed that the supporting role of public authorities in the establishment of a self-regulating institution was inadequately defined in the scenario, GWUAs being perceived as operating in a vacuum. Overall, the serve as a basis for discussion provided the role of the State is clarified (and partly reinforced as regards the adjudication and enforcement of penalties). The workshops also identified one form of trading which might be considered as acceptable (in France) provided land and water property rights remain linked. Overall, the debates highlighted the need to create an institutional culture in which groundwater users are perceived as potential co-managers, not just as users safeguarding their own private individual uses, as already stressed in other contexts [START_REF] Agarwal | Participatory Exclusions, Community Forestry, and Gender: An Analysis for South Asia and a Conceptual Framework[END_REF].

From a methodological point of view, the research presented above has demonstrated farmers' ability to contribute to an exploratory assessment of possible future water-management scenarios, in two contexts where water is not an immediate source of concern for agriculture but only an anticipated future challenge. Significant positive changes in attitudes took place during the participatory process, with farmers opening up to a diversity of possible futures. In France, for instance, some of the farmers who were initially unwilling to consider basic changes, such as the installation of meters and the allocation of individual quotas, have been able to discuss, evaluate, and provide very meaningful comments and suggestions on highly innovative water-management scenarios, e.g., water markets. However, this does not mean that they would accept the implementation of the scenarios, but they were at least able to discuss them.

Farmers, experts, and institutional representatives alike appreciated the opportunity for debate offered by the scenario workshops. Overall, all of the participants said that they found the discussion time valuable. The workshops offered them a unique opportunity to think about the consequences of long-term changes, something which neither farmers nor institutional experts have time to do, being trapped within their present constraints and short-term objectives. Another key motivation for participants was the opportunity offered by the workshop to listen to each other's opinions, in a strategic, behavior-free context, i.e. having the possibility to freely express all their ideas without concern for the defense of specific vested interests. This was emphasized by the institutional experts, who are used to meeting in arenas where they have to defend vested interests and entrenched positions dependent on their institutions, with regard to short-term political and economic issues (Rinaudo et al. forthcoming). Portuguese farmers also appreciated learning from scientists, in particular on issues related to the dynamics of the aquifer, climate change and its effects on cultures, and water-management institutions. The choice of a methodology that consisted in conducted a series of participatory foresight workshop has generating some social learning. It is however unclear if this process will be continued by authorities or farmers themselves after the research project ends.

Concerning the use of predefined scenarios, participants found the three scenarios useful as a basis for discussion, recognizing that exploring possible futures would have been very difficult if not impossible without such support. However, some participants regretted that scenarios were presented as static descriptions of three different possible future situations, without considering possible transition paths from the present to the futures considered. Naturally, this methodology is not flawless, and other types of foresight techniques could be used, explicitly considering transition paths in the scenario description [START_REF] Poumadère | Worst case scenario as stakeholder decision support: a 5-to 6-m sea level rise in the Rhone delta, France[END_REF] or using facilitating tools such as roleplaying games or experimental economics. This type of alternative approach has already been used in another case study conducted in Morocco (ref. to Faysse, this issue), and is currently being tested by the French research team, as an extension of the work presented in this paper [START_REF] Lenouvel | From a blind truncheon to a one-eyed stick: testing in the lab an optional target-based mechanism adapted to groundwater withdrawal[END_REF]).

  which are controlled by dams. The remainder of the irrigated area relies on an estimated 4,500 individual boreholes, only a few dozen of them being officially registered and equipped with the metering devices required by law.Over the last decade, groundwater-management matters have become particularly fraught, for a number of reasons. First, the doubling of the population between 1954 and 2007, and the development of tourism along the coast have created a substantial increase in drinking-water demands. These needs have been met by increasing withdrawals of groundwater, leading to a lowering of the water tables and temporary restrictions on the use of boreholes, especially for agriculture during periods of drought. Long-term predictions of climate change suggest that tensions over water resources are likely to become only more severe. According to the work of the Vulcain Project, irrigation-water needs will increase(Rinaudo et al. forthcoming) and river flows are expected to fall by about 20% to 40% by 2030(Caballero and al. submitted) resulting in increased pressure on groundwater resources.

Roussillon (France) 2 Querença-Silves (Portugal) 3

	Geography and	Coastal plain (700 km²).	Most important groundwater reservoir in
	climate	Rainfall= 570 mm/year	the Portuguese Algarve region (317,85 km²)
		Mediterranean climate	Rainfall= 653 mm/year
			Mediterranean climate
	Water resources	2 coastal aquifers: (a) Pliocene confined	Aquifer built of karstified limestones and
		aquifer (abstraction= 41 Mm3/year) and	dolomites. Total abstraction = 21,4
		(b) Quaternary shallow alluvial aquifer	hm3/year.
		(abstraction= 43 Mm3/year) Large reservoirs supply water to gravity irrigation canals (agriculture)	Large reservoirs supplying water to public water utilities (drinking water use) Multimunicipal public water supply system
			managed by the regional Water Utility
			Águas do Algarve
	Groundwater	Drinking water (52 M m3/year) including	Agriculture: 6816 irrigated ha (from a total
	uses	tourism	area of 32 000 ha), 1784 wells, 30
		Agriculture: 15,000 irrigated ha, 4, 500	hm3/year (average)
		boreholes, abstraction of 23 M m3/year Industry (4 M m3/year)	Drinking water : 10 hm3/ year Several data coexist and detailed data still is
			being constructed by ARHA and experts of
			the University of Algarve.

Threats to groundwater

Risk of seawater intrusion (locally observed). Declining groundwater table in confined aquifer

Risk of brackish water intrusion from the estuary into the karst aquifer Table 1: Main features of the two case study areas.

Acknowledgments

The results presented here were produced as part of AQUIMED, a research project contributing to the ERA Net CIRCLE program. Financial support from Fundação para a Ciência e Tecnologia (Portugal) and the Ministry of Ecology (France) is acknowledged. The research related to water markets presented in this paper has also benefited from financial support from ONEMA (France) as part of the Water Cap & Trade project, funded under the Era Net IWRM Net program. We wish to acknowledge the very useful comments and suggestions of three anonymous reviewers of this paper.

farmers were somewhat uncomfortable with this scenario as they felt that it was driven by the State's desire to shed the burden of managing a common good. This supported the conclusions of previous research work which emphasized the role of higher authorities in effective self-regulation arrangements (Schlager and Lopez-Gunn , Lopez-Gunn and Cortina 2006).

DISCUSSION AND CONCLUSION

Summary of participants' perceptions of scenarios

When asked to describe how water management is likely to evolve in the future, French stakeholders and farmers quoted most of the items contained in the self-regulation scenario (C). They do not believe that the State will intervene in water management as described in regulatory scenario (A), and most of them consider the development of the water markets scenario to be very unlikely (B).

Overall, Scenario C is considered to be highly desirable by more than 90% of the participants.

However 30 to 40% of participants also find that some of the assumptions included in Scenarios A and B should be considered, and included in a hybrid scenario.

For Portuguese stakeholders, the state should continue to play a crucial role in the management of groundwater. Although they call for greater involvement of farmers in groundwater regulation, the participants did not support the self-regulation scenario. The Water Market scenario is categorically dismissed by most participants, who consider it unethical.

In short, the outcome of the participatory foresight conducted in the two countries has enabled the identification of certain key elements which could be taken from the various scenarios and combined to formulate a preliminary basis for discussion with stakeholders. In Portugal, this scenario would be based on the regulatory scenario, farmers being granted a much greater participation in the definition of the rules of use, monitoring, and sanctions. In France, the self-regulation scenario could