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Consists of a detailed description of reel-to-reel tape recorders and their applications in second language instruction. (Text is in French.) (CLK)

Discusses general characteristics, practical uses, advantages and disadvantages of magnetic tapes, including video tapes. (Text is in French.) (AM)

To document the long-term energy and water budget of a fallow land, a complete dataset was collected in the south-west of France. It included climatic data, surface fluxes, vegetation evolution and soil moisture monitoring. The soil hydrodynamic and thermal properties were also determined in situ and through laboratory measurements. The experiment was conducted during 3 years (1995–1997) on a 20 ha field. A natural plant-residue mulch layer was constantly covering the field. Its thickness artificially increased in June 1995 when the farmer unexpectedly cut the grass and left the residue on the ground. Two versions of the simple soil plant atmosphere transfer (SiSPAT) model were compared using the 1995 dataset: the original version which did not take into account the mulch layer; and a modified version where explicit modelling of the mulch was included (SiSPAT-mulch). The prediction of soil temperature, surface fluxes and surface soil moisture was improved when considering a mulch layer. Annual surface evaporation was divided by a factor two to four and transpiration increased by 30–50% when considering the mulch layer. Annual total evaporation decreases by 4–10% according to mulch characteristics, leading to a better simulation of the annual and daily water balance of the site. A sensitivity analysis investigated the robustness of these results and possible causes of inaccuracies on half hour values.

While very little is yet known about the root system depth of sugar cane, the study set out to model the potential root extraction ratio in soil (PRER), by counting root intersections (RI) obtained using the trench?profile method (TPM), to infer the spatial distribution of sugar cane root length densities (RLD) and root distances (RD). The experiment was conducted in a deep clayey cambisol in Réunion. RI values were counted using a grid with a 5 x 5 cm mesh, down to a depth of 4 m over a 1.5 m wide soil profile. The results revealed: i) wide variability in root distribution between the soil surface and root front (4 m), with many competing roots and other rootless zones over distances of more than 5 cm, ii) the PRER, calculated as the ratio of the volume of soil available to a root for water uptake to the volume of soil assigned to that root, ranged from 80% near the surface to values between 30% and 1% between 1 and 3 m, iii) if RLD mean values were used to estimate usable water, as is often the case in crop models, the values were overestimated, with an even greater gap when only depth of the root front was accounted for. It is thought that the PRER approach may be an indicator of crop functioning under water stress and may facilitate the identification of genotypes able to withstand drought, as reported by Nidichao et al., (2011) for oil palm.

International audience; Heterogeneity in soil properties has been identified to impact water transfers at different scales from vertical column, hillslopes to watershed. Thus Distributed physically based hydrological models require distributed hydraulic characteristics to quantify these impacts. To characterize soil properties and their heterogeneity, a multi-scale sampling strategy was proposed based on distributed information including electromagnetic survey maps, topography and land use coverage. Each identified units are characterized by there hydraulic properties including in situ infiltration tests. This strategy was applied over the Ara Catchment (12km2) in northern Benin. It has been instrumented in the framework of the AMMA-Catch experimental network in West Africa, to better determine water resources and to investigate possible hydrological retro-action on monsoon cycle. From hydrological point of view, distributed soil hydraulic properties are supposed to impact water transfers and watershed dynamics all along the monsoon cycle. To document this heterogeneity, an electrical conductivity map and geological survey was used as starting points to identify the ground structures which align with the north-south direction with a dip angle of 20° east. A total of 20 pits have been opened to document the 0-2m horizons, and 2 more for the 0-5m horizons. 3 pits were digged within each geological structure areas at the surface. In each pit, the retention and hydraulic conductivity curves of each pedological horizon were characterized with three replicates. This database is used to document the variability of these properties and to produce soil hydraulic property maps. Using the variability information, we tested their impact with the Parflow-CLM 3D distributed model. It was run in an homogeneous configuration and compared with a data controlled heterogeneous configuration. The latest is prepared using a turning band algorithm to distribute soil hydraulic properties.

International audience; Physically based hydrological models involve a large amount of parameters and data. Any of them is associated with uncertainties because of indirect measurements of some characteristics or because of spatial or temporal variability of others, ... Then, even if lots of data are measured in the field or in the laboratory, ignorance and uncertainty about data persist and a large degree of freedom remains for modeling. Moreover the choice for physical parameterization also induces uncertainties and errors in model behavior and simulation results. To address this problem, sensitivity analyses are useful. They allow the determination of the influence of each parameter on modeling results and allow the adjustment of an optimal parameter set by minimizing a cost function. However, the larger the number of parameters, the more expensive the computational costs to explore the whole parameter space. In this context, we carried out an original approach in the hydrology domain to perform this sensitivity analysis using a 1D Soil - Vegetation - Atmosphere Transfer model. The chosen method is a global method. It focuses on the output data variability due to the input parameter uncertainties. The latin hypercube sampling is adopted to sample the analyzed input parameter space. This method has the advantage to reduce the computational cost. The method is applied using the SiSPAT (Simple Soil Vegetation Atmosphere Transfer) model over a complete year period with observations collected in a small catchments in Benin, within the AMMA project. It involves sensitivity to 30 parameters sampled in 40 intervals. The quality of the modeled results is evaluated by calculating several criteria: the bias, the root mean square error and the Nash-Sutcliffe efficiency coefficient between modeled and observed time series of net radiation, heat fluxes, soil temperatures and volumetric water contents.... To hierarchize the influence of the various input parameters on the results, the study of correlations between several criteria (cost functions) and each parameter is necessary. Correlation coefficients can be chosen as sensitivity factors. The advantage is that they are independent of the chosen range of each parameter (unlike the regression coefficients). Nonetheless, as all parameters are varying simultaneously, total correlation coefficients are not providing the expected information, whereas partial correlation coefficients indicate parameter influence considering the other parameters. The multiple correlation coefficients between a result and parameters indicates the quality of the multiple linear regression. To optimize the parameter set, we use the simulated annealing method. Thus, it is possible to look for a global extremum (while most of other methods allow to find only a local extremum) as it includes a random part to explore a large scale of possibilities. It is its main advantage. The optimal parameter set allows a significant improve of the cost functions. Despite this enhancement, a certain degree of uncertainty still remains on the optimal parameter set.

International audience; This study presents a macroscopic double-porosity dispersion model for unsaturated water flow conditions, developed by asymptotic homogenization method. The model shows a coupling between the concentrations in the micro- and macro-porosity domains (local non-equilibrium), resulting in an early breakthrough and a long tail effects. A series of one-dimensional dispersion experiments of passive tracer under unsaturated steady-state flow conditions in a physical model of double-porosity material, was performed. An excellent agreement between the numerical simulations and the experiments proves good predictive capacity of the developed model.

Sur les petits bassins versants ruraux, la question du fonctionnement hydrologique est intimement liée à la façon dont ces bassins sont aménagés. Le façonnage du paysage par la présence humaine et l'activité agricole (découpage de parcellaire, drainage de certaines parcelles, plantation de haies en bord de parcelles, creusement de fossés) peut modifier profondément les processus hydrologiques, et avoir des conséquences importantes, notamment en termes de pollutions diffuses d'origine agricole (nutriments ou produits phytosanitaires). Si l'on veut répondre à des questions d'aménagement de ces bassins pour par exemple limiter les pollutions, on a besoin de modélisations hydrologiques précises, représentant explicitement les différents éléments du paysage susceptibles d'avoir un effet hydrologique important, y compris dans leurs formes géométriques et leurs positionnements dans l'espace.

International audience; Soil and surface seal hydraulic properties were determined from simulated rainfall experiments by the inverse method applied to the Richards equation. Measurements used for the estimation include the soil water pressure head versus time at two distances from the soil surface, the transient infiltration rate at the soil surface and the drainage rates at the bottom of the soil profile. Seal properties were evaluated using a model that simulates changes in the seal bulk density with respect to time and space. Uncertainties, correlations and sensitivities of the soil and seal parameters were quantified to evaluate the accuracy of the model estimation and to compare the information content of each measurement type to parameter estimations. It appears that the uncertainties related to three seal parameter estimations, namely the parameter related to the dynamics of seal formation, the modelled seal thickness and the initial bulk density, were larger than 50% of the parameter values, because of the low sensitivity of the model to them and their multiple correlations. In addition to seal hydraulic parameter estimation, bulk density profiles of the soil surface were measured after the rainfall simulations using the X-ray method. The exponential-decay shape assumed in the soil surface seal model was found to correctly reproduce the measured distribution of bulk density with depth. However, the measurements showed a less developed seal than that suggested by the bulk density profile estimated from rainfall experiments. Finally, bulk density measurements were used as given input parameters of the model. Setting the initial bulk density and its maximal change over time at the measured values greatly decreased the seal parameter uncertainties. The method proposed could be used to improve the experimental design used to quantify the seal's hydraulic properties using inverse techniques.

A decision-making software tool for monitoring irrigation of small farms in heterogeneous environments (OSIRI) was developed at the request of small-scale sugarcane farmers on the island of Réunion (France) hampered by variable climate and soil conditions. This program, which is based on a simple water balance simulation model coupled with a comprehensive set of decision rules, was designed to provide farmers with customized advice on discrete irrigation units and to simulate irrigation system scenarios so as to optimize their performance. The basic equations and main decision rules of OSIRI, as well as the software features, were given in Chopart et al. (2007). A detailed experimental study was carried out on a 5000 m2 irrigated sugarcane field to compare the performance of this tool with the currently used method based on maximum crop water requirements (control). The results showed that OSIRI reasonably well simulates actual evapotranspiration and drainage below the sugarcane root zone. Moreover, it allowed savings of about 26% in irrigation delivery throughout the crop cycle as compared with the control method, without a significant decrease in yield, and irrigation water productivity increased by 25%. The results of a survey of 25 farmers using OSIRI showed that it is a well accepted valuable decision-making tool.

The general purpose of the study is the quantification of the influence of landscape management on water resources and water quality. Subsurface drainage is a common agricultural practice, and is recognized to influence strongly catchment hydrology. A simplified model for simulating subsurface drainage at the field scale was developed. It relies on two submodules, which were recoded on the basis of already existing models. Submodule SIDRA simulates water flow in the saturated zone through the Boussinesq equation. Its outputs are the water table elevation at mid-drain spacing and the unitary drain flow. Submodule SIRUP handles the vadose zone and allows computation of surface runoff and recharge to the water table, using a capacitive-based approach. SIDRA and SIRUP can be used either separately or coupled together. Both were previously evaluated independently against field data. In order to study more accurately the behaviour of both models and evaluate their coupling, comparative simulations between the SIDRA+SIRUP modelling system and the Hydrus-2D software were performed on appropriate scenarios. The addition of SIRUP improves the quality of simulations, although SIDRA alone gives satisfactory results. Peak flows and water table elevation are well reproduced. Yet slight modifications of the SIRUP submodule lead to more accurate results for drainage recession tails. The next step of the study will be the addition of a solute transport submodule and the test of the resulting coupled model against field data.

Subsurface drainage is a widely spread agricultural management device that deeply influences water path and solute transport. Indeed it generates 2D water flows (vertical in unsaturated zone, radial close to the pipe, horizontal in saturated zone at mid drain spacing). As a consequence solute transport is spatially and temporally heterogeneous. Elution curves usually present a typical pattern with a fast peak and long tail. A dedicated modelling tool was developed from this background. As a coupled Boussinesq/reservoir simplified model is already available for hydrodynamics representation, a conceptual approach is preferred for solute transport. The soil equivalent compartments approach is based on observation of water flow paths in a 2D drained soil profile and the estimation of travel times of solute particles from soil surface to the drain for different hydrodynamical conditions. Then the soil can be divided into several compartments with quite homogeneous travel times. Solute transport is represented in each compartment using an exponential transfer function parametrized by drain flow and travel time. Adsorption and degradation coefficients for reactive solutes are also added. Such a solute transport model was implemented and tested against field data for herbicide Isoproturon. First results are encouraging : after calibration, fast breakthroughs after rainfall events are reproduced correctly and simulated pesticide concentrations in drain flow are quite accurate.

The LIQUID modelling framework was used to design and implement a field-scale model for subsurface tile-drainage, surface runoff and pollutant transport calculation. Three separate modules were developed to simulate the soil water dynamics in both the saturated (SIDRA) and unsaturated (SIRUP) zones as well as the solute transport into drainage and runoff water (SILASOL). Each module was built using the LIQUID template library. The modules are interconnected within the framework, using a slot-and-signal communication system allowing SIDRA, SIRUP and SILASOL to interact and interplay synchronously. The resulting model was compared with a non-modular version of the same code working with a fixed time step and experimental field data as well. The agreement between the LIQUID-based model results and field observations is more accurate, mainly because climatic forcing dynamics are better taken into account. This shows that the application of the LIQUID concepts can easily lead to an operational tool for decision makers.

A number of experimental studies have tackled the issue of solute transport parameter assessments either in the laboratory or in the field. But yet, the behavior of a plume in the field under density driven forces, is not well known due to possible development of instabilities. Some field tracer tests on the fate of plumes denser than native groundwater such as those encountered under waste disposal facilities, have pointed out the processes of sinking and splitting at the early stage of migration. The process of dispersion was widely investigated, but the range of dispersivity values obtained from either experimental tests, or numerical and theoretical calculations is still very large, even for the same type of aquifers. These discrepancies were considered to be essentially caused by soil heterogeneities and scale effects. In the meantime, studies on the influence of sinking and fingering have remained more scarce. The objective of the work is to analyze how transport parameters such as dispersivities can be affected by unstable conditions, which lead to plume sinking and fingering. A series of tracer tests were carried out to study under natural conditions, the transport of a dense chloride solution injected in a shallow two-layered aquifer. Two types of experiments were performed: in the first type, source injection was such that the plume could travel downward from one layer to the other of higher pore velocity, and in the second one, the migration took place only in the faster layer. The results suggest some new insights in the processes occurring at the early stages of a dense plume migration moving in a stratified aquifer under groundwater fluctuations, which can be summarized through the following points: (i) Above a stability criterion threshold, a fingering process and a multi modal plume transport take place, but local dispersivities can be cautiously derived, using breakthrough curves matching. (ii) When water table is subject to some cycling or rising, the plume can be significantly distorted in the transverse direction, leading to unusual values of the ratio between longitudinal and transverse dispersivities. (iii) Under stable conditions, for example in the case of straightforward injection in the faster aquifer layer, longitudinal dispersivity is greater than the transverse component as usually encountered, and the obtained transport parameters are closed to macro dispersivity values, which reach their asymptotic limit at very short distances. (iv) The classical scale effect about the varying dispersivity at short distances could be a process mainly due to the distance required for a plume stabilization. (c) 2005 Elsevier Ltd. All rights reserved.

International audience; The paper deals with modelling of unsaturated water flow in soils which are composed of two distinct sub-domains of very different hydraulic properties (double-porosity soils). Flow in such media is often characterized by the local non-equilibrium of the capillary pressure in the two regions, which cannot be described by the local equilibrium model [4]. Recently, a non-equilibrium model based on the homogenization approach has been proposed [5]. In the present paper, the model is applied to simulate gravitational drainage in a particular case of double-porosity medium. The medium consists of highly conductive sand matrix with weakly conductive spherical inclusions arranged in a periodic manner. The numerical simulations have been performed with the DPOR-1D code [11]. Calculation of the effective parameters of the medium is presented as well as the solution of the macroscopic boundary value problem. The behaviour of the double-porosity medium is compared with the behaviour of single-porosity media, i.e. a homogeneous sand and a sand with impermeable inclusions. The results show the importance of the non-equilibrium effects for the flow in double-porosity soils.

Nous présentons les principes de la plate-forme LIQUID et les illustrons pour le développement du modèle de bassin versant adapté au cas d'étude d'un petit bassin versant aux sol peu profonds.

[Departement_IRSTEA]RE [TR1_IRSTEA]RIE / TRANSFEAU [TR2_IRSTEA]RIE / PHYLEAU; Les plantes recouvrent 70 % des surfaces continentales et régulent la vitesse et l'importance des échanges entre ces surfaces et l'atmosphère. L'évapotranspiration réelle (ETR) est donc l'un des termes essentiels du cycle hydrologique. Sa quantification requiert la connaissance, au cours du développement du couvert, de ses composantes: évaporation du sol et transpiration des couverts végétaux. Notre objectif est d'améliorer la compréhension des interactions biosphère - atmosphère en prenant mieux en compte les différentes sources de vapeur d'eau (végétation, sol). Dans ce contexte, le traçage isotopique est de plus en plus utilisé, dans la mesure où, dans l'arsenal encore limité des outils à notre disposition, c'est le seul à pouvoir caractériser les sources à travers des signatures spécifiques et à quantifier les parts de mélange de vapeur d'origines différentes dans l'atmosphère. Cependant, l'utilisation du traçage isotopique naturel de l'eau doit encore passer par une caractérisation expérimentale des signatures des différents réservoirs intervienant dans le continuum sol plante - atmosphère. Ces données sont nécessaires pour renforcer la qualité de la paramétrisation des modèles qui calculent les flux d'échanges entre réservoirs à partir des traceurs isotopiques et ainsi permettre une analyse plus précise des interactions entre la biosphère et l'atmosphère. L'une des originalités de ce travail consiste donc à vouloir introduire les bilans isotopiques dans les modèles qui décrivent les transferts d'eau entre le sol, la plante et l'atmosphère. Ceci passe par l'approche couplée « traceurs isotopiques-modélisation TSVA (Transfert Sol Végétation Atmosphère) développée au sein de nos laboratoires. Cette détermination s'appuie sur une très forte collaboration interdisciplinaire et différents outils : « réacteurs biogéochimiques (BioEMCo, Thiverval-Grignon), champ expérimental (Observatoire PCBB, INRA, Lusignan), modélisation SiSPAT-Isotope (LTHE-Grenoble/Cemagref, Lyon, ) ». Une très forte complémentarité entre l'utilisation d'expérimentations de laboratoire, d'expérimentations de terrain et la modélisation caractérise donc ce projet. Nous avons étudié expérimentalement les points suivants en y associant un volet modélisation :Détermination, en conditions contrôlées et en régime transitoire, de la signature de la vapeur transpirée d'une part, de la signature de la vapeur issue de l'évaporation d'un sol d'autre part et, enfin du mélange des deux dans un couvert en voie de développement. -Détermination de la partition évaporation, transpiration in situ sur une parcelle de maïs

Dans le contexte de la modélisation de la pollution par les produits phytosanitaires en milieu rural, on présente un modèle local de devenir de solutés en parcelle artificiellement drainée, appelé SI#. Il simule la hauteur de nappe à l'interdrain, les débits drainé et ruisselé, ainsi que les concentrations en produits phytosanitaires dans les eaux de drainage et de ruissellement, pour des événements pluvieux correspondant à la saison de drainage intense dans le contexte pédo-climatique du Grand Ouest de la France. Le support pour son implémentation et application est la plate-forme de modélisation intégrée LIQUID®. SI# est testé par comparaison avec des données expérimentales issues du site Arvalis de La Jaillière (44), pour les produits Isoproturon et Diflufénicanil, aux échelles de l'événement pluvieux et de la saison de drainage intense. Les résultats montrent un bon comportement global de SI#, en termes de débits et de concentrations.

Application of organic chemicals to a newly irrigated sugarcane (Saccharum officinarum L.) area located in the semiarid western part of Reunion Island has prompted local regulatory agencies to determine their potential to contaminate ground water resources. For that purpose, simple indices known as the ground water ubiquity score (Gustafson index, GUS), the retardation factor (RF), the attenuation factor (AF), and the log-transformed attenuation factor (AFT) were employed to assess the potential leaching of five herbicides in two soil types. The herbicides were alachlor [2-chloro-2',6'-diethyl-N-(methoxy-methy) acetanilide], atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-1,3,5-triazine], diuron [3-(3,4-dichlorophenyl)-1,1dimethylurea], 2,4-D [(2,4-dichlorophenoxy) acetic-acid], and triclopyr [((3,-5,6-trichloro-2-pyridyl)oxy) acetic-acid]. The soil types were Vertic (BV) and Andepts (BA) Inceptisols, which are present throughout the Saint-Gilles study area on Reunion Island. To calculate the indices, herbicide sorption (K,) and dissipation (half-life, DT50) properties were determined from controlled batch experiments. Water fluxes below the root zone were estimated by a capacity-based model driven by a rainfall frequency analysis performed on a 13-yr data series. The results show a lower risk of herbicide leaching than in temperate regions due to the tropical conditions of the study area. Higher temper-atures and the presence of highly adsorbent soils may explain smaller DT50 and higher Koc values than those reported in literature concerning temperate environments. Based on the RF values, only 2,4-D and triclopyr appear mobile in the BV soil, with all the other herbicides being classified from moderately to very immobile in both soils. The AFT values indicate that the potential leaching of the five herbicides can be considered as unlikely, except during the cyclonic period (about 40 d/yr) when there is a 2.5% probability of recharge rates equal to or higher than 50 mm/d. In that case, atrazine in both soils, 2,4-D and triclopyr in the BV soil, and diuron and alachlor in the BA soil present a high risk of potential contamination of ground water re-sources.

International audience; Artificial drainage has been subject to widespread criticism because of its impact on water quality and because there is suspicion that it may have detrimental effects on flood genesis. The present work aims at a better understanding of the mechanisms controlling infiltration and surface runoff genesis, particularly in soils with artificial drainage and affected by surface crusting. A field experiment was conducted during one drainage season (November 2003-March 2004) in the Brie region (80 km east of Paris, France) on a subsurface drained silty soil. Water table elevation and surface runoff were monitored above the drain and at midpoint between drains. Soil water pressure head was measured at various depths and locations between the midpoint and the drain. Soil surface characteristics (microtopography and degree of structural and sedimentary crust development) were recorded regularly on the experimental site and on other plots of various drainage intensities. The results show that the first surface runoff events were induced by high water table. However, runoff was higher at midpoint between the drains because water table reached the soil surface at that point, thus considerably reducing infiltration capacity compared to that above the drain. Comparing different plots, the area with older drainage installation (1948) yielded the most surface runoff. Wider drain spacing, smaller dr ain depth and possible plugging may have led to a greater area of saturated soil between drains. During the winter period, the impact of raindrops induced the formation of a structural crust on the soil surface. Furthermore, the development of the sedimentary crust, which was favored by water actually flowing on the soil surface during the high water table periods could be correlated with surface runoff volume. The formation of this crust had a significant impact on runoff occurrence at the end of the winter. Therefore, poorly drained fields presented more favorable conditions for both Horton type runoff and saturation excess runoff. Drainage effectively reduces surface runoff occurrences not only by lowering the water table in winter but also by limiting soil surface sealing.

International audience

International audience

National audience; Cette communication interdisciplinaire à l'Ecole des Houches retrace la démarche et les résultats des volets hydrologique et économique d'une recherche menée sur le site de La Côte Saint-André (Isère). L'objectif final de cette recherche consistait à éclairer les décisions publiques en matière de régulation des pollutions diffuses, d'une part en précisant le contenu et le coût de différents scénarios alternatifs susceptibles de préserver ou d'améliorer la qualité de l'eau souterraine, d'autre part en évaluant l'efficacité et l'impact environnemental de différentes politiques publiques régulatrices. La première partie analyse et propose plusieurs types de modélisation des processus de transfert d'eau et d'azote, à l'échelle de la parcelle et du bassin d'alimentation, dans la situation actuelle et pour différents scénarios alternatifs. La seconde partie montre qu'il est possible, en se calant sur ces résultats, de construire un indicateur synthétique d'impact environnemental permettant de fonder une analyse coût-efficacité pour chacun des scénarios envisagés. On débouche alors sur la simulation économique de différents modes de régulation au niveau du bassin versant, incitant les agriculteurs à modifier leurs pratiques culturales. La troisième partie esquisse un bilan de cette expérience innovante de recherche interdisciplinaire.

National audience; La plaine de Bièvre-Liers (300 km2) entourant le district de la Côte-Saint-André est une zone d'agriculture intensive dans laquelle la concentration en nitrates de la nappe augmente régulièrement depuis le milieu des années 70, menaçant sérieusement de dépasser le seuil de potabilité dans plusieurs captages en eau potable. Dans ce contexte, et dans l'optique du développement d'une activité agricole plus soucieuse de la protection des ressources en eau mais économiquement rentable, un programme scientifique interdisciplinaire, pluriannuel associant organismes de recherche, universités, collectivités territoriales et la profession agricole, a été mis en place en 1991. Les auteurs présentent ici une brève synthèse des principaux résultats obtenus au terme d'un important travail de terrain réalisé pendant quatre années et ce, à différentes échelles spatiales : (i) le m2 et la parcelle (1 ha) pour l'étude fine des transferts dans le système du sol (vu comme réacteur bio-physicochimique) plante-atmosphère ; (ii) l'exploitation agricole pour l'expérimentation agronomique et économique de scénarii préventifs moins polluants ; (iii) l'ensemble du bassin d'alimentation pour la simulation de programmes de préservation ou d'amélioration de la qualité de la ressource en eau souterraine en situation hétérogène, pour l'évolution prévisible de la demande en eau potable et la capacité des consommateurs à supporter le coût d'une eau de qualité.

Diffusion du document : INRA-RA Université Pierre Mendès-France BP 47 38040 Grenoble Cedex 9 (FRA); National audience; La plaine de Bièvres-Liers (300 km2) entourant le district de la Côte-Saint-André est une zone d'agriculture intensive dans laquelle la concentration en nitrates de la nappe augmente régulièrement depuis le milieu des années 70, menaçant sérieusement de dépasser le seuil de potabilité dans plusieurs captages en eau potable. Dans ce contexte, et dans l'optique du développement d'une activité agricole plus soucieuse de la protection des ressources en eau, mais économiquement rentable, un programme scientifique interdisciplinaire et pluriannuel associant organismes de recherche, universités, collectivités territoriales et la profession agricole a été mis en place en 1991. Les auteurs présentent une brève synthèse des principaux résultats obtenus au terme d'un important travail de terrain réalisé pendant quatre années et ce, à différentes échelles spatiales : le m2 et la parcelle (1 ha) pour l'étude fine des transferts dans le système sol (vu comme un réacteur bio-physicochimique) plante-atmosphère ; l'exploitation agricole pour l'expérimentation agronomique et économique de scénarii préventifs moins polluants ; l'ensemble du bassin d'alimentation pour la simulation de programmes de préservation ou d'amélioration de la qualité de la ressource en eau souterraine en situation hétérogène.

Ce rapport fait état des résultats d'une recherche interdisciplinaire, menée en 1996 et 1997 sur la plaine de Bièvre-Liers (Isère). Ce site d'expérimentation et d'observation de 300 km2 recouvre une zone d'agriculture intensive sur laquelle on observe depuis une quinzaine d'années une progression continue des teneurs en nitrates dans la nappe profonde, certains captages dépassant fréquemment la norme de potabilité de 50 mg/litre. Cette recherche a impliqué douze laboratoires appartenant à diverses disciplines : hydrologie, géochimie, climatologie, chimie, physiologie végétale, microbiologie des sols, agronomie et économie. Les résultats de ce travail concernent trois volets différents d'investigation : 1. Une approche expérimentale et théorique sur la compréhension et la modélisation (dans un but prédictif) des mécanismes prépondérants des transferts d'eau, de fertilisants et d'herbicides dans le système atmosphère-plante-sol-nappe et sur l'établissement de bilans précis à l'échelle stationnelle, puis à la parcelle. 2. Une analyse de l'évolution prévisible, au niveau de l'ensemble du bassin d'alimentation, des ressources en eaux souterraines (qualité et quantité) en fonction de l'utilisation des sols, de l'évolution du climat. 3. Une recherche couplée agronomique et économique, portant sur l'expérimentation de scénarios préventifs de lutte contre cette pollution, à l'échelle de la parcelle et de l'exploitation, puis sur la simulation de programmes de préservation ou d'amélioration de la qualité de la ressource, enfin sur l'évolution prévisible de la demande en eau potable à long terme.