Influence of spatial variability of hydraulic characteristics of soils on surface parameters obtained from remote sensing data in infrared and microwaves

The correct interpretation of thermal and hydraulic soil parameters infrared from remotely sensed data (thermal infrared, microwaves) implies a good understanding of the causes of their temporal and spatial variability. Given this necessity, the sensitivity of the surface variables (temperature, moisture) to the spatial variability of hydraulic soil properties is tested with a numerical model of heat and mass transfer between bare soil and atmosphere. The spatial variability of hydraulic soil properties is taken into account in terms of the scaling factor. For a given soil, the knowledge of its frequency distribution allows a stochastic use of the model. The results are treated statistically, and the part of the variability of soil surface parameters due to that of soil hydraulic properties is evaluated quantitatively

Modélisation du transport de solutés dans la zone non saturée du sol

On présente une analyse critique des modèles décrivant les transferts d'eau et de substances chimiques dans les sols partiellement saturés en eau. Pour la clarté de l'exposé, ces derniers sont classés en modèles déterministes (mécanistes et fonctionnels) et stochastiques. Les avantages et limites de ces différentes approches sont discutés. Finalement quelques recommandations et voies de recherche relatives à ce domaine sont suggérées.The importance ofthe unsaturated (vadose) zone as an inextricable part of the hydrologic cycle has long been recognized. Theoretical and experimental studies on both water flow and solute transport in this zone have been further motvated by attempts to manage the root zone of agricultual soils optimally as well as by concerns about soil and groundwater pollution. These studies have greatly increased our conceptual understanding of the many complex and interactive physical, chemical and microbiological processes operating in the unsaturated zone. They have also led to a large number of models which vary widely in their conceptual approach and degree of sophistication, and are strongly influenced by the environment training and preoccupations of their developers.A number of conceptual models for solute transport in partially saturated soils are reviewed and categorized. A key distinction is made between deterministic models which assume that a system behaves in such a way that the occurrence of a given set of events leads to a uniquely definable outcome, and stochastic models, which presuppose the outcome to be uncertain and are structured to account for this uncertainty. A second main distinction is between mechanistic and functional models. Mechanistic implies that the model takes into account the most fundamental mechanisms of the processes, as presently known and understood (e.g. Darcy's law for water flow, combination of mass-flow and difrusion-dispersion mechanisms for solute transport). The term functional refers to models that incorporate simplified treatments of solute and water and make no claim to fundamentality. However, their use requires less input data and computer expertise as compared to mechanistic models.Additionally, it may be useful to distinguish between models that are primarily research tools (most of the mechanistic models) developed either to aid the testing of assumptions or to contribute to better understanding of the physical processes, and those (functional models) that are mainly useful as guides to the management of soil and water resources.Although no attempt is made here at an exhaustive litterature review, the main features, the strengths and weaknesses of these approaches are presented and discussed. This analysis and other reviews published in recent years have revealed progress in many areas. Investigators have uncovered a number of inadequacies with existing models of soil transport processes and have made significant steps toward a better understanding of these phenomena. Some new research areas such as biodegradation modeling, immiscible phase transport, water and solute transfers in structured or swelling soils, and multi-interative ion transport are emerging.A number of challenges still remain for both the theoretician and the practitioners. They include: i) how best to deal with preferential water flow and transport, ii) how best to model the effects of local and regional spatial and temporal variabilities of soil hydraulic properties on solute transport, iii) how to couple multi-component geochemical submodels efliciently with available unsaturated-saturated flow modes, iv) how to improve field methods for estimating vadose zone transport parameters and v) how best to predict the long-term consequences of short-term management decisions.It is apparent that the complexity and variety ofthe physical processes have led to increasing specialization within the area of transport modeling. Soil physicists, soil chemists, soil microbiologists and agronomists have the propensity to limit their consideration and vision to their respective disciplines. A natural consequence of this specialization has been the evolution of scientific jargon specific to each subdiscipline. This may be overcome by reinforcing interdisciplinary cooperation among investigatory by training of students both at the graduate and postgraduate levels and by encouraging topical workshops and publications in interdisciplinary journals. Another general observation gleaned from this review is that very few solute transport models have been exhaustively tested under field conditions. Indeed, the quantitative criteria for validating models do not seem to be clearly identified or universally recognized. It appears very important that such criteria should be established and used to make an objective comparison of the abilities of the various types of model to simulate the results of field experiments. Without such tests and without comparisons between models there is a risk that disagreements between the predictions of different models and the resulting confusion could greatly diminish the usefulness of modeling techniques. While computer codes excalate in number as pressures mount for improved managements strategies it is time for asking the question: should the scientific community continue to develop more and more sophisticated general or even spectific models or should it put an emphasis on field experiments ? Obtaining an answer will probably be ofgreat importance in the near future. As a matter of fact, because of decreasing computer costs and relative increase in the cost of carefully designed field experiments, there is a worldwide trend to « observe the water and solute movements through computer screens » ! The modelers should be aware that without reliable estimates of the input parameters as well as in-situ validation their models will appear more as intellectual games of academic interest rather than as tools to help the practitioners in their management decisions

Evapotranspiration en zone semi-aride de deux couverts végétaux (gazon, blé) obtenue par plusieurs méthodes : 2. Méthodes neutroniques et tensiométriques

International audienc

Evidence of soil pollution by nitrates derived from pig effluent using 18O and 15N isotope analyses

International audienceIn Réunion Island, expanding human populations, urbanization and agriculture during the last 50 years have all contributed to a steady increase in the level of nitrates in drinking water. Various nitrate point sources are responsible for the nitrate contamination around the island including chemical fertilizers, animal effluent applied to pasture and crops, and urban waste such as sewage and domestic waste water. In terms of agricultural fertilizers, pig effluent is the most widely used, but the cumulative effects of slurry applications on soil water and groundwater are unknown. Our objectives were (1) to characterize and follow in situ the fate of nitrogen through the subsurface after application of pig effluent onto a cultivated soil using stable nitrate isotopes, δ15N and δ18O, and (2) to compare the isotopic signatures of Réunion Island's principal aquifers with results from the experimental site to infer potential contamination sources. The study was conducted on an experimental field site planted with maize in the western part of Réunion Island during the rainy season. A control site with no fertilizer application to the maize was compared with the investigation site which had pig effluent applied once a year. The site which had pig effluent applied over one year had an average maximum surface soil water 15N-${\rm NO}_{3}^{-}$ value of +9.0‰ at 0.45 m depth. This signature was significantly more enriched in 15N than the corresponding subsurface soil water 15N-${\rm NO}_{3}^{-}$ value of +3.8‰ at 10 m depth. The control site average maximum surface soil water 15N-${\rm NO}_{3}^{-}$ value of +3.6‰ at 0.45 m is similar to the subsurface pig effluent application plot. This indicates that nitrates derived from pig effluent have not reached 10 m depth in the subsurface, even though over the last 18 months this site was subjected to two effluent applications, each around 200 kg N ha−1, and more than 1900 mm of rain, more than half of which drains directly into the root zone. This slow migration shows that mobilization of nitrates through cultivated soil can take many tens of years before infiltrating and contaminating the saturated zone situated at several tens, and in places, hundreds of meters depth. On an island-wide scale, an isotopic assessment of nitrates from the experimental site's soil water and other drinking water wells highlights a nitrogenous contamination derived primarily from urban and/or agriculture via effluent application

Caractérisation hydrodynamique des sols : analyse simplifiée des essais de drainage interne

International audienc

USCID fourth international conference

Presented at the Role of irrigation and drainage in a sustainable future: USCID fourth international conference on irrigation and drainage on October 3-6, 2007 in Sacramento, California.Includes bibliographical references.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

Caractérisation et modélisation de la dynamique de l'évapotranspiration en Afrique Soudanienne en zone de socle (interaction entre les aquifères et la végétation)

Dans un contexte où des millions de personnes dépendent de la ressource en eau exposée au caprice de la mousson en Afrique de l'Ouest, ce travail de thèse vise à mieux appréhender les processus hydrologiques en zone de socle, et notamment à évaluer le rôle de la redistribution latérale de l'eau dans le sol par les interactions entre les réservoirs souterrains, la végétation et l'atmosphère, par la caractérisation et la modélisation à l'échelle de la parcelle et du versant. Ce travail s'appuie sur le dispositif expérimental déployé dans le petit bassin versant de l'Ara dans le cadre de l'observatoire AMMA-CATCH. La mise en oeuvre du modèle ParFlow-CLM permet de simuler les transferts dans la zone saturée et la zone non saturée par la résolution de l'équation de Richards en 3D, en étant conditionné par un forçage atmosphérique en surface. Après avoir identifié les paramètres influents pour les transferts verticaux, une caractérisation spatiale de ces paramètres a été menée. La configuration du modèle a ensuite été évaluée en 1D. Il est montré que le modèle reproduit de manière pertinente les séries temporelles du bilan d'énergie et la distribution de l'eau dans le sol. L'effet de la variabilité spatiale des paramètres hydrodynamiques est ensuite étudié à l'échelle de la parcelle. Enfin, en incluant les géométries de socle connues et une distribution de végétation, les transferts horizontaux souterrains sont mis en évidence avec la formation de zones sèches ou humides en relation avec des distributions spatiales d'évapotranspiration.In West Africa, millions of people rely on water resources exposed to the monsoon variability. In this context, the aim of this thesis is to better understand hydrological processes in bedrock areas, and more particularly to estimate the role of lateral redistribution of soil water by the interactions between underground reservoirs, vegetation and atmosphere, using hydrogeological prospection and modeling at field and catena scale. This work is supported by the experimental device implemented in the small Ara catchment in the framework of the AMMA-CATCH observatory. The use of the ParFlow-CLM model allows the simulation of transfers in the saturated and the vadose zone by solving the Richards equation in 3D. The model was forced using observed atmospheric forcing at the surface. We first identify influential parameters for vertical water transfers. Then a spatial characterization of these parameters is carried out. The 1D version of the Parflow-CLM model is assessed using observed data. We show that the model provides relevant times series of the surface energy balance and of soil water distribution as compared to the observations. The impact of the spatial variability of the hydraulic parameters at the field scale (<1 ha) is studied. Finally, the bedrock geometry and the spatial distribution of vegetation are taken into account in the modelling. This allows the identification of horizontal subsurface lateral fluxes, which generate wet and dry patterns, which are related to the spatial distribution of evapotranspiration.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF