Modélisation du transfert de l’eau et des sels dans les casiers rizicoles du Delta du Fleuve Sénégal

La salinisation des sols constitue un risque majeur du développement de l’irrigation dans le monde. Dans le Delta du Fleuve Sénégal, l’existence d’une nappe phréatique salée et proche de la surface du sol accentue ce risque. Une modélisation des transferts hydriques a été réalisée pour mieux évaluer ce risque. Elle a été faite à partir du code Hydrus sur l’ensemble du cycle cultural et a pris en compte deux types de sol et deux modes de culture de cette région : la simple et la double culture. Elle a permis d’estimer les flux hydriques descendants pendant la phase d’irrigation par submersion, et les flux hydriques et salins ascendants pendant la phase d’irrigation et de non-irrigation. Cette modélisation indique que : i) le toit de la nappe salée remonte d’environ 50 cm pendant la phase d’irrigation; ii) une zone non saturée de faible épaisseur subsiste entre la surface inondée et la nappe; iii) l’infiltration cumulée est deux fois plus forte en double qu’en simple culture; et iv) la forte évaporation durant la phase de non-irrigation peut faire remonter entre 5,5 à 6 t de sels par ha et par an, selon les sols et les conditions de culture. Cette modélisation, qui s’appuie sur des mesures in situ des paramètres hydrodynamiques, devra être confrontée à des observations in situ de flux. Elle devra intégrer des processus non pris à ce stade tel que le piégeage d’air. Les relevés piézométriques depuis dix ans montrent, par ailleurs, un léger relèvement du niveau de la nappe salée. L’ensemble de ces résultats est suffisamment préoccupant pour préconiser, dans le cas d’une irrigation par submersion, la mise en place d’un drainage superficiel pour éviter le relèvement de la nappe et assurer l’évacuation des sels hors de la nappe et, par là, la pérennisation de l’agriculture dans cette région du Delta du fleuve SénégalSoil salinisation is a major hazard of the extension of irrigation in the world. In the Delta of the Senegal River Valley, a highly saline groundwater close to the soil surface increases this hazard. A water and salt modeling by means of the Hydrus code has been performed to evaluate this hazard. Two soil types and two crop systems, a single and a double one, have been considered. The calculations have been performed on the complete agricultural cycle to determine water flows going down during the irrigation phase, and water and salt flows ascending to the soil surface during the irrigation and non irrigation phases. The modeling shows that: 1) the recharge due to the irrigation period induces an elevation of the water table of about 50 cm; 2) an unsaturated zone persists between the flooded surface and the water table; 3) the cumulative infiltration is two times higher in the double crop rice system than in the single crop system; 4) the high evaporation during the dry phase causes an ascension of from 5.5 to 6 t of salts per ha and per year according to the soil type and the crop system. This modeling, which was based on field measurements of hydrodynamic properties, has to be validated by field observations of the fluxes. Some processes such as air trapping should be tested. Piezometer monitoring over the past 10 years indicates a slight rise in the saline water table. All these results demonstrate the relevance of setting up a surficial drainage system for the flooded irrigation fields, to avoid an elevation of the water table and to evacuate salts from the soil surface, in order to perpetuate agriculture in the Delta of the Senegal River Valley

The fate of nitrogen and sulfur in hard-rock aquifers as shown by sulfate-isotope tracing

Stable SO4 isotopes (delta S-34(-SO4) and delta O-18(-SO4)). and more occasionally delta N-15(-NO3) were studied in groundwater from seven hard-rock aquifer catchments. The sites are located in Brittany (France) and all are characterized by intensive agricultural activity. The purpose of the study was to investigate the potential use of these isotopes for highlighting the fate of both SO4 and NO3 in the different aquifer compartments. Nitrate-contaminated groundwater occurs in the regolith; delta S-34 fingerprints the origin of SO4, such as atmospheric deposition and fertilizers, and delta O-18(-SO4) provides evidence of the cycling of S within soil. The correlation between the delta O-18(-SO4) Of Sulfates and the delta N-15(-NO3) of nitrates suggests that S and N were both cycled in soil before being leached to groundwater. Autotrophic and heterotrophic denitrification was noted in fissured aquifers and in wetlands, respectively, the two processes being distinguished on the basis of stable SO4 isotopes. During autotrophic denitrification, both delta S-34(-SO4) and delta O-18(-SO4) decrease due to the oxidation of pyrite and the incorporation of O from the NO3 molecule in the newly formed SO4. Within wetlands, fractionation occurs of O isotopes on SO4 in favour of lighter isotopes, probably through reductive assimilation processes. Fractionation of S isotopes is negligible as the redox conditions are not sufficiently reductive for dissimilatory reduction. delta S-34(-SO4) and delta O-18(-SO4) data fingerprint the presence of a NO3-free brackish groundwater in the deepest parts of the aquifer. Through mixing with present-day denitrified groundwater, this brackish groundwater can contribute to significantly increase the salinity of pumped water from the fissured aquifer

Catchment-scale analysis of hydrological and agricultural impacts of small reservoirs

International audienceSmall reservoirs are dams built to intercept and store runoff water. Small reservoirs can be a resource for farmers by providing water for crop irrigation. In agricultural areas, small reservoirs are seen as a way to sustain agriculture in times of drought. Changes in rainfall patterns due to climate change, with higher rainfall in some seasons and longer droughts in others, and the need to maintain or even increase agricultural productivity are also prompting some stakeholders to promote the development of small reservoirs. The proliferation of small reservoirs in a catchment can put pressure on the water cycle and have a cumulative impact on river flows and other hydrological components, which in turn can affect other water uses and the quality of downstream aquatic environments (Habets et al., 2018). There is a need to better understand and quantify both the cumulative hydrological impacts and the agricultural benefits of small reservoirs.We present here an analysis of the cumulative impact of small reservoirs on hydrology and crop yield in an agricultural catchment. This analysis is based on the modeling of a 20 km² catchment in southwestern France. We used a new agro-hydrological model called Mhydas-Small-Reservoirs, a model coupling hydrological and crop processes with farmers' water management decisions (Lebon et al., 2022). Several catchment situations were considered. These situations combine different levels of reservoir use (current situation with 26 reservoirs of which only 13 are exploited for crop irrigation, a situation with no reservoirs at all, a situation where reservoirs currently not exploited are used for irrigation) and different climatic years (dry year, wet year, and year with average rainfall). The simulations were analyzed in terms of crop yields and different water balance terms (flow, ET, irrigation withdrawal). From the preliminary results, we show the interest and the need to take into account the interactions between hydrological and agricultural processes to quantify the impacts due to small reservoirs. We also identify the need for observations in agrohydrological modeling applied to catchments with small reservoirs

Diversification from field to landscape to adapt Mediterranean rainfed agriculture to water scarcity in climate change context

International audienceRainfed Mediterranean agriculture (MA) must adapt to water scarcity due to climate change and pressures on water resources. According to recent literature, two adaptation solutions based on the concept of diversification can be explored. The first solution is crop diversification at the field level. Three main cropping systems, namely agroforestry, intercropping, and service crops, have been shown to increase soil water availability and to improve crop water use. The second solution is to consider diversification at the landscape level by diversifying crops and associated agricultural management practices (in number, abundance, and spatial organization) and building small-scale water-harvesting infrastructures (WHI). In order to move toward a sustainable MA, one of the main scientific challenges ahead is to provide knowledge and tools, such as integrated agro-hydrological models, useful to evaluate several spatiotemporal combinations of these solutions in order to optimize soil water availability and crop water use

Sub-chapter 3.2.2. Long term agro-ecosystem observatories in the Mediterranean

Introduction Alleviating the impacts of climate change is a major challenge facing agriculture in the near future. It is however not the sole challenge, since agriculture is experiencing many other pressures, including a 30 percent increase in global world population, changing dietary patterns and intensifying competition for increasingly scarce land, water and energy resources. In the Mediterranean area, all these challenges are at a very high level since this region already faces a shortage..

Using long time series of agricultural-derived nitrates for estimating catchment transit times

International audienceThe estimation of water and solute transit times in catchments is crucial for predicting the response of hydrosystems to external forcings (climatic or anthropogenic). The hydrogeochemical signatures of tracers (either natural or anthropogenic) in streams have been widely used to estimate transit times in catchments as they integrate the various processes at stake. However, most of these tracers are well suited for catchments with mean transit times lower than about 4–5 years. Since the second half of the 20th century, the intensification of agriculture led to a general increase of the nitrogen load in rivers. As nitrate is mainly transported by groundwater in agricultural catchments, this signal can be used to estimate transit times greater than several years, even if nitrate is not a conservative tracer. Conceptual hydrological models can be used to estimate catchment transit times provided their consistency is demonstrated, based on their ability to simulate the stream chemical signatures at various time scales and catchment internal processes such as N storage in groundwater.The objective of this study was to assess if a conceptual lumped model was able to simulate the observed patterns of nitrogen concentration, at various time scales, from seasonal to pluriannual and thus if it was relevant to estimate the nitrogen transit times in headwater catchments. A conceptual lumped model, representing shallow groundwater flow as two parallel linear stores with double porosity, and riparian processes by a constant nitrogen removal function, was applied on two paired agricultural catchments which belong to the Research Observatory ORE AgrHys. The Global Likelihood Uncertainty Estimation (GLUE) approach was used to estimate parameter values and uncertainties. The model performance was assessed on (i) its ability to simulate the contrasted patterns of stream flow and stream nitrate concentrations at seasonal and inter-annual time scales, (ii) its ability to simulate the patterns observed in groundwater at the same temporal scales, and (iii) the consistency of long-term simulations using the calibrated model and the general pattern of the nitrate concentration increase in the region since the beginning of the intensification of agriculture in the 1960s. The simulated nitrate transit times were found more sensitive to climate variability than to parameter uncertainty, and average values were found to be consistent with results from others studies in the same region involving modeling and groundwater dating.This study shows that a simple model can be used to simulate the main dynamics of nitrogen in an intensively polluted catchment and then be used to estimate the transit times of these pollutants in the system which is crucial to guide mitigation plans design and assessment

Water Use Efficiency (WUE) across scales: From genes to landscape

International audienceWater scarcity will be one of the main issues of the 21 st century, because of competing needs between civil, industrial, and agriculture use. While agriculture is the largest user of water, its share is bound to decrease as societies develop. Clearly, agriculture needs to become more water efficient. Improving water use efficiency (WUE) at the plant level is important although there is a long way into translating this at the farm/landscape level. As we move up from a cell/organ/plant scale to more integrated scales such as plot, field, farm system, and landscape, other factors need to be considered, including trade-offs, to possibly improve WUE. These include choices of crop variety/species, farm management, landscape design, infrastructure development, ecosystem functions, where human decisions matter. This review is a cross-disciplinary attempt to analyze ways to address WUE at these different scales where metrics of analysis are defined and trade-offs considered. The equations in this perspective paper use similar metrics across scales for an easier connection and are developed to highlight which levers, at different scales, can improve WUE. We also refer to models operating at these different scales to assess WUE. While our entry point is plants and crops, we scale up the analysis of WUE to farm systems and landscapes