Analysis of long-term trends in piezometric series to assess the effectiveness of substitution reservoirs as a tool for managing water resources in the Marais Poitevin

This study analyses the long-term trends of piezometric time series to evaluate the effectiveness of substitution reservoirs as a water management tool in the Marais Poitevin. The initial step in this research is to undertake a historical analysis of the pumping data, the irrigated area, and the piezometric series. The objective of this analysis is to identify the impact of local water management measures and substitution reservoirs on water tables. The analysis of the piezometric series is based on the use of statistical tests, in particular the Standard Normal Homogeneity test and the Pettitt test, making it possible to evaluate the homogeneity of the series and to detect any significant changes in their average. In a second phase, the study uses historical and future hydrological simulations produced by the ORCHIDEE model, integrating the effects of climate change through four scenarios. The objective is to estimate the long-term evolution of groundwater levels in the absence of irrigation. The model is fed by historical SAFRAN meteorological data, as well as by four climate projections corresponding to the RCP8.5 radiative forcing scenario. Two approaches were used to estimate the level of the water table. The first one is based on soil moisture simulated by ORCHIDEE, which is used as an indicator of the natural level of the water table. The second approach is based on a linear reservoir model, which is fed by drainage and surface runoff data from ORCHIDEE simulations. The findings of this study highlight the positive impact of water management measures on groundwater levels. Furthermore, the findings show that substitution reservoirs have been set up in response to water management measures, in order to maintain irrigation while limiting the impact on groundwater in summer. However, future simulations reveal the limited effectiveness of substitution reservoirs in the future. For three out of the four climate projections studied, the number of days when it is possible to fill up the reservoirs is projected to decrease. Furthermore, the simulation based on simulated soil moisture demonstrates that the crisis threshold has been estimated to be exceeded in a natural state of the water table for the warmest scenario. Substitution reservoirs will therefore not be a solution for raising the level above the crisis threshold in summer. Finally, this study highlights the complexity of water management in the Marais Poitevin. It emphasizes the challenges posed by climate change in achieving a balance between agriculture and the ecological water needs.