Estimating Long-Term Drainage at a Regional Scale Using a Deterministic Model

Ary Bruand : Present Address : Institut des Sciences de la Terre d'Orléans; International audience; This study discusses the use of a deterministic model, at a regional scale, when the soils have small heterogeneity. Long-term soil drainage was simulated on 3500 ha in Beauce (France), taking into account variation in soil thickness, agricultural practices, and climate characteristics for a 32-yr period. The model was based on a numerical solution of Richards' equation, using a finite element method. Hydraulic properties were determined at four sites that are 4 km apart. The results show a small variability of hydraulic parameters, with coefficient of variation (CV) ranges from 1 to 16% for the α parameter, the empirical constant n, the saturated water content θs, and the residual water content θr, and from 60 to 75% for the saturated hydraulic conductivity Ks. Hence, the spatial and temporal variability of drainage are mainly related to soil thickness, agricultural practices, and climate characteristics. Assigning small spatial variability to hydraulic properties, we analyzed the role of soil thickness and initial water content (IWC), induced by agricultural practices and climate characteristics, on drainage prediction. Five IWC were defined: 100, 90, 80, 70, and 55% of field capacity (FC). Four soil thicknesses were studied: 30, 60, 100, and 150 cm. The IWC appeared as a significant factor in drainage prediction for 60% of the 32 yr, regardless of soil thickness. Soil thickness became the most significant factor (20% of the 32 yr) for low IWC. The model was spatially extrapolated by combining the calculated drainage (which varied with soil thickness and IWC) and the spatial distribution of crops and soil thickness classes. Simulated drainage was closely related (r = 0.90) to groundwater recharge during the 32 yr, indicating that the model could be applied to other areas with appropriate parameters.