Résumé
Factors governing recharge in fractured crystalline rock aquifers are poorly understood. It is unclear whether recharge is primarily controlled by the properties of the heterogeneous underlying aquifer or the overlying soil characteristics, which can also be strongly heterogeneous. Further, in semi-arid and arid environments, soil properties vary strongly and non-linearly as a function of soil moisture. Soil moisture is controlled in part by soil water retention properties and rainfall intensity, but also, increasingly, by land use and human influence. Current large-scale hydrological models often do not account for heterogeneity and the nonlinear variability of soil hydraulic properties. The aim of this study is to identify and quantify the driving forces that shape spatial recharge patterns in crystalline rock aquifers, while critically assessing the models used, model uncertainty and parameter sensibility. Reference recharge maps were obtained previously by applying an improved water table fluctuation (WTF) technique to a well-characterized and continuously monitored catchment in South India at a 685x685m scale (a scale at which underground lateral transfers are assumed to be negligible). Land use maps, irrigation practices, soil maps and associated hydraulic properties are well documented from previous studies. The first step of this study was to calibrate a simple soil moisture balance model (SMBM) against WTF recharge estimates, but it quickly became apparent that assuming constant hydraulic properties for the whole observation period lead to over- or underestimation of recharge relative to soil moisture conditions. Thus, in a second stage, a very simple pseudo-2D physically based unsaturated flow model was applied at a 100x100m scale. This permitted the model to incorporate time-variable water retention properties, and smaller-scale land use and soil type patterns. It was found that land use had a strong effect on recharge, more than soil type distribution. Soils which are kept flooded for rice cultivation are generally poorly transmissive, although, counter-intuitively, these act as zones of enhanced recharge. This is due to their high water content, which increases the effective soil permeability. Parameter uncertainty and the possible existence of lateral transfers are also discussed.