On Groundwater Recharge in Variably Saturated Subsurface Flow Models

Groundwater models that simulate only saturated flow use groundwater recharge as an input parameter. In contrast, variably saturated subsurface flow models, including integrated surface and subsurface hydrologic models, can jointly simulate the movement of water in the saturated and unsaturated zones. Instead of recharge, they require climate data such as precipitation and potential evapotranspiration. Given that the latter models represent hydrological processes operating throughout the unsaturated zone and at the water table, one might expect that recharge can be readily extracted from them. In this paper, we demonstrate that it is not the case. When the commonly used definitions of groundwater recharge are implemented in variably saturated subsurface flow models, they do not yield meaningful results. Above all, the problems occur because of the storage dynamics in the capillary fringe above the water table. Despite this difficulty, variably saturated subsurface flow models can provide the information required for water resources management directly.