Extraction of recharge in variably saturated subsurface flow models

Abstract

Variably saturated subsurface flow models have been widely used in the context of water resources management as they conceptualize and simulate water flow in the unsaturated and saturated zone. By solving the Richards equation and using infiltration flux as an input, these models do not require groundwater recharge. As the models simulate the infiltration dynamics through the unsaturated zone, recharge is expected to be reliably extracted from such kinds of models. In this study, we explore to what extent variably saturated subsurface flow models can actually be used to extract groundwater recharge. In this context, we implement numerous definitions of groundwater recharge in a simple, variably saturated 1D model, extract groundwater recharge for a wide range of infiltration and groundwater dynamics imposed through boundary conditions, and assess the reliability of the extracted values. The results show that the value of recharge cannot be uniquely obtained from such kinds of models. The problem is attributed to the storage dynamics in the capillary fringe above the water table. However, it is important to keep in mind that if a variably saturated subsurface flow model of a project area is available, extracting recharge is superfluous as the model is capable of representing all the relevant flux and dynamics.  Keywords: Variably saturated subsurface flow models; Groundwater recharge; unsaturated zone; Water resources management.Reference: Gong, Chengcheng, Peter G. Cook, René Therrien, Wenke Wang, and Philip Brunner. "On groundwater recharge in variably saturated subsurface flow models." Water Resources Research 59, no. 9 (2023): e2023WR034920.