Estimating groundwater recharge through unsaturated flow modelling: Sensitivity to boundary conditions and vertical discretization

Unsaturated flow modelling is increasingly being used to estimate potential groundwater recharge. A review of previous studies found that unit-gradient and fixed water table lower boundary conditions have been applied to models of both constant and variable vertical grid spacing (discretization). In order to provide a general guidance, this work studies the effect of both discretization and boundary conditions on simulation times and estimated fluxes at the water table, using one-dimensional models of 2, 4, 6, and 12 m comprised of sand, sandy loam, loamy sand, and loam. The study uses climatological data from the Boreal Plain of northern Alberta, Canada. Because of the long-term average water deficit and the thick unconsolidated glacial deposits, unsaturated flow is expected to be vertical, both downward and upward, and inter-annual changes in water storage will be important. Long-term simulations (1919–2007) that comprised both wet and dry cycles, reveal that when a variable vertical discretization at both the top and bottom of the columns (varying from 0.1 to 10 cm) is utilized, a balance between simulation accuracy and running time can be achieved. It is also found that whenever the unsaturated flow modelling approach is used to estimate potential groundwater recharge, a fixed-head lower boundary condition should be selected because it also allows upward flux from the water table during dry periods, a situation that prevails on both sub-humid and semi-arid areas, where accurate groundwater recharge estimates are needed the most. However, it should be kept in mind that the use of a fixed water table is a simple representation of the regional water table, which in reality interacts with the regional groundwater flow and surface water bodies (e.g., lakes and wetlands).