An assessment of different methods to determine specific yield for estimating groundwater recharge using lysimeters

Estimation of groundwater recharge is considered crucial for the management of groundwater resources. The groundwater level fluctuation (GLF) method is a widely used approach to estimate groundwater recharge due to its simplicity and ease of implementation. However, the main source of uncertainty is the specific yield for the GLF method. Although there have been a lot of methods for determining specific yield, the performance of specific yield on the estimation of recharge remains unclear. We set up three lysimeters with different water table depths in the Guanzhong Basin, China. Soil moisture content along with the soil profile, water table depths, and rainfall data were measured continuously. These data provide us with accurately observed recharge and allow us to analyze the performance of specific yields from different methods in estimating recharge. The main results are: (1) The constant specific yield from the pumping tests, which is equal to the ultimate specific yield, significantly overestimated the observed recharge rates independent of water table depths. (2) The constant specific yield obtained from saturated soil moisture content minus field capacity tended to overestimate recharge under the shallow water table depths (less than 2 m), and vice versa; (3) The depth-dependent specific yield using the measured soil moisture content along with soil profile can obtain reliable recharge across all water table depths. (4) The accuracy of the depth-dependent specific yield obtained by the soil water retention curve relies on reliable parameters. The parameters α and n have to be taken into account carefully for determing the specific yield. Our results are important for the application of the GLF method to estimate recharge. More importantly, it is valuable for the sustainable management of groundwater recourses.