Modified method for the estimation of groundwater evapotranspiration under very shallow water table conditions based on diurnal water table fluctuations

Abstract

Groundwater evapotranspiration (ETG) is often estimated using the White method from diurnal water table fluctuations. However, under very shallow water table conditions (<1 m in this study), the groundwater replenishment to ET-induced depleted soils during nighttime is significant, but is ignored in the White method. A variably saturated flow model, HYDRUS-1D, was used to elucidate the impact of this omission on the accuracy of ETG under various soils, depths to the water table, climatic conditions, vegetation coverage, and root distributions. We found that the mean ETG was underestimated by 33–69% by the original White method because of the inaccurate estimation of the water table recovery rate during nighttime. The daily hydrostatic equilibrium recovery (Re) term was introduced to modify the original White method by considering the ET-induced disequilibrium in soil moisture. Using the modified method, the estimated ETG approaches its real value, and the mean difference between them is only approximately 2%. The modified method was applied to estimate ETG in a wetland in NW China, resulting in better agreement with potential evapotranspiration measurements than the original White method.