Revisiting the quantitative relationship between groundwater evapotranspiration and water table depth: A numerical study

The relationship between groundwater evapotranspiration (ET_GW) and water table depth (WTD) is critical for estimating groundwater budget. In regions with shallow water tables, which corresponds to the discharge area of a groundwater flow system, ET_GW and lateral groundwater inflow are both important components of groundwater budget. In this study, we set up HYDRUS models with different inflow rates at the lower boundary and varying evaporative demand at the upper boundary. No matter what the initial WTD is, the finally stable WTD is dependent on the inflow rate and the evaporative demand, and ET_GW equals the groundwater inflow rate. We also set up HYDRUS models with a no flow lower boundary to obtain the extinction depth (d_ext) of ET_GW for 12 typical soils. Based on a series of ET_GW versus stable WTD for each typical soil, we propose an equation dependent on d_ext, d_dec (decoupling depth, the limiting depth above which ET_GW equals ET₀) and an empirical parameter p, all of which are dependent on soil texture. We also examined the influence of vegetation on the three parameters for each soil. Our equation is verified by using data of ET_GW versus WTD at a field site in Florida, USA. Additionally, we find previous models would overestimate ET_GW when WTD is very shallow. This is the first time to consider the control of d_ext and d_dec on ET_GW simultaneously. Our improved understanding of ET_GW would promote groundwater resource management in many regions with shallow water table.