Evaluating the different methods for estimating groundwater evapotranspiration using diurnal water table fluctuations

Riparian ecosystems fundamentally depend on groundwater, and thus accurate estimates of daily groundwater evapotranspiration rate (ETg) is curial for understanding the interactions between groundwater and vegetation. Up to now, various methods (i.e., White, 1932; Hays, 2003; Soylu et al., 2012) have been proposed to estimate daily ETg using the water table fluctuations. The objective of this paper was to evaluate and compare their performances under various water table conditions in field environments. For this purpose, we employed a variety of measurements (i.e., water table levels, latent heat flux and soil water contents) at a riparian forest (T. ramosissima) in Northwest China in 2017. Our results showed that the White method is suitable for use during the water table declining periods. Also, the selected time intervals could have impacts on the estimations of ETg. The Soylu method and the Hays method performed well under various water table conditions. Thus, it seems that the Hays and Soylu methods are more suitable for long-term ETg estimates in field environments. In addition, the percentage of plant transpired water from groundwater varied during the growing seasons. The riparian plants mainly use soil water during the early growing stages, and it tended to use groundwater during the late growing stages. In the future, more isotope-based studies were needed to verify the seasonal patterns of water uptake of the riparian forest.