Performance of the White method for estimating groundwater evapotranspiration under conditions of deep and fluctuating groundwater

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration ( ET g ) from groundwater level diurnal signals. Because of a lack of direct measurements of ET g , it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2‐year field observation in a hyper‐arid riparian tamarisk ( Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration ( ET s ) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield ( S y ) and recharge rate of groundwater ( r ) in the White method were investigated. The results showed that the readily available S y determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant S y value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000 h to 0600 h for r estimation for the White method worked best than other time spans. A 2‐day moving average of r values further enhance ET g estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ET g estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ET g in desert riparian forests with deep groundwater depth. Copyright © 2015 John Wiley & Sons, Ltd.