Quantification of groundwater evapotranspiration (ET) rates, particularly in arid regions is a prerequisite for sustainable groundwater resource use and management as well as natural ecosystem protection and restoration. Whereas the accurate estimation of ET remains a challenge in that it is typically subject to uncertainties associated with climatic variables, vegetation parameters, geological variables, and hydrologic parameters.

A research group, led by Prof. YU Jingjie from the Institute of Geographic Sciences and Natural Resources Research (IGSNRR) of the Chinese Academy of Sciences, has developed a new water table fluctuation (WTF) method to estimate ET based on seasonal groundwater level changes.

Researchers investigated ET processes at two sites dominated by phreatophytes (Tamarix ramosissima and Populus euphratica) within hyper-arid desert environments in northwestern China for the period 2010–2012. In both field sites, the presence of diurnal water table fluctuations during the summer may be attributed to groundwater evapotranspiration due to direct root water uptake by plants. 

“Using the WTF method, the average ET rate during the summer months (June-August) of 2010-2012 was estimated to be 0.63-0.73 mm/d and 1.89-2.33 mm/d at the two sites dominated by phreatophytes. The field investigation indicated that the different ET rates are likely due to the differing plant community compositions and plant canopy cover characteristics; the 3-year average NDVI during the growing season (May–September) was 0.172 at the Tamarix ramosissima site, while it was 0.228 at the Populus euphratica site”, Dr. WANG Ping stated.

The results show that the standard deviation of the detrended diurnal water table fluctuations can serve as an important diagnostic indicator of groundwater evapotranspiration. In addition, estimation of ET based on the WTF method is a relatively simple, less expensive technique.

However, this method involves a number of sources of uncertainty. To enhance the performance of the WTF method based on seasonal water table declines, further research on the estimation of lateral flow rates will be performed by using an effective network of groundwater monitoring in the future.

The related work has been published in the Journal of Hydrology（Wang P., Grinevsky SO., Pozdniakov SP., Yu J.*, Dautova DS., Min L., Du C., Zhang Y. Application of the water table fluctuation method for estimating evapotranspiration at two phreatophyte-dominated sites under hyper-arid environments. Journal of Hydrology, 519, Part B: 2289-2300).