Nearly three quarters of a century ago, White  introduced a method for estimating ET based on analysis of diurnal water table fluctuations induced by transpirative uptake by riparian vegetation. Since that time, the method has been used sporadically, with no consensus regarding the reliability of this approach. We present a rigorous analysis of the White method using variably saturated groundwater flow modeling. Results suggest that the method is reliable for coarse-grained sediments, but can be expected to fail miserably when applied in areas with fine grained soils. The method fails because less water is released from storage as the water table drops than would be expected using a traditional definition of specific yield since, 1) in fine grained soils water is released slowly from the vadose zone to the saturated zone and 2) when the water table is very shallow, the vadose zone is too thin to release the volume of water expected using the traditional specific yield model for a given change in the position of the water table. To address these two issues, a method was introduced to estimate the readily available specific yield, which is appropriate for time scales associated with diurnal fluctuations. In addition, a distinction between total ET and ETG (the component of ET from groundwater) was introduced because only the latter is detected with this method.
This is a very practical and inexpensive method that can be easily implemented to quantify ETGin environments with phreatophytic vegetation and a shallow water table. The method results in a continuous record of ETG over the growing season.