On evapotranspiration and shallow groundwater fluctuations: A Fourier-based improvement to the White method

Evapotranspiration (ET) is a significant component of the water and energy balance in wetlands and riparian zones, yet it is also one of the most challenging components to estimate. Diurnal water table fluctuations can be used to directly measure groundwater consumption by phreatophytes, which are often important contributors to the total ET in riparian systems. Although such methods are cost effective, significant uncertainties usually exist, and more accurate techniques continue to be developed. In this study we present a new Fourier method for calculating daily (and longer) groundwater ET consumption using a moving, multiday sine function to capture robust, diurnal water table fluctuations. The technique is tested and calibrated in Tamarix chinensis and Populus deltoids-dominated riparian areas in the Middle Rio Grande region of New Mexico and in a Phragmites australis-dominated riparian wetland in south-central Nebraska (using independent, energy balance estimates of ET). The results show that-at both field sites-the new Fourier technique performs significantly better than the commonly used White method, regardless of the length of the moving window that is employed. The Fourier method presented here provides a step toward increasing the accuracy of ET estimates from diurnal water table fluctuations. Guidelines are defined for applying the new and improved method in the most accurate fashion, based on groundwater hydrographs and solar radiation data (or theoretical clear-sky estimates).