Testing the Ideas of Walter White: An Estimation of Riparian Evapotranspiration Using Groundwater Upwelling

In arid and semi-arid systems the riparian zone boarding a stream has high evapotranspiration rates compared to uplands. Evapotranspiration (ET) is difficult to estimate, but accounts for much of the water loss from landscapes. Walter White (1932) introduced a method using the diurnal fluctuation of groundwater depth to estimate ET. With the advent of accurate data logging pressure sensors, the White method has recently reemerged as an important strategy. A persistent challenge of the White method is knowledge of how much water is released per unit fall in the water table (the “specific yield”). Specific yield is highly variable, and there exists no validated method for its measurement. Our experiment will indirectly measure this term using two methods. The first method utilizes soil moisture probes, spaced 3cm apart along the vertical axis of a soil column to measure the volumetric change in water content accompanying changes in water table. Soil moisture and tension data will be used in conjunction with the Hydrus 1-D numerical model to estimate evapotranspiration. The second method is performing classical pump tests to estimate the in situ specific yield, and using this estimated specific yield term in the Walter White equation to estimate evapotranspiration. We will compare these groundwater based estimations of evapotranspiration against two micrometeorological data based estimations of evapotranspiration. The micrometeorological based methods are the ‘eddy covariance’ method as well as the ‘Penman-Monteith’ method. The deliverable by the end of this project will be a simple method to install and calibrate a groundwater table depth observation well to produce an accurate estimation of evapotranspiration and groundwater upwelling in areas with shallow water tables such as riparian areas.