Estimating potential evapotranspiration over the Edwards Aquifer, utilizing the Priestley-Taylor equation
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Estimating recharge is a critical aspect of groundwater management, when aquifer resources are constrained by multiple users. The Edwards Aquifer, an artesian aquifer underlying Austin and San Antonio, Texas, sustains municipalities, farmers and fragile habitats at discharge locations. Rising municipal demand for Edwards water supports the need for effective conservation over time to maintain the well-being of all users. Predicting recharge is a valuable tool for determining future available resources. Evapotranspiration (ET) accounts for a majority of water loss following precipitation, significantly affecting recharge. Developing a method for accurate regional estimates of ET is complicated by aquifer characteristics, expensive instrumentation, and a variable climate. This study investigates a specific method for estimating regional potential ET (ETp), by combining the Priestley-Taylor equation with data primarily retrieved from the Moderate-Resolution Imaging Spectroradiometer. Improved resolution and timing of satellite measurements provides greater regional specificity for variables related to ET calculations. ETp is then estimated for 2004 and 2005, utilizing data from MODIS, aboard NASA’s Aqua and Terra satellites. Land surface temperature, leaf area index and albedo retrieved from MODIS replace in situ measurements, which are often nonexistent in a regional context. Incoming radiation, a direct input in the Priestley-Taylor equation, is retrieved from the National Center for Environmental Prediction’s North American Regional Reanalysis Model (NARR). Results show methods overestimate ET between 400% to over 1000% when compared to actual ET (ETa) at two locations in the northeast portion of the aquifer. Correlation is improved when ETp is treated as an instantaneous rate rather than daily. During months of above average precipitation, which are more representative of potential conditions, instantaneous ETp exceeded ETa by an average of 81%, with a root mean squared error of 1.15 mm/30min and an average positive bias of 2.84 mm/30min. Considering the soil moisture limited conditions throughout Central Texas, a positive bias is not surprising. Incorporating a calibrated Priestly-Taylor could improve accuracy, but estimating regional ETp remains restricted by available daily data necessary for calculations and comparison.