USING EDDY COVARIANCE TO INVESTIGATE ECOSYSTEM AND CLIMATE CONTROLS ON EVAPOTRANSPIRATION (ET) ACROSS THE EDWARDS AQUIFER RECHARGE REGION OF CENTRAL TEXAS

The Edwards Aquifer of central Texas is a prolific artesian aquifer that serves almost two million people. It is karst-dominated, defined by abundant sinkholes and conduits, with recharge occurring through a combination of focused and diffuse mechanisms. Understanding the water budget of this system is vital, especially the diffuse mechanisms that are poorly characterized and parameterized in hydrologic models. In this study, the eddy covariance (EC) technique has been used to directly measure fluxes of water vapor (i.e., evapotranspiration; ET) at five locations across the Edwards Aquifer recharge region since 2016, representing different climates and ecosystems across the recharge area.

From 2016-2018, three stations were maintained at Camp Bullis, north of San Antonio, one each in oak savanna, oak-ashe juniper woodland, and grassland ecosystems. Similar average ET rates were observed at each site during the study period (~2.0 ± ~1.1 mm/d), but with seasonal differences; specifically, ET rates from the savanna site were higher in summer and lower in winter than the woodland site, while the savanna and grassland sites were better correlated throughout the year. To understand the control of climate variability on ET across the recharge region, two stations were moved in 2021 to the more humid eastern (Cibolo) and more arid western (Uvalde) portions of the aquifer recharge area. The Cibolo station has collected data in an oak-ashe juniper woodland from 2021-present, while the Uvalde station collected data in a mesquite woody savanna from 2021-2023. Overall, from March 2021 through February 2023, the average daily ET was greater at Cibolo (1.63 ± 1.1 mm/d) than Uvalde (1.38 ± 1.2 mm/d). Both sites experienced seasonal differences in ET rates, with increased ET following precipitation events, and ET rates declining more rapidly at the Uvalde site after precipitation had ceased. Precipitation in excess of ET on a seasonal basis (a first-order estimate of the potential for recharge) ranged from +186.8 mm in spring 2021 at Cibolo to -100.0 mm in summer 2021 at Uvalde (positive numbers indicate potential for recharge). Future work will correlate ET data with satellite-derived vegetation indices and ET estimates to upscale the point measurements and apply the results across the Edwards Aquifer recharge region.