NGWA Event and Education Recordings Archive

Prior to the 1970’s it was generally believed that a relatively large portion of precipitation recharged the Edwards Aquifer because of its highly karstic landforms and high transmissivity. In the 1980’s stream flow loss studies were interpreted to indicate that evapotranspiration was relatively high (85% of precipitation for the Barton Springs Segment), that only 6% of precipitation over the combined contributing and recharge zone, and that less that 1% of precipitation recharged the intervening areas of the Edwards Aquifer between major creek channels. Even though other parts of the world measured a much higher portion of precipitation recharging karst areas, this surprising stream flow loss was not questioned until a new generation of recharge studies were conducted.

Beginning in 1996, successful long-distance groundwater tracing delineated the groundwater basins across the Barton Springs Segment differently than those assumed in the 1980’s stream-floss loss studies. In 1998, Central Texas reported new site specific actual evapotranspiration climate towers, using eddy covariance and Bowen ratio methods and coupled with surface runoff monitoring, measured about 70% of the precipitation budget as opposed to about 15 to 20% measured in stream flow loss studies. Beginning in 2004, new continuous gauging stations were installed based in groundwater tracing results and stream flow gauging. Water balances using both site specific eddy covariance actual evapotranspiration and runoff could be compared to refined stream flow loss recharge results.

About 26-28% of precipitation recharges directly from the recharge zone as opposed to about 1% reported in 1984.  The new recharge values fall within the range of values found worldwide and are corroborated with regional measurements of evapotranspiration. This study demonstrates the utility of introduced groundwater tracers for delineating groundwater basins prior to creating water balances based on stream flow loss.