SUSTAINABLE YIELD OF JACOB’S WELL SPRINGSHED, MIDDLE TRINITY AQUIFER, WIMBERLEY, TEXAS

Jacob’s Well (JWS) is a karst spring issuing from the Middle Trinity Aquifer in central Texas, sustaining habitat and recreational resources along Cypress Creek and the Wimberley Valley area. Historically JWS was perennial and contributed up to 25% of the baseflows to the Blanco River, even during the 1950s drought of record. The Blanco River provides allogenic recharge to the Edwards Aquifer downstream. Over the past 15 years, increased pumping from the Trinity Aquifers has resulted in capture of JWS springflow and ultimately cessation of flow during drought. This study characterizes the hydrogeology of JWS and helps constrain the sustainable yield of the JWS springshed, defined here as maintaining measurable flow during severe drought conditions.

The JWS springshed is estimated by integrating hydrologic data with methods published by Lanini et al., 2016 and Bonacci and Andric, 2015. Estimated average annual effective recharge is about 30% of rainfall. During the drought of record, JWS flow was measured at 2.6 cubic feet per second (cfs) in March 1955 and estimated as low as 0.2 cfs in August 1955. Monthly average springflow since 2005 is 8.8 cfs. Four large-scale production wells in the JWS springshed pump a monthly average of 0.3 cfs. Numerous residential wells (n=650) pump an estimated 0.2 cfs. Thus, the minimum springflow during severe drought conditions of 0.2 cfs is exceeded by total monthly average pumping of 0.5 cfs. The result is cessation of springflow during recent droughts that are less severe than the 1950s drought of record.

Reducing pumping by up to 90% within the springshed during drought could result in continuous springflow and a sustainable yield. Although this much reduction in pumping would be unrealistic, approaches to achieving reductions could include a special management zone that can focus on demand reduction through conservation, education, additional drought curtailments, and other regulatory rules. Alternative supplies could include rainwater, the Lower Trinity Aquifer, aquifer storage and recovery, and temporary interconnections to other water sources outside the springshed. The sustainable yield of JWS Springshed is a worthy long-term goal, but may be difficult to achieve; thus preservation of flow may need to focus on increasing the percentiles of flow during all conditions for JWS.