Characterizing Stream-Aquifer Interactions in the Pedernales Watershed in Central Texas
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Understanding the nature of stream-aquifer interaction is important for understanding the degree of hydraulic connection between streams and adjacent aquifers within any given watershed. Such information is necessary before the effects of groundwater pumping on streamflow, or the effects of surface-water runoff on aquifer recharge, can be assessed. Traditional techniques to analyze stream-aquifer interaction are based on digital groundwater flow models; however, aquifer parameters for model calibrations are generally unavailable or difficult and costly to obtain. Recession curve analysis is an alternative approach to determining stream-aquifer interaction Basic assumptions in recession curve analysis include: (1) no regulation on the stream, (2) stream fully penetrates the aquifer, (3) the watershed is underlined by impermeable rocks, (3) aquifer systems have uniform hydraulic conductivity and transmissivity, (4) aquifer that sustains stream flow is a one-dimensional flow regime (neglects vertical interaction with deeper aquifers), and (5) watershed has uniform storage and recharge. This study sought to investigate changes in precipitation, streamflow, baseflow, and hydrologic properties of the Pedernales watershed in central Texas, as well as identify the primary aquifer contributing flow to the Pedernales River between 1940 and 2014. The USGS Ground Water Tool Box RECESS program was used to extract meaningful segments of streamflow recession and the slope (K) of such segments (recession curve index, RCI). Man-Kendall Monotonic trend (MK) tests were used to assess changes in climatologic and hydrologic conditions during the study period. The results of annual trend analysis of precipitation, streamflow, baseflow and RCI showed no significant changes over the study period. Values of the stream-aquifer property T/a2S (where T is the transmissivity of the aquifer, a is the average distance between the stream and watershed divide, and S is the storativity of the aquifer was estimated as 0.0403/d and the watershed K value was 23.15 days/log cycle. It is concluded that the Hensel Sand (rock formation) of Cretaceous age is the primary source of baseflow to the Pedernales River above the Johnson City gage. Results of this study are relevant to water resource management in the study area to satisfy the needs o fa growing population while maintaining the ecological integrity of the stream-aquifer system. The approach used in this study is transferable to other watersheds as method requires only streamflow hydrograph and recession analysis.