Abstract

Synoptic gain/loss studies have been a primary method for determining surface water/groundwater interactions in Texas Hill Country streams. The Meadows Center has participated in numerous studies in recent years and has learned scientific and logistical lessons during these studies.

Very little historical spatial or temporal water discharge and quality data exists for many of the medium- and small-sized watersheds. Sparse data sets are sometimes available for larger streams. For many streams, these recent studies represent the only comprehensive data available. As the population of the Hill Country increases, these studies help create a baseline for future studies.

As most lands in Central Texas are privately owned, many landowners are reluctant to permit researchers on their property. Gaining access to live water on private lands with suitable monitoring locations and wells is the most time-consuming task in these studies. One often needs to visit landowners several times to gain trust and access and develop networks of landowners. This process needs to start at the beginning of the project. Performing several synoptic gain/loss studies under varying base flow conditions can be very revealing when flows are low. Higher flows tend to obscure some gaining and losing stream reaches. Dye trace studies can aid in determining direct recharge and loss areas.

Surface water/groundwater interactions are happening in every stream, creating base flow, and are directly related to geology and geologic structure. The measurement of groundwater levels in wells can be as important as stream flow discharge measurements in determining surface water/groundwater interactions. In Central Texas, groundwater flows tend to follow regional structural dips. Springs often occur due to fracturing and faulting, creating vertical flow paths. Base flow water quality is generally good. The lack of large-scale regional development and lack of phosphorus in the aquifer equates to clear flowing streams.