How Much Water in the Guadalupe? A Baseflow Analysis




Wierman, Douglas A.
Walker, Jenna
Udita, Tasnuva Shabnam

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Over the past several years, the Meadows Center’s “How Much Water is in the Hill Country?” research efforts have focused on developing baseline groundwater-surface water interaction and water quality data on Onion Creek, and the Blanco and Pedernales Rivers to gain a clearer understanding of the complex hydrogeology of Hill Country rivers, aquifers, and springs. The limited geographic focus in the Hill Country was by design since the groundwater/surface water interactions were poorly understood and undocumented. The implications of our findings to date have helped quantify how much of the surface flows of the rivers come directly from groundwater and vice versa. These findings have direct relevance to many communities that rely on Hill Country streams and rivers as the source of their drinking water and livelihood, as well as for aquatic organisms living in the river. Once we had a better understanding of the groundwater-surface water dynamics of the Blanco, Onion, and Pedernales Rivers, the Meadows Center sought to expand our research using the same methodology in the GRB from the headwaters to the tide waters. The first phase of investigating the Guadalupe River was a desktop study named “How Much Water is in the Guadalupe? A Preliminary Data Analysis and Gap Analysis” (Wierman, 2019). This study is referenced as the Preliminary Report throughout this report. The findings of the Preliminary Report confirmed that surface water/groundwater interactions are dominated by the flow contribution of several major springs, including the Plateau Edwards headwaters spring system, Comal Springs, San Marcos Springs, Hueco Springs, Pleasant Valley Springs, and Jacob’s Well. An analysis of United States Geological Survey (USGS) stream gages with long periods of record (over 70 years) indicate relatively flat linear trends in discharge. Since 2000, all gages in the basin have indicated decreasing discharge trends. The cause of the declines may be from increased withdrawals and/or climate change due to increasing temperatures and decreasing precipitation. Researchers have predicted continuing declines in discharge from major springs due to climate change (Wierman, 2019). Potentially declining groundwater levels in shallow aquifers can result in declining baseflow. The Preliminary Report identified several data gaps for further study in this next phase of “How Much Water is in the Guadalupe? Headwaters to Gulf.” Major data gaps addressed in this report include: • Is declining discharge due to declines in baseflow or storm runoff, or both; • Have diversions from the river changed since 2000 and are they effecting flow; • Are manmade discharges into the river significantly altering stream flow; • Have there been significant land use changes in the basin that could have an impact on flow; and • What are the population trends and projected water use in the basin?



Guadalupe River Basin, precipitation patterns, population growth, streamflow


Wierman, D., Walker, J., & Udita, T. S. (2019). How much water in the Guadalupe? A baseflow analysis (Report No. 2019-11). Texas State University, San Marcos, Texas.


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