Blanco River Aquifer Assessment Tool: A Tool to Assess How the Blanco River Interacts with Its Aquifers: Creating the Conceptual Model
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The headwaters of the Blanco River are in northern Kendall County, TX. The river flows eastward for about 87 miles across the Texas Hill Country to join the San Marcos River just southeast of San Marcos, TX (Figure 1). Along the way, the river flows both above and below ground and interacts with the Trinity and Edwards aquifers (Ferguson 2017). The Blanco River watershed provides water to supply spring flow at iconic central Texas springs including Barton Springs, San Marcos Springs, Pleasant Valley Springs, and Jacob’s Well Spring.
The Blanco River basin includes some of Texas’ and the nation’s fastest growing counties. With increases in population come increased demands on the water resources in the basin. Increased extraction of water to meet growing demand has the potential to reduce flow in the Blanco River and discharge from Barton Springs, San Marcos Springs, Pleasant Valley Springs, and Jacob’s Well Spring (Gary et al. 2019).
One way to plan for increased water demand and to effectively manage water resources is to employ a numerical, computer model to simulate the changes in amount of water in Blanco River basin over time given projected changes in water demand with continuing growth. A computer model provides a means to test the impact of economic development hypotheses on the water resources in the basin and to analyze the utility of water-management strategies.
The Texas Water Development Board (TWDB) developed a numerical, computer model of groundwater flow in the Trinity Aquifer in the Hill Country. However, this model: 1) simulates groundwater flow and does not explicitly simulate or account for surface water considerations; and 2) is regional in scope with the main purpose of estimating available groundwater volumes for the entire Trinity Aquifer, which extends beyond Blanco River basin to the north, west, and south.
A new tool needs to be developed that is specific to the Blanco River basin and explicitly accounts for surface water in the basin. The purpose of the new tool is to allow local landowners, communities, and groundwater conservation districts to better understand and manage groundwater resources in the Hill Country by providing understanding and quantification of the interaction of groundwater and surface water in the Blanco River basin. "Surface water" as used in this report comprises water in surface water bodies such as streams and lakes (lumped together in this report as "streamflow"). In this study, surface water also includes water flowing at or near the land surface outside of such water bodies, typically referred to as "runoff". Runoff and other near-land surface processes are included in the "land-surface processes" category in this study.
Although this new tool will also be a numerical, computer model, this new, more local model would not replace TWDB’s groundwater availability model. Instead, it will supplement the TWDB model with more detailed data that local groundwater conservation districts can use to not only inform local management decisions but to inform decisions on desired future conditions and to improve subsequent updates of the regional model.
Development of a numerical, computer model to test hypotheses relating to potential impacts of increased pumping and water extraction on groundwater levels, springs, and river flows is a substantial undertaking. Consequently, the development effort is divided into two phases.
- Phase 1: Create a conceptual model of the Blanco River system and use this to generate a blueprint for the numerical, computer model.
- Phase 2: Develop, calibrate, and validate the numerical, computer model.
The new tool is called the Blanco River Aquifers Tool for Water and Understanding Resiliency and Sustainability Trends (TRATWURST). It is likely that there will be subsequent phases to test hypotheses related to future pumping and changes in future weather patterns on the Blanco River system.
The purpose of this document is to present the results of Phase 1: Creation of the Conceptual Model.