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Item A Tool to Assess How the Blanco River Interacts with Its Aquifers: Creating the Numerical Model: Phase II(2019-08) Mace, Robert E.; Wierman, Douglas A.There is and continues to be a very high level of economic development and growth along the I-35 corridor between Austin and San Antonio as well as west of I-35 in ecologically sensitive areas of the Hill Country. A significant by-product of this growth is the stress and demand placed on natural resources, foremost of which are water resources in the Blanco River watershed. Increased demand for water leads to more wells and more pumping which in turn affect springs and baseflow to rivers and streams. For example, water levels in the Middle Trinity Aquifer declined between 3 and 54 feet between 1980 and 1997 (Jones and others 2011). A historical observational well near Wimberley shows a decrease in water levels of more than 100 feet since the mid-1980s (TWDB 2018a). Water wells near the river also show substantial water-level declines during drought when pumping is typically higher. For example, a well in Blanco near the river showed more than 80 feet of water-level decline during drought periods (TWDB 2018b). Water-level declines decrease spring and baseflow which in turn affect iconic springs; flows in the river for the environment, recreation, and water supplies; and recharge to the Edwards Aquifer (which affects other iconic springs). As a result, a new groundwater model—specific to the aquifers in the Blanco River watershed—will be developed to create the tool needed by local landowners, communities, and groundwater conservation agencies to better understand and manage groundwater resources in the Hill Country. This new, more local model would not replace Texas Water Development Board’s groundwater availability model; instead, it would supplement the Texas Water Development Board’s model with more detailed data that local groundwater conservation agencies 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. The project will be a collaborative effort involving numerous stakeholders and experts. These will include, but are not limited to, Southwest Research Institute©, The Meadows Center for Water and the Environment, Barton Springs Edwards Aquifer Conservation District, Blanco-Pedernales Groundwater Conservation District, Cow Creek Groundwater Conservation District, Hays Trinity Groundwater Conservation District, and Edwards Aquifer Authority.Item Blanco River Aquifer Assessment Tool: A Tool to Assess How the Blanco River Interacts with Its Aquifers: Creating the Conceptual Model(2019-09) Martin, Nicholas; Green, Ronald T.; Nicholaides, Kindra; Fratesi, S. Beth; Nunu, Rebecca R.; Flores, Mauricio E.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.