Climate Projections for Drought Assessment

Introduction

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The Edwards Aquifer provides drinking water to more than 2 million people in San Antonio and surrounding areas in south-central Texas. This vital resource supports municipal, agricultural, and industrial uses, and is essential for sustaining habitats for several threatened and endangered species (EAA, n.d.). Classified as a karst aquifer, the geological characteristics of the Edwards Aquifer include permeable limestone, conduits, caves, sinkholes, and springs. In a karst system, these dissolution features at and near the surface allow groundwater to be replenished by rainfall more quickly than aquifers composed of gravel or sandstone (USGS, 2021). However, rapid recharge can make the aquifer more susceptible to contamination, and constant discharge from large springs makes it susceptible to drought events (EAA, n.d.).

 In 1993, the Texas State Legislature established the Edwards Aquifer Authority (EAA) to protect the aquifer and ensure its sustainable use for all who depend on it. EAA responsibilities include regulatory and hydrogeologic monitoring, and management of permitted groundwater withdrawals. Data collection activities include volumes pumped from permitted wells, meteorological data, water levels, water quality, and discharges from major springs in addition to data collected for research, such as evapotranspiration and soil moisture data (EAA, n.d.). During drought conditions, which are triggered by low water levels or low spring flows, the EAA enforces groundwater withdrawal reductions to protect the habitats of threatened and endangered species at the major springs and to ensure availability of water for all users. Additional mitigation and conservation measures have been developed and implemented to ensure the sustainability and drought resilience of the aquifer and its habitats.

The EAA recognizes the importance of using climate projections to estimate future aquifer conditions and to assess the effectiveness of existing drought mitigation and conservation strategies under future climates. However, publicly available projected climate data must be scaled down to capture local details before they can be used to inform management decisions. With different global climate models, downscaling techniques, and emission scenarios, many factors must be considered when selecting the appropriate climate projections for management and planning (Wooten et al., 2024). The EAA partnered with a climatologist from the South Central Climate Adaptation Science Center (South Central CASC) to generate customized downscaled climate projections for the Edwards Aquifer Region. This collaboration produced a better understanding of potential future climate conditions and their impacts on the aquifer, and provided a robust and technically defensible process for EAA’s estimation of future aquifer water availability and environmental flows.

Key Issues Addressed

Global Climate Models (GCMs) are key tools the EAA can use to assess future climate conditions. GCMs represent components of Earth’s major climate systems, and their projections can be downscaled to predict temperature and precipitation patterns for specific regions. Projections are not forecasts, but they provide valuable insights into climate change-driven risks. Although much GCM data is publicly accessible, the downscaling process can be costly and time consuming because it requires advanced software and expertise to select the most accurate and representative GCMs for a given region.

The EAA needs region-specific downscaled climate projections to evaluate the Edwards Aquifer’s response to changes associated with future climate conditions. An important question, for example, is how recharge in the semi-arid western portions of the region will be affected by increasing temperatures and variations in precipitation. An important reason for the climate assessment is the renewal of the Incidental Take Permit for the Edwards Aquifer Habitat Conservation Plan, which requires evaluation of climate impacts on the aquifer’s major spring systems. 

EAA implements a Critical Period Management (CPM) plan to ensure adequate spring flow during drought conditions. CPM consists of permitted withdrawal reductions and pumping forbearance leases that are implemented in stages as water levels decrease. Renewal of the Incidental Take Permit will rely in part on an evaluation of whether the current CPM practices will be adequate to protect the spring systems under future climate conditions. Planning for management of the aquifer over the next three or four decades will depend on an understanding of the risks of more frequent or more intense droughts in the region and the effects of intense rain events of aquifer recharge.

Project Goals

• Identify global climate models and downscaling techniques that best represent temperature and precipitation patterns in the Edwards Aquifer Region.
• Downscale selected GCMs to produce high resolution climate projections for the Edwards Aquifer Region through the year 2100.
• Summarize projected climate model output from multiple models and emission scenarios to assess potential climate change spatially and temporally in the Edwards Aquifer region.
• Examine how future climate conditions may impact recharge and affect drought risk for the Edwards Aquifer.

Project Highlights

• Select Methodologies, Models, and Emission Scenarios for Regionally Scaled Climate Projections: The EAA partnered with a climatologist from the South Central CASC to downscale climate projections for the Edwards Aquifer Region.The climatologist helped select the best methods and models to represent regional conditions. Oak Ridge National Laboratory’s Daymet precipitation and temperature dataset was used as training data for statistical downscaling. This allowed the researchers to downscale models to the Great Plains scale, based on annual rainfall and temperature patterns specific to the Edwards Aquifer Region (Wootten et al., 2024).
• Partnerships Enhance Understanding of Possible Future Climate Conditions: The collaboration between the EAA and the South Central CASC led to 19 sets of climate projections downscaled to the Edwards Aquifer Region that show daily precipitation and temperature under high and intermediate emission scenarios (Wootten et al., 2024). With a high spatial resolution of 1 km, these models allow EAA planners to evaluate how future conditions may impact aquifer recharge, groundwater levels, and spring flows. Projections show that through the end of the century, the Edwards Aquifer Region can expect to face drought conditions similar to those it has experienced since the 1950s.
• Evaluate Management Outcomes to Enhance Drought Planning: The project team developed and used Counterfactual AI to assess what would have happened in past droughts if Critical Period Management had not been used (Sharma et al., 2024). Results showed that current drought mitigation and conservation measures have made a positive impact on the sustainability of the aquifer and spring flows.

Lessons Learned

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As a regional groundwater management agency, the EAA has limited technical resources to develop region-specific climate projections. This project enabled EAA to work closely with an experienced climatologist to select appropriate models and downscaling approaches for the Edwards Aquifer Region and constrain the number of models needed to make efficient use of financial and computing resources in the time allotted for the project.

Throughout this project, EAA water resources managers enhanced their ability to integrate projected climate data and associated uncertainties into the EAA’s management plans. Additionally, EAA and Edwards Aquifer Habitat Conservation Plan stakeholders had opportunities to ask questions and engage in discussions with the South Central CASC climatologist during public meetings. This raised awareness and provided education that was essential to build support for climate-informed decision making. Access to South Central CASC expertise enables the EAA to develop technically defensible approaches to future climate assessments and to evaluate current and future approaches to its mitigation and conservation plans.

While some downscaled climate data is publicly available, EAA planners and South Central CASC partners required customized climate data sets to ensure accurate representation of the impact of future climates on the Edwards Aquifer system. Financial support from the U.S. Geological Survey (USGS) provided the South Central CASC climate scientist with the resources, time, and data necessary to develop reliable downscaled projections to support its drought planning efforts. Collaborative funding efforts overcome barriers, driving greater project success.

Next Steps

• Update the climate projections used for EAA drought projection and planning as newer climate and emission scenario data become available.
• Utilize NASA evapotranspiration data to gain a deeper understanding of regional water balance and drought projections for the Edwards Aquifer Region.
• Analyze how recharge in the Edwards Aquifer Region may evolve in response to a changing climate and provide data to expand existing land easements to ensure continued protection of these vital recharge areas.

Funding Partners

• Edwards Aquifer Authority
• South Central Climate Adaptation Science Center
• U.S. Geological Survey

Sources Referenced

• Edwards Aquifer Authority. (n.d.). “About the Edwards Aquifer.”
• Sharma et al. (2025). “Efficacy of mitigation strategies for aquifer sustainability under climate change.” Nature Sustainability 8: 44-53.
• Thornton et al. (2022). “Daymet: Daily surface weather data on a 1-km grid for North America, version 4 R1.” ORNL Distributed Active Archive Center.
• U.S. Geological Survey. (2021). “Karst Aquifers.” Water Resources Mission Area.
• Wooten et al. (2024). “Customized Statistically Downscaled CMIP5 and CMIP6 Projections: Application in the Edwards Aquifer Region in South‐Central Texas.” Earth’s Future 12 (10).

Contacts 

• Adrienne Wooten, South Central Climate Adaptation Science Center: amwootte@ou.edu
• Hakan Başağaoğlu, Edwards Aquifer Authority: hbasagaoglu@edwardsaquifer.org  

CART Lead Author

• Jessica Zimmerman, Case Study Author, CART: jnzimmerman3@gmail.com  

Suggested Citation

Zimmerman, J., N. (2025). “Climate Projections for Drought Assessments in the Edwards Aquifer Region.” CART. Retrieved from