NewsDrop-Oct_Nov-2025

AQUIFER MANAGEMENT SERVICES

By: Mark Hamilton Executive Director for Aquifer Management Services

evaluated for impacts to sustained springflows during a hypothetical repeat of the DOR. The objective of the “Bottom Up” analysis is to assess whether the targeted minimum springflow requirements of the EAHCP would be met during a repeat of the DOR. To be considered effective, the combined conserva tion measures had to be sufficient to produce model results that showed a minimum flow of 30 cfs at Comal Springs, and 45 cfs at San Marcos Springs during the peak of the DOR and under maximum per mitted withdrawals for the aquifer conditions. Result from the groundwater modeling provided guidance on the timing and magnitude of certain conservations measures. For example, the goals for VISPO leasing and the timing of SAWS ASR use were adjusted to ensure the springflow targets could be met. Modeling relies on data. For the EAA models, regional water levels, pumping, springflow, recharge, and other aquifer data are used for a period of record dating back to the late 1940’s and extending through 2022.

The EAA has invested heavily in both improved and more extensive data sets over the last twenty-plus years. These data are and will continue to be integral to future modeling efforts as well as in managing the resource. Managing the resource will provide contin ued certainty to EAA’s permit holders while complying with the EAHCP and related efforts to maintain flows for threatened and endangered species at Comal and San Marcos springs. Conclusion Critical Period Management and other conservation measures are necessary for sustainable manage ment of the aquifer system. The permitting system, combined with extensive data collection and mod eling efforts allow permit holders to maximize use, while aquifer levels are also maintained sufficiently to sustain springflows. Without a system in place to measure and limit withdrawals, aquifer levels would be far lower during extended periods of drought than without these measures. Modeling data indicates

that without the management system summarized in this document, Comal Springs would periodically go dry due to increased demand for water in the region, especially when combined with the frequency of short duration, high intensity droughts. For example, pumping from the aquifer increased from approximately 170,000 ac-ft/yr in the 1940’s, to about 540,000 ac-ft/yr in the 1980’s. Since the implementation of EAA’s permitting system, CPM, and other conservation measures, pumping has averaged about 350,000 ac-ft annually for the period 2013 2023. Management of the resources through the measures discussed herein has proven to be a suc cess. The success is largely due to the administrative implementation of a permitting system, collection and analyses of many years of aquifer-related data, and complex groundwater modeling efforts.

This measure is achieved via leak detection programs, upgraded irrigation systems, public awareness/education, water conserving plumbing fixtures, and similar efforts. • San Antonio Water Systems (SAWS) Aquifer Storage and Recovery (ASR) – the ASR provides for long term storage of water in the Carrizo Aquifer. Edwards Aquifer water is conveyed to the ASR via the SAWS distribution network. This stored water can be used in times of critical drought to reduce demand on the Edwards Aquifer by retrieving stored water and introducing it back into the distribution system when needed most. Through the EAHCP and contrac tual agreements with SAWS, the EAA has stored 126,000 ac-ft of water for use in filling the ASR. ASR water use is triggered when the five-year rolling average of annual aquifer recharge drops below 500,000 ac-ft and aquifer levels at J-17 fall below 630 ft-msl.

• CPM Stage 5 – this measure is the most stringent of the CPM stages and is reserved for drought condi tions that are severe and have a likelihood of signifi cantly impacting springflows. The permit reductions associated with Stage 5 total 44 percent. Meaning, a groundwater use permit of 100 ac-ft would be reduced to 66 ac-ft if stage 5 reductions were in effect for an entire year. These conservation measures are necessary for the protection of springflows should a repeat of the DOR occur. Because it is enacted early in a drought sequence, CPM provides a first line of defense in maintaining aquifer water levels and springflows by managing permitted withdrawals from the system. The CPM-related permit reduc tions and the other conservation measures were extensively evaluated using multiple versions of a numerical groundwater flow model developed for the EAA. The EAA’s original groundwater model was completed in 2004 and was used extensively

to evaluate and develop management practices and conservation measures.

This initial model was focused primarily on a model ing period between 1947 and 2000. Over time, the conceptual understanding of the aquifer system has continued to improve through collection and analy ses of new data. As a result, the original model was revised in 2017 to incorporate new high-quality data and to include newer understanding of the aquifer’s characteristics. The revised groundwater model was calibrated for the years 2001 to 2011 and validated using data through 2015. The most well known modeling effort related to conservation measures is often referred to as the “Bottom Up” analyses, in which each of these con servation measures are layered into scenarios across the modeling period (which includes the drought of record). The measures are included in the groundwa ter model essentially in the order they are listed and

8

9