NewsDrop-Oct_Nov-2025

AQUIFER MANAGEMENT SERVICES

By: Mark Hamilton Executive Director for Aquifer Management Services

BEHIND CRITICAL PERIOD MANAGEMENT THE HISTORY & SCIENCE INTEGRAL TO MANAGING THE AQUIFER IS REGULATING HOW MUCH WATER CAN BE WITHDRAWN ANNUALLY. MANAGING WITHDRAWALS IS ACCOMPLISHED THROUGH A PERMITTING PROCESS, IN WHICH USERS OF THE RESOURCE MUST HAVE A GROUNDWATER PERMIT.

Figure 2: Current Summary Charts of CPM Trigger Levels for the San Antonio and Uvalde Pools .

Developing CPM and other Conservation Measures CPM is one of the primary management tools that ensures protection of flows at Comal and San Marcos springs in a repeat of the historical 1950’s drought, or drought of record (DOR). Other management tools, generally referred to as conservation measures, are utilized in addition to CPM to maintain water levels and springflows during severe drought as well. The other measures are: • Voluntary Irrigation Suspension Program Option (VISPO) – this is a program designed to compen sate holders of irrigation permits that enroll in the program. Compensation is either for enrollment (standby payment) or forbearance of water, result ing in a larger payment to participants. VISPO is triggered in the calendar year following any year in which water levels at J-17 are at or below 635 ft-msl on October 1. When triggered, VISPO accounts for approximately 40,000 ac-ft of forbearance from use to maintain aquifer water levels. • Regional Conservation is another conservation measure in which 10,067 ac-ft of water is con served regionally for the duration of the EAHCP.

through 2008, the San Antonio Pool CPM triggered when aquifer levels, as measured at the Bexar County index well (J-17), dropped below 650 ft-msl, or when flows at Comal or San Marcos springs went below 220 cubic feet per second (cfs) or 110 cfs respectively. The Uvalde Pool CPM triggered when water levels at the Uvalde County index well (J-27) dropped below 845 ft-msl. The Uvalde Pool in this early inception of the rules was limited to Stage III and Stage IV; no permit reductions were required until the Stage III level was reached. These initial rules are summarized in figure 1. In May of 2007, the legislature amended the cap from 400,000 ac-ft to 572,000 ac-ft to accommo date the total number of applications for permits, based on historical use. This resulted in a rule change for CPM, which was enacted in 2008 by the EAA. CPM trigger levels for the San Antonio and Uvalde pools as well as the percentages of permit reductions were changed. Beginning in 2008, CPM was triggered when the ten-day average water level at J-17 reached 660 ft-msl (rather than the previous 650 ft-msl value). Permit reductions started with a 20 percent reduction at Stage I and increased to a

40 percent reduction in Stage IV. Springflow triggers were also amended and started at 225 cfs and 96 cfs at Comal and San Marcos springs, respectively. Sim ilarly, trigger levels in the Uvalde Pool were amended. Starting with CPM reductions at a Stage II level, when the 10-day average at J-27 drops below 850 ft-msl and increasing to a 35 percent reduction in Stage IV. Critical Period Today In 2013, CPM was amended again under the Edwards Aquifer Habitat Conservation Plan (EAHCP). CPM stage 5 was added and resulted in an additional 4 percent permit reduction for a total of 44-percent in both the San Antonio and Uvalde pools. Stage 5 CPM triggers when the San Antonio Pool reaches a ten-day average level of 625 ft-msl at J-17, or flows at Comal Springs drop below a ten-day average of 45 cfs, or a three-day average of 40 cfs., Stage 5 CPM trig gers in the Uvalde Pool when J-27 drops below 840 ft-msl. Current CPM trigger levels are summarized graphically in figure 2. Today, groundwater permit holders are familiar with the five CPM stages and the associated cutbacks of their permits; however, many stakeholders may not be familiar with how these metrics were developed.

Figure 1: Initial CPM Trigger Levels Critical Period as Amended in 2007

Critical Period – In the Beginning CPM was initially adopted in 2002 as a tool for managing groundwater withdrawals during periodic droughts. One of the goals of CPM is to maintain springflow at Comal and San Marcos springs suffi cient to protect threatened and endangered species during drought. Initially, CPM consisted of four stages of groundwater permit reductions ranging from 5 to 23-percent in the San Antonio Pool, and 15 to 23-percent in the Uvalde Pool. For the period 2002

The total volume of permitted withdrawals is capped at 572,000 acre-feet per year (ac-ft/yr) of water. Permit ted withdrawals (water withdrawn from the aquifer associated with a permit) are subject to reductions depending on aquifer levels. This summary discusses the history and science behind the EAA’s Critical Period Management (CPM) process, which is the regulatory program associated with reduction of per mitted withdrawals and a primary conservation measure used to manage the aquifer.

One acre-foot of water is equivalent to approximately 325,851 gallons of water or about the size of a football field filled with water one foot in depth.

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