GMA-9 and the DFC: Informing the MAG for Individual GCDs


Regional Aquifer Planning for Optimal Future Aquifer Conditions.


As the State’s population grows and agricultural, business, and industry water requirements begin to outpace locally available supplies; the Legislature created Groundwater Conservation Districts (GCDs) to manage groundwater resources. GCDs are charged with a variety of protective mandates including the development of science, education of users, management of use through permitting, and long-range regional aquifer planning.


For aquifer planning purposes, Texas is divided into 16 distinct planning groups based on major aquifer boundaries and made up of the GCDs within those boundaries. The Hays Trinity GCD belongs to Groundwater Management Area 9 (GMA-9) along with our neighbors within the Hill Country Trinity Aquifer. The GMA Planning Committee is made up of representatives from each GCD in the GMA and is responsible for creating a document called a GMA Explanatory Report that details all of the criteria needed to establish a GMA-wide Desired Future Condition or DFC.

What is the GMA-9 DFC?

The official GMA-9 DFC (Desired Future Condition) is 30-feet of geographically averaged drawdown over time by the year 2060. Each GCD’s portion of that DFC will vary based on existing use, population growth, and geology. During drought conditions, the average drawdown may exceed 30-feet, and during wet periods, the aquifer may rebound above baseline levels.

What is the Hays Trinity Groundwater Conservation District’s DFC?

The official DFC (Desired Future Condition) in western Hays County is no more than 19-feet of average aquifer drawdown by 2060. The DFC drawdown data is gathered through an array of from 59regularly monitored wells (our windows into the aquifer) that are dedicated for that purpose.

How is that decision made?

In a series of public meetings with local community and GCD input, the GMA is tasked with balancing the aquifer’s volume with historic and current usage and the needs of future populations. The community must balance its expectation of current water usage against its expectations of water availability in the future.

How do we translate aquifer drawdown (DFC) into a pumpable volume?

The Texas Water Development Board (TWDB)uses the DFC to calculate The Modeled Available Groundwater volume, or MAG. This is the maximum (annual) volume of water that is available to use without violating the DFC.

Exactly how is the MAG calculated?

The TWDB uses a groundwater availability model (GAM) to calculate the maximum volume of groundwater than can be pumped from the aquifer yearly without exceeding the DFC. The TWDB has been using the theHill Country Trinity GAM to calculate MAG volumes since 2004. The latest MAG report for the HTGCD is TWDB GAM Run 21-014: Hays Trinity Groundwater Conservation District Groundwater Management Plan, January 2020.

What is our MAG?

The TWDB’s official MAG for western Hays County is 2,963,288,994 gallons per year (9,094 acre-feet).

What is the best estimate of how much we currently use for domestic and agriculture (exempt from permitting uses) plus what we have permitted for commercial use?

Domestic and agriculture wells are not required to be metered in the Hays Trinity Groundwater Conservation District, so that volume is estimated. The HTGCD estimates the total volume of groundwater pumped by all exempt and non-exempt users to be 2,535,772,482 gallons per year (7,782 acre-feet).

How does this aquifer planning process inform the 50-year vision of the State Water Plan?

The State Water Plan uses the MAGs as the future groundwater by volume available to a county. If there is a future shortfall, the State Water Plan must designate a Water Management Strategy (such as pipeline, desalination, or new reservoir) to supplement the future overall water requirements each County.

How can I contribute my opinion to this long-range planning process?

All Hays Trinity  GCD and GMA-9 meetings are publicly posted, open to the public, and ready for public comment. Please check your local GCD website for the next meeting. You may also fill out and submit this  GMA-9 written comment form that will be considered by GMA-9 in the planning process.


DFC – Desired Future Condition is the maximum average drawdown and/or minimum average springflow that the local community decides is acceptable in 50-years.

GAM – Groundwater Availability Models determine how much water is or will be in an aquifer at a specific time.

GCD – Groundwater Conservation Districts provide local control of local groundwater resources.

GMA — Groundwater Management Area is an aquifer-wide planning group made up of individual GCDs for the purposes of cooperation and aquifer-wide planning.

MAG – Modeled Available Groundwater is the volume of water authorized by the TWDB based on the local GCD’s DFC.

TCEQ – Texas Commission on Environmental Quality is a state regulatory agency that carries out the Federal Environmental Protection Agency mandates. In this context, they can be thought of as the administrators of water quality protections.

TWDB — Texas Water Development Board is a state agency that oversees the state’s water science, planning, and financing.


2021 DFC Explanatory Report
2016 DFC Explanatory Report
HTGCD DFC current Status
GMA-9 Public Comment forms
Official GMA-9 Website
Texas Hill Country Water Resources Groundwater Management Page
2022 Texas State Water Plan


2016-2021 Planning Cycle Support Documents