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 creates Groundwater Conservation Districts (GCDs) to manage aquifers. GCDs are charged with a variety of protective mandates including the development of science, education of users, and management of use through permitting.

The official DFC in western Hays County is no more than 19-feet of average aquifer drawdown by 2060.

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.

The TWDB uses the DFC to calculate The Modeled Available Groundwater volume, or MAG. This is the maximum amount of water that is available to use without violating the DFC.

The TWDB uses a groundwater availability modeling (GAM) program to calculate a specific volume. The GAM that calculated the HTGCD MAG is called the TWDB GAM Run 19-026: Hays Trinity Groundwater Conservation District Groundwater Management Plan, January 2020.

The TWDB’s official MAG in western Hays County is 2,963,288,994 gallons per year (9094 acre-feet).

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

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.

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