Groundwater Availability Modeling August 2012

In 1999, the TWDB began developing a groundwater availability model to help water planners working on their regional water plans assess how much groundwater was in a portion of the Trinity Aquifer. Largely due to the success of that model, in 2001 the Texas Legislature provided funding for the TWDB to develop additional models for the state's aquifers and TWDB's Groundwater Availability Modeling program began in earnest.

Known as GAMs in TWDB parlance, groundwater availability models are three-dimensional, numerical computer models based on hydrogeologic principles, various aquifer measurements and stakeholder guidance. Because groundwater is difficult to observe and measure, water planners need assistance in gauging how much groundwater is and will be available under various conditions. These models provide that critical information.

The first models proved so important to the water planning process that the legislature later mandated the development of models for all the aquifers in the state. There are nine major and 21 minor aquifers in Texas, and all but nine of the minor aquifers have now been modeled. Of those remaining nine, studies are underway for seven aquifers. These numbers, however, do not tell the whole story. Because many of the state's aquifers are very large and complex, they required more than one model. Since 1999 our modeling team and other collaborators have developed more than 38 models for Texas aquifers, with an additional three scheduled for completion this year.

In 2005, the groundwater availability models became even more essential to water planning efforts when the legislature established a new process for managing groundwater. The process requires that regional water planning groups and groundwater conservation districts use modeled available groundwater values based on the desired future conditions of aquifers. Groundwater availability models are used to determine those values.

In the first round of this new process, our modeling staff assisted groundwater conservation districts by conducting numerous model runs on various aquifers to estimate groundwater availability values. All told, our staff performed 166 model runs to assist the districts with decisions on the desired future conditions for their groundwater resources. Since the program began in 2001, our staff has completed 450 model runs and related model exercises. In almost all cases, the GAMs are the default tool for estimating modeled available groundwater values from desired future conditions.

TWDB's groundwater availability modeling program is the only one of its kind in the United States. No other state has attempted to model all of its aquifers. Given the size of Texas, the accomplishments of the modeling program are even more impressive. The models, though, are more than just nifty scientific tools. Armed with the models' predictions, Texans can make more informed decisions about their water supplies and the fiscal resources necessary to manage them.

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