GW AVAILABILITY AND THE MAG

Responsible and proper management of groundwater resources requires a basic understanding of how much groundwater is “available” from an aquifer. The Merriam-Webster dictionary defines availability as “the quality or state of being accessible or obtainable”. If presented with a five-gallon container of water and a practical method for accessing it, most people will state that the amount of available water in the container is five gallons. Groundwater availability might also be thought of in this way: the amount of groundwater physically available is simply the amount of groundwater that can be withdrawn. Physical availability is limited only by the constraints imposed by the hydraulic characteristics of the aquifer and the technologies used to withdraw water from it. Obviously, it may not be practical or desirable to withdraw the amount physically available in an aquifer, but it is important that the magnitude of the resource is understood and considered in the management equation.

In contrast, regulatory “availability” is the amount of water that can be withdrawn while still meeting a specified limit on aquifer response. Using the simple analogy of the five-gallon container of water, the regulatory “availability” of water is one gallon if it is determined that a maximum 20% storage decline is acceptable. Because this is a self-imposed decision to use only a portion of the water that is physically available, groundwater “availability” in a regulatory sense is more properly termed a “response-based limit on withdrawals” or, more simply, a “regulatory limit”. In Texas groundwater planning, a Desired Future Condition (DFC) corresponds to the specified limit on aquifer response, while the Modeled Available Groundwater (MAG) is the portion of the total available water that can be withdrawn without exceeding the chosen constraints on aquifer response.

In general, the existing MAGs do not correspond to the amount of physically available groundwater. However, this fact is not widely understood, even by those involved in the ongoing DFC/MAG joint planning process. It is important to maintain a clear distinction between physical availability and the MAG so that water planners and stakeholders can make informed choices concerning limits that may be placed on groundwater use in the future.