REGIONAL GROUNDWATER MANAGEMENT: CONSIDERING FLOWS TO SPRINGS AND STREAMS

Groundwater conservation districts in Texas are required to work together to define desired future conditions for their shared aquifers. State law requires that districts consider “impacts on spring flow and other interactions between groundwater and surface water” as part of this process. The Texas Water Development Board recently quantified regional groundwater – surface water relationships in the 9 major aquifers and 21 minor aquifers that extend beneath 81 percent of the land area of Texas. The results can be used by districts and policymakers to inform their groundwater management decisions.

We used “hydrologic landscape regions” to estimate groundwater flow to surface water based on data from nearly 600 U.S. Geological Survey stream gauging stations. This approach yielded a statewide average net groundwater flow to surface water of 9.3 million acre-feet per year, or about 30 percent of all surface-water flows in Texas. This average historical flow does not address the inherent variability of hydrologic processes over time or across regions. Groundwater contributions to surface water are greatest in east Texas and around major springs in the Hill Country and west Texas. The Gulf Coast Aquifer discharges the most groundwater to surface water, with an estimated flow of 3.8 million acre-feet per year. The Edwards (Balcones Fault Zone) Aquifer discharges the greatest volume of baseflow per square mile of aquifer area. Springs and seeps in west Texas also contribute locally significant baseflow to streams. About half of Texas aquifers contribute less than 50,000 acre-feet per year to surface-water flows.

Three major aquifers—the Edwards (Balcones Fault Zone), Edwards-Trinity (Plateau), and Pecos Valley aquifers—contribute more than 50 percent of the baseflow of streams flowing across their outcrop zones. Eighteen major and minor aquifers contribute between 20 and 50 percent of the flow to streams flowing over their outcrop zones. Eight minor aquifers contribute between 14 and 20 percent of the flow to streams flowing over their outcrop zones. One minor aquifer, the Rita Blanca Aquifer, contributes zero percent to streamflow and is classified as non-tributary. Each of the state’s aquifers has local areas that may differ from the regional, aggregate designation.