Researchers from the Texas-based Southwest Research Institute published a report in August showing a link between surface water and groundwater in the Devils River watershed. The Devils River runs southwest through a semi-arid landscape from its headwaters in Crockett, Schleicher and Sutton counties to Val Verde County and Lake Amistad. The watershed overlies a portion of the Edwards–Trinity Aquifer.
The intent of the report was to conduct a watershed-level analysis of the area. In previous research, the Devils River headwaters were studied only as part of regional-level analyses that “did not provide insight on individual watersheds.” A study of the lower Devils River watershed, that was included in the Val Verde County water resource assessment, showed that discharges to Amistad Reservoir and the Rio Grande River outpace precipitation. This suggests that a large amount of the discharged water comes from outside of the county.
To conduct their analysis, researchers considered different models. They found that a model which looks only at groundwater flow “does not adequately reflect baseflow under low-flow or drought conditions.” Integrated models, however, presented additional challenges due to differences between how surface water and groundwater are modeled. In addition, the model needed to address varying rates of permeability. The watershed is characterized by preferential flow paths—meaning some areas drain faster than others. The researchers conceptualize this as straws in a sponge. To address these challenges, they developed a coupled surface water/groundwater model.
Researchers noted that the river was gaining through most of its reaches. Their analysis found that the karstic pathways of the aquifer follow the same channels as surface water in the watershed—which creates springs that sustain the river. They also found that even modest pumping can impact a river in a semi-arid watershed.
More specific findings include:
- The morphology of the area, rather than the hydraulic property of the rocks, controls groundwater flows;
- Modest pumping that started in the 1960s dewatered Beaver Lake and shifted the start of the headwaters about 10 miles south;
- Groundwater pumping reduces baseflow, rather than surges in surface water flows; and
- Pumping will cause a proportional reduction of the Devils River flow.
This was the first time modeling showed a strong relationship between groundwater pumping and spring discharge. The researchers see their results as meaningful to water managers.
“This study provides, for the first time, the means to evaluate the relationship between groundwater pumping and spring discharge in the semi-arid environment encountered in the Edwards Plateau,” said Ronald Green, the researcher who led the study. “These tools allow for water managers to evaluate water resource strategies when administering the stewardship of this valuable resource.”
Written by Marta L. Weismann