Decisions are being made today that will have far-reaching and, in some cases, potentially irreparable impacts on the people of South Texas and northern Mexico, as well as on the area's sensitive ecology.
The Edwards-Trinity Aquifer is a karst limestone aquifer that provides the majority of the water flowing in the Devils River. Due to the unique and complex hydraulics of karst aquifers, a clear understanding of the means and mechanisms by which water is conveyed from the headwaters of the watershed to the points of discharge is important for the effective management of these valuable resources.
The architecture of a karst aquifer is dominated by conduits and other features that allow water to pass through the limestone. The Integrated Flow Model expands the understanding of how different parts of the karst system relate to each other (and how pumping from one area will affect the others). Understanding the aquifer’s intricate structure – how water is moving through underground conduits, flowing from springs into streams, and seeping underground through caves and small openings in the limestone is a useful tool when making water management decisions.
The Edwards-Trinity Aquifer provides about 76 percent of the source water for the Devils River (Green and Bertetti, 2010; Green et al., 2014). So pumping tens of billions of gallons of water, as has been proposed by water marketers, would have a significant negative impact on the Devils and numerous other waterways and springs, many of which provide drinking and irrigation water to towns throughout the region.
The accurate assessment of water availability in areas where resources are limited and stressed is of critical importance, the Integrated Flow Model provides this much needed information.
The Devils River Integrated Surface Water and Groundwater Flow Model links the intricacies of the groundwater and surface water systems. This model provides water managers the opportunity to evaluate strategies and fully understand impacts of pumping on the shared resource.
Below is the Executive Summary of the Southwest Research Institute's recent report, Water-Resources Management of the Devils River Watershed (or you can read the full report here).
Origin of the Project
This project began in 2009 with reconnaissance work that included the Devils River. SwRI followed that up with a preliminary groundwater flow model solely looking at the Devils River watershed. SwRI had marginal success in the development of the Devils River groundwater flow model but were unable to replicate discharge from the Devils to Amistad Reservoir at low flow. From this it became apparent that a surface water component was needed to accurately predict discharge from the Devils River at low flow. In this phase of the project SwRI aimed to produce an Integrated Surface Water and Groundwater Flow Model. The focal question in the development of the Integrated Flow Model was ‘What happens to surface water when groundwater is removed?’ and the goal was to accurately predict the effects of groundwater removal on surface water flows.
Separate Treatment of the Devils Basin
SwRI treated the Devils River Basin as a separate system from the surrounding Basins. There is strong justification that this watershed does not have hydraulic communication particularly in the upland areas with the adjoining watersheds. What happens in the Devils watershed stays there and is not impacted by surrounding watersheds. This is very different from the existing models such as the Edwards Trinity Model that covers 23 counties and treats the system as a single entity by showing pumping from one side of the aquifer impacts the other side. In reality that will not happen.
Upon review of data provided by TWDB’s well map it was determined that all of the high capacity wells in the Devils Basin fall within a 1.5 mile radius of the major river channel. This discovery instigated the inquiry into preferential flow paths incorporated in the development in the initial Devils River Groundwater Flow Model. Preferential flow paths developed over geologic time by the exhumation of the Edwards Plateau and as it exhumed the rivers formed and over time the rainfall enhanced the permeability along these river channels. These are not river channels that meander like the Mississippi; once they begin they are literally carved in stone. Literature cited in the final report supports the conceptualization of preferential flow paths aligned with river channels.
Where a groundwater well is placed determines the capacity of the well, if it is placed near a preferential flow path it will have a high capacity, if placed up on the plateaus it will be a low capacity well. The corroborating evidence is listed in detail in the final report and includes the Sutton County GCD Dye Tracer Test, geophysical surveys using resistivity tracing to determine location of preferential flow paths in relation to the Devils River channel, water chemistry assessment of the watershed and a gain/loss study.
Groundwater Model Refinement
In the original groundwater flow model SwRI treated the matrix with uniform properties and assigned conduits with lower values in the upper regions and higher values in the main tributaries. The original product of this model did not align with the reported discharge at low flow of the river. In the development of the Integrated Flow Model SwRI undertook the enhancements and refinements needed to reconcile the groundwater model with recorded river discharge data.
Surface Water Model Development
The surface water model works by taking rainfall data and applying a loss method which removes anything that is not direct runoff. The purpose of the surface water model is to determine recharge to the groundwater and recharge is determined through the loss method described in detail in the final report. The primary output of the model is infiltration which is exported as recharge to couple the groundwater model.
Performance of the Completed Coupled Model
The reported base flow at Pafford crossing is closely aligned with the model output for base flow at Pafford proving the functionality of the model (see figure 24 in the Final Report). With the model output in close agreeance with reported base flow SwRI is now able to run scenarios and accurately predict groundwater pumping effects on surface water flows. Three scenarios have been run thus far, one pre-groundwater extraction, the second of current conditions (as of July 2017) and the third with a well field in Juno. These scenarios examine the discharge of the Devils River at Pafford under the above listed conditions. Figure 25 in the Final Report shows the spring locations for each of these scenarios thus exhibiting the effects of groundwater pumping on surface water discharge to the Devils. Figure 26 in the Final Report shows a scenario where 8,000 gallons per minute of groundwater are removed from the basin at Juno and the effects on base flow discharge at Pafford.
The development of this model is groundbreaking. There are integrated surface water and groundwater models but it has never been applied to a karst system. This tool has the possibility to positively affect the water management practices in the Devils Basin.