Background
During this project we explored methods to fully integrate surface-water flow models with groundwater flow models in semi-arid karst watersheds. This will allow for accurate assessment of the effect of groundwater management practices (i.e., large-scale pumping) on river flow and the reciprocal effect of changes in surface-water flow on groundwater. Precipitation and recharge in arid and semi-arid climates, and a conduit-dominated flow groundwater regime indicative of a karst carbonate system, need to be accommodated to evaluate the water resources of western Texas, in particular, and all other arid and semi-arid environments. Although integrated surface-water/groundwater models developed for less-challenging environments have met with some success, application of a fully coupled surface-water/groundwater model to a karst aquifer with a very high ratio of evapotranspiration to precipitation has not been achieved.
The primary research element of this project was to investigate and simulate the dynamic hydraulic interactions between a surface-water flow regime and groundwater flow regime in response to varying precipitation (i.e., droughts) and water-management actions, such as developing and implementing large-scale groundwater well fields. This investigation culminated in the development of a fully coupled surface-water/groundwater model suitable for arid and semi-arid environments, and karst carbonate aquifers.
Approach
The project team conducted an in-depth review of software packages that profess to accommodate coupled surface-water/groundwater flow and interaction. After an initial phase of research into the documented capabilities of these packages, packages were evaluated by constructing a groundwater model and a surface-water model of the Devils River watershed. The coupled models were then calibrated to water levels and stream hydrographs.
Accomplishments
We selected and refined protocols for creating a coupled surface-water/groundwater model. The ideal protocol or software depends on the precise needs of the project; however, the widely available groundwater modeling software MODFLOW is the most desirable package for the groundwater component. Furthermore, MODFLOW includes modules that can, in some cases, stand in for the surface-water model without additional coding. The most successful model produced during this project was completed and calibrated using MODFLOW and a simple runoff based surface-water model that incorporates evapotranspiration. The final model from this project successfully replicated the stream hydrograph and the transient water levels to a degree not previously achieved for this watershed.