Sensitivity of Texas Groundwater Availability Models (GAMs) to Variations in Recharge Rates

Quantifying the groundwater availability is of great importance nowadays in Texas' aquifers, furthermore how changes in certain inputs, such as recharge, change said availability. The selected aquifers are an important source of groundwater for agriculture (Ogallala) and metropolitan usage (Edwards-Trinity and Gulf Coast), and the changes of recharge will have a profound impact in the groundwater availability in the future. Said changes were done in the MODFLOW based GAMs commissioned by the Texas Water Development Board. Their objective is to model the aquifers responses and future behavior in a similar fashion to the Sustainable Groundwater Programs carried out in California.

The scope of this analysis is to determine how and where are said aquifers more sensitive to recharge change. Aquaveo's GMS software (10.4) was used to model the aquifer's sections with variated recharge values (from 50% to 150% the normal recharge in 10% increments). The model's output was then processed in MATLAB to obtain plots with the behavior of the aquifer's sections when subjected to the changing recharge values.

A linear correlation between the recharge variation and the mean head, and river baseflow was hypothesized and the model's output confirmed said trend. However, while the great majority of the cells in the GAM's experienced a moderate increase in head with the increase in recharge, some sections of the aquifers demonstrated a higher increase. Such is the case of the Edwards-Trinity and Carrizo-Wilcox-North that show a difference of 300 ft and 200ft in the highest head values from the mean head in the aquifers respectively (150% normal recharge in both). In general, the gain of average head in the aquifers varies from 0.8% (Ogallala) to 9% (Edwards-Trinity) when comparing 50% and 150% normal recharge at the end of the simulations. The linear nature of the reactions to recharge shows that it is important to obtain a proper estimation of recharge for the sensitive areas in order to better comprehend how the groundwater availability will change over time.