Impacts of Variable Climate and Effluent Flows on the United States-Mexico Transboundary Santa Cruz Aquifer

The Transboundary Santa Cruz Aquifer (TSCA) is located in Northwestern Mexico and Southwestern United States (U.S.). Groundwater from the transboundary aquifer is being shared by the states of Arizona in the U.S. and Sonora in Mexico; particularly by the cities of Nogales, Arizona and Nogales, Sonora. The Arizona-Sonora border region is subject to climate uncertainties, limited water availability, and water quality issues. The objective of this study is to assess the impacts of changes in groundwater demand, effluent discharge, and climate uncertainties within the TSCA. Groundwater recharge in the TSCA is highly sensitive to climate uncertainties and physical water and wastewater transfers from both the U.S. and Mexico. Perennial flows in the area depend on the effluent discharges from both the U.S. and Mexico. Population growth and residential construction have increased groundwater demand in the area, in addition to wastewater treatment and sanitation demands. These human activities, coupled with climate uncertainties and possible reductions to effluent discharge, influence the hydrology of the area. We use a conceptual water budget model to analyze the long-term impact of the different components of potential recharge and water losses within the aquifer, including changes in projected climate that are based on three downscaled CMIP5 RCP8.5 Global Climate Models. Water budget model simulations for most effluent discharge and groundwater pumping scenarios reflected groundwater deficit. Additionally, climate projections showed variations that range from severe long-term drying to positive wetting. This research improves the understanding of the impact of climate uncertainties and water management decisions on water sustainability, with an accessible methodology that can be globally applied.