The United States-Mexico Transboundary Aquifer Assessment Program in Arizona and Sonora: The Upper Santa Cruz and Upper San Pedro Basins

Most aquifers shared by the United States and Mexico have been understudied. Drought, declining water tables, contamination, and high population growth rates make research on these aquifers urgent. In 2006, Congress passed the U.S.-Mexico Transboundary Aquifer Assessment Act authorizing the USGS and Water Resources Research Centers in Texas, New Mexico, and Arizona, “to develop and implement an integrated scientific approach to “…assess priority transboundary aquifers…” and “…provide the scientific information needed by water managers and natural resource agencies.” In Arizona, the binational Santa Cruz and San Pedro aquifers were designated for study. Challenges include inadequate knowledge of aquifer geometry, and water quality and movement, especially in Mexico. The effects of climate change are also poorly understood (for example, regional precipitation appears to be shifting toward a more summer-dominated regime). There are also non-technical, but equally important challenges. These include the bilingual, binational, bicultural environment with an often less than harmonious shared history. Our approach to overcome these challenges and fulfill the objectives of the Act is to build a binational team with expertise not only in the physical and biological sciences, but also in policy, institutions, and water management. We are actively engaging stakeholders to better understand the institutional framework and we are developing proposals for hydrologic studies in each basin. Priorities include increased monitoring of ground-water storage and quality, stream discharge, vegetation, and precipitation. We are proposing integration of existing transboundary ground-water models, development of coupled ground- and surface-water models, and analysis of uncertainty. As an example of the type of multi-stakeholder projects we are supporting, we are cooperating with the local San Pedro watershed group to conduct an ephemeral channel runoff and infiltration study to improve estimates of urban-enhanced recharge and to help understand the role played by ephemeral channels in maintaining flow in the San Pedro River.