In 2017, the University of Arizona Water Resources Research Center started working on a Five-Year Transboundary Aquifer Assessment Program (TAAP) effort funded by the federal government (Award No. G17AC00439). This bi-national effort came after the signing of the Transboundary Aquifer Assessment Act (US Public Law 109–448, TAA-Act) in 2006, the signing of the "Joint Report of the Principal Engineers Regarding the Joint Cooperative Process United States-Mexico for the Transboundary Aquifer Assessment Program" (TAAP Cooperative Framework) in 2009, and years of collaboration with the United States Geological Service (USGS), the International Boundary and Water Commission (IBWC), Universidad de Sonora (Unison), and the Mexican National Water Commission (CONAGUA). The objectives of both, the TAA-Act and the TAAP Cooperative framework, are to improve the knowledge base on the agreed-upon Aquifers of Focus, the Santa Cruz, San Pedro, Mesilla, and Hueco Bolson aquifers (Figure 1), and other transboundary aquifers if designated and approved by the two countries.