Water Resources in the Borderlands of the Colorado River Basin: Climate Uncertainties, Anthropogenic Impacts, and U.S.-Mexico Binational Agreements

Abstract

The U.S.-Mexico border extends over 3,218 kilometers and comprises different arid and semiarid environments. This study focuses on the borderlands of the Colorado River Basin (CRB), particularly the Arizona-Mexico portion of it. The CRB borderlands are subject to water quality and quantity issues. Population growth, industrialization, increase of agricultural lands, changes in land use, and lack of binational groundwater management policies have contributed to decreasing groundwater levels, dewatering of aquatic ecosystems, pollution, and habitat loss and fragmentation. These problems have been aggravated by climate uncertainties and consecutive droughts. Ecological systems in this region, particularly aquatic and riparian, are greatly affected by the aforementioned issues. For decades, the increasing water demands of the growing border population have put the water needs of the environment at risk. Despite efforts by the governments of the United States and Mexico to allocate environmental flows in the Colorado River Delta, there is no formal strategy for establishing and allocating water for the environment in the U.S.-Mexico borderlands. Furthermore, the allocation of groundwater resources in the CRB faces similar uncertainties. The 1944 Water Treaty for the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande is the main water-allocating mechanism between the two nations, but it leaves groundwater unmentioned. This study assesses the anthropogenic impacts on land and water resources and their relation to environmental flow science, the effect of climate uncertainties and binational land and water management decisions on groundwater recharge, and the lack of binational groundwater management agreements between the United States and Mexico. The present work comprises a three-article dissertation format (Appendix A, Appendix B, and Appendix C) in which each article addresses a specific component of the binational groundwater challenges in the CRB. Appendix A examines the gap between understanding and allocating water for the environment by analyzing environmental flow projects in the Arizona-Mexico portion of the CRB. This article presents and discusses stakeholder interviews regarding engagement mechanisms and antecedent social conditions for the development of environmental flow projects, barriers to gathering available data and implementing such projects, and the current status of environmental flow regulations in the study region. Findings indicate that environmental flow projects have been completed based on voluntary agreements, mandatory agreements, and even a combination of the two. The paucity of data, limited funding, over-allocation of water resources, and lack of a regulatory framework that specifically allocates water for the environment represented the main barriers to providing water for the environment. It was also found that actors contributing to successful environmental flow projects include members of governmental agencies, non-governmental organizations, and private institutions, as well as stakeholders. This analysis helped to determine the most important features of success for environmental flow projects, and it provides lessons that can be applied to future restoration projects in the borderlands of the United States and Mexico. Appendix B demonstrates the use of a conceptual water-budget model as a tool for assessing the long-term impact of effluent discharge, climate uncertainties, and groundwater pumping in a transboundary aquifer located in the borderlands of the CRB. The Transboundary Santa Cruz Aquifer (TSCA), shared by the states of Arizona and Sonora, is highly sensitive to climate uncertainties and physical water and wastewater transfers from both the United States and Mexico. Water budget model simulations for the TSCA for most effluent discharge scenarios and groundwater pumping projections reflected a 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 natural and anthropogenic variables on water sustainability with a simple methodology that can be applied to other transboundary aquifers within the CRB borderlands and elsewhere. Appendix C evaluates the Joint Report of the Principal Engineers Regarding the Joint Cooperative Process United States-Mexico for the Transboundary Aquifer Assessment Program (TAAP Cooperative Framework) as a guide for binational groundwater collaboration. The Transboundary Aquifer Assessment Program (TAAP) is a joint effort by the two nations to assess the U.S.-Mexico transboundary Santa Cruz, San Pedro, Mesilla, and Hueco Bolson aquifers. In order to identify whether the TAAP Cooperative Framework can guide assessment in areas that have not entered into formal agreements for binational collaborative studies, the program’s elements of collaboration were used to conduct a comparison between the framework’s policy and six transboundary groundwater management agreements. Appendix C finds evidence of successful outcomes within the TAAP Cooperative Framework consistent with available transboundary groundwater management agreements, demonstrating that the approach is suited to serve as a model for others engaged in transborder aquifer assessments worldwide. Furthermore, the TAAP elements of collaboration can help to establish the meaningful and robust binational cooperation necessary for the development of U.S.-Mexico groundwater management agreements at the aquifer level. This present work contributes to the improvement of available knowledge of water resources in the borderlands of the CRB by providing a comprehensive approach to address different surface and groundwater management issues in three different dimensions. It discusses issues related to the allocation of water for the environment in a binational basin, the impact of transboundary water management decisions and climate uncertainties on water resources, and a framework that can promote binational groundwater collaboration between the United States and Mexico. The analysis of U.S.-Mexico collaboration on surface and groundwater resources alongside the information gained from the conceptual water budget modeling efforts for the TSCA provide lessons that can be applied in other regions of the CRB borderlands and elsewhere.