Groundwater is an important source of fresh water for populations and the environment. Fresh water represents only 2.8% of the total water resources in the world, with 70% of fresh water composed of polar ice layers and continental ice, 1% from surface watercourses, and 29% from groundwater [1
]. Transboundary rivers, lakes, and aquifers are home to over 70% of the world’s population and supply water for around 60% of global food production [2
]. Approximately 600 transboundary aquifers have been identified around the world [3
]. However, only six of them have formal binational or multinational mechanisms of cooperation: (1) the Guaraní Aquifer System in Brazil, Argentina, Paraguay, and Uruguay; (2) the Franco-Swiss Genevese Aquifer System in France and Switzerland; (3) the Northwestern Sahara Aquifer System in Algeria, Libya, and Tunisia; (4) the Iullemeden Aquifer System in Mali, Niger, and Nigeria; (5) the Nubian Sandstone Aquifer System shared by East Libya, Egypt, Northeast Chad, and North Sudan, and; (6) the Al-Saq/Al-Disi Aquifer System in Jordan and Saudi Arabia (Figure 1
Although geographically widespread, these aquifers represent only 1% of identified transboundary aquifers, a proportion that is quite different from the proportion of transboundary river basins with international basin agreements. While there are 310 transboundary river basins around the world, a total of 688 transboundary basin agreements have been signed between 1820 and 2007 [4
]. These agreements apply to 133 river basins, representing 36% of the identified transboundary basins [4
]. The reasons for such a disparity between the number of basin agreements and the number of groundwater agreements include the “invisible” nature of groundwater [7
], limited and dissimilar groundwater data [9
], and the lack of institutional capacity for groundwater governance [10
Groundwater collaboration between the United States and Mexico is similar to other transboundary settings around the world. Efforts by the two countries to understand and manage groundwater resources have been scarce and sporadic [11
]. The two countries have a surface water agreement, the 1944 Water Treaty Regarding the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande (1944 Treaty); however, groundwater was left unmentioned. Only Minute 242 was approved in 1973 by the International Boundary and Water Commission (IBWC), one of many interpretations of the 1944 Treaty includes a provision that is relevant to groundwater. The IBWC is the international body that oversees the application of U.S.-Mexico treaties related to boundary demarcation, national ownership of waters, sanitation, water quality, and flood control in the border region [12
]. Challenges in the management of groundwater resources in the U.S.-Mexico border region include rapid urbanization and industrialization, agricultural intensification, contamination of surface and groundwater resources, increase in surface and groundwater demands, and climate uncertainties [13
]. These challenges indicate the need for binational transboundary collaboration to secure water for populations and the environment. Such a collaboration could take the form of a binational agreement for the management of groundwater resources. However, scholars have recognized that the assessment of shared aquifer systems is a necessary antecedent to the development of any groundwater management agreement [8
]. For example, Kirstin I. Conti [18
] indicated that scientific research is an enabling factor for groundwater cooperation, along with existing legal mechanisms, regional institutions, high institutional capacity, funding mechanisms, strong political will, previous water cooperation, and third-party involvement.
The Joint Report of the Principal Engineers Regarding the Joint Cooperative Process United States-Mexico for the Transboundary Aquifer Assessment Program (TAAP Cooperative Framework) [20
], guides the joint effort between the United States and Mexico to improve the knowledge base of transboundary aquifers. The program began in 2006 with the Transboundary Aquifer Assessment Act (U.S. Public Law 109–448, TAA-Act). The TAA-Act authorized the United States Geological Survey (USGS) and the Water Resources Research Institutes (WRRIs) of Arizona, New Mexico, and Texas to work with Mexican counterparts on the development of transboundary aquifer assessments. The TAA-Act authorized U.S. involvement in binational studies of the Santa Cruz and San Pedro Aquifers, shared by the state of Arizona in the United States and the state of Sonora in Mexico, and the Mesilla and Hueco Bolson aquifers, shared by the states of Texas and New Mexico in the United States and the state of Chihuahua in Mexico (Figure 1
). These priority aquifers were selected based on their proximity to highly populated areas, increasing groundwater demands, and water quality issues [21
]. The binational TAAP was formally initiated in 2009 upon the signing of the TAAP Cooperative Framework by the principal engineers of the U.S. and Mexican sections of the IBWC. The two countries agreed upon the TAAP aquifers of focus consistent with the TAA-Act priority aquifers (Figure 1
). According to the TAAP Cooperative Framework, either of the two countries can propose an aquifer of focus, but both countries must agree to develop a joint assessment.
The TAA-Act and the TAAP Cooperative Framework offer a foundation for collaboration to study shared groundwater resources through an effective partnership among federal agencies, academic institutions, and federally established water resources research institutes [21
]. The TAAP can also be considered a climate and water adaptation initiative for the western U.S.-Mexico border [13
], a transboundary regional initiative that has the potential to build adaptive capacity [15
], an activity that can support decision-making processes related to groundwater management in each country [23
], and a precedent for a binational partnership that can promote and implement a new binational aquifer assessment [9
]. However, the relevance of the TAAP Cooperative Framework as a model mechanism for groundwater collaboration has not been fully addressed in the literature.
The TAAP Cooperative Framework is limited to assessment only, with four transboundary aquifers studied to date. The Map of Transboundary Aquifers of the World [3
] includes 11 shared transboundary aquifers along the border between the United States and Mexico. Yet, a review of technical studies, reports, and publications on U.S.-Mexico transboundary aquifers suggest that at least 36 transboundary aquifers are shared by the two countries [24
]. Clearly, additional study opportunities exist, and the activities undertaken by the TAAP can serve as the basis for assessment that goes beyond the current TAAP aquifers of focus and that can even guide future dialogue regarding groundwater governance and management [8
]. The primary objective of the study is to determine whether the elements of the TAAP Cooperative Framework can serve as a model for others wishing to engage in transboundary aquifer assessment. Expert interviews and lessons learned from evaluating six existing international groundwater agreements helped to determine whether the objectives, framework/process, funding, principles, and communication arrangements of the TAAP Cooperative Framework can guide further groundwater cooperation.
Currently, there is no groundwater treaty between the United States and Mexico. The 1944 Water Treaty regarding the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande (1944 Treaty) is the primary surface-water-allocating mechanism for the two countries. The treaty, however, does not mention groundwater. The Joint Report of the Principal Engineers Regarding the Joint Cooperative Process United States-Mexico for the Transboundary Aquifer Assessment Program (TAAP Cooperative Framework) is a case of groundwater collaboration for the assessment of the U.S.-Mexico transboundary aquifers of focus: the Santa Cruz, San Pedro, Mesilla, and Hueco Bolson aquifers. However, at least 36 transboundary aquifers shared by the United States and Mexico have been identified so far [25
Relevant studies on U.S.-Mexico groundwater governance have analyzed (1) the intranational institutions for the management of shared groundwater resources [22
]; (2) the importance of institutional asymmetries for transboundary aquifer assessment [20
]; (3) the institutional assessment of the Transboundary Santa Cruz and San Pedro Aquifers [21
], and; (4) the management perspectives for the shared aquifers of the United States and Mexico [25
]. While most of these studies discussed the outcomes, advantages, and disadvantages of the TAAP Cooperative Framework and the program itself, the components of the TAAP Cooperative Framework have not been analyzed as a model for groundwater collaboration.
This study analyzed the TAAP Cooperative Framework as a guide for furthering scientific assessment in areas that have not entered into formal agreements for binational collaborative studies. Through literature review and analysis of existing transboundary groundwater management agreements, we found that common elements of collaboration between the TAAP Cooperative Framework and existing groundwater management agreements include provisions for the exchange of data, concurrence for binational aquifer assessment, the establishment of technical advisory committees and technical groups, and respect for the legal framework and jurisdictional requirements of each country.
The TAAP exhibits several features that enable groundwater collaboration: existing legal mechanisms, previous water collaboration, third-party involvement, scientific research, and funding mechanisms. Additionally, the framework is consistent with four UN Draft Articles on the Law of Transboundary Aquifers, findings that may indicate the readiness of the two countries to move on to a next step: to implement additional aquifer assessment along the U.S.-Mexico border or to initiate dialogue toward the development of groundwater management mechanisms.
Some scholars have argued that the TAAP marginalizes issues such as groundwater rights and management [60
] and lacks a binding capacity (personal communication, 2020), and we agree with this premise. The information generated through the TAAP is “solely for the purpose of expanding knowledge” [20
]. However, the present study has found that scientific assessment is a prior step for the development of groundwater management agreements. In fact, interviews with experts on transboundary waters explored two ways in which the TAAP could guide groundwater management: through local agreements for the management of specific aquifer systems or through a regional agreement that guides the use of groundwater resources in the border region. In any case, lessons from the TAAP Cooperative Framework and the program itself remain as a model of robust binational groundwater collaboration with principles that have the potential to guide future groundwater assessment and management not just along the U.S.-Mexico border, but across the world.
The United States and Mexico share rivers, basins, and aquifers. Yet they do not share a water management agreement that suits the needs of the border communities that completely rely on groundwater resources. Challenges for managing shared groundwater in the region include population growth, industrialization, increase in agriculture, contamination, increase in surface water and groundwater demands, and climate uncertainties. These challenges indicate that some sort of binational arrangement is needed to protect and manage the shared groundwater resources. However, the topic has only been mentioned twice since the IBWC was created. Almost five decades after the signing of Minute 242 and three decades after the development of the Bellagio Draft Treaty, there has been no effort to establish a comprehensive groundwater agreement. Meanwhile, efforts toward increasing understanding of the U.S.-Mexico transboundary aquifers have taken place. This study analyzed the TAAP Cooperative Framework as a guide for furthering scientific assessment in areas that have not entered into formal agreements for binational collaborative studies. To achieve this, we compared the elements of collaboration present within the TAAP Cooperative Framework and six transboundary aquifer agreements around the world.
From this analysis, we found that five elements were particularly relevant as common features of collaboration that align with the TAAP Cooperative Framework: (1) the presence of data exchange provisions, (2) the concurrence for binational aquifer assessment, (3) the establishment of technical advisory committees, which occurred with all of the aquifers, (4) the presence of technical groups, and (5) respect for the legal framework and jurisdictional requirements of the involved countries. Expert interviews also served to identify lessons learned from the TAAP and global challenges for groundwater collaboration, which included the importance of trust-building between border communities sharing water resources, groundwater assessment, and a pre-existing framework for collaboration. It was also suggested that the TAAP principles are general enough to be used as a guide to promoting additional groundwater collaboration for the assessment of other transboundary aquifers in Mexico and the United States and around the world. Yet, the applicability of the TAAP Cooperative Framework will depend largely on the unique circumstances of the involved countries.
We conclude for several reasons that the transboundary aquifer assessment efforts following the TAAP Cooperative Framework represent a model for others wishing to engage in transboundary aquifer assessment. The TAAP Cooperative Framework is a concisely written and readily available document that has been successfully approved and signed by two countries. It has promoted productive scientific collaboration between the United States and Mexico in a manner consistent with the Draft Articles on the Law of Transboundary Aquifers (UN Draft Articles). Its elements are also consistent with the information gathering portion of successful groundwater management agreements around the world. It includes funding and communication provisions that are uncommon in existing international agreements but that facilitate groundwater cooperation, as made evident by the collaboration to date. Finally, according to the TAAP Cooperative Framework, either of the two countries can propose an aquifer of focus, meaning that there is no need to develop a new cooperative framework for assessing additional transboundary aquifers shared by the United States and Mexico.
The present study 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 wishing to engage in transborder aquifer assessments worldwide. Furthermore, the principles of the TAAP Cooperative Framework include elements that promote trust between the United States and Mexico (e.g., data sharing, development of binational aquifer assessment activities, the establishment of technical advisory committees, and establishment of technical groups). These and the rest of 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.