Evaluating Historical and Future Compliance with the 1944 US–Mexico Water Treaty under Changing Climate and Use Conditions

About half of the earth’s land surface is composed of international transboundary basins, half of which are managed by formal agreements. However, climate change, population growth, and other changing conditions are straining these agreements, leading to conflicts as riparian countries seek to protect historical water rights. The 1944 Water Treaty between the United States (US) and Mexico, which outlines specific allocations for Colorado River and Rio Grande waters, is one example. For the Rio Grande (known as the Río Bravo in Mexico) the Treaty prescribes Mexico’s mandated delivery of water to the US, and for the Colorado River it prescribes the US’s mandated delivery of water to Mexico. Overallocation of the Colorado River prompted negotiations that resulted in amendments to the Treaty, adapting water allocations to the basin’s conditions. Despite parallel overallocation of the Rio Grande and ongoing challenges to meet delivery obligations, the allotment of Rio Grande waters has not been revised. The Treaty’s inability to cope with the system’s variability exerts pressure on local communities as Mexico balances its local needs with its international duties amidst ongoing drought. This study evaluates the conditions necessary to uphold the Treaty by conducting a historical resimulation of treaty deliveries under various streamflow scenarios. An alternative delivery strategy was developed to assess the benefits of flexible deliveries. Results indicate that prolonged periods of above-average streamflow are necessary to meet mandated deliveries without disproportionately impacting Mexican communities. We also find that flexible deliveries increase treaty compliance and distribute the impacts of decreased flow between both countries.