- Climate change and growth are straining a long-standing water treaty.
- Rio Grande deliveries face greater challenges than the Colorado River.
- New research tests whether current flows can meet treaty obligations.
- Flexible delivery rules could reduce pressure on border communities.
Friday, January 16, 2026 — In 1944, the United States and Mexico signed a treaty to divide the waters of the Colorado River and the Rio Grande, known in Mexico as the Río Bravo. At the time, negotiators relied on optimistic assumptions about river flows, population growth, and the length of droughts. Eight decades later, those assumptions no longer reflect conditions on the ground.
A January 2026 study published in the Journal of Water Resources Planning and Management
by Vianey Rueda and Andrew D. Gronewold examines whether the treaty can still be met under modern climate and water-use conditions. The research focuses on the Rio Grande portion of the treaty, where Mexico is required to deliver water to the United States from six tributaries over five-year accounting cycles.
Why the Rio Grande Is Different.
The 1944 treaty treats the Colorado River and the Rio Grande very differently. For the Colorado River, the United States delivers water to Mexico, and treaty amendments known as “minutes” have introduced flexibility during drought by linking deliveries to reservoir levels at Lake Mead.
For the Rio Grande, Mexico delivers water to the United States, averaging about 350,000 acre-feet per year over five-year cycles. While the Colorado River rules have been updated to reflect scarcity, the Rio Grande allocations remain largely unchanged, even as flows decline and droughts last longer.
The study notes that this imbalance places disproportionate pressure on Mexican communities, particularly during prolonged dry periods.
Testing the Treaty Against Reality.
Rather than debating legal theory, the authors tested the treaty against real hydrologic data. They reconstructed historical streamflows from 1953 through 2021 for the six Mexican tributaries used to calculate treaty deliveries. They then ran multiple scenarios, including:
- Extended wet periods.
- Extended dry periods.
- Heavy storm years such as 2008 and 2010.
- Situations where only one major tributary provides most of the flow.
This approach allowed the researchers to evaluate whether treaty deliveries could be met without causing persistent deficits.
Key Findings from the Modeling.
The results are sobering. The study finds that treaty compliance without severe local impacts is only possible when the basin experiences prolonged periods of above-average streamflow. Average conditions are not sufficient.
Two tributaries, the Rio Conchos and the Rio Salado, provide most of the water used to meet treaty obligations. When either experiences sustained low flows, deficits quickly accumulate. Short bursts of heavy rainfall, including major hurricanes, can temporarily ease shortages but do not offset long dry periods.
In several modeled scenarios, even historically rare storm events were not enough to compensate for years of reduced flow.
The Role of Flexibility.
The authors also tested an alternative delivery strategy that introduces flexibility similar to what already exists on the Colorado River. Under this approach, deliveries would be proportionally reduced during drought, rather than forcing full compliance regardless of conditions.
The modeling shows that flexible delivery rules reduce long-term deficits and spread the impacts of scarcity between both countries, instead of concentrating them on Mexican communities and ecosystems.
Implications for the Future.
The study does not call for abandoning the 1944 treaty. Instead, it highlights a growing mismatch between fixed legal obligations and a river system shaped by climate change, population growth, and limited storage capacity.
Without updated mechanisms that reflect current hydrology, treaty compliance on the Rio Grande increasingly depends on rare wet periods or emergency measures that strain local water users. The findings suggest that future negotiations will need to balance international commitments with the physical limits of the river itself.
Citation; Download the Paper.
Evaluating Historical and Future Compliance with the 1944 US–Mexico Water Treaty under Changing Climate and Use Conditions
Authors: Vianey Rueda, Andrew D. Gronewold, Ph.D., P.E.
Publication: Journal of Water Resources Planning and Management
Volume 152, Issue 1
https://doi.org/10.1061/JWRMD5.WRENG-6884
Frequently Asked Questions
What does the 1944 Water Treaty cover?
The treaty governs how the United States and Mexico share waters from the Colorado River, the Rio Grande, and the Tijuana River.
Who delivers water under the Rio Grande portion of the treaty?
Mexico is required to deliver water to the United States from six tributaries of the Rio Grande over five-year cycles.
Why is the treaty under stress now?
Climate change, declining river flows, longer droughts, and rapid population growth have reduced water availability compared to conditions assumed in 1944.
What did the study test?
Researchers modeled historical and hypothetical streamflow scenarios to see whether treaty deliveries could be met under different conditions.
What is the main conclusion of the research?
Treaty compliance is only consistently possible during extended wet periods. Average or dry conditions lead to deficits unless delivery rules are made more flexible.
Does the study recommend specific policy changes?
The research highlights the benefits of flexible delivery mechanisms but does not prescribe legal outcomes. It provides data to inform future negotiations.
When was the study published?
The study was published in January 2026 in the Journal of Water Resources Planning and Management.
