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February 2, 2022

A new study finds that peak runoff on the Rio Grande could arrive earlier in the season by the end of the century, negatively impacting a watershed where demand already exceeds supply.

U.S. Geological Survey scientists paired a unique streamflow model that they developed in 2020 with projected climate data to understand how climate change may affect future water supply in the Upper Rio Grande Basin. What they discovered may have major impacts to both humans and ecosystems. Streamflow projections from this study can be used by water managers to better understand the impacts of climate change in the Upper Rio Grande Basin and to plan for the imbalances between water supply and demand. 

“Understanding how climate change is affecting streamflow and snowmelt in an intensely managed area is challenging but crucial to determining future water availability,” said Dave Moeser, a USGS hydrologist and lead author of the study. “For example, if earlier peak flows no longer line up with the growing season, then managers may need to store water longer in reservoirs where it’s more prone to loss through seepage to groundwater and evaporation.” 

Upper Rio Grande Water Operations Model Map, NMWSC
Upper Rio Grande Water Operations Model Map, NMWSC

The Rio Grande flows nearly 2,000 miles from its headwaters in the San Juan Mountains in southwestern Colorado to the Gulf of Mexico. In the Upper Rio Grande watershed, which includes portions of Colorado, New Mexico and far west Texas, approximately 75% of Upper Rio Grande streamflow comes from seasonal snowpack in the headwaters and high-elevation areas of Colorado and New Mexico with the rest falling as monsoonal rain in the southern and low elevation areas. The Rio Grande is the primary source of water for more than 13 million agricultural, municipal and industrial water users in Colorado, New Mexico, Texas and northern Chihuahua, Mexico.  

Streamflow in the Upper Rio Grande Basin is heavily managed through a system of dams and diversions. Water managers use the Upper Rio Grande Water Operations Model, a model collaboratively developed by multiple agencies, to help make decisions. The Bureau of Reclamation’s San Juan-Chama Project imports water annually, from the San Juan Basin into the Middle Rio Grande, and annually distributes up to 96,200 acre-feet to project contractors to help meet demand there. By the time water reaches the southernmost portion of the Upper Rio Grande Basin near Fort Quitman, Texas, it has undergone a 95% reduction in flow with some stretches going dry intermittently, impacting downstream water users. 

In the new study that modeled streamflow under different climate projections, scientists from the USGS New Mexico Water Science Center and the South Central Climate Adaptation Science Center at the University of Oklahoma discovered that peak spring runoff on the Rio Grande will continue to arrive earlier and by 2099, could arrive a month earlier than the historic average if temperatures continue to warm. They also found that warming temperatures could affect the volume, type, and distribution of precipitation across the Upper Rio Grande Basin. In the mid- to high-elevation regions of New Mexico, for example, more precipitation would fall as rain instead of snow, which is detrimental because snow typically acts as a natural reservoir. When peak streamflow occurs earlier, it also affects the ability of reservoir managers to store water to meet needs later in the year. If water is arriving earlier, less may be available later in the summer season when demand is highest.   

“The Rio Grande already flows through arid to semiarid regions where, in addition to removal by humans, evaporation has a major impact on water supply,” said Shaleene Chavarria, a USGS hydrologist and co-author of the study. “Knowing when peak runoff will occur and how much water will flow through the Rio Grande is crucial for water users like farmers, major contributors to the New Mexico economy, who need to know how much water is available and when it will arrive.” 

This study was funded by the Department of the Interior South Central Climate Adaptation Science Center, Reclamation, and the United States Army Corps of Engineers. For more information about water in the Upper Rio Grande Basin visit: 

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