More rain seems like it would increase water flow in our region's rivers, but not when it is coupled with less snow. Researchers found that increased temperatures in the Colorado River Basin have lowered the basin's streamflow -- further evidence that climate change is impacting water resources.

Researchers believe, however, that rising temperatures have resulted in increased rain frequency and earlier snowmelt.

The Colorado River's water flow is important to water resource managers, who need accurate forecasts to plan system operations. Runoff from snowmelt in the high mountains in the upper part of the basin is the primary water source for the Colorado River.

"When rain occurs in place of snow, streamflow peaks earlier in the year and can make it challenging for water managers to assess resource availability," said Tom Piechota, UNLV director of sustainability and multidisciplinary research and professor in civil and environmental engineering.

Using historical temperature, precipitation, and natural runoff data, researchers identified a consistent increase in temperatures in the basin, increased river flows in the late fall and winter months, and decreased river flows during the peak runoff season (April through July).

The study does not explicitly compare snowpack and streamflow trends. Researchers believe, however, that rising temperatures have resulted in increased rain frequency and earlier snowmelt.

The Colorado River Basin includes portions of seven states -- Colorado, Wyoming, Utah, New Mexico, California, Nevada, and Arizona.

The study appeared in the October issue of Journal of Hydrometeorology. Piechota conducted the study with W. Paul Miller, a graduate student and hydrologic engineer student trainee with the U.S. Bureau of Reclamation in Boulder City. Funding came from the Bureau of Reclamation, National Science Foundation, and National Oceanic and Atmospheric Administration.