May 10, 2016--Groundwater a huge factor in sustaining Colorado River flows (Summit Voice)

Resource managers grappling with the vexing question of how to allocate Colorado River water to the thirsty cities, ranches and farms of the Southwest have some new food for thought. A new U.S. Geological Survey study published this week in the journal Water Resources Research shows that more than half the streamflow in Upper Colorado River Basin originates as groundwater. The information is especially important in the context of how climate change — through increased temperatures and evaporation, as well as changing precipitation patterns — will affect the river. One recent study, for example, showed thatwarmer spring temperatures are reducing flows independently of winter precipitation. In 2014, another study found that groundwater depletion threatens the sustainability of the Colorado River. The Colorado River Basin provides water for about 50 million people right now, and by some estimates, that number could grow by more than 20 million in the next couple of decades. Quantifying base flows is important because the Colorado is already one of the most over-allocated rivers in the world, and the imbalance between water supply and demand are growing quickly, with a major water gap projected by 2050.

About 90 percent of the river’s total streamflow originates in the Upper Basin, above Lees Ferry, Arizona. Along with providing water supplies for people, the river sustains wildlife, mining, and recreation. The researchers studied long-term records of water chemistry and streamflow data at 146 sites in the Upper Colorado River Basin in Colorado, Utah, New Mexico and Arizona. These data were then analyzed to create a model to predict and map where streamflow originates in the basin. On average, 56 percent of the streamflow in the basin originated from groundwater. “These findings could help decision makers effectively manage current and future water resources in the Colorado River Basin,” said Matthew Miller, a USGS scientist and the lead author of the study. “In light of recent droughts, predicted climate changes and human consumption, there is an urgent need for us all to continue to think of groundwater and surface water as a single resource.” The model estimates the amount of water lost during stream transport to the Lower Colorado River Basin, which is due largely to withdrawals for irrigation and evaporation to the atmosphere. In the high elevation headwaters of the Colorado River Basin, there is a greater percentage of snowmelt and precipitation contributing to the surface-water streamflow. As water flows further into the basin at lower elevations, a greater percentage of streamflow is from groundwater. To view the full article visit the Summit Voice.