Study Concludes Colorado River Reservoirs Could Bottom Out from Warming, Water Management Business-as-Usual
All reservoirs along the Colorado River might dry up by mid-century as the West warms, according to a new study led by researchers at the University of Colorado, Boulder. The probability of such a severe shortage by then runs as high as one-in-two, unless current water-management practices change, the researchers report in a paper to be published in the American Geophysical Union (AGU) journal Water Resources Research.
The study’s coauthors looked at the effects of a range of reductions in Colorado River stream flow on future reservoir levels and at the implications of different management strategies. Even under the harshest drying caused by climate change, the large storage capacity of reservoirs on the Colorado might help sustain water supply for a few decades. However, new water management approaches are critical to minimize the chances of fully depleting reservoir storage by mid-century.
This study, along with others that predict future flow reductions in the Colorado River Basin, suggests that water managers should begin to re-think current water management practices during the next few years, before the more serious effects of climate change appear.—Balaji Rajagopalan, lead author
Water supply from the massive reservoirs on the Colorado River (currently about 60 Million Acre Feet (MaF) or roughly four times the annual average flow in the river) has historically rendered the water supply reliable even when taxed by severe drought such as the early 1990s. This capacity, the researchers note, has been the backstop supporting population growth and economic expansion in the southwest United States. Roughly 30 million people now depend on the Colorado River for drinking and irrigation water.
The annual basin deliveries have risen in recent decades and now approach the annual average river flow. With the projection for further demand increase, the reliability of the water supply becomes increasingly dependent on reservoir storage.—Rajagopalan et al.
The Colorado River system is enduring its tenth year of a drought. The river system entered the drought in 2000 with the reservoirs at approximately 95% of capacity. The reservoir system is currently at 59% of capacity, about the same as this time last year, says Rajagopalan.
The research team examined the future vulnerability of the system to water supply variability coupled with projected changes in water demand. They found that through 2026, the risk of fully depleting reservoir storage in any given year remains below 10% under any scenario of climate fluctuation or management alternative. During this period, the reservoir storage could even recover from its current low level, according to the researchers.
But if climate change results in a 10% reduction in the Colorado River’s average stream flow as some recent studies predict, the chances of fully depleting reservoir storage will exceed 25% percent by 2057, according to the study. If climate change results in a 20% flow reduction, the chances of fully depleting reservoir storage will exceed one in two by 2057, Rajagopalan says.
On average, drying caused by climate change would increase the risk of fully depleting reservoir storage by nearly ten times more than the risk we expect from population pressures alone. By mid-century this risk translates into a 50 percent chance in any given year of empty reservoirs, an enormous risk and huge water management challenge.—Balaji Rajagopalan
Implementing more aggressive management practices—in which downstream releases are reduced during periods of reservoir shortages—could lead to only a two-fold increase in risk of depleting all reservoir storage during this period, according to the study.
The magnitude of the risk will ultimately depend on the extent of climate drying and on the types of water management and conservation strategies established.
The study was conducted with support from the Western Water Assessment, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration (NOAA), as well as CADSWES and the Bureau of Reclamation.
Other study authors included James Prairie of the Bureau of Reclamation, Martin Hoerling and Andrea Ray of NOAA, Joseph Barsugli and Bradley Udall of the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU-Boulder, and Benjamin Harding of AMEC Earth & Environmental Inc. of Boulder.
Balaji Rajagopalan et al., Water Supply Risk on the Colorado River: Can Management Mitigate? Water Resources Research, in press.