Reservoir evaporation a big challenge for water managers in West

December 28, 2015 •

Water managers in Colorado and the West scrambling to meet the growing demand for increasingly scarce water supplies caused by large populations far from water resources, climate change and drought need to focus more effort on conserving water, including addressing reservoir evaporation, say University of Colorado Boulder researchers.

While reducing water consumption has been successful in places like Denver and much of California, the loss of water from reservoir evaporation is an issue already affecting the growing population of the West, said CU-Boulder Associate Professor Katja Friedrich. The reservoir water loss is becoming even more important as broad uncertainties in precipitation projected by climate change and early snowmelt require more reservoir storage, she said.

“Evaporation of water from open reservoirs in the arid western U.S. cannot be neglected any more, especially with the possibility of precipitation decreases occurring as a result of a changing climate,” said Friedrich, a faculty member in the Department of Atmospheric and Oceanic Sciences (ATOC). “We need to try to plan for both short-term needs and to make sure we have enough water over the coming decades.”

A recent workshop on campus convened by researchers at CU-Boulder and the Desert Research Institute (DRI) in Reno, Nevada, brought together experts in atmospheric science, hydrology, land use and water resource management from the western U.S. and Canada, said Friedrich.

Water managers have little information on evaporative loss, relying on outdated methods like “pan evaporation,” developed in the 1920s and still in use today. In pan evaporation, a 4-foot-in-diameter, 10-inch deep pan is set next to selected reservoirs where water managers fill the pan and measure water evaporation in 24-hour increments and extrapolate the results to corresponding reservoirs. The method is used today in many Colorado reservoirs as well as major Colorado river impoundments.

The problem in part is not all reservoirs are equal in terms of location, elevation, shape or evaporation. Attendees of the CU-Boulder-hosted reservoir evaporation workshop in October proposed the use of high-resolution weather models coupled with sophisticated reservoir models, which could be used not only to estimate evaporation but also to forecast it, a method not previously considered by water managers.

Little research has been done on quantifying evaporation with instrumentation and numerical models, Friedrich said. “We need to better understand evaporation, which will require continuous measurements of wind direction and speed, air and reservoir temperatures, humidity, solar radiation and vegetation at individual reservoirs.”

Evaporation is a large and continuing problem in the Colorado River basin, including Lake Mead and Lake Powell where about 500 billion gallons of water evaporate annually, according to CU-Boulder Assistant Professor Ben Livneh of the Department of Civil, Environmental and Architectural Engineering.

This represents roughly 10 percent of the total natural flow of the Colorado River Basin said Liveneh, who also is also a fellow at the Cooperative Institute for Research in Environmental Sciences -- about five to 10 times the amount of Denver’s annual water use.

“We can no longer afford to lose this amount of water. Once it is lost it is gone,” said retired ATOC scientist Bob Grossman, who helped organize the conference on reservoir evaporation. “The neglect of evaporative loss as the cost of doing business in a water-abundant world will likely cut into the bottom line as scarcity looms.”

Proposed “geo-engineering” techniques for reducing reservoir evaporation include covering surface water with thin films of organic compounds, reflective plastics or extremely lightweight shades. Other proposals include moving reservoir water underground into new storage areas or aquifers or relocating or building new storage reservoirs at higher elevations where less evaporation occurs.

“One thing we do know is that you can only reduce evaporation and not eliminate it unless you store it underground,” said Friedrich. “But that has its own set of problems. Our intention is to help water managers reduce evaporation for current and future reservoirs.”

To study evaporation differences in different reservoirs, a team of scientists, water managers and federal and state agency representatives led by DRI researcher Justin Huntington deployed high-tech buoys at reservoirs in California, Idaho and Nevada to better understand the water evaporation process. In addition, there is ongoing research on evaporation from the Great Lakes by CU-Boulder geography Professor Peter Blanken and his Canadian and U.S. colleagues.

Participants in the October evaporation workshop included a number of universities and federal and state agencies like the U.S. Bureau of Reclamation, the Colorado Water Conservation Board, Environment Canada and the National Center for Atmospheric Research. The researchers hope to test new techniques and tools related to evaporation on a Front Range reservoir starting next year, said Friedrich.