The mystery of the Colorado River’s missing water has finally been solved by scientists from the University of Washington, revealing a crucial shift in the hydrological cycle in the American West.
For the past several decades, hydrological managers have used winter snowpack measurements to predict how much water will be available in reservoirs during the summer months, but since 2000, managers have been overestimating streamflow forecasts based on winter snowpack measurements. New research conducted by scientists revealed that warmer and drier springs are responsible for this phenomenon.
Spring precipitation is critical in helping to deliver water from snowmelt to the river, but when spring rain does not occur, mountain plants will draw from snowmelt before it reaches the river. This physiological effect is enhanced by clearer skies and enhanced solar radiation, which together account for approximately 70 percent of the basin’s water deficit. Capturing this ecological interception of water from the Colorado River Basin is critical for the 40 million people who live and work in the basin and who depend on diminishing water supplies.
Solve the mystery of missing water in the Colorado River
The contrast between abundant snowfall in winter and dry riverbeds during spring and summer was entirely attributable to evaporative losses. New research from Geophysical Research Letters now shows that the main reason for this phenomenon is that plants use up a large amount of melted snow before it reaches large reservoirs downstream such as Lake Mead and Lake Powell.As monthly amounts of precipitation decrease in the spring, plants (from wildflowers to those found in upland forests) become “thirsty” early in the season.
With more sunlight and less rain, these plants use more of the melting ice as a food source.
Why rising temperatures permanently reduce river flows
The loss of this water is primarily due to a structural shift from drought to drought known as the “Millennium Drought” that began in 2000. According to data from the US Geological Survey (USGS), the region is currently experiencing a condition called aridity, meaning that rising temperatures are permanently reducing river flows and that the changes are occurring despite the amount of precipitation falling.
Warming increases the amount of moisture that can be retained, and this leads to a greater vapor pressure deficit, causing plants to draw more on soil moisture and melting snow.
Why does 100 percent snow no longer guarantee full stream flow?
Research shows there is about a 7 percent decrease in spring precipitation throughout the upper Colorado River Basin. Low-elevation ponds have been significantly affected by earlier snowmelt, thus providing a longer period of time for plants to extract water during their growing season.
Research funded by the National Science Foundation also shows that even if winter snowfall is 100% of normal, a dry, sunny spring can lead to a “significant runoff deficit” with only 50% of the expected amount of water reaching the river.
Traditional snow accumulation metrics are insufficient for predictive modeling
Current management practices, based primarily on April 1 snowfall reports, are often governed by the “River Code.” The Bureau of Reclamation and the National Oceanic and Atmospheric Administration (NOAA) have now been asked to innovate their modeling capabilities to include not only plant phenology but also methods for predicting spring weather. If 70 percent of the water is lost to these “biological pumps,” the states and Mexico that share the river will continue to face increasingly inaccurate water allocation and shortages in available supplies.