Colorado River mystery solved: Why billions of liters of water disappear before reaching reservoirs World News

University of Washington scientists have finally solved the mystery of the Colorado River’s water shortage, revealing a key shift in the hydrological cycle of the western United States. For decades, hydrologic managers have used winter snowpack measurements to predict the amount of water available in summer reservoirs, but since 2000 managers have been overestimating river flow forecasts based on winter snowpack measurements. New research by scientists suggests that a warm and dry spring is to blame. Spring precipitation is critical in helping move snowmelt water to rivers, but when no spring precipitation occurs, mountain vegetation absorbs water before melting snow reaches rivers. This physiological siphon effect is enhanced by clearer skies and increased solar radiation, which together account for almost 70% of the water shortage in the basin. Capturing ecologically trapped water in the Colorado River Basin is critical to the 40 million people who live and work in the basin and rely on dwindling water supplies.

Colorado River water shortage mystery solved

The contrast between heavy snowfall in winter and dry river beds in spring and summer used to be blamed solely on evaporative losses. New research in Geophysical Research Letters shows that the main reason for this phenomenon is that plants consume large amounts of melted snow before reaching large downstream reservoirs such as Lake Mead and Lake Powell.As monthly precipitation decreases in the spring, plants (from wildflowers to those found in high-elevation forests) become “thirsty” earlier in the season. As sunlight increases and rainfall decreases, these plants consume more of the melted snow as their food supply.

Why rising temperatures could permanently reduce river flows

The water loss is largely due to the structural shift from drought to aridification that began in 2000, known as the “Millennium Drought.” According to the U.S. Geological Survey (USGS), the region is currently experiencing a state called aridification, which means rising temperatures will permanently reduce river flow, and the changes are occurring despite increased precipitation. A warming atmosphere increases the amount of moisture it can hold, which results in an increased vapor pressure deficit, resulting in greater uptake of soil moisture and snowmelt by plants.

Why 100% snowpack no longer guarantees adequate water flow

Research shows that spring rainfall has decreased by about 7% throughout the upper Colorado River Basin. Lower elevation basins are most severely affected because snow melts earlier, giving vegetation a longer window to extract water during the growing season. Research funded by the National Science Foundation further shows that even if winter snowpack is 100% of normal, a dry, sunny spring can result in a huge “runoff deficit,” in which only 50% of the expected water flows into the river.

Traditional snow cover metrics are insufficient for predictive modeling

Current management practices are primarily based on the April 1 snowpack report and are generally governed by the “river code.” The Bureau of Reclamation and NOAA are now being asked to innovate their modeling capabilities to include not only plant phenology but also methods for forecasting spring weather. If 70 percent of the water is lost to these “biological pumps,” states and Mexico that share the river will continue to face increasingly inaccurate water allocations and shortages of available supplies.