The mystery of the Colorado River’s missing water has finally been solved by University of Washington scientists, revealing a significant change in the hydrological cycle of the American West. For the past several decades, hydrological managers have used winter snowpack measurements to estimate how much water will be available in reservoirs during the summer months, but since the year 2000, managers have consistently underestimated stream flow predictions based on winter snowpack measurements. New research conducted by scientists has revealed that hot and dry springs are responsible for this phenomenon. Precipitation in the spring is important to help transport water from melting snow to the river, but when there is no spring precipitation, mountain vegetation will draw water from snowmelt before it reaches the river. This physical siphoning effect is enhanced by clear skies and increased solar radiation, which accounts for about 70 percent of the water loss in the basin. Capturing this ecological hold of water from the Colorado River Basin is critical to the 40 million people who live and work in the basin, who depend on dwindling water supplies.

Mystery of Colorado River’s missing water solved

Evapotranspiration losses were attributed entirely to the difference between abundant snowfall in winter and dry river beds during spring and summer. New research in Geophysical Research Letters now shows that the main reason for this phenomenon is that plants are using large amounts of the melting ice cubes before they reach large downstream reservoirs like Lake Mead and Lake Powell.As the monthly amount of spring rainfall decreases, plants (from wild flowers to high altitude forests) become ‘thirsty’ early in the season. With increased sunlight and less rainfall, these plants use more of the melting snowpack as a food supply.

Why are rising temperatures permanently reducing river flows?

This water loss is primarily caused by the structural change from drought to desiccation, known as the ‘Millennium Drought’, which began in 2000. According to data from the United States Geological Survey (USGS), the region is currently experiencing a condition called desiccation, which means rising temperatures are permanently reducing river flows and changes are occurring regardless of the amount of precipitation received. A warming atmosphere increases moisture content, which increases vapor pressure loss, causing plants to draw more moisture from the soil and snow to melt.

Why does 100 percent ice no longer guarantee full flow?

Research shows that spring precipitation has decreased by about 7 percent across the entire Upper Colorado River Basin. Lower elevation basins are most affected due to earlier melting of snow and therefore vegetation has a longer period to extract water during its growing season. Research funded by the National Science Foundation further shows that even if winter snowfall is 100 percent of normal, a dry, sunny spring can result in a large ‘runoff deficit’, causing only 50 percent of the expected amount of water to reach the river.

Traditional snowpack metrics are inadequate for predictive modeling

Current management practices, which are based primarily on snowpack reports as of April 1, are often governed by the ‘law of the river’. The Bureau of Reclamation and NOAA are now requested to renew their modeling capabilities to include not only plant phenology but also spring weather forecasting methods. If 70 percent of the water is wasted in these ‘biological pumps’, the states that share the river and Mexico will continue to face misallocation of water and a shortage of available supply.