USGS water study details evapotranspiration rates

Mapping may help water managers prepare for climate change

Large parts of the arid intermoountain West lose more than half the precipitation that falls to evapotranspiration. Map courtesy USGS.

By Summit Voice

FRISCO — At least 80 percent of the precipitation that falls in the hot and dry American Southwest is lost to evapotranspiration, U.S. Geological Survey scientists said in a new report that will help resource managers plan for the future.

The study, published in the Journal of the American Water Resources, is the first to map average evapotranspiration rates across the continental United States. Knowing those rates is important because ir’s part of the equation for determining the amount of water available for people and ecosystems.

Evapotranspiration is the amount of water lost to the atmosphere from the ground surface.  Much of this loss is the result of the “transpiration” of water by plants, which is the plant equivalent of breathing.

To arrive at their findings, the scientists compared total annual average precipitation with the long-term discharges from 838 watersheds across the U.S. Then they plugged other climate data into the equation to determine the evapotranspiration rates.

“Since evapotranspiration consumes more than half of the precipitation that happens every year, knowing the evapotranspiration rates in different regions of the country is a solid leap forward in enabling water managers and policy makers to know how much water is available for use in their specific region,” said Bill Werkheiser, associate director for water at the USGS. “Just as importantly,” he added, “this knowledge will help them better plan for the water availability challenges that will occur as our climate changes since transpiration rates vary widely depending on factors such as temperature, humidity, precipitation, soil type, and wind.”

“The map of the long-term average annual evapotranspiration rates for different areas should be immensely helpful for ensuring the long-term, sustainable use of water in different regions, especially since forecasted climate change will, in many places, change the amount of precipitation and evapotranspiration that occurs,” Sanford said. “This tool, for example, allows water managers to quantify surface water runoff to reservoirs or water recharge to aquifers. It will also enable natural resource planners to understand the water needed for healthy-functioning ecosystems.”

One interesting finding illustrated in the maps is that in certain regions of the United States, such as the High Plains and the Central Valley of California, evapotranspiration exceeds the amount of precipitation because water is imported from other regions.  The map also shows that the Pacific Northwest has many areas with low evapotranspiration to precipitation rates because of the area’s very high rainfall and low-to-moderate temperatures.  In contrast, counties in the arid Southwest have evapotranspiration rates that usually exceed 80 percent of precipitation.

The research was published this week in the Journal of the American Water Resources Association.  To read the article and see the maps, click here.



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