Why are evapotranspiration rates important to know? The amount of water available for people and ecosystems is the amount of annual precipitation (snow or rain) minus the amount of annual evapotranspiration. 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.
Evapotranspiration has been difficult to measure accurately on a regional or continental scale. To produce these maps, USGS scientists examined satellite imagery for climate and land-cover data and streamflow data for more than 800 watersheds. This information allowed them to generate a mathematical equation that can be used to estimate long-term evapotranspiration in the continental United States.
One interesting finding illustrated in the maps is that in certain regions, 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.
You can find the original USGS news release here→.