One of the most important factors for water availability is evapotranspiration (ET), a figure that combines evaporation and plant breathing, known as transpiration. Although most of the water that falls from the sky goes toward ET, it has historically been difficult to calculate with accuracy. But a new model, developed by the United States Geological Society (USGS), gives water managers a better picture of how much water their area and region is losing.
The model shows that huge swaths of the Western U.S., including nearly all of the farmland West of the Mississippi River, is losing most of or all of the total precipitation to ET. If temperatures continue to trend upward, Central California, the High Plains, and Northwest Montana are some of the places that could see even more water scarcity than they do now.
Ward Sanford, a hydrologist with USGS and lead author of the study that appeared in the February issue of the Journal of the American Water Resources Association, says climate data improved significantly over the past decade. More information on precipitation, even in places where it changes rapidly was particularly important, as was the spread of GIS (geographic information system) technology, which allowed hydrologists to work faster, more cheaply and over larger areas than they could with traditional methods.
"To calculate ET, it's a very difficult thing to do. There's a lot of uncertainty," says Karl Taboga, a hydrogeologist with the Wyoming Geological Society, which is compiling groundwater studies to updates the state's river basin plans. Traditionally, many water managers would do just a water-balance approach to calculate ET. If they know the precipitation and then subtract the water stored in wells and flowing out of streams, the missing water is ET. Another approach is to measure the radiant energy needed to turn the water to gas as it evaporates at the surface, but it is usually only done over small areas because it is labor intensive.
Credit: Sanford et. al.
The new USGS model takes 30 years of historical precipitation data and water-balance information, and then combines them with climate and land cover regression models, which estimate the conditions via statistical analysis. "Climate variables are the most influential in determining ET, with the land cover adding a small but finite additional effect," Sanford says. He first developed the model when doing water studies for the state of Virginia, "then I realized we could so this for the whole country," he adds.
Some places aren't doing so badly. Taboga's home state of Wyoming has relatively healthy aquifers, for example. But around the country, Sandford's USGS model shows just how hard the drought has hit, especially across the Midwest, traditionally known as America's breadbasket. In regions such as the High Plains and the Central Valley in California, the ratio of ET to precipitation is greater than 1, meaning that evapotransiration is greater than the amount of precipitation?the area is losing water. In many of the areas with high rates of ET, such as the High Plains, water is imported to sustain agriculture. "We're engineering those areas to be that way; they can't be that way naturally," Sanford says. "How long is this sustainable?"
To help answer this question, the files for the model, which are free online, can be used to predict how water availability will change if average temperatures ratchet higher. Knowing how the water is being depleted, resource managers can take action to decrease the depletion and plan for the future, Taboga says, whether that's limiting or reallocating water rights or suggesting irrigation efficiency to legislators. He says the model will be especially important in places like the Ogallala Aquifer that stretches under the Great Plains region, which covers many states with competing interests. In water-scarce regions like the Middle East, models that can be used across the entire region are even more critical.
Sandford's next project is to combine his modeling with satellite data for an even more robust picture of ET across large areas, especially as climate shifts. "Even if precipitation goes up in the next 10, 20 years," Sanford says, "if the temperature goes up, it's going to leave even less water available."
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