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Research Shows Large-Scale Irrigation Reduces Local Precipitation
By Shawna Richter-Ryerson
A University of Nebraska-Lincoln researcher has shown that widespread irrigation has resulted in a net moisture loss in Nebraska, a finding that could have worldwide water conservation implications if substantiated by further research.
Across Nebraska, runoff rates have generally dropped by a little more than a tenth of an inch per decade between 1979 and 2015, said Joe Szilagyi, a research hydrologist with the Conservation and Survey Division in the School of Natural Resources. During that same period, statewide precipitation rates have increased by about a tenth of an inch per decade, contradicting a scientifically accepted relationship between the two variables.
That law, called the Budyko curve, states that when an area becomes more humid, a higher proportion of precipitation will turn into runoff. When an area becomes drier, a higher proportion of precipitation will return to the air as evapotranspiration.
"Nebraska is in clear defiance of the Budyko law," Szilagyi said. "The explanation lies in the about 50 percent increase in irrigated land area over the study period, making Nebraska the leader in irrigated acreage totals within the United States."
Distribution of the 1-square-kilometer cells where irrigated land in 2012 exceeded 50 percent of the cell area (blue-green dots) in Nebraska. Pink corresponds to declining annual, while yellow corresponds to declining irrigation-season (May-July) precipitation rates over the 1979-2015 period. The brownish color marks their spatial overlap.
Evapotranspiration rates, fueled by generally increasing air temperatures and expanding irrigation across Nebraska, have grown at double the rate of the precipitation increase, leading to dropping runoff rates, Szilagyi said. Moreover annual precipitation rates have decreased over the most heavily irrigated regions, while increasing in the other areas of the state.
"The picture is similar when we look at the precipitation rates of the irrigation season only, typically between May and July," Szilagyi said.
Satellite-derived average monthly surface temperatures, in degrees Fahrenheit, for 2000-2009. Long after the typical irrigation season of May-July in Nebraska, the irrigated areas, due to their high soil-moisture content, still remain significantly cooler even in September than the surrounding non-irrigated land
Szilagyi theorizes that the irrigation-enhanced evaporation has a cooling effect over a region, making the overlying air more stable, similar to that of the Great Lakes during the spring and early summer. The air cooled by the evaporating surface becomes denser and less buoyant, forming fewer rain-producing clouds. Once the air leaves the irrigated fields, it becomes more buoyant, eventually dropping its surplus moisture somewhere downwind. That moisture has been reported falling as far away as Ohio and Indiana, Szilagyi said.
Though this wind-driven moisture export from Nebraska has been known for a while, the local suppression of precipitation over large-scale, expanding irrigation projects has not been researched much. But if further research confirms Szilagyi's finding, it could present a "double-whammy for sustainable large-scale irrigation and water conservation all over the world," he said.
"Not only does the extra moisture that is released by the irrigated crops leave the area and form a net loss of water, but this loss is made even worse by further reduced local precipitation rates triggered by the large spatial scale over which the irrigation takes place," he said.
That means the human-driven disturbance of the hydrological cycle could have lasting impacts for extensively irrigated areas, Szilagyi said.
The study was published in the February issue of the Journal of Hydrology.