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ET measurement is key to irrigation managementNebraska To better manage irrigation, producers need to know how much water evaporates from the field and how much transpires through the crop's leaf pores, said a University of Nebraska-Lincoln specialist. "Once we determine the first irrigation date, then we can use the evapotranspiration information to check water balance and make better irrigation decisions for our crops," said Suat Irmak, UNL water resources engineer. When a crop reaches complete canopy cover, evaporation decreases, Irmak said. It doesn't go away entirely, but it is minimal and transpiration becomes the dominant factor. ET is cyclical and the "off-season" is just as important as the active growing season for water resources planning and management for large scales. That's the time to assess the incoming water supply and predict whether or not there will be enough water in the spring to get a crop started. ET information is as important to state and federal regulators to do regional or watershed modeling as it is to growers who need to manage irrigation water, Irmak said. A number of environmental factors like wind speed, solar radiation, temperature and humidity affect a crop's evapotranspiration rate. Wind speed increases the ET rate until it's so extreme that the plants close their leaf pores to stop water loss. Increased humidity will result in decreased transpiration because of a decreasing vapor pressure deficit between the surrounding air and the crop surface. High solar radiation is usually associated with increasing heat and ET, Irmak said. The highest evapotranspiration rates usually occur in mid-summer, July and August. The same information applies to pasture and forages, he said. "We are in the process of measuring ET for both irrigated and dryland pasture, irrigated and dryland corn, disk tilled and no till irrigated soybeans, winter wheat, and other surfaces in the south central part of the state," he said. For more on evapotranspiration, including Irmak's ET measurements, go to http://bse.unl.edu/faculty/IrmakS.shtml. To learn more about ET, irrigation and producing more with less water, be sure to register for the Great Plains Conference on Improving Crop Water Productivity on Sept. 18, 2008, at UNL's West Central Research and Extension Center. 8/25/08 Date: 8/19/08 Advertisement
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