Evapotranspiration (ET) is a major part of the water cycle. Reliable measurements or estimates of ET can greatly improve quantitative forecasts and hindcasts of water demand by crops, horticulture, and natural vegetation, and also help to manage and conserve water resources. Direct measurement of ET requires not only specific devices such as eddy covariance instruments, but also well-trained research personnel to collect accurate data. As a result, a variety of indirect methods for estimating ET have been developed in recent decades. Among them, remote sensing methods have proved cost-effective in providing accurate regional and global coverage of ET.
In Sussex County, Delaware’s leading county in crop production, the ET distribution for the 2016 growing season was estimated using the Mapping Evapotranspiration at high Resolution with Internalized Calibration method, an energy-balance based ET mapping tool that utilizes satellite images and weather data. The estimated result was compared with field measurements using an eddy covariance instrument. The total estimated ET during Sussex County’s growing season (May-September) in 2016 accounts for 77 to 87 percent of historical-averaged annual ET in this region. The model-simulated seasonal ET for agricultural land is about 33 percent higher than urban/suburban areas and about 22 percent lower than forested areas. This study shows that when forestlands are converted to urban/suburban uses, significant amounts of water are diverted from ET and are then available to run off and/or infiltrate. Given that urban/suburban land has impervious surfaces in the forms of rooftops, roads, driveways, parking lots, sidewalks, etc., much of the water not lost to the atmosphere through ET presumably becomes part of the surface runoff portion of the water budget, thus underscoring the need for adequate storm-water management systems for urban/suburban lands. The results also imply that the practice of ET-based irrigation scheduling could be valuable in Sussex County and throughout the 20 percent of Delaware farmland that is irrigated.
University of Delaware
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