|Hunsaker, Douglas - Doug|
Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2011
Publication Date: 12/12/2011
Citation: Glenn, E.P., Neale, C.M., Hunsaker, D.J. 2011. Vegetation-index-based crop coefficients to estimate evapotranspiration by remote sensing in agricultural and natural ecosystems. Hydrological Processes. 25(26):4050-4062. Interpretive Summary: Evapotranspiration, or ET, is the measure of crop water use that provides key information needed for efficient water resource management in irrigated agriculture and natural ecosystems. A method widely used by water managers for determining the crop or vegetation ET, multiplies reference ET data, available from a local weather station, by a crop coefficient (Kc), available in the literature. In recent years, methods have been developed, which use a remotely-sensed vegetation index (VI) of the crop or vegetation to determine the actual Kc. The use of the VI methods has been shown to provide more accurate estimates of ET than that using Kc from the literature. This manuscript provides current information on the state-of-the-art methods available for estimating ET with VI and remote sensing. The overall goal is to describe the methods in sufficient detail so that they can be understood and applied by an increasing number of water resource managers and researchers. Wider application of these techniques is expected to increase water use efficiencies in irrigated agriculture and improve water management of natural ecosystems. This research will be of interest to irrigation farm managers, irrigation consultants, federal and state government agencies, and the irrigation industry.
Technical Abstract: Crop coefficients were developed to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions. Starting in the 1980s, crop coefficients developed through lysimeter studies or set by expert opinion began to be supplemented by remotely-sensed vegetation indices that measured the actual status of the crop on a field-by-field basis. Vegetation indices (VI) measure the density of green foliage based on the reflectance of visible and near infrared (NIR) light from the canopy, and are highly correlated with plant physiological processes that depend on light absorption by a canopy such as ET and photosynthesis. Reflectance-based crop coefficients have now been developed for numerous individual crops, including corn, wheat, alfalfa, cotton, potato, sugar beet, vegetables, grapes and orchard crops. Other research has shown that VIs can be used to predict ET over fields of mixed crops, allowing them to be used to monitor ET over entire irrigation districts. VI-based crop coefficients can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows instead of to a modeled crop growing under pristine conditions. Recently, the concept has been applied to natural ecosystems at the local, regional and continental scales of measurement, using time-series satellite data from the MODIS sensors on the Terra satellite. VIs or other visible-NIR band algorithms are combined with meteorological data to predict ET in numerous biome types, from deserts, to arctic tundra, to tropical rainforests. These methods often closely match ET measured on the ground at the global FluxNet array of eddy covariance moisture and carbon flux towers. This review surveys methods developed for agricultural and natural biomes and sources of error or uncertainty in their application.