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Crop evapotranspiration is a function of both weather (reference ET) and the growth stage of the plant. The ratio between reference ET and crop ET is the crop coefficient, which changes during the season with crop physiological changes. At the time of planting, most ET is evaporation from the bare soil surface, but as the canopy matures ET is predominantly transpiration from the crop (Fig. 6.1). The single crop coefficient combines evaporation and transpiration. A theoretically more accurate approach divides the evaporation and transpiration into two separate coefficients (dual crop coefficient). Both methods are extensively described in the Food and Agriculture Organization Irrigation and Drainage Paper No. 56 (FAO 56). Crop coefficients are normally calculated based on days after planting, but they can also be calculated based on heat units after planting. Heat units are the integrated temperature and time after planting. This chapter also includes a Fourier series approach to modeling crop coefficients and weather. Finally, the chapter includes a brief introduction to the use of remote sensing for irrigation scheduling.
KeywordsCrop evapotranspiration Crop coefficient Heat unit Irrigation scheduling Irrigation frequency
- Allen RL, Pereira DR, Smith M (1998) Crop evapotranspiration guidelines for computing crop water requirements – FAO Irrigation and drainage paper 56. FAO – Food and Agriculture Organization of the U. N http://www.fao.org/docrep/X0490E/x0490e00.htm
- Martin E, Slack D (2001) Arizona Irrigation Scheduling System (AZSCHED). The University of Arizona. http://cals.arizona.edu/cgi-bin/formhandler/formhandler.cgi