ABSTRACT
Crops and vegetation on the earth’s surface vary in height, amount of leaf area, amount of soil shaded, color, amount of stomatal control to evaporation, and amount of soil wetness beneath the canopy. All of these factors affect, to some degree, the amount of evapotranspiration (ET) from the crop or vegetation. Rather than assigning parameters for all of these terms during the process of predicting ET from a specific type of vegetation using an ET equation, as covered in the entry on Evapotranspiration Formulas, the impacts of these variables are often lumped into a single parameter, termed the crop coefficient, Kc. This approach is done to reduce the complexity and time requirement for predicting ET for each type of crop or vegetation, and relies upon a common ‘‘reference ET’’ for a defined type of reference vegetation to represent the change in ET caused by variation in weather parameters. Kc is defined as the ratio of ET from a crop or soil surface to ET from the reference surface. Reference ET is the ET from a fully vegetated surface covering the soil, and normally represents ET from clipped grass (termed ETo) or alfalfa (termed ETr).
