Dominant crop and environmental conditions need to be considered
to obtain accurate estimates of Et for a specific crop. Meteorological
conditions determine the evaporative demand while the crop canopy and
soil moisture conditions determine the extent to which that demand will be
met. Evapotranspiration for a particular crop can be estimated if measurements
or estimates of a potential or reference E, are available. These
measurements or estimates represent the meteorological demand. Crop
coefficients represent the crop and soil ability to meet the demand.
Extensive research has been conducted on reference E, methods and
crop coefficients because of their use in irrigation scheduling and water
resources allocation, management, and planning. The available methods
for estimating reference E, when properly used with reliable crop curves
permit estimating crop E, within the accuracy of most field-irrigation
systems to deliver water (Jensen et al., 1971; Jensen and Wright, 1978;
Wright and Jensen, 1978).
Various procedures have been used during the past three decades to
obtain the experimental crop and reference Et data needed to develop Et
crop coefficients. Several sets of curves derived from these data have been
published (Burman et al., 1980; Doorenbos and Pruitt, 1977; Jensen, 1974;
Pruitt et al., 1972, l987a; Wright, 1979, 1981, 1982). It is important that
empirically derived-crop coefficients be used with the appropriate reference Et.
The climatic adequacy of the methods, the necessary data, and the
time scale all need to be understood and carefully applied if accurate
estimates of crop water requirements are to be obtained
