account by the Penman method, the associated crop coefficients are
essentially independent of climate. The seasonal variations in crop coef-
ficients correspond roughly to incomplete soil cover, full canopy, and
senescing crop growth stages outlined previously (Jensen, 1968; Dooren-
bos and Pruitt, 1977). These crop coefficients thus depend on physiologi-
cal stage of crop growth and degree of canopy coverage.
In applying Equation 20 to predict ETfor a crop, estimates of ETp are
required. Several methods have been presented for calculating ETp.
Since values of ETp vary with method (Table 7), crop coefficients will also
vary and must be developed for use with a specific method for calculating
ETp.
Examples of crop coefficients for citrus, pasture and turfgrass, and corn
are presented in Figures 10a, 10b, and 11, respectively. The kc values
shown by solid lines were taken from Soil Conservation Service Technical
Release No. 21 (1967). These kc values were developed from a modified
Blaney-Criddle formula. Also on each graph are crop coefficients (k')
calculated from data presented in this bulletin and based on the Penman
method for calculating ETp (a = 0.05, k, = 0.7). The curves may not be
directly comparable, since the curves from the Soil Conservation Service
8
7 E-T- ft.. -
6
1 -
Date
Fig. 9. Evapotranspiration (ET) for peanuts (reported by Stansell et al. (1976)
for Tifton, Georgia).
NOTE: See Note, Fig. 5.
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