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Fraction of Thermal Units as the Base for an Evapotranspiration Crop Coefficient Curve for Corn

Saint Rose School, 607 Seventh Ave., Belmar NJ 07719
Dep. of Agronomy, Throckmorton Hall
Dep. of Physics, Cardwell Hall, Kansas State Univ., Manhattan, KS 66506.

^* Corresponding author.

A need exists to determine if evapotranspiration (ET) crop coefficient curves, based on a fraction of thermal units concept, could be used across cultivars that require various thermal unit totals from emergence to physiological maturity. Our objective was to develop such a curve for corn (Zea mays L.) and then to test the ability of this curve to estimate ET of cultivars of various maturity lengths. A basal crop coefficient curve for corn was developed by using field data obtained from 1974 through 1982 near Manhattan, KS and during 1981 and 1982 near Tribune, KS. The method involved calculating the ratios of measured ET rates adjusted for surface evaporation to reference ET rates, adjusting these ratios for the amount of available soil water, and then performing regression of the adjusted ratios against fraction of thermal units. The developed basal crop coefficient curve (n = 48, RMSE = 0.11, and R² = 0.88) was then used to estimate the ET rate of three corn cultivars grown near Manhattan, KS in 1983 and 1984. We then performed linear regression of estimated ET rates against measured ET rates (water balance technique) for each cultivar. All regression equations and coefficients were significant at P < 0.001. The hypotheses that intercept equals zero and slope equals 1 could not be rejected for any of the cultivars. The use of fraction of thermal units as the crop coefficient base scale allowed for a general and accurate use of the basal crop coefficient curve across corn cultivars requiring various thermal unit totals from emergence to physiological maturity.

Received for publication February 16, 1988.