Abstract
Field studies were conducted in central Arizona to develop basal crop coefficients (Kcb) for an earlymaturing,upland cotton. Cotton evapotranspiration (ETc) rates were determined from soil water depletion measurementsmade during the season in both small and large level basins in 1993 and 1994. Values for Kcb were developed from theETc data and an estimated grass-reference evapotranspiration (ETo), based on weather data. The basal cropcoefficient procedure presented in the recently published Food and Agriculture Organization (FAO), Irrigation andDrainage Paper No. 56 was used to estimate the soil water evaporation coefficient (Ke) following periods of irrigationand rain and, thus, quantify the amount of soil evaporation. A separate analysis was conducted to evaluate the waterstress coefficient (Ks) for conditions of low soil water. The developed Kcb data were used to derive two Kcb polynomialcurves as functions of days past planting (DPP) and cumulative growing degree days (CGDD). A third curve was derivedfrom the data using the FAO straight-line method. When used in irrigation management procedures, any of the threeKcb curves presented should result in good estimates of daily ETc for an early-maturity cotton, grown under climaticconditions similar to those for these studies.
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