BASAL CROP COEFFICIENTS AND WATER USE FOR EARLY MATURITY COTTON

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 measurements made during the season in both small and large level basins in 1993 and 1994. Values for Kcb were developed from the ETc data and an estimated grass-reference evapotranspiration (ETo), based on weather data. The “basal crop coefficient” procedure presented in the recently published Food and Agriculture Organization (FAO), Irrigation and Drainage Paper No. 56 was used to estimate the soil water evaporation coefficient (Ke) following periods of irrigation and rain and, thus, quantify the amount of soil evaporation. A separate analysis was conducted to evaluate the water stress coefficient (Ks) for conditions of low soil water. The developed Kcb data were used to derive two Kcb polynomial curves as functions of days past planting (DPP) and cumulative growing degree days (CGDD). A third curve was derived from the data using the FAO “straight-line” method. When used in irrigation management procedures, any of the three Kcb curves presented should result in good estimates of daily ETc for an early-maturity cotton, grown under climatic conditions similar to those for these studies.