Crop evapotranspiration (ETc) was measured as evaporative heat flux from drip-irrigated coffee (Coffea arabica L. cv. Yellow Catuai) fields at different stages of canopy development using the Bowen ratio-energy balance technique. Irrigation requirements were determined by comparing the ETc values obtained against reference values (ET0) derived from a modified Penman equation, and expressed as the ETc/ET0 ratio, or crop coefficient (Kc). In 1991, the average Kc was 0.75 to 0.79 for fields containing 2- to 4-year-old plantings. This ratio was 0.58 for a field containing a 1-year-old planting. Crop coefficient was 30% lower in 1992 due to higher ET0 values and lower stomatal conductance. Measurements made between July and August and again between September and November 1991 suggested that Kc may vary seasonally. Crop transpiration (T), determined with the stem heat balance technique, comprised from 40% to 95% of ETc as the leaf area index increased from 1.4 to 6.7. Behavior of Kc and T during a 25-day soil drying-reirrigation cycle indicated that the crop was able to maintain relatively high levels of gas-exchange activity during periods of severe water deficit.
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