ESTIMATING WATER-USE AND IRRIGATION REQUIREMENTS OF COFFEE IN HAWAII

Crop evapotranspiration (ET(c)) was measured as evaporative heat flux from drip-irrigated coffee (Coffea arabica L. cv. Yellow Catuai) field s at different stages of canopy development using the Bowen ratio-ener gy balance technique. Irrigation requirements were determined by compa ring the ET(c) values obtained against reference values (ET0) derived from a modified Penman equation, and expressed as the ET(c)/ET0 ratio, or crop coefficient (Kc). In 1991, the average Kc was 0.75 to 0.79 fo r fields containing 2- to 4-year-old plantings. This ratio was 0.58 fo r a field containing a 1-year-old planting. Crop coefficient was 30% l ower 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 transpir ation (T), determined with the stem heat balance technique, comprised from 40% to 95% of ET(c) as the leaf area index increased from 1.4 to 6.7. Behavior of Kc and T during a 25-day soil drying-reirrigation cyc le indicated that the crop was able to maintain relatively high levels of gas-exchange activity during periods of severe water deficit.