Crop coefficients for a pear orchard (Pyrus communis L.) obtained using eddy covariance

Crop evapotranspiration estimates are frequently obtained using an approach that combines reference evapotranspiration and empirical crop coefficients, disseminated by the Food and Agriculture Organization of the United Nations (FAO). Several recent studies made in orchards showed that the crop coefficients (Kc) proposed by the FAO methodology and those obtained experimentally may not agree for Portuguese conditions. The Kc values obtained locally were consistently lower. Therefore, for such conditions, a verification of the FAO crop coefficients might be needed, in order to obtain more rigorous estimates, especially if the goal is to minimize water consumption. For a crop such as pear (Pyrus communis L.) cv. 'Rocha' in the Oeste region of Portugal (95% of the total national pear production), water savings can have economical and environmental benefits. The experimental work took place during the summer of 2004, in the Oeste region of Portugal (Vale de Maceira, Alcobaca) in a 2.5 ha plot, part of an area of 34 ha of pear and apple orchards, with daily drip irrigation. This study presents some results on Kc values obtained with direct measurements of evapotranspiration using eddy covariance method. The closure error of the surface energy balance equation was 10%. The data were selected according to footprint analysis. The results, combined with calculated reference evapotranspiration, allowed the determination of orchard Kc values. The evapotranspiration rates were on average 1.9 mm/day and the mean Kc (for the mid-season stage) was 0.5, below the tabled Kc for pear orchards with no ground cover (0.95) or with active ground cover (1.2), confirming the need for adjusting published values for local conditions.