Evapotranspiration of a drip-irrigated clementine citrus tree in a weighing lysimeter

A large weighing lysimeter (4x4x1.5 m) with forced suction at the bottom was installed in a 2 ha plot of drip-irrigated. Clementine citrus trees (Citrus clementina, Hort. ex Tan., cv. Clementina de Nules). A four year old tree was transplanted to it by the end of 1989, where it has grown since then normally. The design, calibration and performance of the lysimeter during 1991-1995 are described. The measured values of crop evapotranspiration, estimates of soil evaporation E-s, in selected periods and crop coefficients, K-c calculated in relation to several reference evapotranspiration methods are presented. Monthly K-c showed a distinct seasonal trend with maxima in autumn and minima in spring, and the inverse tendency was observed for canopy resistance. This trends presumably reflected changes of mean physiological age of the canopy along the season, as well as the effects of increased E-s by rainfall in autumn and of leaf area reduction produced by pruning in spring. Annual K-c values increased as the tree canopy grew and showed a good linear correlation with the percentage of ground cover.