Evapotranspiration and crop coefficients of Golden Delicious/M793 apple trees in the Koue Bokkeveld

Irrigation scheduling is key to efficient irrigation water use. The objective of this research was to determine the evapotranspiration (ETc) and crop coefficients (Kc) of Golden Delicious/M793 apple (Malus domestica L.) trees in a semi-arid climate to promote optimal usage of limited water resources in South Africa. The ETc and Kc values for trees irrigated at 50% plant available water depletion throughout the season were determined from 2005/06 to 2007/08. Over three seasons, ETc amounted to c. 870 mm between October and April, whereas reference evapotranspiration was c. 1262 mm. The ETc during vegetative growth and fruit cell division (Phase 1), fruit cell enlargement until end of shoot extension growth (Phase 2), ripening until harvest (Phase 3) and postharvest (Phase 4) contributed c. 15%, c. 41%, c. 20% and c. 24% to the seasonal water requirements of the trees, respectively. Since the seasonal ETc amounts exceed the current annual water allocation of 800 mm per hectare, accurate irrigation scheduling is crucial to ensure efficient irrigation water use. Means to estimate Kc values for the development and late stages are provided. A Kc of 0.79 applies during the mid and late stages until the onset of leaf fall.