Measurement and modelling of water use by high yielding apple orchards and orchards of different age groups in the winter rainfall areas of South Africa

Abstract

Apple production for the export market is in South Africa entirely dependent on irrigation. In recent years, high performing orchards yielding up to 120 t of fruit ha-1 are becoming common in a country where the average yield is between 60 and 80 t ha-1. This raises important questions regarding the sustainability of the exceptionally high yields given the limited availability of water for irrigation. Moreover, there is also no accurate quantitative information published on water use by apple orchards of different age groups and this compromises efficient irrigation scheduling. The aim of this study was to quantify water use by high yielding apple orchards and orchards of different age groups. Data were collected in four commercial orchards, two planted to non-bearing ‘Cripps’ Pink’ and ‘Golden Delicious’ apples and another two high-yielding full-bearing orchards of these cultivars. Transpiration in the full-bearing orchards was measured using the heat pulse velocity sap flow method. Granier probes were used on the young non-bearing trees. Orchard evapotranspiration (ET) was measured using eddy covariance systems during selected periods. Ancillary data which included the orchard microclimate, stomatal resistance, soil water content and soil evaporation were also collected. The full-bearing ‘Golden Delicious’ orchard (22-year-old) had the highest seasonal transpiration of 785 mm, followed by the fullbearing ‘Cripps’ Pink’ (9-year-old) which transpired 587 mm. The non-bearing ‘Cripps’ Pink’ (3-year-old) transpired 272 mm compared to 198 mm for the nonbearing ‘Golden Delicious’ (2-year-old). The data were used to validate a dual source ET model based on the Shuttleworth-Wallace method. Transpiration of full-bearing orchards was accurately predicted by the model with the RMSE of 0.55 mm d-1 for ‘Cripps’ Pink’ and 0.70 mm d-1 for the ‘Golden Delicious’ orchards. Improvements to the substrate evaporation sub-model are required to account for various orchard floor management practices.