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 highyielding
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.
