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

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.The research reported here formed part of a bigger project entitled “Quantifying water use by high performing apple orchards in the winter rainfall areas of South Africa” project no WRC K5/2398.The Water Research Commission (WRC), with additional funding from Hortgro Science, through the South African Apple and Pear Producers Association (SAAPPA) in South Africa.http://www.actahort.orgam2017Plant Production and Soil Scienc

Estimating the water requirements of high yielding and young apple orchards in the winter rainfall areas of South Africa using a dual source evapotranspiration model

Exceptionally high yielding (>100 t ha−1) apple orchards (Malus domestica Borkh.) are becoming common in South Africa and elsewhere in the world. However, no accurate quantitative information currently exists on the water requirements of these orchards. Information is also sparse on the water use of young apple orchards. This paucity of data may cause inaccurate irrigation scheduling and water allocation decisions, leading to inefficient use of often limited water resources. The aim of this study was therefore to investigate the dynamics of water use in eight apple orchards in South Africa planted to Golden Delicious and the red cultivars i.e. Cripps’ Pink, Cripps’ Red and Rosy Glow in order to understand how canopy cover and crop load influence orchard water use. Four of the orchards were young (3–4 years after planting) and non-bearing, while the other four were mature high yielding orchards. Transpiration was monitored using sap flow sensors while orchard evapotranspiration (ET) was measured during selected periods using eddy covariance systems. Scaling up of ET to seasonal water use was done using a modified Shuttleworth and Wallace model that incorporated variable canopy and soil surface resistances. This model provided reasonable estimates in both mature and young orchards. The average yield in the two mature ‘Cripps’ Pink’ was ∼110 t ha−1 compared to ∼88 t ha−1 in the ‘Golden Delicious’ orchards. However, average transpiration (Oct-Jun) was ∼638 mm for the ‘Cripps’ Pink’ and ∼778 mm in the ‘Golden Delicious’ orchards. The peak leaf area index was ∼2.6 and ∼ 3.3 for the mature ‘Cripps’ Pink and ‘Golden Delicious’ orchards. So, canopy cover rather than crop load was the main driver of orchard water use. Transpiration by the young orchards ranged from 130 to 270 mm. The predicted seasonal total ET varied from ∼900 to 1100 mm in the mature orchards and it was ∼500 mm in the young orchards. Orchard floor evaporation accounted for ∼18 to 36% of ET in mature orchards depending on canopy cover and this increased to more than 60% in young orchards.The Water Research Commission of South Africa (Project no WRC K5 2398//4), the South African Apples and Pears Producers Association and the South African Parliamentary Grant to the CSIR on Water Security (Project number ECHS043).http://www.elsevier.com/locate/agwat2019-09-30hj2018Plant Production and Soil Scienc