Quantifying the Water Saving Benefits of Fixed White Protective Net in Irrigated Apple Orchards Under Mediterranean-Type Climate Conditions

Apple (Malus domestica Borkh.) producers are increasingly using nets to address climate and pest-related challenges. In the water-scarce Western Cape region of South Africa, with its winter rainfall climate, all commercial apple orchards are irrigated. Quantitative information on the water saving benefits of nets in apple orchards and possible adjustments needed in irrigation practices, is scarce. This lack of knowledge may result in over irrigation in netted orchards, unnecessary production costs and decreased water productivity, and may hamper the mitigation of reduced water availability during droughts. This study quantified the water use over two seasons of commercially-grown high yielding ‘Rosy Glow’ apple trees under fixed white net compared to trees in the open (control). The adjacent orchards for the two treatments were irrigated throughout the growing season using micro sprinkler irrigation and were scheduled separately. Transpiration was quantified using the heat ratio method of monitoring sap flow, while orchard evapotranspiration (ET) was determined using soil water balance modelling. Microclimate was measured using automatic weather stations placed within the control and netted orchards. The net treatment reduced solar radiation by 12% compared to the control, in both seasons, while the reference ET declined by 9%. Total transpiration in the first season under the net amounted to 5 330 m3 ha-1, an 8% reduction compared to 5 810 m3 ha-1 in the control. In the second season, a seasonal total transpiration of 5 360 m3 ha-1 was estimated under the net, a 14% reduction compared to 6 250 m3 ha-1 in the control. In this season, the seasonal total ET was 8 314 m3 ha-1 for the net treatment compared to 8 649 m3 ha-1 for the control. The orchards had a crop water productivity (WPc) of 13.4 and 13.2 kg m-3 for the net and control treatments, respectively. Furthermore, the mid-season crop coefficient (Kc) was 1.07 and 1.13 for the net and control treatments, respectively. Apple growers in warm and water-scarce climates should consider the use of nets not only for the control of sunburn and hail damage, but also to reduce orchard water use by reducing transpiration and evapotranspiration. Furthermore, where protective nets are installed, separate soil water monitoring and irrigation scheduling must be provided since water requirements will differ from open orchards.