Be realistic about water savings under nets

Nets protect apples from sun, hail, sleet, and wind. But do they reduce water use? Preliminary data suggests yes, but less than elsewhere in the world. By Engela Duvenage.

Hortgro is co-funding a project with the Water Research Commission to study the effect of fixed or draped nets on water use in South African apple orchards.

The project has been led by Dr Sebinasi Dzikiti of the Department of Horticultural Science at Stellenbosch University since January 2021. The research team includes doctoral student Edward Lulane, also from the Department of Horticultural Science at Stellenbosch University, and collaborators from the Council for Scientific and Industrial Research, and the Agricultural Research Council.

Lulane has already collected data for two seasons from a Rosy Glow orchard under white fixed netting in the Witzenberg Valley. Data from one Golden Delicious orchard in the Witzenberg Valley and a second near Villiersdorp, both under black draped nets, have been collected for one season.

Aspects such as tree water use, soil-water dynamics and evaporation, and tree physiology were recorded, while the specific microclimate inside and outside the nets was assessed.

Lulane’s study leader, Prof. Stephanie Midgley, Climate Change and Risk Scientist at the Western Cape Department of Agriculture, says that international literature suggests that reduced water use of up to 30% can be expected under netting under certain conditions.

“We did not get near that,” she notes. Preliminary results show that seasonal transpiration under the white fixed nets was reduced by 9%–14% and seasonal evapotranspiration by 8%–10%. “Higher water savings during certain stages of the season are possible, but some data issues still require further analysis before we can say for sure,” adds Midgley.

During the first study season, water use was reduced by 15%, and evapotranspiration by 13% under black draped nets.

Midgley stresses that these percentages are not nothing. “Any potential saving is never trivial in a water-stressed production region.”

The preliminary findings show that reduced transpiration was the result of lower exposure to solar radiation under nets. Water loss was 12%–15% less for trees under fixed netting than for ones growing in an adjacent open orchard. Netting also reduced the wind speed by up to 56%.

Only about a 1% difference in the average air temperature was measured between the netted and the open orchard. No difference in relative humidity or vapour pressure deficit was observed.

Black draped nets reduced sunlight by 30% during the months these were used, compared to sunlight exposure of trees in open control rows. This led to a decrease in the reference potential evapotranspiration of at least 30% in the trees under nets.

The project is set to continue until 2022.

Image: Data collection in an apple orchard.

Supplied by Stephanie Midgley | Western Cape Department of Agriculture.