Quantifying water saving benefits of fixed white protective netting in irrigated apple orchards under Mediterranean-type climate conditions in South Africa

Abstract

• Transpiration based crop water productivity (WP) is higher under nets compared to open orchards. • The water saving benefits of fixed nets are smaller than expected when considering ET rates possibly because of wetter soils under the nets. • There is merit in adjusting crop coefficients for irrigation scheduling under nets given the significant differences in tree transpiration rates. 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, all commercial apple orchards are irrigated. Based on limited information on tree water relations under protective nets, we hypothesised that significant water savings and improvements in crop water productivity could be achieved under nets. The lack of accurate quantitative information on water use under nets may perpetuate sub-optimal irrigation practices partly due to inaccurate crop coefficients. This study quantified tree transpiration (T) using the heat ratio sap flow method and orchard evapotranspiration (ET) using the soil water balance approach in mature irrigated ‘Rosy Glow’ apple orchards under fixed white net compared to an open (control) over two seasons. Differences in water use between treatments were analysed by comparing microclimates, plant water status, root zone soil moisture, and irrigation levels. Averaged over the two seasons, the nets reduced daily total solar radiation by ∼ 12%, wind speed by more than 36%, and reference evapotranspiration by ∼ 12%. Seasonal T was 11% lower under nets, while ET was only 4% lower. The nets reduced irrigation water requirement by 4%. Peak mid-season basal crop coefficients (K cb ) were 0.54±0.02 and 0.59±0.02 while there were no differences in the single crop coefficients (K c = 1.07±0.04 and 1.13±0.06) for the nets and control treatments, respectively. Transpiration based crop water productivity (WP) was higher under nets (20.7 kg m − 3 ) compared to 18.2 kg m − 3 in the open. There were no differences in the ET-based WP values (13.4 and 13.2 kg m − 3 ) for the nets and control treatments, respectively. This study suggests that water saving benefits of fixed nets are smaller than expected when considering ET rates because of more active ground cover under nets. But there is merit in adjusting crop coefficients for irrigation scheduling under nets given the much lower transpiration.