Evapotranspiration of a rain-fed sweet cherry orchard in Eastern Free State, South Africa

Evapotranspiration measurements of fruit trees can assist in improving irrigation scheduling, and consequently for sustainable water resources management of orchards. The purpose of this study was to assess how evapotranspiration and precipitation variability are influencing harvesting dates of sweet cherry (Prunus avium L.) trees. The study was conducted in Eastern Free State, South Africa on a 25-year old rain-fed sweet cherry orchard over two dry seasons (2017/2018). The reference evapotranspiration (ETo) was computed using the Penman-Monteith equation. Measurement of crop evapotranspiration (ETc) rates were done using an open path eddy covariance system (OPEC). The latent (LE) and sensible (H) heat flux data were selected according to the footprint analysis and to the energy balance closure error. The total amount of rainfall for 2018 was 37 mm much higher than that of 2017. However, the monthly average ETo estimation increased by 12mm. Below normal rainfall was observed during the two periods as compared to the long-term mean rainfall of the area. The ETc for October and November ranged between 2.2 to 3.2 mm day(-1) for 2017 season, and 2.4 to 3.49 mm day(-1) for 2018 season, with an averaged value for the two seasons together of 2.8 mm day(-1). The daytime ratio of H/LE increased from 1.05 to 1.35 (2017) and 1.23 to 1.74 (2018), while the relationship between ETc and ETo decreased from 0.30 to 0.20 in both growing seasons. The harvesting date for 2018 was delayed by one week (started on 24 October) as compared to 2017. Therefore, there is a need for introducing irrigation during the early stage of fruit development for below-average rainfall conditions with frequent heatwaves.