Estimating evapotranspiration and determining crop coefficients of irrigated sweet potato (Ipomoea batatas) grown in a semi-arid climate

An open-path eddy-covariance system containing energy balance sensors was installed in a 1.3 ha field to estimate ET of orange-fleshed sweet potato. The study was conducted during the 2014/15 and 2015/16 growing seasons. This is the first study of its kind in South Africa, and first on locally developed OFSP cultivar ‘Bophelo’. The average seasonal ET obtained was 356 mm. The time-average Kc values were 0.47 during the initial stage (39 days), 0.47 – 0.97 during the development stage (25 days), 0.97 during middle stage (25 days) and 0.44 during late growing stage (25 days). A heat unit based thermal time equation was successfully developed and validated using three-day running average Kc values. [Display omitted] •Daily sweet potato water use was highly affected by changes in canopy cover and weather conditions.•Time-average crop coefficients were derived for each growing period.•A heat unit based thermal time – crop coefficient equation was successfully developed and tested. Accurate quantification of crop water use or evapotranspiration (ET) is crucial in agriculture for improved irrigation scheduling and water resource planning across a wide range of farming conditions. The aim of this study was to quantify ET dynamics for irrigated sweet potato (Ipomoea batatas) in order to determine the FAO-type single crop coefficients (Kc) that can be used in models to estimate crop water use under a wider range of semi-arid climatic conditions. An open-path eddy-covariance system containing energy balance sensors was installed in a 1.3 ha field to estimate ET of orange-fleshed sweet potato. The study was conducted during the 2014/15 and 2015/16 growing seasons. Daily ET varied between 0.5 and 5.5 mm (linked closely to canopy cover and weather conditions), with total seasonal measured ET of 361 and 347 mm for the 2014/15 and 2015/16 seasons, respectively. Time-averaged Kc values of 0.47 for the initial stage, 0.97 for the mid-season stage and 0.44 for the late growth stage were derived from this study. In addition, a heat unit based Growing Degree Days - Kc equation was successfully developed and validated in this study to adjust Kc values of sweet potato to specific climates. These Kc values can be used in combination with the FAO-56 reference evapotranspiration (ETo) to estimate site-specific sweet potato ET, which is vital for improving water management of irrigated sweet potato cropping systems growing under semi-arid conditions.