Quantifying water use and nutritional water productivity of two sweet potato (Ipomoea batatasL.) cultivars grown in South Africa

Abstract

The most accurate way of quantifying water use or evapotranspiration (ET) is achieved through direct physical measurements. Therefore, quantifying water use of sweet potato using the eddy covariance (EC) system which is one of the direct methods, and calculating crop growth parameters under optimal crop growing conditions, will improve our understanding and crop management practices. Also water scarcity is becoming more and more of a threat to food and nutritional security. Therefore, it is important to produce higher nutrients per unit of water used in order to alleviate malnutrition and to conserve natural water resources. Thus, it is important to do field trials for this crop in order to measure ET and calculate crop coefficient (Kc) to increase transferability of information generated to other scenarios. This information will help to improve agricultural production and livelihoods in arid and semi-arid areas mainly through improving nutritional water productivity (NWP), and irrigation water management within the sweet potato industry. The aim of this study was to: (a) quantify ET dynamics for sweet potato (Ipomoea batatas) in order to determine FAO-type Kc which can be used to estimate crop water use for a range of growing conditions, and (b) to investigate the effects of deficit irrigation on storage root yield, water use efficiency (WUE), nutritional content (NC) nutritional yield (NY) and NWP of commonly cultivated sweet potato cultivars in South Africa (SA) [one orange-flesh (OFSP) ‘Bophelo’ and one white-flesh (WFSP) ‘Blesbok’]. An eddy covariance (EC) system containing energy balance (EB) sensors was installed in a 1.3 ha field with only OFSP variety ‘Bophelo’ in order to quantify the water use. A second trial was composed of 18 plots of 5 m × 4 m (20 m2), with a 2 m border between plots. Six treatment combinations of OFSP and WFSP and three water levels, termed full irrigation (FI), supplementary irrigation (SI) and rainfed (RF) were arranged in a randomized complete block design (RCBD) with three replications at the Agricultural Research Council–Vegetable and Ornamental Plants (ARC–VOP) Gauteng Province, SA. Daily ET varied between 0.5 to 5.5 mm (linked closely to canopy cover and weather conditions), with total seasonal ET measured at 361 and 347 mm for the 2014/2016 and 2015/2016 seasons, respectively. Averaged values of Kc were 0.46, 0.92 and 0.57 during the initial, middle and late growth stages, respectively. Final storage root yields were 32 t ha-1 (2014/2015) and 29 t ha-1 (2015/2016), with WUE of 89 and 85 kg ha-1 mm-1, respectively. The study was conducted during 2014/2015 and 2015/2016 seasons, from January to May in both seasons.