Assessing evapotranspiration and crop coefficients of potato in a semi-arid climate using Eddy Covariance techniques

Abstract

A correct estimation of crop coefficients (KC) is essential to assess water requirements of crops from weather variables and thereby optimize irrigation management. KC depends on crop type and varies with crop growth stage, and to a limited extent, with climate. KC values are often assumed to be transferable between locations and climates. KC values have not been determined yet for modern potato cultivars grown under South African, semi-arid climatic conditions. The objectives of this study were (i) to quantify evapotranspiration and water use efficiency (WUE) of potato crops in a semi-arid climate under irrigation, (ii) to estimate KC values for these crops at different growth stages, and (iii) to assess the usefulness of the Penman-Monteith equation to estimate evapotranspiration and irrigation requirements. An eddy covariance (ECV) system was used in potato fields planted with the variety Mondial in two production regions of South Africa: Limpopo (crop growing in winter) and North West (spring-summer crop). Additional sensors were added to the system to measure relative humidity, near surface soil temperature, solar radiation, rainfall and irrigation. Decagon 10 HS capacitance sensors installed at varying soil depths were used to measure the change in soil moisture content of the potato rooted zone. Accuracy of the ECV measurements was evaluated by following the energy balance closure method. Seasonal mean crop evapotranspiration (ETC) was 3.2 mm d−1 for the crop in Limpopo and 5.7 mm d−1 in North West. The reference evapotranspiration (ETO) correlated well with the daily patterns in ETC for most of the season. ETO thus serves as a useful indicator of ETC and can be used for irrigation scheduling of potato. Seasonal mean KC value was 0.99 for the crop in Limpopo and 0.78 in North West. While the KC value in Limpopo likely represented that for a crop free of water-stress, the crop growth in North West was likely somewhat limited by water availability. The winter crop had the highest WUE of 3.55 kg dry potato tuber m−3 of water evapotranspired, whilst WUE for the spring-summer crop was 3.03 kg m−3. This difference could be explained by differences in mean vapour pressure deficit between the growing seasons. To optimize WUE of potato growing in semi-arid, water scarce regions such as South Africa, it is advisable to grow crops in the coolest available growing season, outside the frost-prone period.