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A correct estimation of crop coefficients (K_C) is essential to assess water requirements of crops from weather variables and thereby optimize irrigation management. K_C depends on crop type and varies with crop growth stage, and to a limited extent, with climate. K_C values are often assumed to be transferable between locations and climates. K_C 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 K_C 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 …