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 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 (ET_C) was 3.2 mm d⁻¹ for the crop in Limpopo and 5.7 mm d⁻¹ in North West. The reference evapotranspiration (ET_O) correlated well with the daily patterns in ET_C for most of the season. ET_O thus serves as a useful indicator of ET_C and can be used for irrigation scheduling of potato. Seasonal mean K_C value was 0.99 for the crop in Limpopo and 0.78 in North West. While the K_C 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.

作物系数 (K _C )的正确估计对于根据天气变量评估作物的需水量至关重要，从而优化灌溉管理。K _C取决于作物类型并随作物生长阶段而变化，并在一定程度上随气候变化。K _C值通常被假定为可在地点和气候之间转移。在南非半干旱气候条件下种植的现代马铃薯品种的K _C值尚未确定。本研究的目的是 (i) 量化灌溉条件下半干旱气候下马铃薯作物的蒸发蒸腾和水分利用效率 (WUE)，(ii) 估计 K _C这些作物在不同生长阶段的值，以及 (iii) 评估 Penman-Monteith 方程在估计蒸散量和灌溉需求方面的有用性。在南非的两个产区种植了 Mondial 品种的马铃薯田，使用了涡流协方差 (ECV) 系统：林波波省（冬季作物生长）和西北地区（春夏作物）。系统中增加了额外的传感器来测量相对湿度、近地表土壤温度、太阳辐射、降雨和灌溉。Decagon 10 HS 电容传感器安装在不同的土壤深度，用于测量马铃薯生根区土壤水分含量的变化。通过遵循能量平衡闭合方法评估 ECV 测量的准确性。季节性平均作物蒸散量 (ET _C) 是林波波省作物的3.2 mm d ^-1和西北地区的5.7 mm d ^-1。参考蒸散量 (ET _O ) 与大部分季节的ET _{C 中}的日模式相关。因此，ET _O可作为 ET _C的有用指标，可用于马铃薯的灌溉调度。林波波省作物的季节性平均 K _C值为 0.99，西北地区为 0.78。虽然林波波省的 K _C值可能代表了没有水分胁迫的作物，但西北地区的作物生长可能在某种程度上受到可用水量的限制。冬季作物的 WUE 最高，为 3.55 kg 干马铃薯块茎 m ^-3蒸发蒸发的水量，而春夏作物的 WUE 为 3.03 kg m ^-3。这种差异可以用生长季节之间平均蒸汽压差的差异来解释。为了优化在南非等半干旱缺水地区种植马铃薯的 WUE，建议在霜冻期以外的最凉爽的生长季节种植作物。