Evapotranspiration and crop coefficients of potato in a semi-arid climate (South Africa)
Correct estimation of crop coefficients is essential to accurately assess water requirements of crops and thereby optimize irrigation management. The objectives of this study were (i) to quantify evapotranspiration and water use efficiency of potato crops grown in different locations and seasons 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 system (ECV) was used in potato fields in three production regions of South Africa: the western Free State (summer crop), North West (spring crop) and Limpopo (winter crop). An IRGASON integrated open-path CO2/H2O gas analyzer - the ECV system (Campbell Scientific) was used to measure the H2O vapour fluxes above the crop canopy. It was integrated with a sonic anemometer, which measures the three-dimensional wind speed. Additional supporting sensors (fine wire thermocouple, NR-Lite net radiometer, silicon pyranometer, krypton hygrometer, CS616 reflectometer, Hukseflux heat flux plates) and a tipping bucket rain gauge were added to the system. Data were sampled at a frequency of 10 Hz, processed using EasyFlux DL software (Campbell Scientific), and recorded using a CR3000 datalogger. Seasonal mean crop evapotranspiration (ETc) was 4.6, 4.4 and 2.8 mm/day, respectively, for the summer, spring and winter crops. The reference evapotranspiration (ETo) correlated well with the daily patterns in ETc (r = 0.89, 0.56 and 0.76 for winter, spring and summer potato crops, respectively) and can be used for irrigation scheduling of potato. Seasonal mean Kc values varied considerably between the winter (1.00) and the spring (0.69) / summer (0.79) crops. This range is within the range reported from other parts of the world. The winter crop had the highest WUE of 3.2 kg dry potato tuber m3 of water evapotranspired, whilst WUE for the spring and summer crops was 2.9 and 2.2 kg/m3. The corresponding dry matter tuber yields were 12.0 t/ha (winter crop), 14.1 t/ha (spring crop) and 9.1 t ha/1 (summer crop). Our results suggest that ETo, which can be relatively easily estimated based on data from a weather station, can be used for irrigation scheduling of potato, but Kc values used to estimate crop evapotranspiration and irrigation requirements may need to be adjusted depending on the cropping season. Water use efficiency of potato was higher for the crop growing in the cooler winter compared to the crops growing in spring / summer. To optimize water use efficiency of potato in water-scarce areas that rely mostly on irrigation water for potato production, it is advisable to grow potato crops in the cooler season, outside the frost-prone period if possible.
Steps to reproduce
IRGASON integrated open-path CO2/H2O gas analyzer ECV system (Campbell Scientific) - data sampled at 10Hz frequency EasyFlux DL software (Campbell Scientific) AccuPAR LP-80 ceptometer (Decagon Devices) Automatic weather station (Campbell Scientific) Penman-Monteith equation for calculating reference evapotranspiration Pearson's correlation coefficient Energy balance closure to evaluate accuracy of ECV measurements