Water availability, demand and reliability of in situ water harvesting in smallholder rain-fed agriculture in the Thukela River Basin, South Africa

摘要

Water productivity in smallholder rain-fed agriculture is of key interestfor improved food and livelihood security. A frequently advocated approachto enhance water productivity is to adopt water harvesting and conservationtechnologies (WH). This study estimates water availability for potential in situWH, and supplemental water demand (SWD) in smallholder agriculture in SouthAfrica's Thukela River Basin (29 000 km2, mean annual precipitation550–2000 mm yr−1). The study includes process dynamics governingrunoff generation and crop water demands, quantification of predictionuncertainty, and an analysis of the reliability of in situ WH.The agro-hydrological model SWAT (Soil and Water Assessment Tool) wascalibrated and evaluated with the Sequential Uncertainty Fitting algorithmagainst observed discharge (at ten stations) and maize yield (the dominantcrop type) for the period 1997–2006. The water availability was based onthe generated surface runoff in smallholder areas. The SWD was derived froma scenario where crop water deficits were met from an unlimited externalwater source. The reliability was calculated as the percentage of years inwhich water availability ≥SWD. This reflects the risks of failureinduced by the temporal variability in the water availability and the SWD.The calibration reduced the predictive uncertainty and resulted in asatisfactory model performance. For smallholder maize yield, the Root MeanSquared Error was 0.02 t ha−1 during both the calibration and theevaluation periods. The width of the uncertainty band was reduced by23% due to the calibration. For discharge during the calibration(evaluation) period, the ten-station range in the weighted coefficient ofdetermination (Φ) was 0.16–0.85 (0.18–0.73), and in the coefficientof determination (R2) 0.42–0.83 (0.28–0.72). The calibration reducedthe width of the uncertainty band by 25% on average.The results show that the smallholder crop water productivity is currentlylow in the basin (spatiotemporal median: 0.08–0.22 kg m−3, 95%prediction uncertainty band (95PPU)). Water is available for in situ WH(spatiotemporal median: 0–17 mm year−1, 95PPU) which may aid inenhancing the crop water productivity by meeting some of the SWD(spatiotemporal median: 0–113 mm year−1, 95PPU). However, thereliability of in situ WH is highly location specific and overall ratherlow. Of the 1850 km2 of smallholder lands, 20–28% display areliability ≥25%, 13–16% a reliability ≥50%, and4–5% a reliability ≥75% (95PPU). This suggests that the risk offailure of in situ WH is relatively high in many areas of the basin.