Potential impacts of water harvesting and ecological sanitation on crop yield, evaporation and river flow regimes in the Thukela River basin, South Africa

• Published in: Agricultural water management 2011-05, Vol.98 (7), p.1113-1124
• Main Authors: Andersson, Jafet C.M., Zehnder, Alexander J.B., Rockström, Johan, Yang, Hong
• Format: Article
• Language: English
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; Animal, plant and microbial ecology; Biological and medical sciences; canopy; crop yield; Dry-spell; Dry-spell Evapotranspiration Water productivity Resilience; ecosystems; Evaporation; Evapotranspiration; Fertility; Freshwater; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Harvesting; Maize; nitrogen; nutrient availability; Resilience; risk; River basins; River flow; rivers; sanitation; Soil (material); Soil and Water Assessment Tool model; soil fertility; Soil moisture; soil water; space and time; transpiration; urine; Water balance and requirements. Evapotranspiration; water harvesting; Water productivity; watersheds
• ISSN: 0378-3774; 1873-2283; 1873-2283
• DOI: 10.1016/j.agwat.2011.02.004

► Soil fertility mainly constrains smallholder maize yield in the Thukela River basin ► Fertility enhancements increase yield magnitudes but also the yield variability ► Soil moisture enhancements improve the yield stability ► Human urine fertilization increases yield and transpiration throughout the basin ► In situ water harvesting is beneficial in some locations and for the lowest yields. In this study we explore the potential impacts of two strategies, namely in situ water harvesting ( in situ WH) and fertilisation with stored human urine (Ecosan), to increase the water and nutrient availability in rain-fed smallholder agriculture in South Africa's Thukela River basin (29,000 km 2). We use the soil and water assessment tool (SWAT) to simulate potential impacts on smallholder maize yields, river flow regimes, plant transpiration, and soil and canopy evaporation during 1997–2006. Based on the results, the impacts on maize yields are likely to be small with in situ WH (median change: 0%) but significant with Ecosan (median increase: 30%). The primary causes for these effects are high nitrogen stress on crop growth, and low or untimed soil moisture enhancement with in situ WH. However, the impacts vary significantly in time and space, occasionally resulting in yield increases of up to 40% with in situ WH. Soil fertility improvements primarily increase yield magnitudes, whereas soil moisture enhancements reduce spatial yield variability. Ecosan significantly improves the productivity of the evaporative fluxes by increasing transpiration (median: 2.8%, 4.7 mm season −1) and reducing soil and canopy evaporation (median: −1.7%, −4.5 mm season −1). In situ WH does not generally affect the river flow regimes. Occasionally, significant regime changes occur due to enhanced lateral and shallow aquifer return flows. This leads to higher risks of flooding in some areas, but also to enhanced low flows, which help sustain aquatic ecosystems in the basin.