The effects of rainwater harvesting on agrohydrological ecosystem goods and services in the smallholder farming community of Potshini, South Africa
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Water harvesting techniques offer subsistence farming communities the ability to enhance water supply and management, leading to increased food and water security and health. Though the benefits seem obvious, a lack of understanding surrounds the local effects of these activities on natural systems, and the increasing prevalence of water harvesting practices invites a closer look. This study employs agrohydrological modeling to simulate changes in land use and water management and uses the outputs to assess the potential impact of upscaling rainwater harvesting on selected agricultural and hydrological ecosystem goods and services. Potshini, a well-studied rainwater harvesting community in the foothills of the upper Drakensberg range in KwaZulu-Natal, South Africa, was used as a case study. A detailed land cover map was created in order to accurately simulate the small scale of the Potshini catchment (9.643 km2) and subcatchment boundaries were delineated according to streamflow monitoring instrument locations. Model scenarios were developed according to current and realistic future expansion of rainwater harvesting practices which were used to assess the effects on ecosystem goods and services. Selected goods and services include water supply and water regulation (in terms of quantity, quality, location, and timing), food production (by harvested rainwater fed vegetable gardens), and raw materials (natural grassland primary production of grass roofing materials). Indicators included water flow rates, timing, and distribution, crop yields, irrigation demand, plant stress factors, reservoir storage, and net primary production of grasslands. Results suggest that water supply is enhanced for crops while environmental water flows are reduced. Water quality indicators imply a decline in surface and shallow ground water resources, though harvested rainwater provides an alternative and flexible option for fresh water. Distribution of water is altered to increase relative surface flow, though overall decreases also suggest additional evapotranspiration and soil water retention. Timing of extreme flow events is adjusted and dry season low flow periods are extended, although rainwater storage enables flexibility in irrigation schedules. Food production is increased, while production of grass roofing materials is slightly reduced. In several cases indicators of water supply and regulation under rainwater harvesting trend towards those of the historical undeveloped scenario, suggesting a hydrological regime shift approaching the natural environmental state. Contributions are made towards a more accurate understanding of small catchment impacts of rainwater harvesting, and the tools and methods developed hold potential for future use in investigating other aspects of natural processes or alternative future scenarios in Potshini, as well as application to other locations with under similar conditions.