Rainwater harvesting systems and their influences on field scale soil hydraulic properties, water fluxes and crop production

Abstract

South Africa, in common with many parts of Sub-Saharan Africa, is facing increasing water shortages. Limited available water arising from a low and poorly distributed rainfall, must supply domestic, agricultural, industrial and ecosystem needs. Agricultural activities of smallholder farmers, who largely occupy arid to semi-arid areas, are rainfall-driven as they do not have the capacity to develop conventional water sources, such as boreholes and large dams. This situation has led to persistent food shortages, low income and a lack of investments, resulting in high dependency levels of which examples include over reliance on social grants, household crop production that largely relies on external inputs and availability of cheap unskilled labour. A growing global perception that water for agriculture has low value relative to other value uses could further jeopardize the already over exploited agricultural water. Developing economies such as South Africa are likely to favour, in terms of water allocation, e.g. electricity generation through steam turbines relative to irrigation needs because industry plays a more significant role in the economy. While substantial scientific research has resulted in enhanced yields through in-situ water harvesting and soil and water conservation, as well as crop and soil fertility management and plant breeding, less work has been done to assess the impact of intermittent dry spells on crop yield, particularly with regard to smallholders. Indeed, the interventions that have been promoted to smallholders may provide little buffer against such events. In addition, the increase in yield from many such efforts has been marginal and inconsistent, leading some to conclude that semi-arid environments are hydrologically marginal, have no significant agricultural potential and any attempts to intensify agricultural activities would lead to severe environmental degradation. This study investigated the rainwater harvesting and storage potential among rainfed farmers in a summer-rainfall region of South Africa. The influences of this practice on soil hydraulic properties, water fluxes and crop production is detailed in subsequent chapters. Using historical meteorological data, this study commenced with an investigation of the factors that influence the length of maize (Zea Mays L.) growing seasons notably the prevalence of early season dry spells and late season low temperature which could be responsible for persistent low maize yields amongst smallholder rainfed farmers (Chapter 2). An increasing trend of dry spells was observed which was found to influence sowing dates and the length of the growing season. The influence of no-tillage (NT) as an intervention to secure more root-zone soil moisture was investigated in comparison to conventional tillage (CT) practices. Field experiments, with the aim of quantifying the extent to which water productivity and yields can be improved among smallholder rainfed farmers in the Potshini catchment, Thukela basin; South Africa (Chapter 8), were conducted during both the dry and growing seasons from 2005/06 – 2007/08 seasons at four sites with similar soil textural properties and slopes. Each site was developed as a runoff plot and was fitted with moisture and runoff measuring devices. Meteorological parameters were measured from a weather station installed nearby. A snapshot electrical resistivity survey was used to compliment soil moisture profiling. The analyses of the different measurements provided information on various water flow paths and potential downstream hydrological effects (Chapter 3). The average cumulative runoff was 7% and 9% of seasonal rainfall in NT and CT treatments over the three seasons. Changes over time in soil hydraulic properties due to tillage were examined at two depths through infiltration tests and determination of their bulk densities. These included changes in steady state infiltration rate and hydraulic conductivity (Chapter 4), interaction between soil infiltration and soil characteristics (Chapter 5) and water conducting porosity and water retention (Chapter 6). In 50% of the sites, NT treatments showed significantly higher hydraulic conductivity compared to CT treatments. In response to an unexploited opportunity identified to produce vegetables in winter, an assessment of the potential for runoff water harvesting systems using polyethylene lining as an alternative cost-effective construction method for underground rainwater storage systems, particularly in areas where groundwater levels fluctuate rapidly was undertaken (Chapter 7). The process from conceptualization through design, construction and utilization of the stored water is described and recommendations for the design and construction of such systems made. Finally, various case studies which highlight the potential impact of improved soil profile moisture storage, the additional benefits of water stored in tanks and recommendations for tailored policies to support household food and income generation are made