Exploring the potential for the use of remote sensing technology and GIS to aid the upscaling of rainwater harvesting in Sub-Saharan Africa.

Abstract

Increased strain on water resources across the globe, and particularly in Sub-Saharan Africa, has resulted in increased vulnerability of those communities who rely directly on rainfall to sustain their livelihoods, through crop production, water for drinking and domestic purposes and other economic activities. This dynamic interface between people and the environment is central to the current decadal research theme of the International Association of Hydrological Sciences (IAHS) “Panta Rhei” – everything flows, emphasises that greater recognition and understanding of the interconnection between human action and water resources, and how in order for development plan to be sustainable they must take greater cognisance of the dynamic interface between people and the environment. Applying this philosophy to the subject of RWH suggests an alternative approach to the traditional guidelines for assessing RWH suitability approach. A review of the conditions under which RWH currently take pace was done and found that guidelines often only prescribe optimal conditions for RWH which results in many sites which may be suitable being over looked. Results show that RWH is taking place under a much broader range of conditions than those recommended by the guidelines. An alternative approach was investigated which rather aims to assess how much water a selected RWH system can supply in any location, applied at a regional scale across the whole of South Africa, under both present and shifting climate conditions as well as optimising the water storage tank to secure a certain level of supply. Results showed that the eastern portions of South Africa were best suited to RWH with supply being secured for 100 -200 days of the year. However this also highlighted that a multiple source water supply system, which can dynamically adjust to supply water from different sources depending on water supply, will be more sustainable. This will allow water demand for different uses to be satisfied for different supplies, rather than a conventional piped water supply system which provides one quality of water, often drinking water standard, for domestic consumptions where up to 70% of water use is not used for direct consumption. In order to design a dynamic sustainable system, continuous monitoring is needed to understand the constant changes in the system. One such monitoring tool gaining popularity in water resources is remote sensing (RS). RS technology was used to calculate total evaporation (ET) and the normalized difference vegetation index (NDVI) as indicators of the current implementation of RWH. This allows for a census technique to monitor the extent and uptake of RWH systems as well as evaluate the performance of different systems in increasing soil water or water available to plants. Results show that large scale techniques such as the spate irrigation in Tanzania or mass implementation of smaller techniques, such as the “Zai Pits” and contour bunds in Burkina Faso were visible from calculated ET maps. The contour bunds were the most successful in storing water, in the soil profile, for plant use with higher ET being measured from the bunded system compared to the surrounding landscape well into the dry season. However, the fields irrigated by micro-basin plastic storage tank systems in South Africa were not visible from ET maps but were visible from NDVI maps in summer. RS is also used to monitor the extent of less transient factors, such as slope and soil types, which influence the runoff potential that can be generated and then stored. Using RS at a catchment or sub-catchment scale will allow planners to evaluate the runoff potential of a landscape and design a RWH system that can sustainably capture and utilise that runoff. RS can also be used to monitor the impacts of the RWH system on the landscape by continuously monitoring the changes in ET, NDVI, soils and slope over time. RS provides a cost and time effective method for doing this from a remote location.