Geographical information system (gis) mapping: guides for rainwater harvesting potential These maps are guides for the "best bet" selection of areas suitable for the implementation of different rainwater harvesting technologies. NB: other factors, such as socio and economic, have also to be taken into account. Please download the full report:

Rainwater harvesting: in situ In-situ RWH refers to all activities in which rain water is harvested and stored within the soil profile for crop production. It may include open-sky RWH systems such as terraces, pitting methods such as zai, bunding methods such as majaluba and large external catchment systems such as trapezoidal bunds. Since water is stored in the soil, the main criteria for mapping is that the area may be in need to soil moisture replenishment, at some point within the year. After lengthy deliberations with peer reviewers, it was agreed that wet areas could still benefit from in-situ RWH due to climatic variability. Thus, areas with annual rainfall in the range 200-1200 mm rainfall were considered in need of in-situ RWH. By removing protected areas and areas with saline soils, it was assumed that the rest of the land had agricultural potential and was therefore eligible for in-situ RWH as there was no reliable data showing agricultural areas for all of Africa. Topography was considered a non issue, since it can be altered by terracing or bunding. This means that large areas in Africa can adapt in-situ RWH.

Rainwater harvesting map: landuse It was not possible to get full coverage at good resolution for land use/cover for full Africa coverage, but Africover data were available for some of the countries. Land use/cover maps were used for delineating agricultural areas and those areas used for special purposes, e.g. forests. This layer was used in developing domains for in-situ RWH. Africover data was used for individual country maps.

Rainwater harvesting map: Population Africa-wide digital data on population were obtained from FAOSTAT (resolution 5 km), and a classification scheme was developed that allowed a minimum of one household per sq. km as the minimum population density for RWH. This decision was arrived at after several trials with higher values. It was noted that the nature of population density in Africa is such that with the exception of mining and urban areas, population in the rural areas is concentrated in the wetter, better agriculturally endowed areas in most countries. Wet areas are also likely to be well served with basic infrastructure, including piped water, and irrigation schemes. On the contrary, it is the drier areas, where perennial rivers are scarce and far between, ground water is usually expensive or difficult to exploit (usually saline too), and where the most viable source of water becomes RWH, but where population density is sometimes lowest. The nature of ASALs is such that settlements are scattered, each holding only a small population and therefore, supply of piped water can be uneconomical and difficult- the very reason RWH becomes the most viable alternative. ASALs also hold large numbers of livestock that require watering, and this may not be evident from the human population statistics. There are limitations in using population density to prioritize areas for RWH interventions because the areas with the lowest population (ideally the lowest priority) tend to be the most disadvantaged and where RWH would have the greatest impact.

Rainwater harvesting map:Rainfall is the main ingredient in RWH. Ideally, seasonal rainfall would have been more useful but due to the heterogeneity of the continent, available continent-wide spatial data was on mean annual rainfall, which was utilized. In terms of classification, annual rainfall below 200 mm shows deserts with low population and high risks of production. Due to the need to target areas where there is huge incremental benefit in RWH, areas receiving 400-1200 mm are considered most optimal. Above 1200 mm, RWH for crop production is not a necessity except for drinking water.

Rainfall/ET index Agro-climatic zone (ACZ) maps give an indication of the inherent risks to rainfed crop production and therefore provide some basis for setting criteria for mapping in-situ RWH systems, sand dams, and even pans/ponds. This is because ACZ maps have been created by combining important climatic variables (rainfall, relief, temperatures). However, this database could not obtain reliable ACZ maps for Africa except for some of the countries and only ET digital data were available for Africa scale. Thus Rainfall/ET were calculated and used. By setting Rainfall/ET of 60% as threshold for wet areas, this index was used in delineating areas considered too wet to contain sand rivers, and hence used in delineating areas suitable for sand/sub-surface dams.

Rainwater harvesting map: Rooftop RWH is one of the easiest ways of providing drinking water at household level. For instance, for rural households in Africa, in terms of rainfall availability for roof water harvesting, an area receiving just 200 mm annual rainfall has as much potential (and higher priorities) as one receiving 2,000 mm. For example, simple arithmetic assuming per capita rural water consumption at 20 litres/day shows an annual water demand of 7.3 m3 per person/year, which could be supplied by a roof catchment of 36.5 m2, if only 200 mm of rainfall per annum were available. Therefore all that is required is the presence of roofs to provide the necessary catchments. In countries where settlements have been mapped such as Kenya, it is possible to show where rooftop RWH can be targeted. By applying mapping masks (a mask hides unwanted information, e.g. settlements having piped water), it is possible to prioritize where rooftop RWH would be most opportune. In countries spatial lacking data on settlements, it was assumed that all areas have a potential for rooftop RWH as long as annual rainfall is at least 200 mm. High rainfall areas do not necessarily preclude the need for rooftop RWH because of poor levels of development, and neither do low population areas, where scattered settlements may mean centralized piped systems are uneconomically viable.

Rainwater harvesting map: The potential of RWH and storage in small ponds and pans refers to collection of runoff from open surfaces, such as roads, home compounds, hillsides, open pasture lands and may also include runoff from watercourses and gullies. Therefore this is an intervention that could be implemented almost anywhere, so long as local site conditions are permit. For the GIS mapping, only areas with steep slopes (> 8%) and areas with very low rainfall (< 200 mm) were removed. In general, runoff harvesting into ponds depends also on soil type and geology, especially to avoid seepage problems. As there was no continent-wide spatial data showing the relevant soil properties, e.g. low permeability, the aspect of soil types could not be incorporated in the GIS analyses, and therefore the maps obtained are rather too inclusive. Since seepage can be controlled in water pans/pond through different interventions, and this fact should be acknowledged in using the maps.

Rainwater harvesting map: The potential of sand/subsurface dams in an area is a function of the availability of sand rivers, topography that allows construction of weirs, geology to suit storage structures and the presence of a population to make use of the water. Sand and sub-surface dams are small-sized RWH structures, whose location in ephemeral sand-river beds demands detailed surveys, preferably with thorough ground truth. Ideally, there is no realistic way to show the suitability of their location in low resolution GIS databases but since this database is for awareness creation, it is possible to show that sand/subsurface could be applicable along various watercourses on a map, by removing those water-courses and land units where the intervention may not be applicable. Moreover, most sand rivers tend to be in ASAL areas where population density is very low. Site selection is based on availability of settlements rather than population density. Thus, in this database, the potential for sand/subsurface dams was shown by a combination of Rainfall/ET index (<60%) and presence of ephemeral rivers. It was noted that much smaller streams did not show on the database due to scale limitations. This mapping also generally depicts the potential for flood runoff harvesting from watercourses by any other type of storage, including weirs, small earthen dams and ponds in the dry areas.

At the Africa scales, data were available for ephemeral rivers for some of the countries, although it does not distinguish which ones may be sand rivers. At country scale, the maps for Kenya and Tanzania when queried were found to represent sand rivers relatively well on removing humid areas. However, for Ethiopia, some of the ephemeral rivers lie above 2000 m.a.s.l, and may not be sand rivers. It was not possible to use soil maps to identify “sandy soils” as suitable catchments, and therefore areas likely to host sand dams as this was found to be unreliable. This is because the sand in a sand-river is the product of a long journey along the river profile and may not necessarily come from the surrounding land. The idea to use sub-basins to identify areas suitable for sand dams had been suggested by some members of the team. However, it should be noted that sub-basins simply comprise smaller basins containing the full lengths of rivers that drain to a major river. Many sub-basins emanate from the head-waters, usually at mountain water towers, and therefore, a sub-basin would include from top to bottom of a given river/stream, including areas where river profile does not contain sand. Consequently, use of sub-basins would inadvertently introduce errors and was abandoned, in favour of the rainfall/ET index to delineate ASAL areas.

Rainwater harvesting map: The slope of land is important in site selection and implementation of all ground-based RWH systems, especially ponds, pans, weirs and in-situ RWH. Due to the need for continent-wide and country scales of the mapping, 90-m resolution digital elevation models (DEMs) obtained from SRTM were used. Slope steepness was also determined in GIS analyses and used for showing areas preferable for runoff harvesting from open areas.