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Strategic Water Source Area grid cells were identified from a mean annual runoff grid for South Africa (mm/year for each 1 x 1 minute grid cell) using all grid cells with a value of = 135 mm/year. The grid data layer, MAR_SWSA is also on BGIS, and was based on disaggregating the Water Resources Assessment 2005 mean annual runoff data (Middleton and Bailey 2009), which represents the most commonly-used national mean annual runoff data used by the Dept of Water Affairs for water resources planning and management. This data layer was used to identify Strategic Water Source Areas for South Africa. Strategic Water Source Areas are those areas that supply a disproportionate amount of mean annual runoff to a geographical region of interest.

This map uses the Water Resources of South Africa data (WR2005) for Mean Annual Runoff at a quaternary catchment scale (Middleton and Bailey 2009), which was then disaggregated to a 1 x 1 minute grid resolution using a set of generalised rainfall-runoff relationships provided by Pitman (1996) and used in the WR90 project (Midgley et al. 1994). The disaggregated 1 x 1 minute data thus generated were summarised to a quaternary catchment scale so that it could be compared to the quaternary catchment mean annual runoff of both the WR90 and WR2005 national water resource assessments (Midgley et al. 1994; Middleton and Bailey 2009). The results were favourable, with r-squared values of 0.87 and 0.95 for WR90 and WR2005 respectively. An adjustment factor was then applied to the 1 x 1 minute data so that the mean annual runoff matched the WR2005 data at each quaternary outlet. This essentially means that the rainfall-runoff curves were used to disaggregate to WR2005 mean annual runoff to a 1 x 1 minute resolution. The final map of Strategic Water Source Areas was produced by grouping areas generating mean annual runoff across South Africa, Lesotho and Swaziland into percentile categories, where areas representing 50% of the mean annual runoff in the region were considered to be strategic water source areas for the country. These areas in South Africa, Lesotho and Swaziland together contribute 50% of the region’s water supply in less than 8% of the land surface area.

Available documentation:

Nel, J.L, Colvin, C., Le Maitre, D.C., Smith, J. and Haines, I. 2013. South Africa’s Strategic Water Source Areas.

CSIR Report CSIR/NRE/ECOS/ER/2013/0031/A, CSIR, Stellenbosch, South Africa.

SANS 1878

FGDC-STD-001-1998