Surface water SWSAs are found in areas with high rainfall and produce most of the runoff. The SWSA-sw study identified 22 areas that were significant at the national level and a further nine that are significant at a sub-national level. They are important because they contribute considerably to the overall water supply of the country. These multi-purpose landscapes are key ecological infrastructure assets for South Africa, supporting growth and development needs. The effective protection of surface water SWSAs areas is vital for national security because a lack of water security will compromise national security and human well-being.

The current boundaries for surface water SWSAs depend on a mean annual runoff (MAR) surface derived from a rainfall surface at 1’ x 1’ spatial resolution and rainfall-runoff relationships developed to quantify water resources for water resource planning. The use of a raster (gridded) data set to determine the MAR thresholds resulted in numerous scattered grid cells. This problem was addressed by consolidating the high MAR areas and removing the isolated cells.

The first step was to select pixels or cell-level MAR thresholds of =135 mm and =75 mm. All pixels were converted to points. The points with MAR =135 mm were selected from all catchments in South Africa, Lesotho and Swaziland. Then the points with =75 mm in the Upper Vaal (Secondary catchments C1 and C8), Upper Usuthu (W5) and Waterberg (A4 and A5) were added to this dataset. All the other point values were set to zero to ensure that only the high MAR points were include in the next stage.

The point MAR data set was converted to a density surface using the Kernel Density function in ArGIS and converted to a contoured surface with one surface for each density sampling radius (kernels). Density contours were generated from this surface and then individual contours were extracted and converted to polygons. These polygons were then split, where necessary, using catchment boundaries and the nationally strategic polygons (SWSAs) were selected and put into a new data set. Various combinations on density kernel radiuses and density thresholds were tested to identify the set of nationally strategic SWSA polygons which captured >50% of the MAR for the smallest included area. A kernel density radius of 8 km and a kernel density threshold of 0.12 was selected as providing an appropriate balance between inclusion and exclusion of high MAR areas.

Available documentation:

Le Maitre, D.C., Seyler, H., Holland, M., Smith-Adao, L.B., Nel, J.L., Maherry, A. and Witthüser. K. (2018). Identification, Delineation and Importance of the Strategic Water Source Areas of South Africa, Lesotho and Swaziland for Surface Water and Groundwater. WRC Report No TT 754/18, Water Research Commission, Pretoria.

Nel, J.L., Le Maitre, D.C., Roux, D. J., Colvin, C., Smith, J. S., Smith-Adao, L.B., Maherry, A. and Sitas, N. (2017). Strategic water source areas for urban water security: Making the connection between protecting ecosystems and benefiting from their services. Ecosystem Services, 28: 251-259. doi: https://doi.org/10.1016/j.ecoser.2017.07.013.

Nel J., Colvin C., Le Maitre D., Smith J. and Haines I. (2013). South Africa’s Strategic Water Source Areas. CSIR Report No: CSIR/NRE/ECOS/ER/2013/0031/A, Natural Resources and the Environment, CSIR, Stellenbosch.

WWF. (2013). An introduction to South Africa’s Water Source Areas. World Wildlife Fund South Africa, Cape Town.

SANS 1878

FGDC-STD-001-1998