3.1 Climate
SER B is the largest of the three regions, covering about 640,570 km2, or about 78% of the area of Namibia. This region contains approximately 60% of the population, although it is generally spread out, with the exceptions of a few major towns such as Mariental, Lüderitz, Walvis Bay, Karasburg, Keetmanshoop and Maltahohe. The precipitation gradient increases from the south-western corner to the north-eastern corner, rising from a mean annual value of 50 mm to 550 mm. Similarly, the temperature gradient increases from south-west to north-east, exhibiting a mean annual
minimum temperature of 13.3°C to 15.5°C in the month of July, and a maximum mean
annual temperature of 22.3°C to 25.5°C in the summer. Figure 2 shows the average
annual temperature and precipitation by month for SER B.7
40 20 60 80 100 120 140 0 mm
oC Lower South Atlantic
0 10 20 30 40 50 60 70
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
Figure 2. Relative aridity and humidity shown by average annual temperature and precipitation by month for the Lower South Atlantic Basin – SER B.
3.2 Watershed characteristics
SER B is comprised primarily of the following watershed basins: Upper South Atlantic, Lower South Atlantic, Fish, South Interior, and Coastal Drainages. The Upper and Lower South Atlantic basins represent an aggregation of the western flowing ephemeral river basins, with the exception of the Kuiseb and Swakop river basins. The Fish river basin is the most important tributary to the Orange River in Namibia. The south interior basin is dominated by the Kalahari Desert, and contains only ephemeral rivers.
The Fish river basin has four major dams on its river and tributaries: the Nawaspoort, Naute, Merensky and Hardap dams. The largest dam in Namibia, the Hardap dam, lies directly on the Fish River, and was built in 1972. It primarily provides irrigation and
domestic water for the town of Mariental. Namibia currently has access to an agreed 0.5 km3 of water per annum from the Orange River.
The Upper South Atlantic basins contain the major ephemeral rivers: Hoarusib, Omarur, Ugab and Khan rivers. These rivers flow from the central highlands down the escarpment to the Atlantic Ocean. The Hoarusib’s safe yield is approximately 20 MCM per annum. The Omarur’s safe yield is approximately 12 MCM per annum and the Ugab’s safe yield is 8 MCM.
3.3 Groundwater resources
There are several important aquifers in SER B that supply water for domestic and other purposes. There are three main types of aquifers currently being exploited: alluvial aquifers on the western edge of Namibia, the Grootfontein Karst aquifers, and the Stampriet artesian aquifers in the Orange basin.
The western alluvial aquifers which contain well fields, include the Omdel, the Neis- Neis, and the Omaruru aquifers, all located on various stretches of the Omaruru River. These three aquifers supply Henties Bay, Rossing Mine, Arandis, Swakopmund, Uis Village, Tin Mine, Omaruru and irrigation farmers with water. The Spes-Bona, Khan- Kranzberg, and Khan River aquifers are all located on the Khan river. These aquifers supply Karibib, Usakos, and Rossing Mine with water. The Koichab Pan aquifer provides water to the town of Lüderitz, via a 100 km long pipeline from the north.
The Karstveld, about 2,500 km2 in extent, contains massive underground water
stores. This aquifer is recharged directly from rainfall. Groundwater is abstracted from this aquifer for domestic, irrigation and other purposes for the entire Tsumeb-Otavi- Grootfontein region. There is a canal connecting the Karstveld to the Omatako dam, however it is not presently being used. This water may one day be used as a supplement to water from the ENWC to Windhoek (Joint Venture Consultants 1995).
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SER C – Khomas
4.1 Introduction
The central region, SER C, is characterised by the Khomas highlands to the west and the Berg Auas mountains in the south. The capital city, Windhoek, originally attracted settlers in the late1800s because of natural springs in the mountains. As the population has grown, however, the water resources required to sustain the city have far outstripped the available local supply. In fact, the existing three dam supply structure (Von Bach, Omatako, Swakop) is not expected to provide enough water for Windhoek as the city grows in the next ten years. One of the main interbasin transfers, the Eastern National Water Carrier, transfers water from the Omatako dam, which lies approximately 165 km to the north of Windhoek, to Von Bach dam.
4.2 Climate
The annual average precipitation for Khomas increases from about 50 mm to 375 mm moving from west to east. The majority of rainfall occurs in the summer months, from December to March. Figure 3 shows the average annual temperature and precipitation
by month for SER C.8 The average annual minimum temperatures vary from 15.5°C in
the east to 13.4°C in the west, and a maximum temperature range of 24°C to 25°C.
40 20 60 80 100 120 140 0 mm oC Kuiseb 0 10 20 30 40 50 60 70
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
Figure 3. Relative aridity and humidity shown by average annual temperature and precipitation by month for the Kuiseb Basin – SER C.
4.3 Watershed characteristics
The Khomas region is comprised primarily of the following watershed basins: Swakop, Kuiseb, and Orange. As mentioned above, these watersheds do not provide Khomas with all of its water supply, as some of the water is piped from as far away as Omatako dam. The Kuiseb and Swakop basins are characteristic of the ephemeral western flowing river basins that occur all along the coast of Namibia. The Orange basin, as labelled in this analysis, denotes that part of the Orange River basin, outside of the Fish river basin, which lies within Namibian borders.
The Swakop river basin contains three dams, the Swakop, the Von Bach, and the Avis, and the Goreangab reclamation works. The safe yield from the Swakop River is 6 MCM per annum, or approximately 3% of the estimated safe yield from all of the western ephemeral rivers. The assured safe yield is a measure of the long term sustainable quantity of water that can be obtained from the river.
The Kuiseb river contains the Friedenau dam, which is currently not operational. The safe yield of the Kuiseb River is 5 MCM per annum, or slightly less than 3% of the total estimated safe yield of the western flowing rivers. The Kuiseb river is ecologically important as it limits the northward migration of the large sand dunes of the Namib sand sea. It protects the Walvis Bay town and the lagoon.
Finally, the portion of the Orange River Basin lying within SER C contains the Otjivero main and silt dams. Outside of the Khomas boundary, this basin contains the Oanab, Dreihuk, Bondels, Daan Viljoen and Tilda Viljoen dams. The south-eastern flowing Nossob River and its tributaries flow towards the Orange River, but never reach it.
4.4 Groundwater resources
The western flowing rivers are characterised by alluvial aquifers, which often provide groundwater supply to villages when the river is dry. Recharge occurs from intermittent floods in the riverbeds. There are important well fields which currently exploit the Lower Kuiseb and Swakop rivers. The Lower Kuiseb River supplies Walvis Bay, Rossing Mine, Arandis, and Swakopmund with water. The aquifer has been sub-divided into the Rooibank and Swartbank compartments and includes the Kuiseb Delta area known as the Dorop aquifer.
The Osana aquifer is a riverbed alluvial aquifer in the vicinity of the confluences of the Swakop and Okahandja rivers. This well field supplies water to the population of Okahandja, and to irrigation farmers. This water augments supply from the Von Bach dam. The use of water from these well fields is currently considered unsustainable, as the water table is being drawn down. There are plans to complete a desalinization plant by the year 2002, which will augment supply to coastal towns Swakopmund and Walvis Bay, and the Rossing uranium mine. Although the concept of a pipeline from the coast providing desalinated water to Windhoek has been debated, it is at present considered to be impractical due to energy and capital cost considerations.9
The Windhoek aquifer used to flow freely at the surface, but has ceased to flow as the aquifer has been taxed by increasing pumping rates from the increased water demands. The aquifer is underlain by schist and quartzite rocks, and receives the bulk of its recharge from precipitation over the Auas mountains. The aquifer is presently a major supply source, providing Windhoek with 2.34 MCM per annum. In time of drought, the production from the boreholes can be increased to augment dwindling surface water supplies.
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IIASA Regional Level SER Analysis
The IIASA PDE project goal is to get an understanding of the dynamics between the human population and its surrounding environment. Particularly in the case of Namibia, where water resources are scarce, this project is oriented towards policies that might help mitigate future problems with water shortages. A brief description of Namibia’s modelled water supply by basin and SER is given below. Also described are the scenarios that were applied in our analysis of the sustainability of Namibia’s water resource.
9
Personal communication with Martin Harris, NAMWATER, Manager Infrastructure Planning and Design, March 2000.