Difference between revisions of "Hydrogeology of Botswana"
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'''Dr Roger Key''', UK
'''Dr Roger Key''', UK
Bibliographic reference: Upton, K, Ó Dochartaigh, B É,
Bibliographic reference: Upton, K, Ó Dochartaigh, B É, Key, R . 2016. Africa Groundwater Atlas: Hydrogeology of Botswana. British Geological Survey. Accessed [date you accessed the information]. http://earthwise.bgs.ac.uk/index.php/Hydrogeology_of_Botswana
==Terms and conditions==
==Terms and conditions==
Revision as of 16:37, 3 May 2017
The textual information on this page was taken from a number of different sources, which are listed in the References section, below. If you have more information on the hydrogeology of Botswana, please get in touch.
- 1 Compilers
- 2 Terms and conditions
- 3 Geographical setting
- 4 Geology
- 5 Hydrogeology
- 6 Groundwater status
- 7 Groundwater use and management
- 8 References
- 9 Return to the index pages
Dr Kirsty Upton and Brighid Ó Dochartaigh, British Geological Survey, UK
Dr Roger Key, UK
John Farr, Botswana
Bibliographic reference: Upton, K, Ó Dochartaigh, B É, Key, R and Farr J. 2016. Africa Groundwater Atlas: Hydrogeology of Botswana. British Geological Survey. Accessed [date you accessed the information]. http://earthwise.bgs.ac.uk/index.php/Hydrogeology_of_Botswana
Terms and conditions
Botswana is a land-locked country. It is predominantly flat, forming a gently rolling plateau with an average elevation of 1100 m. Up to 70 per cent of the country - most of the centre, west and south - is covered by the Kalahari Desert. In the north-west is the Okavango Delta.
|Estimated population in 2013*||2 011 100|
|Rural population (% of total) (2013)*||43.1%|
|Total surface area*||566 730 sq km|
|Agricultural land (% of total area) (2012)*||45.7%|
|Border countries||South Africa, Namibia, Zimbabwe, Zambia|
|Annual freshwater withdrawal (2013)*||194 million cubic metres|
|Annual freshwater withdrawal for agriculture (2013)*||41.2%|
|Annual freshwater withdrawal for domestic use (2013)*||40.1%|
|Annual freshwater withdrawal for industry (2013)*||18.0%|
|Rural population with access to improved water source (2012)*||92.8%|
|Urban population with access to improved water source (2012)*||99.3%|
* Source: World Bank
Botswana’s climate is mostly semi-arid. Rainfall is low - less than 250 mm/year over most of the country, and less than 100 mm/year in the Kalahari - and is unevenly distributed and highly variable from year to year. Drought is a recurrent phenomenon. The east and north of the country receive the highest rainfall, but very few areas receive more than 600 mm/year.
Potential evaporation is around 2000 mm/year (United Nations, 1989).
These maps and graphs were developed from the CRU TS 3.21 dataset produced by the Climatic Research Unit at the University of East Anglia, UK. For more information see the climate resource page.
Many rivers in Botswana are ephemeral, flowing only during wet seasons. The major perennial river systems in the north of the country are the Chobe river, which is a tributary of the Zambezi river, and flows north into Zambia; and the Okavango river, which drains into the Okavango Delta. The east of Botswana lies in the Limpopo river basin, and a number of tributaries in Botswana flow into the Limpopo river. The Notwane river in the south provides water to the capital city through the Gabarone Dam. The Molopo river forms the border with South Africa to the south.
There are a number of dams in the country.
|Much of the country is covered with arenosols typical of arid, desert areas. Only in the east is there widespread development of other soil types.|
|Up to 70 per cent of the country is covered by the Kalahari Desert, with grassland and savannah vegetation that is often sparse. The east of the country has more forest cover. The north-west of Botswana lies in the Okavango Delta, with extensive wetlands.
This section provides a summary of the geology of Botswana. More information is available in the Africa Groundwater Literature Archive.
The geology maps below show a simplified version of the geology at a national scale (see the geology resource page for more details).
Much of Botswana is covered by unconsolidated sediments of Tertiary to Quaternary age, related to the Kalahari Desert, and known as the Kalahari system.
The pre-Kalahari geology of the country includes sedimentary rocks - largely sandstones - of Karoo type, sometimes capped by volcanic rocks; and large areas of Precambrian rocks. The Precambrian rocks are divided into sedimentary basins, which are also sometimes capped with volcanic rocks, dolomites, and basement-type crystalline rocks.
Extensive tectonic activity in different eras has affected the structure of all the geological units in Botswana, with more recent uplift of ancient erosion surfaces to form the current high plateau topgraphy superimposed on ancient regional structural trends. Deep weathering also affects many of the geological units.
The country is rich in mineral resources, including coal, diamonds, and metals such as nickel and copper.
|Kalahari||Tertiary to Quaternary||Predominantly terrestrial deposits that vary in thickness from 10 to st least 150 m (Farr et al., 1981), and over 400 m in the Okavango Basin (Jones, 2010). Includes a Tertiary lower sequence of fluvial and lacustrine deposits, and a Quaternary upper sequence dominated by of aeolian sands (Jones, 2010). Lithologies also include calcretised sands, calcretes, silcretes and marls (Farr et al., 1981). The upper Kalahari sands cover much of Botswana, up to 50 m thick generally.|
|Karoo sedimentary rocks - uncovered||Late Carboniferous to Jurassic||The Karoo system comprises a predominantly terrestrial, arenaceous sedimentary sequence, partially capped by volcanic basaltic lavas (Farr et al., 1981). The maximum thickness of Karoo rocks in Botswana is unknown, but is thought to be of the order of 1000 to 1200 m in the axial regions of the Central basin and parts of the South-west basin (Farr et al., 1981). From oldest to youngest, the main units are the Dwyka; the Ecca Group; the Beaufort; the Red Beds; the Cave Sandstone; and the Stormberg Basalts. Parts of the Karoo system have been subject to deep weathering (Jones, 2010).
The Dwyka group consists largely of tillites, shales and siltstones associated with glacial and glaciofluvial deposition (Farr et al., 1981).
The Ecca group passes upwards from siltstones to, successively, coarse-grained arkosic sandstones, finer-grained sandstones, shales, coals, and argillaceous siltstones and mudstones. Basal conglomerate units also occur where middle Ecca strata lie directly on pre-Karoo basement. Impure limestones are recorded rarely. Finer-grained sandstones passing into shaley sandstones and sandy shales and siltstones are often interbedded with the coarser units (Farr et al., 1981).
The Beaufort group has little coals or carbonaceous matter, but has conglomerates with calcareous concretions, and mudstones and siltstones with calcareous cement (Farr et al., 1981).
The Red Beds consist of a sequence of mudstones and siltstones, red shales, marls and fine-grained calcareous sandstones.
The Cave Sandstone is the most widespread Karoo unit in Botswana, comprising largely aeolian sandstones, usually massive, and often loosely cemented. The sandstones are mostly fine grained, sometimes almost siltstone grade, but includes extensive coarse-grained horizons. They include non-aeolian silt beds, calcareous concretions and thin limestones and chert nodular horizons (Farr et al., 1981).
The youngest Karoo unit in Botswana comprises volcanic rocks of the Stormberg Basalts. Much of the older Karoo rocks are covered by these relatively flat-lying plateau basalts, formed of lava flows that vary in thickness from less than one metre to several tens of metres. The junctions of individual lava flows are often marked by fracturing. Tuffs are rare and impersistent, but occur near the base of the basalts, along with minor intercalations of Cave Sandstone and, higher in the sequence, occasional thin intraformational sandstones, siltstones, marls and/or palaeosoils (Farr et al., 1981).
|Metasedimentary basins||Late Precambrian - early Palaeozoic||Significant basins of metasedimentary rocks of Precambrian age occur across Botswana, and include sandstones, conglomerates, limestones and quartzites (United Nations, 1989).|
|Dolomites of the Transvaal system||This series includes dolomites, cherts, quartzites and schists, but dolomites are dominant, and widespread across the country (United Nations, 1989).|
|Crystalline basement||Granitic and metamorphic rocks, including amphibolites, quartzites and schists (United Nations, 1989).|
This section provides a summary of the hydrogeology of the main aquifers in Botswana. More information is available in the Africa Groundwater Literature Archive.
The hydrogeology map shows a simplified version of the type and productivity of the main aquifers at a national scale (see the Hydrogeology Map resource page for more details).
|Named Aquifers||Period||General description||Water quality|
|Kalahari sands; alluvium and other unconsolidated deposits||Tertiary - Quaternary||Largely aeolian and alluvial sands and gravels. These cover large areas of Botswana. They typically form moderately to highly produtive aquifers, variously local to regional in scale. Alluvium in river valleys typically forms local aquifers, which are, however, an important water source in rural areas, as groundwater levels are shallow. Aeolian Kalahari sands can form a regional aquifer, such as in the Okavango Delta. Locally, low permeability beds (clays, silts, etc.) can reduce groundwater potential. Borehole yields in the range 2 to 10 litres/second (l/s) are seen (Jones, 2010).
Recharge is by direct rainfall infiltration and, in river valleys, by indirect infiltration of river flow.
|Groundwater typically has low levels of mineralisation, with total dissolved solids (TDS) values of less than 100 mg/l (Jones, 2010).|
Sedimentary - intergranular and fracture flow
|Named aquifers||Period||General description||Water quality|
|Karoo system||The Karoo system forms a stratified aquifer which can be moderately productive. More permeable layers (dominantly sandstone) are separated by layers with low permeability. A large proportion of groundwater storage and flow, particularly in the Ecca sandstone, is via fractures in the aquifer layers. The more loosely consolidated Cave sandstones show more dominant intergranular flow. Semi-vertical fault zones and fractures form hydraulic connections between aquifer layers.
The highest potential aquifers within the Karoo system are fractured sandstones with high transmissivity and storage capacity, particularly the Ecca and Cave sandstones. Typical transmissivity values are at least 350 m²/day, with relatively low storage coefficients of 2.5 to 3.5 per cent. Typical borehole yields from the Cave sandstones are around 2.5 litres/second (l/s) (United Nations, 1989). Other information suggests typical borehole yields from Karoo sandstones generally are in the range 2 to 10 l/s (Jones, 2010).
Recharge is from direct rainfall infiltration and, in river valleys, by infiltration of river water, particularly during storm events (Jones, 2010).
|Available information suggests that modern groundwater (recently recharged) in the Karoo has relatively low levels of mineralisation, with TDS values of less than 100 mg/l. Groundwater in the Ecca sandstone is considered to receive little modern recharge and to be many thousands of years old (Jones, 2010), with higher levels of mineralisation.|
Sedimentary - fracture flow (including karstic)
|Named aquifers||Period||General description||Water quality|
|Dolomites||Precambrian||These rocks form karstic aquifers, with high potential where karstic features are well developed - where they are not well developed, yields are likely to be low. Transmissivity values of at least 250 m²/day are common, and values of more than 550 m²/day are seen occasionally, with storage coefficients of 0.1 to 6 (United Nations, 1989).|
|Volcanic rocks||These can form moderately to highly productive aquifers. Stormberg lavas typically have transmissivity values of around 25 m²/day. Typical borehole yields are around 2.5 l/s (United Nations, 1989).|
|Metasedimentary rocks||Precambrian||Quartzites can have moderately high yields.|
|Named aquifers||Period||General description||Water quality|
|Precambrian||Small, low productivity local aquifers are formed in fractured zones and/or weathered basins. Yields are generally low. Recharge is largely from direct rainfall infiltration. Low storage capacity in the small weathered basin aquifers can limit recharge potential.||Weathering processes can cause moderately to highly mineralised groundwaters, with evidence of TDS values of 300 to 400 mg/l in some areas (Jones, 2010).|
There have been a number of estimates of the volume of Botswana’s groundwater resources. One puts the total groundwater resource at around 100 billion m³, with an average annual recharge of 1600 million m³/year (Department of Surveys and Mapping, Government of Botswana, 2001); another gives a much lower estimated sustainable yield of 96 million m³/year. These different figures illustrate that further groundwater exploration work is needed to estimate the sustainable amount of groundwater in the country (Department of Water Affairs, Ministry of Minerals, Energy & Water Resources, 2013)
Only a small part of the estimated available groundwater resources can be economically abstracted, due to high abstractions costs, poor water quality and remoteness of productive aquifers in relation to water demand (Department of Water Affairs, Ministry of Minerals, Energy & Water Resources, 2013).
Groundwater use and management
Groundwater is the main water source in Botswana. It abstracted for use in rural water supply; industry (including mining); energy (by power plants); irrigation; and urban water supply.
Groundwater management falls under the Ministry of Minerals, Energy and Water Resources (MMEWR). The Hydrogeology Division of the MMEWR has responsibility for, among other things, monitoring groundwater exploration programmes undertaken by the private sector; maintaining an up-to-date database of all boreholes and wells in Botswana, including a register of drilled water boreholes; carrying out groundwater resource assessments; and monitoring groundwater levels and quality, including installing and operating a national monitoring network of groundwater abstraction.
Water policy is guided by the Botswana National Water Master Plan (NWMP) developed in 1991, and reform recommendations made in subsequent reviews (Department of Water Affairs, Ministry of Minerals, Energy & Water Resources, 2013).
Botswana shares large areas of aquifers with other countries. These include (Altchenko and Vilholth, 2013):
- the Ramostwa and the Pomfret-Vergelegen (or Khakhea/Bray) dolomite aquifers, shared with South Africa
- the South West Kalahari/Karoo aquifer, shared with South Africa and Namibia
- the Northern Kalahari/Karoo Basin, shared with Angola, Namibia and Zambia
- possibly groundwater in the Lokalane-Ncojane Basin, shared with Namibia
- the Tuli Karoo Sub-basin, shared with South Africa and Zimbabwe
- the Eastern Kalahari/Karoo Basin, shared with Zimbabwe and
- the Nata Karoo Sub-basin, shared with Namibia and Zimbabwe.
For further information about transboundary aquifers, please see the Transboundary aquifers resources page
The following references provide more information on the geology and hydrogeology of Botswana.
Many of these, and others, can be accessed through the Africa Groundwater Literature Archive
Useful web resources
Department of Surveys and Mapping, Government of Botswana. The Botswana National Atlas
Altchenko, Y, and Vilholth, K G. 2013. Transboundary aquifer mapping and management in Africa: a harmonised approach. Hydrogeology Journal Vol. 21, Issue 7, 1497-1517.
Department of Water Affairs, Ministry of Minerals, Energy & Water Resources. 2013. Botswana Integrated Water Resources Management & Water Efficiency Plan, Volume 1: Main Report. (L Dikobe, Ed), Gabarone, Botswana: Government of Botswana.
Farr, J L, Cheney, C S, and Baron, J H. 1981. GS10 Project: Evaluation of underground water resources: Final report. Republic of Botswana, Department of Geological Survey.
Jones, M J. 2010. The Groundwater Hydrology of the Okavango Basin. Internal Report prepared by M J Jones (Consultant) for FAO; Okavango River Basin Transboundary Diagnostic Analysis Technical Report, Biophysical Series; EPSMO.
United Nations. 1989. Groundwater in Eastern, Central and Southern Africa: Botswana. United Nations Department of Technical Cooperation for Development. Natural Resources / Water Series No. 19, ST/TCD/6