Terrain Evaluation and the Botswana Environment

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BotswlJlfll. Noles mr4 Records, Volume 36

Terrain Evaluation and the Botswana Environment

By Julius R. Atlhopheng*

Abstract

The paper discusses the various uses to which terrain attributes are put to in Botswana. Noted are the natural landscape components that aid in siting dams, wind energy generation, eco-tourism landforms, shrink-swell (cracking) clays as they affect buildings, and the role of landforms in gravitational positioning of water reservoirs and telecommunications equipment. Botswana soils discussion highlights the limited agricultural output because of the low nutrient status of the soils and the low, erratic and seasonal rainfall. The environmental considerations to further our understanding in resources exploitation are examined. The article is the fIrst on Botswana terrain evaluation and its related importance.

Introduction

The general conflguration of the landscape, its geology, climate, and other natural features that are less than 6m deep, defme terrain. Landscape is similar in meaning to terrain. Evaluation refers to the valuation or worthiness of the landscape. Three stages are realised in terrain evaluation, these being: analysis (simplifying the complex natural environment); classiflcation (landscape data extraction to characterise and distinguish land units); and f1nally appraisal (interpretation and assessment for pragmatic uses).

The contribution of terrain evaluation is realised in many applications, like data prior to construction, re~nse to consultancies on terrain uses, and general resource exploitation. Regolith resources and their attributes are at the core of terrain evaluation. Historically, terrain evaluation was used for military purposes like terrain-following missiles, landslides activated to bury the enemy, and mountains used as vantage points.

Some civilian uses of terrain are in fIelds like hydrology, geology, geography, botany, ecology, pedology and others. In the applied sciences, civil engineering, agriCUlture and landscape design are some relevant examples.

An example using terrain attributes is that of road design, where rock materials (aggregates) are used in road construction. Natural weathering processes like disintegration (rock breakdown without much mineral alteration) and decomposition (chemical alteration of the rock) may affect their durability. For instance, in South Africa, road surfacing using basic igneous rocks, which are mainly Karoo dolerites, works well in the west but performs poorly in the east. This is due to disintegration being more pronounced in the west, and decomposition dominating in the east. Precipitation and temperature trends are the forces driving these processes.

In Botswana there are four problematic road building materials. These are Kgalagadi sand, expansive soils, dispersive soils, and weathered igneous rocks.

The Kalahari sand, with its open structure, tends to include clays. These materials in combination tend to become collapsible when wet and loaded with weight, such as vehiculor traffic. The weathered igneous rocks (basalt and dolerite for Botswana) dominate the eastern fringe. Although these pass standard laboratory tests (which are physical) for road construction, some durability problems have been encountered. This is mainly due to chemical alteration and

• Department of Environmental Science, PlBag UB 0704, Gaborone. E-mail: [email protected]

d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e Pu bl is he r ( da te d 20 09 ).

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Botsw_ Notes and Records, Vohfme 36

decomposition of minerals within the rock or aggregate. Thus tests that address the mineralogy and crystal chemistry are alsQ necessary. With expansive soils, the main culprits are the seasonal dry/wet changes and water Ulble movements that influence the volume of the soil. When wet, these soils expand, and this :expansion occurs at a different rate to that of the road surfacing, hence the road fails. Cracks:On the surface are common symptoms of such failure. Dispersive soils have high exchangeablt sodium that disperses the clay fraction when wet. The dispersion creates high run-off during tains, hence erosion and drainage concerns after roads have been completed.

Another problem

ot

soils used for road works is the presence of saline water. Soluble salts in the water tend to migrate to the top layers, where they crystallize, hence weakening the surface. This is a problem with both underground and surface waters that are saline. Salts cause a browning or staining of bit1imlinous road surfaces, looseness of the priming coat, and/or a loss of the bond between the surtjl,cing and the base, including cracking or powdering of the prime coat. In. areas where roads lraverse pans, which tend to collect salts, similar problems are

encountered. '

Thus the Botswana experience in road construction has improved with testing and using the various locally available materials. Cost is one of the major reason for using local materials, e.g. using local gmvels as opposed to blasting rock and crushing it (quarrying) for aggregate.

Table 1. Various disciplines' interpretations of regolith uses. Discipline Geomorphology Soil scientists Building materials Construction engineering 1. Dam Siting Regolith uses

Study details of weathering, both in the field and laboratory for deposition and erosion assessments and material type.

Soil is central to vegetation and agricultural output, e.g. release of nutrients and human impacts like pollution and irrigation.

Destructive effects of weathering on building materials, both natural and artificial, and their durability in various environmental settings.

Engineering assessment on properties of natural rocks and artificial structures, like strength, particle size, permeability and porosity.

Landform Applications

Botswana is a relatively flat, /lemi-arid country. Its semi-arid nature has implications for water resources. Most of the rains ~ome in summer, thus this period is co-incident with river flows.

In winter, during the dry se~n, surface water sources dry up. There is thus a need to extend the seasonally sourced surface water, which comes in summer. To cater for the winter deficit, Botswana has invested in building dams.

The dams straddle most major rivers, with the principal water use being human consumption. The main chanbnge is maintaining adequate supplies for a growing population and its growing water use, agflinst a drought-dominated climatic system.

38 ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e Pu bl is he r ( da te d 20 09 ).

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The various dams in the country are built on a terrain that is flat to gently undulating. The ideal sites, disregarding hydrological parameters, would be areas of steep terrain. In such rugged terrain, deep dams can be built and can hold water for long periods.

For the Botswana setting, however, the gentle to flat terrain results in dams that are wide and shallow. The ratio of depth to surface area is such that we have dams that lose a lot of water to evaporation. Dams in Botswana have relatively large surface areas and less depth than the ideal situation. The evaporation levels in Botswana, which are four times greater than precipitation, exacerbate this.

Thus, damming of seasonal surface waters, which otherwise flow away in rivers, faces high evaporation levels as a result of the country's terrain attributes.

2. W'md Energy Mapping

Botswana, a developing nation, has the potential to tap wind energy. Unfortunately, the wind speeds and their timing in the terrain are not consistent.

Botswana, as part of the Southern Hemisphere, is affected by a continental high-pressure cell system in winter. The lull conditions lead to no rain and very low wind (almost breeze) conditions, The onset of summer, with its cyclogenesis (low pressure cell system that creates instability in abnospheric conditions), initiates windy conditions, especially during the period of July/August, where strong to gusty conditions may be experienced (Ramothwa and

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Minja 2(01). Other periods of strong winds are associated with the rainy periods and months, e.g. OctoberlNovember and March/April. Such episodes of strong winds are normally short lived, and would only prevail during rainfall.

Jain et al. (2001) allude to Botswana's limited wind energy potential, with average wind speeds of 1.5-3.5m/s, thus making wind energy useful for wind mills for pumping water and battery charging only.

A generalised mapping of wind speeds and their probable uses in generating energy remains a potential area to explore. Wind turbines can be used to generate electricity for domestic and industrial use if a viable potential is realised. Upland (exposed) zones of the landscape, as well as valleys (where there may be a channelling effect) could be used. The challenge that remains is to map wind speed measurements in more detail, in various localities at different times of the year. Wmd, like our rainfall, may be tapped as a seasonal resource.

Already in the Kgalagadi, in the area from Tsabong to Bokspits where there is an abundance of dunes, windmills have been set to tap wind energy.

In

the Kgalagadi Transfrontier Conservation Area (between the countries of Botswana, South Africa and Namibia), windmills are used to draw groundwater to water wildlife during the dry seasons and the drought years. High wildlife mortality is common in Botswana during the droughts.

Despite its renewable nature, and hence desirable use, .wind energy turbines covering large areas have been known to affect bird life. Wmd turbines located on bird migratory routes may lead to huge losses of bird life. These fast turbines are not seen by the birds as they fly past, and they may fly into them and be killed. More environmentally friendly machines can be installed, with the rotating blades covered so as to eliminate loss of biodiversity in the form of birds.

3. Eco-tourism

Botswana's landscape is endowed with attractive natural features. The scenic river system and its ecology, the hills (castle kopies), caves, pans, the Okavango Delta and sand dunes are among our best resources that can be tapped in a non-consumptive way. The new term for such sustainable use is eco-tourisin, or ecological tourism, which deviates from conventional mass (high impact) tourism.

Botswana is also blessed with plenty of sunshine. Sunshine is available several hours each day all year round. When designing holiday packages, most tourist operators include the number of sunshine hours fot the tourist destination. This sunshine is not only an attractive asset for tourists, but can also be tapped for household use. Photovoltaics and solar water heaters are some of the immediate uses of solar energy. Buildings in Botswana may be designed using passive solar design, which ensures savings on electricity costs.

3.1 The Hills

Botswana's castle kopies owe their origin to past conditions, when they formed underneath the landscape. Wearing down and erosion of the landscape has exposed them to what we see today. Rock fractures explain the rock boulders, while flat massive outcrops had minimal or no weakness in their structure, and still stand solid today. These special outcrops are a fascination to people from other landscapes or climatic zones. They continue to serve as high points on the landscape, and harbour the unique hilly vegetation like Moologa and Mongololo trees and Maritamatshwene shrubs, with the depressions and hollows in the rocks acting as rainwater catchment systems. The hollows that exist on some of the massive rock outcrops are so large they can be utilised as reservoirs.

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Botswtma Nous and Reconls, lbIanre 36

Some of the bigger hills include Tsodilo, where some of Botswana's rock paintings are found.

People who used to live in the area several thousand years ago made the paintings. In addition

to being archaeological sites, they are a history bank, as it is believed they show the cultural belief systems and lifestyles of at time. Manyana hill (close to Gaborone) is another site with some paintings.

3.2 Caves

Some of Botswana's popular caves include Drotsky cave and the Lobatse caves (Cooke 1975; Thomas and Shaw 1991; Holmgren and Shaw 1999). Some of the caves may owe their origin to groundwater dissolving carbonate structures. This results in the cave formations.

Non-carbonate bedrock dissolution along fractures has also been documented by Gieske and Selaola

(1988), and by Thomas (1997); this mechanism relies on groundwater movement.

These caves have been of interest to scientists and nature lovers alike. The dating of cementing agents in them has given some insight into the overall landscape changes in recent geological times. Episodes of aridity and high rainfall have been elucidated (Holmgren and Shaw 1999).

Tourists and other nature lovers marvel at the existence of such features, and these caves are tapped for eco-tourist purposes in addition to the scientific investigations.

3.3 Pans and Sand Dunes

Pans in Botswana occur both in the hardveld and the sandveld. Pans owe their origins to a variety of factors including structural erosional and water-related events. Structural causes include faulting and down warping, whereas erosional causes indicate the removal of material by wind or water to create a hollow or depression. Water (mainly groundwater) may discharge

at certain spots, ultimately creating a pan environment (Thomas 1997; Thomas and Shaw 1991).

For the Botswana setting, groundwater discharge zones and tectonic controls are believed to be major conditions in pan formation.

Near Mochudi are the Mabe (pan) depressions and Lekwatsing depressions. Both these pans are clay floored and non-saline. The pans formed as a result of rock fracturing or faulting, which created a localised depression. Lekwatsing and Mabe are important water reservoirs for

watering livestock in the area. In addition, Mabe is used as a recreational site by the residents

of Mochudi, Gaborone and nearby settlements. Mochudi people affectionately refer it to as the 'waterfront'. Its recreational value and proximity to Mochudi would necessitate control of littering and promote appropriate use practices.

The bigger pans like Makgadikgadi, which is saline (rich in sodium chloride), are

currently being used for commercial purposes. Salt is commercially produced from drying the pan salts. This helps diversify the country's economy, and brings in much needed revenue and

employment. In addition, the Makgadikgadi pans are a unique habitat for flamingo birds. To be

environmentally sensitive, the company mining the soda ash in Makgadikgadi opted for underground electricity cables, as opposed to the conventional overhead type, and thus help protect the unique flamingo bird colonies of the pan.

Pans in the Kgalagadi are vital watering points for wildlife. Of more importance,

however, is settlement location in the Kgalagadi. Towns like Tsabong are clearly located within a pan. This would have provided surface water in the rainy season, while still affording shallow groundwater in the dry season. These conditions prevail in most pan-related settlements.

Sand dunes may be found in association with pans, for example the pan-surface excavation deposits on the sides of pans that form lunette dunes. However, most dunefields in

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Area

formed more visitors are expected to tour the area.

The existence of one dune form - nebkhas or vegetation-stabilised mounds - has been associated with degradation. These were understood to indicate topsoil loss, hence degradation of the sites involved. However, Dougill and Thomas (2002) have shown that nebkhas are a redistribution of nutrients/sediments in neighbouring interdune areas, and thus are not an indicator of degradation.

42

ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e Pu bl is he r ( da te d 20 09 ).

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BotswaIuz Notes and Records, VoliuINr 36

3.4 The Delta and River System

The Okavango River flows into Botswana from the Bihe Plateau in Angola. The river used to flow past un-interrupted until 2 million years ago, when seismic activities induced a fault system that created a depression within the river course, the current Okavango Delta. The Delta landform is a unique site that harbours a diversity of wildlife and ecology. It is one of the most sensitive systems in the world. Its alluring beauty has seen a persistent influx of international tourists visiting the area. Appropriate resource harvesting techniques remain a concern in areas of such high visitation, in addition to having to meet the local people's livelihood needs.

Botswana's river system generates the waters that fill up the dams, as well as being used as sandbed aquifers. Aggregate resources are also harvested from the rivers for the building industry.

The river system in Botswana is also vital in biodiversity terms. Certain plants occur only along river plains. Shrubs like Teledimo, and trees like Letlhajwa, Mogodiri and Mookana are prominent in riverine conditions. Some of these species, like Mookana (Acacia Karoo), have been found to have a lot of uses: leaves as forage, bark used as an antidote or emetic, and the roots used for healing some ailments.

The next necessary step would be to tap into the traditional knowledge system, to fmd out what the detailed uses of these trees are. Already some veld products, like the grapple plant, are ~ommercially available as natural healing products.

The challenges to our river system are pollution and urbanisation. Used oil, sewage and other pollutants have unfortunately been disposed of in the rivers that provide the water we need most.

River sand extraction is another big challenge. In addition to inducing erosion and creating gullies in the surrounding landscape (as a result of base level lowering), sand extraction affects the sandbed aquifer. The sand aquifer harbours water for people and animals in rural areas, where wells are dug by hand. The exact environmental impacts of such widespread sand extraction are not yet fully understood: for example, it is not known whether or not the river system is a focus of recharge in the overall landscape.

At a regional level, treaties like the ZAC Plan (Zambezi River Action Plan) between some SADC members to share water and related resources highlight the importance of rivers. One of the major benefits of such a joint venture is to avoid conflict arising from resource exploitation. In addition, environmentally benign resource exploitation methods would be promoted. This remains crucial as most rivers in the region are shared or even delineate international boundaries.

4. Shrink-sweU Cklys (Vertic Properties)

Land subsidence, or the lowering of the landscape (ground surface) due to natural or human factors, has been noted in several areas around the world. Areas of subsidence are usually in regions where subterranean resources like oil, water, coal and salt have been heavily extracted. Compaction of sediments arising from drainage or irrigation, the dissolution of solids like sulphur and salt, and vibrations of un-cemented granular sediments during earth movements are documented examples leading to subsidence. Fluid withdrawal reduces fluid pressure (pore pressure), with the result that the effective stress (grain to grain stress) increases, leading to compaction.

Compaction may either be elastic or non-elastic (permanent). Elastic materials regain their pore pressure once the fluid is re-introduced. Hydro-compaction is one compaction

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lJotBw_ NoUr mul Records, Volute 36

method. The regolith products (vertisols) swell when moisture-laden and shrink as they dry up. Most of the vertic properties involve cracks that occur at some period in the year. The cracks are typically 1-50 cm wide, and about 30 cm to 2 m deep, in some cases going all the way up to the surface horizons.

The shrink-swell process has led to damage of property like buildings, roads, pipelines and other infrastructure. Here in Botswana, shrink-swell complications have been reported in Letlhakeng and Lobatse. In both cases, damage to property worth millions of Pula occurred. The extent of damage loss

in

Letlhakeng has led to the abandonment of ~ulti-million pula government buildings, including offices and schools.

In Lobatse, most affected houses had to be re-built using new methods, where several compacted layers were introduced after the in-situ clay material was removed.

5. Water Reservoirs

and

Tet.communieations

Most population centres in Botswana use water that first undergoes treatment. This is to ensure that the water is fit for Iannan consumption, in accordance with WHO (World Health Organisation) standards. After treatment, the water is then ready to be distributed to households. For areas with no natural high point, artificial structures are constructed, into which water is pumped to then flow out to serve the population.

Where there are high terrain features like hills, the reservoir would be located on the

hill. Gravitational energy is then used to distribute to the individual households. Examples where such systems are in use include centres like Thamaga, Tsabong and Moshupa.

The same mecllaJmm is used in telecommunications, where antennas are put on hilltops to boost the signals. Radio, TV and telephone communications need such high points. In some parts of the Kgalagadi, where the landscape tends to be more flat, towers are built to boost the signals.

6.

Soil Resources

Soils are central to vegetation growth and agricultural output. The four broad categories of soils are alluvial (erosional deposits), lacustrine (dominant in pan depressions), the sandveld (Kgalagadi sand stratum), aDd the hardveld (typified by the eastern fringe of the country). Of the sixteen soil units, Arenosols (soils derived from sand deposits that originated from the weathering of medium grained rocks) make up over 72% of the total soil units in Botswana (Moganane et al. 2001).

The soils in Botswana have limited agricultural potential (Moganane et al. 2001, Whiteman 1971, and NAMPAAD 2002). Climatic limitations (rainfall amounts, intensity, duration, timing and droughts), edaphic limitations (soils that are poor in nutrients/organic matter, susceptible to eroding agents and have low water storage capacity), and biotic limitations (where the rich modiversity of the country has a negative impact on crops) all limit farming productivity. Crop damage resulting from weeds and pests accentuates the otherwise limited arable production in Botswana. Thus the economic and social aspirations of ensuring food security and creating employment in rural areas faces a major hurdle by relying on an otherwise risky interprise. A small percentage of the country is viable for crop production, and may be orgarnised into three priority zones: a) first priority areas - Barolong, BaletelI'lokweng, Tutume, Chobe, Ngwaketse South, Tonota, Tali, Kgatieng, Machaneng and Mahalapye; b) second priority areas - Serowe, Ngwaketse Central and North, Palapye and Bobonong; with c) Kweneng North, South and West being the third priority areas (NAMPAAD 2002).

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Botswana Note8 tuUl Reeonls, Volume 36 Productivity zone demarcation involves both the soil nutrient status and the moisture regime (rainfall). All places that lie far away from the 450-500mm rainfall isohyets coincide with the sandveld, which experiences the most moisture stress.

Issues of rangeland degradation have been encountered across the country, especially during drought years. Thus the prosperous beef industry is at risk from reduced grass fodder. In addition, the veldt products that are supported by various soil units need protecting, as some are commercially exploited, e.g. grapple plant as mentioned previously and thatching grass.

Thus, soils have a very crucial role to play in sustaining both the social and economic well-being of Botswana. Deliberate policies and practices to maintain the sustenance of the soil productivity system are thus vitally important.

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Botswana is endowed with landforms that are highly scenic. In addition to their natural beauty, some economic value has been derived from these natural features.

The dam siting parameters point to a landscape that is flat to gently undulating. The dams thus constructed are wide and shallow, in high evaporating conditions, and hence act like large evaporating ponds.

Wmd energy has created spectacular sand dunes in the Kgalagadi (Bokspits area). Windmills are used to draw water as one way of harvesting the wind energy. There is a need to

assess the wind energy characteristics and come up with a map of areas that have the potential to run turbines that could generate electricity.

Most landfQrms in Botswana (hills, pans, sand dunes, the Okavango Delta, and river

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BotswQlUJ Notes and Records, Volume 36

systems) have played a vital role for man in the past and continue to do so in the present. It has been shown that some of these act as eco-tourist resources.

The vertic soils in Lobatse and Letlhakeng were discussed, outlining their negative impacts. Mapping of such soils, and relating them to the weathering mechanisms of the underlying geology, is one way of avoiding the multi-million Pula loss to the economy that occurs when buildings have to be abandoned.

Botswana soils in general have limitations that are climatic, edaphic and biotic. Most agricultural production in· the country is low due to these. In addition, agriCUltural intensification may lead to bush encroachment and grass removal (Dougill and Trodd 1999), Dougill et al. (2002) have indicated that, to abate soil degradation, integrated nutrient

management that uses both compound fertilizer and manure additions was beneficial for arable farming even in the infertile soils of the Kgalagadi sand.

High points in the landscape are used to locate water reservoirs and telecommunication stations. Where such high points do not exist, artificial structures are built to boost TV and radio signals, or for water distribution (gravity) systems.

From the discussion it can be seen that most of the landforms in Botswana are put to good use, and are an integral part of the economy. The appropriate use of resources, especially where background understanding is still needed, requires caution. Botswana should be able to use its rich resources and biodiversity well into the future if decisions are based on accurate data and careful analysis.

Acknowledgements Maps by Mr G. Koomtwe.

References

Agnew, C. and Anderson, E. (1992) Water resources in the arid realm. London,Routledge.

Atlhopheng, J., Molebatsi, C., Toteng, E. and Totolo, O. (1998) Environmental issues in Botswana - a handbook. Gaborone, Lightbooks Publishers.

Bechuanaland Protectorate Records of the Geological Survey Department (1956). Lobatse. Botswana Roads Department, Volume III - standard specifications for road and bridge works

(1983). Gaborone; MinistrY of Works and Communications.

Carney, J.N., Aldiss, D.T. and Lock, N.P. (1994) The Geology of Botswana. Bulletin 37, Geological Survey Department. Gaborone, Government Printer.

Botswana National Atlas (2001) Gaborone, Department of Surveys and Mapping.

Cooke, H.J. (1975) The Lobatse Caves, Botswana Notes and Records, 7: 29-34.

Dougill, A.J. and Thomas, A.D. (2002) Nebkha dunes in the Molopo Basin, South Africa and Botswana: fonnation controls and their Validity as indicators of soil degradation.

Journal of Arid Ehvironments 50: 413 - 428.

Dougill, A. and Trodd, N. (1999) Monitoring and modelling open savannas using multisource information: analysis of Kalahari studies. Global &ology and Biogeography, 8:

211-221.

Dougill, 1.A., Twyman, C .• Thomas, D.S. G. and Sporton, D. (2002) Soil degradation assessment in mixed farming systems of southern Africa: use of nutrient balance studies for participatory degradation monitoring. The Geographical Journal, 168 (3):

195-210. 46 ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e Pu bl is he r ( da te d 20 09 ).

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BotswfUUJ Notes and Records, Volume 36

Holmgren, K. and Shaw, P. (1999) A late Pleistocene palaeoenvironmental record from Lobatse II Cave, Botswana Notes and Records, 31: 73-81.

Hyde, L.W. (1971) Ground-water supplies in the Kalahari area, Botswana. Proceedings of the conference on sustained production from semi-arid areas, October 1971, Gaborone. Botswana Notes and Records, Special Edition No.1: 77-87. Jain, P., Leipego, A., Kerekaog, K., Mathangwane,

F.

and Molosiwa, K. (2001) Energy,

Botswana National Atlas. Gaborone, Surveys and Mapping 149-156.

Kreimeyer, R. and Siamisang, T.L. (1990) The aggregate and building stone resources of

Botswana. Geological Survey Department. Gaborone, Government Printer ..

National Master Plan for Arable Agriculture and Dairy Development (NAMPAAD). Paper No. 1,2002. Gaborone, Ministry of Agriculture.

Moganane, B., Totolo, 0., Mafoko, T., Ringrose, S. and Kwerepe, R. (2001) Botswana

National Atlas. Gaborone, Surveys and Mapping.

Pike, J.G. (1971) Rainfall over Botswana. Proceedings of the conference on sustained production from semi-arid

areas,

October, 1971, Gaborone. Botswana Notes and

Records, Special Edition No.1: 69-76.

Ramothwa, G. and Minja, W. (2001) Weather and climate, in: Botswana National Atlas. Gaborone, Surveys and Mapping 31-44.

Sebego, R.J.G. (1999) The ecology and distribution limits of Colophospermum mopane in southern Africa, Botswana Notes and Records, 31:53-72.

Timberlake, J.R. (1980) Handbook of Botswllllf,l acacias. Gaborone, Ministry of Agriculture. Thomas, D.S.G. (1997) Arid zone geomorphology: process, form and change in drylands

(edited). Chichester, John Wiley and Sons.

Thomas, D.S.G. and Shaw, P. A. (1991) The Kalahari environment. Cambridge, Cambridge University Press.

Whiteman, P.T.S. (1971) Limitations to crop production in the Kalahari. Botswana Notes

and Records, Special Edition No.1: 114-121.