The Limpopo River Basin is situated in the east of southern Africa between about 20 and 26 °S and 25 and 35 °E. It covers an area of 412 938 km2. Figure 4 shows the basin in relation to major physical features of the subcontinent. The basin straddles four countries: Botswana, Mozambique, South Africa and Zimbabwe. Figure 5 shows the main overland transport routes, urban centres, rivers and nature conservation areas in the basin.
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FIGURE 4
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FIGURE 5
The Limpopo River Basin in relation to
transport routes and urban centres
Climate conditions vary considerably in southern Africa, as the subcontinent lies at the transition of major climate zones. The climate in the Limpopo River Basin is influenced by air masses of different origins: the equatorial convergence zone, the subtropical eastern continental moist maritime (with regular occurrence of cyclones), and the dry continental tropical and marine west Mediterranean (winter rains) (Bhalotra, 1987b; GOB-MMRWA, 1991; Schulze, 1997; Unganai, 1998).
According to the Köppen Classification (Köppen, 1918; Rosenberg, 1999), the basin is predominantly semi-arid, dry and hot (BSh in Figure 6). The central river valley is arid, dry and hot (BWh). Here, the average rainfall is less than 400 mm with likely crop failure in 75-90 percent of years (Reddy, 1985; 1986). The South African highveldt part of the basin is temperate with summer rainfall and cool to hot summers (Cwc and Cwa). The Mozambique coastal plain is mainly warm-temperate with no dry season and hot summers.
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FIGURE 6
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The Limpopo River Basin is a region of summer rainfall, generally with low precipitation. The overall feature of the mean annual precipitation is that it decreases fairly uniformly westwards from the northern reaches of the Drakensberg Escarpment across the interior plateau. However, rainfall is highest on the Drakensberg Escarpment because of its orographic effect. There is also a north-south rainfall gradient towards the Limpopo River.
Rainfall varies from a low of 200 mm in the hot dry areas to 1 500 mm in the high rainfall areas. The majority of the catchment receives less than 500 mm of rainfall per year (Figure 7). The hot dry areas receiving about 200-400 mm of annual rainfall are located mostly within the main Limpopo River Valley itself.
FIGURE 7
Average seasonal total rainfall in the
Limpopo River Basin
Source: UNCTAD (2003).
Rainfall is highly seasonal with 95 percent occurring between October and April, often with a mid-season dry spell during critical periods of crop growth. It occurs on a few isolated rain days and isolated locations, seldom exceeding 50 rain days per year. Rainfall varies significantly between years, with maximum monthly rainfall being as high as 340 mm compared with mean monthly rainfalls of 50-100 mm for January, February and March.
The Limpopo River Basin generally experiences short rainfall seasons, except for some of the outer limits of the basin that have higher rainfall and longer seasons. The rainfall concentration index is 60 percent and above, and this limits crop production because most of the annual rainfall is received in a short period of time.
A rainfall concentration index of 100 percent implies that a location receives all its rainfall in a single month. The rainfall season usually begins in early summer (late November to early December) for the southernmost parts of the basin and in mid-summer (mid-December to January) for the central parts of the basin around the Limpopo River itself. The rainfall season lasts an average of four months.
Rainfall in Botswana is caused mainly by convection thunderstorms, which typically occur as localized events with a high spatial and temporal variability. The annual rainfall in the Botswana part of the Limpopo River Basin varies from 350 mm in the northeast to about 550 mm in the southeast.
Zimbabwe experiences a single annual rainy season of five months (November-March), associated with the summer movement of the Inter-Tropical Convergence Zone over southern Africa. Within the Zimbabwe part of Limpopo River Basin, the mean annual rainfall varies from slightly more than 600 mm in the southern highveldt (Bulawayo) to less than 400 mm in the southeastern lowveldt (Tuli and Beitbridge). The annual variability is considerable, with a coefficient of variation (CV) of about 40 percent. The probability of receiving more than 500 mm of rainfall in any year is less than 60 percent in the southern highveldt and less than 30 percent in the southeastern lowveldt (with less than 10 percent in Beitbridge).
In Mozambique, the generalized rainfall pattern shows a sea-to-land gradient with a CV of about 40 percent in the Limpopo River Basin. Along the coastal strip, the mean annual rainfall is 800-1 000 mm, declining to less than 400 mm in the dry interior bordering Zimbabwe.
Rainfall generally has to exceed a minimum threshold of 20-30 mm before any runoff occurs, owing to high temperatures, low humidity and flat terrain. Many rainstorms are less than this and hence the flow regimes of rivers vary considerably. This results in high storage requirements for dams in order to deliver the yields that are required. Increased storage is costly and causes increased evaporation losses.
Evaporation within the Limpopo River Basin varies from 1 600 mm/year to more than 2 600 mm/year. The highest evaporation occurs in the hot Limpopo River Valley. High levels of evaporation mean that the soil dries up quickly and this reduces the amount of water available for plant uptake. This results in crops being more prone to drought.
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FIGURE 8
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Evaporation from open water in Botswana varies from about 1 900 mm/year to 2 200 mm/year. Slightly lower annual figures of about 1 500 mm are derived from evapotranspiration calculations based on the Penman method. Daily figures range from about 2 mm to 5 mm. Evaporation is highest during the rainfall season, and it significantly reduces effective rainfall, runoff, soil infiltration and groundwater recharge. Evaporation loss from dams is significant owing to the high storage-yield relationship and flat dam basins.
Dryland subsistence farming is generally not viable given the variable rainfall, high evaporation and high evapotranspiration. Figure 8 shows the estimated length of growing period (LGP) derived from satellite imagery. Figure 9 shows the average crop water availability for selected months.
Summers in the Limpopo River Basin are generally warm, and winters are mild. In summer, daily temperatures may exceed 40 °C, while in winter temperatures may fall to below 0 °C. The general figures for air temperature are related closely to altitude, and also to proximity to the ocean. The mean maximum daily temperature in most of the Limpopo River Basin, notably South Africa, Botswana and Zimbabwe, varies from about 30-34 °C in the summer to 22-26 °C in winter. The mean minimum daily temperature in most areas lies between 18-22 °C in summer and 5-10 °C in winter.
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FIGURE 9
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Source: E. Mellaart, personal communication (2003).
The eastern and northern parts of the Limpopo River Basin are frost-free while the southern and western areas experience winter frosts. Frost does not occur in Mozambique and it occurs only occasionally in the southern highveldt of Zimbabwe, associated with an influx of cold dry air from the southeast. Frost-free areas also exist in the lowveldt of South Africa and along the Limpopo River in the Messina area.
Most of the higher-lying areas in South Africa and Botswana within the Limpopo River Basin experience frost, occurring most severely in the southwest of the basin. This may be very moderate in the areas of Tzaneen (Limpopo Province of South Africa) or Mahalapye (Central District of Botswana), but increases to 90-120 days of frost in Lobatse (southeast Botswana) or Mafeking (North West Province of South Africa). The average number of days with heavy frost in these areas is about 30 days. This does not imply that frost occurs over a short uninterrupted period. On the contrary, single or clusters of frost days may occur over a long period, usually between May and September. This may create a problem for late-planted crops.
Relative humidity is generally higher on the eastern side of the Limpopo River Basin, and decreases inland. The relative humidity varies from less than 50 percent in September and October in the hot western parts of the basin in South Africa, to about 65 percent in January and February. Humidity in the lowveldt in South Africa varies only slightly (65-70 percent) in the same period.
Relative humidity in Botswana is comparatively low, with daytime averages of about 30 percent in winter and 40 percent in summer. However, much higher values are reached in the morning, nearing 60 percent in winter and more than 70 percent in summer. Humidity also increases before rainstorms, and is therefore highest between January and March. The dry western parts of Botswana record the lowest humidity.
There is considerable spatial and temporal variation in the rainfall regime in the Limpopo River Basin, as in most dryland areas, as much of the rainfall occurs in a limited number of rain events. A prerequisite to effective agriculture is a description of the rainfall regime in response to questions (Dennett, 1987) such as:
When is rain likely to occur?
What is the probability of a dry spell greater than a certain length?
What is the probability of receiving a daily rainfall greater than a particular amount?
Probabilistic rainfall models to address these questions have been developed by Stern, Dennett and Dale (1982), and Stern and Coe (1982). These models have been applied to rainfall data from Gaborone and Tshane, two Botswana stations, 350 km apart. Model results calculated over a ten-year period indicate that as much as 50 percent of the total rainfall occurs in the 10 percent wettest days, and 80 percent in the 23 percent wettest days. The pattern is apparent in both dry and wet years. In addition, days with high rainfall are clustered. Understanding of such patterns is of prime interest because they determine the length of the growing season.
Reddy (1985, 1986) reported that the Limpopo River Basin in Mozambique presents a high risk of agricultural drought, depending on the type of dryland cropping systems in place. Here, there is high variation in terms of both commencement and cessation times of effective rains; that is, the risk associated with planting time. In terms of reliability, the erratic rainy season may begin any time from November to February. Therefore, average planting dates are only 50-percent reliable. Only 25 percent of the rainy seasons have 120 crop days. These begin on the average date for the rainy season, in December, whereas 25 percent of the years have 120 crop days starting later than this. Half of the remaining years have rainy seasons of more than 60 crop days.
Moreover, Reddy (1986) classified the upper Limpopo River Basin extending to the Zimbabwe border as a very high-risk area with probable crop failure in 75-90 percent of years. The dry semi-arid zone of the middle Limpopo River Basin extending to the lower Limpopo just off the coastline was assessed as a moderate to high-risk dryland agricultural zone where crop failure is expected in 45-75 percent of years. Kassam et al. (1982) determined the pattern of growing period zones in Mozambique. The interior of the Limpopo River Basin permits one growing period per year in 30 percent of the years, two growing periods per year in 45 percent of the years, and three growing periods per year in 25 percent of the years. The mean total dominant LGP for the middle and upper Limpopo River Basin was calculated at less than 120 days, compared with a gradient from 120 to 270 days at the coast (Figure 8).
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FIGURE 10
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Using gridded SADC-RRSU data, Mellaart (personal communication, 2003) illustrates the distribution of rainfall over ETo in the basin area in Figures 9-12. The class R < 0.5 ETo denotes non-arable conditions. The class R = 0.5-1.0 ETo indicates marginally arable to arable conditions. The availability of good soils with favourable water-holding characteristics determines the agricultural potential of these areas. The class R > ETo is restricted to mountainous areas of the eastern escarpment receiving high orographic rainfall. These generally steep areas are generally under plantation forestry.
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FIGURE 11
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FIGURE 12
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Source: E. Mellaart, personal communication (2003).
Predicting drought is very complicated and has so far been unreliable, although there are strong indications for cyclic occurrence of drought, notably in southern Africa. In general, cycles of drier years are followed by successive seasons with opposite conditions. However, after two dry years in a recognized drought cycle, there is no guarantee that the third year will also be a drought year.
El Niño/La Niña phenomena and the Southern Oscillation
In a study on the impact of El Niño - Southern Oscillations (ENSOs) on the climate and crop production in Zimbabwe, Deane (1997) found that ENSO events (Box 4) do affect the subcontinent and that these provide instruments for assessing climate events at the natural region level. Deane concludes that research on the ENSO phenomena has the potential to result in improved management of the risk posed by weather through enabling potential drought years, as well as years with very good rainfall, to be better prepared for.
Recurrent droughts have put strong political pressure on meteorological services and early-warning systems to produce reliable forecasts.
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BOX 4 El Niño - La Niña events and the Southern Oscillation El Niño refers to the large-scale warming of the equatorial eastern and central Pacific Ocean due to a disruption of the ocean-atmosphere system (Figure 13). El Niño events occur irregularly at intervals of 2-7 years, although the average is about once every 3-4 years. They typically last 12-18 months. They have important consequences for weather and climate around the globe, including lower than normal rainfall for South Africa accompanied by higher than normal rainfall for central-east Africa. La Niña refers to unusually cold ocean temperatures in the equatorial Pacific. The impacts of La Niña tend to be opposite to those of El Niño. Various indices of sea surface temperature deviation are obtained by taking the average deviation over some specified region of the ocean. Figure 14 shows the Sea Surface Temperature Index for the NINO 3.4 area. (For assessing widespread global climate variability, NINO 3.4 is generally preferred, because the sea surface temperature variability in this region has the strongest effect on shifting rainfall). El Niño/La Niña events are accompanied by swings in the Southern Oscillation. The Southern Oscillation Index (SOI) is defined as the normalized difference in barometric pressure between Tahiti (French Polynesia) and Darwin (Australia). It is intimately related to the ocean temperature changes mentioned above and is a measure of the strength of the trade winds. SOI values (Figure 15) generally vary between +30 (La Niña) and -30 (El Niño). Together, these phenomena are referred to as ENSO (NOAA, 1994; University Corporation for Atmospheric Research, 2001; Pacific Marine Environmental Laboratory, 2003; Commonwealth Bureau of Meteorology, 2003) |
Although substantial progress in the ENSO interpretation has been made, actual climate conditions in recent years have, to a large extent, not corresponded with the predicted outcomes. This raises concern about the reliability of the early-warning information used and applied, in particular in the southern African region. Until the mid-1990s, the general practice of declaring drought was based on the actual occurrence of drought. The severe drought of the 1991/92 season in southern Africa was only recognized officially as such as late as January 1992, well into the agricultural season.
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FIGURE 13
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Source: NOAA (1994).
The first time that drought was forecast in a very early stage - on the basis of global interpretations of the effects of El Niño - was in June 1997, when severe drought was predicted for the 1997-98 season (SADC, 1999). This led to actions by governments in the SADC region towards information dissemination and providing planting advice to farmers. Recommendations to farmers ranged from the planting of drought-tolerant and early-maturing varieties to destocking (Box 5). Even with improvements in the reliability of the climate forecasts, the occurrence of recurrent drought and related risks have to be accepted and integrated into land use systems sustainable under the present climate conditions. The prospect of accelerated global warming, and associated regional changes in climate, reinforces the need for the consideration of the longer-term constraints that future climate may place on developments in the region. Recent studies conducted on climate variability and change in the region give strong indications of regional temperatures rising in coming decades (Hulme, 1996; Hulme and Sheard, 1999). Rising temperatures could change the rainfall regime in the coming decades, resulting in changes in natural vegetation, as well as agriculture and range conditions and water resources.
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FIGURE 14
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Source: International Research Institute for Climate Prediction (2002).
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FIGURE 15
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Source: Commonwealth Bureau of Meteorology (2003).
Africa is considered highly vulnerable to climate change. The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) notes a warming of about 0.7 °C over most of the African continent during the twentieth century based on historical records (UNEP, 2002). It was found with respect to Zimbabwe that the diurnal temperature range is decreasing. There are more hot days and fewer cold days over time. Night-time minimum temperatures increased at twice the rate of daytime maximum temperatures. Precipitation deviations from a long-term mean are stated to have increased during the last century. While the exact nature of the changes in temperature or precipitation and extreme events are not known, there is general agreement that extreme events will become worse, and trends in most variables will change in response to warming. The expected warming is greatest over the interior semi-arid margins of the Sahara and central-southern Africa. Figure 16 illustrates anomalies during the past 100 years in mean surface temperatures in Africa. A notable upward trend/cycle is shown for the past 25 years. This rate of warming is similar to that experienced globally (UNEP, 2002).
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BOX 5 Farmers and climate forecasts, Zimbabwe In Zimbabwe, only 3 percent of farmers use climate information for planning purposes. Some of the reasons given are that the information is not received in time and that farmers do not trust the meteorological information. Although farmers listen to climate forecast from radios, the poor and marginalized farmers prefer to use their traditional knowledge systems as a control. When contemporary climate forecasting deviates from traditional forecasts, the farmers’ inclination is towards indigenous information for reasons that it blends well with the culture, has been tried and tested over the years, and is in a language that the farmers understand. There is often a striking similarity between indigenous and contemporary climate indicators. Some indicators are the same in both systems, such as wind direction, clouds and temperature. In addition, indigenous climate predictions are also based on plant and animal behaviour. Farmers associate heavy production of tree leaves with a good season while high fruit production is a sign of a poor season. The reasoning behind this observation is that high fruit production implies that people will be living on fruits for lack of alternative foods. The production of white flowers by a local tree called mukuu is also a signal for a dry season, while flower production on top branches of a tree called mukonde indicates a good rainy season. Other indigenous signs of an imminent drought include: heavy infestation of most tree species by caterpillars during springtime; late bearing and lack of figs in July-September of a tree called mukute; late maturing of acacia trees along valleys; and drying off of chigamngacha fruit between September and early November. One of the most important animal indicators is the behaviour of spiders. When spiders close their nests, an early onset of rain is expected because spiders do not like any moisture in their nests. When a lot of crickets are observed on the ground, a poor rainy season is expected. The movement of elephants is associated with occurrence of rainfall because they need a lot of water. A stork flying at very high altitude is associated with a good season. Observing a bird singing while facing downwards from the top a tree is a good indicator that it is about to rain, while a lot of birds is a sign of heavy rain. The wind blowing from west to east, and from north to south, is assumed to bring a lot of moisture and a good rainy season. The prevalence of a strong wind from east to west during the day and at night between July and early November is an indicator of drought. |
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FIGURE 16
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Source: UNEP (2002).
Physiography relates to the physical features of the earth, and it is used here to describe the landscapes of the Limpopo River Basin. The physiographic features of a region commonly affect its climate patterns and tendencies (e.g. rainfall intensity and distribution) and water drainage patterns (surface and subsurface). An important application of physiographic classification is the provision of a physical framework for land use planning in general and catchment management in particular. In addition, the physiography, together with the climate, forms the basis of agro-ecological zoning (AEZ), which is discussed in a later section.
Landform and soil development are linked to geological and tectonic development at a subcontinental scale. Much of the present landscape of the Limpopo River Basin reflects recent geological events - in geological terms - following the break-up of Gondwanaland (Moon and Dardis, 1988). Tankard et al. (1982) and McCourt and Armstrong (2001) recognized the sequence of crustal evolutionary stages in southern Africa.
The Limpopo River Basin is located on the northeast edge of the Kaap-Vaal or Kalahari craton (KvC in Figure 17) and extends onto the southern part of the Zimbabwe craton (ZwC in Figure 17). The Limpopo mobile belt (granulite facies; shown in centre of the basin in Figure 17) and the Bushveld Igneous Complex (not shown in Figure 17) separate the two. The cratons constitute a stable shield, predominately of igneous and metamorphic rocks, at the base of the continental crust. The Kaap-Vaal craton is mostly covered sedimentary rocks. The genesis of the Basement Complex covers a period of 1 000 million years, falling within the Archaean period.
Granite and gneiss are the dominant rock types of the Basement Complex on the highveldt and escarpment of South Africa, with quartzites, granodiorites, and various slightly to moderately metamorphosed sedimentary rock occurring subordinately. The southern part of the Limpopo River Basin within the highveldt is characterized by the occurrence of Karoo sediments (Vryheid Formation), including sandstones, claystones, shales, and coal deposits. Karoo sediments and basalt also occur in a strip from northeast Botswana through southern Zimbabwe. Similar Cretaceous sediments (sandstones, grits and conglomerates) border Zimbabwe with Mozambique. The eastern strip of the lowveldt and the Lebombo Ridge are also dominated by similar Karoo formations, with subordinate occurrence of dolerite intrusions.
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FIGURE 17
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Source: McCourt and Armstrong (2001).
Cycles of erosion have shaped the present landscapes of southern Africa. There is general agreement with respect to the major phases, but opinions differ regarding the more complex subdivisions (King, 1976; Partridge and Maud, 1987). Most of the Limpopo River Basin shows relatively advanced eroded conditions, and often shows younger and shallower soils as compared with less-eroded surrounding areas.
Erosion cycles during the early Tertiary period formed the African denudational surface at high or medium plateau level, such as the highveldt in South Africa. Its major occurrence is southwest of the southern divide of the Limpopo River Basin, the high-level plateau zone in Zimbabwe, the elevated areas near Polokwane (Pietersburg) in South Africa, and the flat-topped hills in eastern Botswana and Limpopo Province in South Africa.
Further erosion in the late Tertiary period formed the Post-African denudational surface. Various phases of this surface are dominant in the Limpopo River Basin. The most recent erosion was active during the Quaternary period, primarily downstream of the main rivers and tributaries in the basin area.
Most of the land within the Limpopo River Basin in Mozambique was formed by aggradational surfaces during the Quaternary and Tertiary periods, except for a band of Cretaceous rocks occurring north of the Save River to the border with Zimbabwe. Extensive, well-developed alluvial formations occur in the middle and lower reaches of the Limpopo River, and in the watercourses of the non-perennial rivers entering the Limpopo River. The oldest formations (Palaeocene and Eocene rocks) are of marine facies and correspond to the calcareous sandstones and conglomerates, which are disconformably overlain and border the effusive Karoo formations on the western border with South Africa.
The main unit of the Quaternary cover is a thick, homogenous mantle of yellowish-brown, saline, sodic, calcareous, sandy clay loam extending over the vast interior of Gaza Province west of the Limpopo River. It builds large, slightly sloping plateaus called Mananga developed over sedimentary, coarse and siliceous rocks. Near the incised valleys, the basal gravels have been exposed after erosion of the Mananga cover. Different cycles of weathering and landscape lowering reworked the resistant gravels into basal gravel floors. The highest gravels correlate to the red sandstones and conglomerates of the late Tertiary. The higher, lower and young gravels are associated with the respective Mananga platforms.
The three major physiographic units of the interior are the South African Plateau (or Cape-Transvaal highveldt), the Zambia-Zimbabwe Plateau and the Kalahari Basin (Bridges, 1990). The Great Escarpment separates these units from the coastal ranges and coastal plains.
Figures 18 and 19, derived from digital elevation modelling, illustrate the basin structure with elevated remnants in places. The Limpopo River Valley separates the plateau areas of South Africa and Zimbabwe, and is bounded to the west by the Kalahari Basin. Towards the east, the Elephants River and several other smaller tributaries of the Limpopo River (see also Figure 7) traverse the Lebombo Ridge before joining the Limpopo River on the coastal plain in Mozambique.
At a generalized level, plains at various altitudes are the dominant landforms of the basin. These are interspersed with low-gradient hills, locally incised valleys and medium-gradient mountains (e.g. the South African Waterberg plateau and the Soutpansberg mountain range). The morphology of the basin, in particular the position of the mountain ranges, has a strong influence on the climate and rainfall pattern in the basin.
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FIGURE 18
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FIGURE 19
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Source: FAO - ISRIC (2003).
Botswana
The land systems of Botswana (De Wit and Bekker, 1990) follow the original land systems concept developed in Australia. This is a hierarchical system based on the subdivision of larger land units into smaller land units, using various criteria linked to major landforms, geomorphological forms, and geology (lithology). This approach is useful for exploratory and reconnaissance mapping, but is of limited use in regional correlation exercises given the use of local nomenclature and non-standard terminology. The Limpopo River Basin falls within the major land division of the hardveldt, a mainly flat to undulating surface with occasional hills and ridges that developed on the Basement Complex. The associated soils vary strongly, especially in depth and clay content, and hence in water-holding capacity and sensitivity to drought.
Mozambique
In Mozambique, the Limpopo River Basin is almost flat, with a gentle slope in a northwest-southeast direction. The Limpopo River crosses a fluvial plain with terraces 1-3 km wide before the confluence with the Elephants River, widening to 2-5 km after the confluence. All basins in Mozambique lie below 400 m above sea level. The Changane River Valley is unusual in that it is situated along an old beach line and flows intermittently.
The main physiographic features of the basin in Mozambique are captured on a generalized landscape map at scale 1:1 million whose units are derived from geology, geomorphology and soils. Landscape patterns are described based on landform, and topography and slope subdivisions, of which 12 classes are applicable to the Gaza Province in which the Limpopo River Basin is located (Figure 20).
South Africa
The terrain morphology map of southern Africa (Kruger, 1983) depicts six broad terrain divisions subdivided according to relief, topography, slopes, and drainage density. The map provides a useful pattern of the landscapes of South Africa, but lacks systematic definitions of landforms and other terrain units, and is thus difficult to correlate with approaches used by other countries. At a more detailed level, the procedure for terrain description employed by the national land type survey (Turner and Rust, 1996) describes the terrain or relief of an area quantitatively by means of two parameters: percentage of level land and local relief (Table 3 and Figure 21).
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FIGURE 20
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FIGURE 21
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Source: ARC-Institute for Soil Climate and Water (2004).
Zimbabwe
Lister (1987) describes four geomorphic provinces within Zimbabwe: the Eastern Highlands, the Limpopo-Save Lowlands, the Zambezi Valley, and the Central Axis (including highveldt, middleveldt and Kalahari sandveldt). The Limpopo River Basin takes up parts of the Limpopo-Save Lowlands and parts of the highveldt and middleveldt subdivision of the Central Axis. Other than a relief map at a scale of 1:1 million (GOZ-Surveyor-General, 1984), no systematic physiographic inventory of Zimbabwe is available.
A description of the physical resources of the communal lands by Anderson et al. (1993) includes a generalized account of the landform of the mapped land units (1:500 000), e.g. “the landform is characteristically almost flat to gently undulating with slopes mainly less than 2 percent”. The land units described in this study cover 42 percent of the country.
TABLE 3
Terrain parameters used in the South African
Land Type Survey
|
Percentage level land1 |
Local relief (m)2 |
Terrain type description3 |
||
|
Symbol |
Class |
Symbol |
Class |
|
|
A |
> 80 |
1 |
0-30 |
Level plains |
|
2 |
30-90 |
Level plains with some relief |
||
|
3 |
90-150 |
Plains with open low hills or ridges |
||
|
4 |
150-300 |
Plains with open high hills or ridges |
||
|
5 |
300-900 |
Plains with open low mountains |
||
|
6 |
> 900 |
Plains with open high mountains |
||
|
B |
50-80 |
1 |
0-30 |
Rolling or irregular plains with low relief |
|
2 |
30-90 |
Rolling or irregular plains with some relief |
||
|
3 |
90-150 |
Rolling or irregular plains with low hills or ridges |
||
|
4 |
150-300 |
Rolling or irregular plains with high hills or ridges |
||
|
5 |
300-900 |
Rolling or irregular plains with low mountains |
||
|
6 |
> 900 |
Rolling or irregular plains with high mountains |
||
|
C |
20-50 |
1 |
0-30 |
Open low hills or ridges with low relief |
|
2 |
30-90 |
Open low hills or ridges |
||
|
3 |
90-150 |
Open hills or ridges |
||
|
4 |
150-300 |
Open high hills or ridges |
||
|
5 |
300-900 |
Open low mountains |
||
|
6 |
> 900 |
Open high mountains |
||
|
D |
< 20 |
2 |
30-90 |
Low hills or ridges |
|
3 |
90-150 |
Hills or ridges |
||
|
4 |
150-300 |
High hills or ridges |
||
|
5 |
300-900 |
Low mountains |
||
|
6 |
> 900 |
High mountains |
||
1 Land with slope of less than 8 percent.
2 Average difference between the highest and lowest point in the landscape as measured per 7.5 by 7.5 minute sampling area.
3 After Kruger (1973; 1983).
Source: ARC-Institute for Soil Climate and Water (2004).
The following main landforms occur: plateau, hills, escarpment and plains.
Plateau
The plateau (flat to undulating, 600-1 500 m above seal level) includes the highveldt area of Botswana, Zimbabwe and South Africa. Although the Limpopo drainage system has eroded deeply into the overall plateau, it has not formed a distinct valley and thus a separate landform unit. Slopes towards the rivers are generally gradual. The plateau includes subordinate occurrence of groups of hills and ridges, which can be distinguished at more detailed scales.
In Botswana, the Limpopo River Basin starts within the transition boundary with the Kalahari sands. At Serowe, a distinct escarpment is formed in Karoo sandstone. The majority of the plateau is flat to gently undulating, in places undulating to rolling with kopjes. The main rock type is granite or granitic gneiss. Sandstones occur south of Mahalapye, and basalt dominates the eastern tip of Botswana, with some subordinate occurrences at Serowe.
Several groups of relatively small hills occur in the southern and central parts of the Botswana hardveldt, in particular near Gaborone and Palapye, often with flat tops at levels of about 1 200 m above seal level, corresponding with the African planation surface. These hills of medium relief (200-400 m above the base) consist mostly of sedimentary rock, but are also formed of dolerite (Shoshong) and other rock. The hills, e.g. the granite hills near Mahalapye, are often associated with pediments.
The plateau developed on the Basement Complex continues on the eastern side of the Limpopo River, in South Africa, also with a flat to gently undulating topography. The occurrence of Karoo sediments (sandstones and shales), including coal deposits near Middelburg and Witbank, characterizes a large area of the southern highveldt.
North of the Limpopo River, the plateau includes parts of the southern highveldt region of Zimbabwe (above 1 200 m above seal level, with Plumtree-Bulawayo as the main catchment) and the adjacent southeast middleveldt region (600-1 200 m, near Gwanda and surroundings). Tributaries of the Shashe and Limpopo Rivers, which run in a north-south direction, dissect the middleveldt. Rocks of the Basement Complex with greenstone belts dominate the geology.
Hills
In South Africa, quite large hilly areas (rolling, 400-600 m above sea level) and ridges occur in the southwest half of Limpopo Province. These hills of the Bushveldt Complex include the Waterberg Hills - which could also be described as a plateau - and also groups of hills towards the southern edge of the catchment (Pilanesberg and Magaliesberg). The lithology of these hills differs from the granite of the basement, and includes quartzite and resistant rock types.
Escarpments
The escarpment zone is a complex landscape consisting of steep hills and mountains (600-1 500 m), forming the transition from the highveldt or Transvaal Plateau to the coastal plains of the lowveldt. The escarpment at the southeast divide of the Limpopo River Basin forms the watershed with the Komati River. Some parts rise to more than 1 500 m above sea level (medium relief class).
The Drakensberg Mountains form the highest part of the escarpment, rising above 2 300 m. Low, east-west mountain ridges (e.g. Soutpansberg and Strydpoortberg) arise above the plateau and link up with the escarpment. The escarpment is characterized by a complex of steep slopes between low and high levels, dissected plateaus and plateau remnants, with associated hills, valleys and basins.
Plains
The plains (gently undulating to undulating, 0-600 m) are the lowveldt of South Africa and Zimbabwe, and the coastal plains of Mozambique. The higher western part (300-600 m) forms the piedmont zone of the escarpment, consisting of eroded foot slopes, developed in mainly granite. Dolerite intrusions occur throughout the lowveldt.
The South African and Mozambican plains are separated by the Lebombo Ridge, which is a cuesta, or a tilted plateau with a steep escarpment bordering the lowveldt and a gradual dipslope of about 5 percent descending east into the coastal plains of Mozambique. This ridge, consisting of rhyolite, is more prominently developed towards the south, outside the Limpopo River Basin.
West of the Lebombo Ridge, north-south zones can be distinguished by rock types. In the Kruger National Park, there is a distinct zone of Karoo basalts, followed by Karoo sediments of the Ecca series (shales and sandstones) lying west.
The southeast lowveldt of Zimbabwe is a broad pediplain with elevations of generally less than 600 m and an almost flat to gently undulating topography. The transition to the middleveldt is gradual. The pediplain of the southeast lowveldt is developed in paragneiss of the Limpopo and Zambezi mobile belts, Karoo volcanics, and for a small part in Karoo sedimentary rocks.
In Mozambique, landforms in most of the interior basin east of the Limpopo River comprise flat to gently undulating plains, valleys, minor valleys and plateaus, not exceeding 5-8 ° in slope and 100 m in elevation. This unit coincides with the dominant soil characteristics belonging to the Mananga group of soils. Coastal dune and plain formations, extending inland from the coast to border the Changane River, dominate the landscape in the lower Limpopo River Basin. Extensive areas of floodplains exist along the Limpopo and Changane Rivers, with hills and minor scarps enclosing the middle-upper reaches of the Limpopo River as well as the Elephants River, the latter above 1 200 m.
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