The Swaziland Environment Action Plan (SEAP)

Contents | Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 | Chapter 7 | Chapter 8

2.0 The Physical Environment

2.1 Country Location and Physiography

Swaziland lies between latitudes 25 and 28 degrees south and 31 and 32 degrees east in the South Eastern part of Africa. The country is landlocked and covers an area of 17364 sq Km. It is bounded by south Africa in the north, west and south and by Mozambique on the east. It lies within the Maputoland centre of plant diversity an area reported to have the greatest biodiversity in Southern Africa.

Swaziland is located between the Transvaal plateau (reaching over 1500 metres) and the coastal plains of Natal, Mozambique and NE Transvaal. Thus the western part of the country lies in an escarpment area, and the eastern part in the zone of the coastal plains. Separating the Swaziland coastal plains from the Mozambique coastal plains, is the Lebombo ridge. Following is a general description of these main physiographic regions (Remmelzwaal, 1993) with an indication of the percentage each makes of the total land area of the country.

With its divergent geology, climate and subsequent landforms, the physiographic regions within the country's boundaries are very distinct. Swaziland's physiography has recently been reclassified primarily on the basis of landforms and elevations and secondly on geology and land forms. A distinction is now made between six physiographic regions, viz, highveld, upper middleveld, lower middleveld, western lowveld, eastern lowveld and the Lubombo Range.

The Swaziland Highveld (33%) is the upper part of an overall escarpment, consisting of a complex of steep slopes between low and high levels, dissected plateaux, plateau remnants, and associated hills, valleys and basins.

The Upper Middleveld (14%) consists of a strongly eroded plateau remnants and hills at an intermediate level of the overall escarpment. It also contains structurally defined basins in relatively protected positions, which are only weakly eroded.

The Lower Middleveld (14%) is basically the piedmont zone of the escarpment, characterized by generally strongly eroded foot slopes. The overall slopes are predominantly moderate and the zone classifies at the first level as a plain.

The Lowveld plain consists of sedimentary and volcanic Karoo beds versus the igneous and metamorphic rocks of the Highveld and Middleveld. The Lowveld is subdivided into the higher Western Lowveld (20%) on sandstone or claystone, and the lower Eastern Lowveld (11%) on basalt.

The sixth zone is the Lebombo Ridge (8%), a cuesta with a steep escarpment bordering the Eastern Lowveld and a gradual dipslope of about 1:20 descending east. As a major landform the Lebombo qualifies as a plateau.

Table 1: PHYSIOGRAPHIC ZONES AND MAJOR LANDFORMS IN SWAZILAND

Source: A Remmelzwaal (1993)

2.2 Climate

The overall climatic characterization of Swaziland is subtropical with summer rains (75 percent in the period from October till March) and distinct seasons. The physiographic zones show clearly different climatic conditions, ranging from subhumid and temperate in the Highveld to semi-arid and warm in the Lowveld. Swaziland lies at the transition of major climates zones, as it is influenced by air masses from different origin: equatorial convergence zone, subtropical eastern continental moist maritime (with occasional cyclones), dry continental tropical and marine west Mediterranean (winter rains, with occasional snow).

The table below gives an overview of some of the most relevant climatic conditions based on long term averages. The mean annual rainfall ranges from 1450 mm in the Highveld to 550 mm in the Lowveld, but conditions vary considerably from year to year. Years with lower than normal rainfall occur frequently, especially in the Lowveld, leading to drought. Drought has always been an inherent characteristic of the semi-arid climate. There are indications that dry periods with more frequent drought occur in cycles of approximately 17 years. These changes cannot be considered as part of a more permanent climatic change. Rainfall figures of the zones are overlapping, which is caused by the overall higher rainfall in the northern part of the country.

The mean temperatures given in the table below are zonal averages. Significant variation occurs only in the Highveld and Lebombo zones. Mbabane is representative for the Highveld average; Nhlangano and Usutu are 1-2 degrees warmer and colder respectively. Highest January mean maximum temperatures are recorded in the Eastern Lowveld (34^oC for Lavumisa at 200m above sea level), and lowest in the Highveld (22^oC for Usutu at 1450m). The lowest July mean minimum temperature of 5^oC occurs at Usutu, the highest of 10^oC at Lavumisa. Frost is recorded in all physiographic zones, but most frequently in the Highveld. In most zones frost is an annually occurring phenomenon, strongly controlled by local conditions and the position in the landscape, such as depressions and valley bottoms.

Table 2: Climatic and agro-climatic data

Koeppen classification

According to Koeppen's classification (see table above) the Highveld has a Cwb climate, which is defined as warm temperate rainy (C), with a dry season in winter (w) and a cool summer (b: warmest month below 22^oC). Upper Middleveld, Lower Middleveld and Lebombo fall into a Cwa climate, same as Highveld but with a hot summer (a: warmest month over 22^oC). The Western and Eastern Lowveld have a BSh climate, a dry-hot steppe climate. Further north along the Mozambican coast the climate changes to tropical (Aw), further south in KwaZulu Natal to rainfall all year round (Cf).

Agro-climatic characterization

The agro-climatic characterization provides essential climatic information for crop production but is also relevant to the occurrence of natural vegetation zones. The agro-climatic conditions are mapped and described by means of moisture zones and thermal zones (Van Waveren and Nhlengetfwa, 1992b). The moisture zones are classified on basis of the annual rainfall and the length of growing period (LPG). The LPG is a simple water balance based on rainfall, evapotranspiration and soil moisture storage capacity and provides an useful indication of the amount of water available to crops. In view of the high inter-annual variation in rainfall, the rainfall and length of growing period are expressed at a 80% reliability level as dependable annual rainfall and dependable LPG.

Swaziland is classified into six moisture zones: one Humid, two Subhumid, two Moist Semiarid and one Dry Semiarid zone. The dependable annual rainfall ranges from 1200 mm in the Humid zone in the western Highveld to 400 mm in the Dry Semiarid zone in the southeastern Lowveld. The corresponding LPG ranges from 290 days to 100 days. The important boundary between the Subhumid and Semiarid zones set at a dependable annual rainfall of 700 mm and a LPG of 180 days more or less follows the boundary between the Upper and Lower Middleveld.

• Humid (H). LPG 270-290 days; dependable annual rainfall 1000-1200 mm; mean annual rainfall 1250-1450 mm. Minor zone limited to highest parts of northern Highveld (3% of Swaziland)
• Subhumid (SH2). LPG 225-289 days; dependable annual rainfall 850-1000 mm; mean annual rainfall 1000-1250 mm. Covers the larger part of the Highveld (15% of Swaziland)
• Subhumid (SH1). LPG 180-224 days; dependable annual rainfall 700-850 mm; mean annual rainfall 850-1000 mm. Comprises the larger part of the Upper Middleveld, with parts of the Lebombo and Highveld (27% of Swaziland)
• Moist Semiarid (MSA2). LPG 150-179 days; dependable annual rainfall 550-700 mm; mean annual rainfall 725 -850 mm. Typifies the Lower Middleveld, but also includes the drier parts of the Lebombo and Upper Middleveld (21% of Swaziland)
• Moist Semiarid (MSA1). LPG 120-149 days; mean annual rainfall 625-725 mm; dependable annual rainfall 450-550 mm. Comprises northern and western parts of Lowveld (23% of Swaziland)
• Dry Semiarid (DSA). LPG 100-119 days; dependable annual rainfall 400-450 mm; mean annual rainfall 550-625 mm. Covers the southeastern Lowveld, the driest part of the country (11% of Swaziland).

The classification of the thermal zones is based on the mean annual temperature and the mean temperature over the growing period. Five thermal zones were mapped in Swaziland with mean annual temperatures ranging from 14^oC in the highest parts of the country to 22.5^oC in the Lowveld and mean temperatures over the growing period ranging from 15^oC to 25^oC.

• Cool (C). Mean temperature over growing period 12.5-15^oC; mean annual temperature less than 14^oC. Minor zone confined to the highest parts of the country (0.1% of Swaziland)
• Moderately cool (MC2). Mean temperature over growing period 15-17.5^oC; mean annual temperature 14-16^oC. Minor zone confined to higher parts of Highveld (5% of Swaziland)
• Moderately cool (MC1). Mean temperature over growing period 17.5-20^oC; mean annual temperature 15-17.5^oC. Covers the larger part of the Highveld (20% of Swaziland)
• Moderately warm (MW2). Mean temperature over growing period 20-22.5^oC; mean annual temperature 17.5-20^oC. Comprises the larger part of the Upper Middleveld and the highest parts of the Lebombo Range (19% of Swaziland)
• Moderately warm (MW1). Mean temperature over growing period 22.5-25^oC; Mean annual temperature 20-20.5^oC. Covers the Lower Middleveld, Lowveld, and the lower parts of the Lebombo (56% of Swaziland)

2.3 Geology

Main rock types of the physiographic zones

The major rock types of the six physiographic zones are indicated in table 1 above. Highveld and Middleveld are composed of the igneous and metamorphic rocks of the Archean basement complex, whereas the Lowveld and Lebombo are characterized by the sedimentary Karoo formations.

The dominant rock type in the Highveld is granite. The granites originate from different igneous events, of which the Mswati granite is the youngest magmatic phase. One of the Mswati plutons is most impressively outcropping as the Sebebe hills north of Mbabane. The metamorphosed sedimentary rocks of the Onverwacht group (ocean floor volcanics, flysch and molasse) and other metamorphic rocks (gneiss and quartzite) occur subordinately. The Upper Middleveld is characterized by granodiorite (igneous rock less acid than granite) and granite, with gneiss and shale subordinate. The most commonly occurring rock type of the Lower Middleveld is the Ngwane gneiss, followed by granites and granodiorites.

The Western Lowveld is made up by the sandstones, claystones, coal and other sedimentary rocks of the Karoo Ecca series, with subordinate dolerite intrusions. The Eastern Lowveld consists of the Karoo basalts (basic volcanic rock), which may be up to 5km thick. The Lebombo Ridge consists of the youngest Karoo rock type of rhyolite (volcanic rock more acid than basalt). The rhyolite formation is described as ignimbrite, a deposit resulting from glowing clouds or avalanches.

Economic geology and environmental aspects

Minerals of economic importance from the Archean formations include asbestos (chrysotile), iron, talc (soapstone), serpentinite, gold, tin and various other base metals such as copper, nickel and chromium. Exploitation, however, is not always economically viable. From the Karoo formations only the coal in the Ecca series is of economic importance. Alluvial reworked diamonds are found derived from kimberlite pipes. Kaoline is an alteration product of other rock types. Quarried stone is processed for road construction. Sand and gravel is extracted from riverbeds and other sources.

The mining of minerals has significant ecological consequences (see also Fakudze, 1996). In Swaziland there is a general lack of rehabilitation and ecological protective measures. Some of the most common environmental effects of mining include: (1) the destruction of landscape and ecosystems by open cast mining; (2) waste accumulation; (3) groundwater contamination by leachates from waste; (4) lowering of the ground water level; (5) toxic concentrations of elements such as copper, nickel, zinc and chromium; (6) environmental health threats through unsafe mining operations in general, and specific minerals (e.g. asbestos) in particular.

2.4 Soils

Characterization of the soils and soil properties of Swaziland is necessary to determine the present status of the soil and its potential for various land uses. Soils reflect environmental changes and it is relevant to monitor these changes.

Soils in Swaziland have developed over long periods and their occurrence falls into two main groups. The first group consists of old soils formed on deeply weathered rock (saprolite), often tens of metres deep, whereas the second is relatively young and has formed on eroded rock or alluvial deposits.

The deeply weathered old soils are found in the Highveld and Upper Middleveld. Over long periods they have formed on the weathering mantle or saprolite which has developed under warm and humid climatic conditions required for high intensity chemical weathering. The weathering period dates back to the Cretaceous, or some hundred million of years (see table 2; see also Mushala et al., 1995). Denudation of the higher crest areas in the weathered zone took place during arid and erosive periods in both the Cretaceous and Tertiary.

In places these deeply weathered and clay-rich soils that developed over this long period have survived the major erosion cycles, especially when occurring in favourable and protected positions such as basins like the Ezulwini valley. However, as a result of local cycli of soil formation, erosion and sedimentation, complex patterns of deposits and soils developed, as evidenced by quartz stone lines, palaeosols and other relict features. The polygenetic profile structure of many of the colluviated soils is reflected by their fabric and other characteristic features resulting from processes active in the past, such as illuviation, dissolution, mineral transformation and translocation of materials. Present soil formation is mainly characterized by ferralitization and kaolinitization.

Younger and less weathered soils characterize the Lower Middleveld, Lowveld and Lebombo. There are two reasons why soils in the lower eastern part of the country are younger. The first is that this part has been strongly eroded by geological erosion cycles which had only little influence on the higher western part. Thus, if any deeply weathered soils have existed in the Lowveld, they have been eroded away. The second reason is that conditions at present and in the recent past are not conducive to strong weathering and formation of saprolite. In fact also in the Highveld these conditions no longer exist, as higher rainfall and temperatures are required for progressive saprolite formation.

Soil classification

The sustainable use of different major soil groupings and specific soil types requires differential management. Soil classification is an important part of the scientific base to facilitate transfer of information relevant to soil resources in comparable environments. The international soil classification system used in Swaziland is the FAO system (FAO-Unesco-ISRIC, 1990). This system also forms the basis for World Reference Base for Soil Resources which is in the process of being finalized (ISSS-ISRIC-FAO, 1994). Comprehensive description, mapping and classification of the soils of Swaziland has taken place in the sixties (Murdoch, 1970). Correlation between the Murdoch Swaziland soil series and the FAO as well as the USDA Soil Taxonomy systems has been established (Remmelzwaal and Masuko, 1994).

The deeply weathered soils of the Highveld and Upper Middleveld are classified as Ferralsols and Acrisols, characterized by an acid soil reaction, high clay contents, a low cation exchange capacity (CEC) of the clay, a low base saturation and mostly deep red colours. Other soils of the Highveld include Leptosols, Regosols and Histosols on eroded land, as well as Fluvisols and Gleysols on alluvial deposits.

The soils of the Lower Middleveld and Lowveld are generally only moderately weathered and show a wide range of soil characteristics, depending on parent material, position, erosion, etc. They include Vertisols, Planosols, Solonetz, Lixisols, Luvisols, Phaeozems, Cambisols, Arenosols, Regosols and Leptosols. All these soils have a neutral or basic soil reaction, a high base status and medium or high CEC clay values. Textures, however, and some other properties such as colour and structure show a wide variation. In some specific locations, such as on the Lebombo plateau, soils are found which show more intensive weathering and soil formation than generally found in the eastern part of the country. These Nitisols of the Lebombo (Lomahasha series), one of the best soils in the country, are characterized by shiny structural ped faces, intermediate CEC, relatively high base saturation and high clay contents.

Environmental aspects of soils

Soil characterization helps to understand the problematic and environmentally sensitive aspects of soils. Depletion of soil resources is caused by unsustainable management practices in agriculture, forestry, mining, industry and road construction. Chemical, biological and physical degradation appears through loss of nutrients, trace elements, organic matter, soil flora and fauna, soil structure etc. It may also become apparent through concentration of substances such as salts, acids, heavy metals and other toxic elements, as caused by mining, waste disposal, use of fertilizers and pesticides, dipping chemicals, irrigation, leaf litter from plantations or acid rain. Soil compaction is a degradation phenomenon caused by machinery and cattle traffic, reducing permeability and water holding capacity. All these aspects of degradation can be monitored in soils.

An relevant example in Swaziland is the formation of Solonchaks, characterized by high salinity, as a result of inappropriate irrigation methods. The sugar industry has improved management of these soils and is now closely monitoring and controlling salinity levels.

Several soil types are highly erodible because of their poor structural resistance towards erosion. The Solonetz and Planosols of the Lower Middleveld and Lowveld have poor subsoil structure due to sodicity. They are easily eroded and require adequate management. The highly weathered Ferralsols (especially the red clayey Malkerns series) of the Upper Middleveld and Highveld are also highly susceptible to erosion. This is aggravated by their saprolite subsoils which have very low resistance to erosion (see also sections 1.2.2 and 2.4.2).

Weathering and formation of soil resources may have taken as long as hundred million years, but human induced destruction may occur in a hundred years, which means a million times faster.

2.5 Vegetation

Sweet and Khumalo (1994) provide a detailed description of the vegetation in Swaziland, which is now classified into 22 units within the physiographic zones. In the Highveld, where short grassland with forest patches is the dominant vegetation type, there are five vegetation units. The Upper Middleveld, characterised by tall grassland with scattered trees and shrubs, has six vegetation units. Three units are located in the Lower Middleveld, which is typically broad-leaved savanna. In the Western Lowveld, dominated by a mixed savanna, there are three vegetation units. In the Eastern Lowveld, there are two vegetation units with Acacia savanna as the main vegetation type. Lubombo is characterised by hillside bush and plateau savanna and has three vegetation units.