5. THE STUDY AREAS
5.2 Biophysical characteristics
5.2.2 Topograph y, Geology and Soils
The study area by virtue of its large size cuts across a wide range of contrasting topographical features, vegetation types, soils, climate and human settlements. Generally, the land rises from sea level in a series of plateaux steps to the highest point in Kenya, Mount Kenya, which
~ The study
area
ETHIOPIA
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TANA RIVER
Figure 5.1 The study areas
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reaches an altitude of 5 200m. The general altitude of the central highlands around Mt. Kenya ranges from 1 500m to 2 500m, with three major land masses rising above the plateaux: Mt.
Kenya, the Abardare Range and the Kikuyu Escarpment. Similar elevated features, in many cases remnants of higher eroded surfaces, are found between the central highlands and the coast. This relatively simple land form has dictated the present drainage system with all major rivers flowing radially from the elevated interior. These include rivers Tana, Athi, Ewaso Ngiro, Sabaki and others, which form the main drainage basins in Kenya (Figure 5.2).
Mt. Kenya fonns the main source of these rivers, and therefore activities which destroy vegetation within this catchment affects the rest of the country. Other important forests and their importance in maintaining river flows in Kenya are shown in Figure 5.2. These rivers generally flow through the dry areas and the water is used for domestic, agricultural, energy and industrial purposes. The raised land fonns are also associated with low temperatures owing to the high altitudes, and to high rainfall. The majority of forests and major vegetation formations are usually concentrated in the mountains. Indeed in Kenya, 70% of forests occur in mountains, hilly or footslope areas (Wass, 1995). The central highlands around Mt. Kenya, Chyulu Hills, Shimba Hills and others are presently some of the main sources of prime carving wood.
For the region around the central highlands, the economic potential is greatly influenced by the existing topographical features and rock formations. For example, soils have been well developed on higher altitudes where lava out pourings was common and poorly developed at low altitudes where lava flows are absent. According to KIFCON (1994), there are three broad categories of soils of the mountain depending on the altitude, ranging from below 2 400m, between 2 400-4 OOOm and above 4 OOOm. Although soils above 2400m are rich in organic matter and generally fertile, low temperatures is the limiting factor to their agricultural uses and they are now mainly used as National Parks. Below 2 400m dark reddish clay soils occur, which are well drained, deep, loamy and very fertile. Major forest formations and economic activities are found on this zone, and Brachylaena huillensis and Olea europea sub sp africana are found.
The greater part of Machakos, Kitui, Mwingi and the central part of Makueni districts consist of a basement complex of metamorphosed sedimentary rocks with some granite and basic igneous. Areas of high elevation have well developed and fertile soils characterized by
_ Forests
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. SOURCE: KIFCON ANALYSIS -FOREST [lOUNDARIES FROM't.DSURVEY SECfION DRAINAGE BASIN BOUNDARLES FROM jlCA / WATER MASTER PLAN
Figure 5.2: Drainage basins in Kenya
granitic and volcanic rocks and black cotton soils. These are the regions where Olea europea sub sp. africana and Combretum schumanii naturally occurs. Lowlands are characterized by red clay soils and sandy soils where Dalbergia melanoxylon is found.
Most districts at the Kenyan Coast, particularly those which border the ocean have four common topographical features with marked geological, soil and rainfall characteristics which dictate the resource use potential and land use patterns. These are the coastal plains (altitude below 60m) the foot plateau (60 - 135m altitude), coastal uplands range (150 - 420m) and Nyika Plateau (300 - 180m). The Arabuko Sokoke Forest Reserve and Shimba Hills are traditional sources of prime carving wood (Brachylaena huillensis. Cobretum schumannii, Spirostachys ufricana) to the carvers along the Coast and deserve a special mention here. The Eastern part of Arabuko Sokoke Forest Reserve lies on a flat coastal plain at an altitude of about 45m, comprising deep loose white sands with the remainder s on the foot plateau with red loam sands derived from tertiary seuiments on higher ground (KIFCON, 1991; Mogaka, 1992). Shimba Hills rises upruptly from the coastal plain to form a plateau at an altitudes of 150 - 450m. Part of the reserve is composed of upper Duruma Sandstones covered with Magarini Sands while the rest is composed of Shimba Grit and Mazeras Sandstone (Davies, 1993). In general, most of these soils are of marginal quality for agriculture or plantation forestry (Thomson and Ochieng, 1992).
The Coastal Plains also have unique features that are a tourist attraction. These include attractive creeks, marine estuaries, estuarine swamps with mangrove forests and sandy beaches. In general, the Coastal Range has high rainfall and deep permeable soils. Hence, this area supports farming and tree growing activities in the region.
5.2.3 Climate
The contrasting topographical features of the entire study areas is associated with contrasting climatic conditions across the study area (Figure 5.3). The central highlands experiences an equatorial type of climate and gets rainfall throughout the year, particularly the high plateaus at altitudes of over I 500m, where rainfall is typically between 1 500mm to 2 300mm during the main rain seasons.(March to June). These are represented by the humid to semi-humid zones in Figure 5.3, particularly areas around Mt. Kenya with highest altitudes. The low lying areas within the central highlands generally receive a bi-modal pattern with some rain
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ZONE A NNUAL RAINFALL
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1. Humid to Semi-humid (High) . above 900 mm2.Semi-llJmidtoSemi-arjd (Medium) 600-1100 mm
" 3. Semi-arid (Medium) 450 - 900 mm
4. Arid (Low) 300 - 350 m m
VerY-Clrid (Very low) 150 - 350 mm
Figure 5.3: Agro-climatic zones of Kenya
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I Afalling in October to December. The total amount of precipitation decreases fairly regularly towards the top of the mountains and the lowlands.
Temperatures ar~ generally inversely related to aspect and altitude, with the mean annual temperature being less than
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on the higher ground. Tables 5.1 and 5.2 show the mean monthly rainfall and mean monthly temperatures respectively, for selected locations within the study area.Table 5.1 Mean monthly rainfall of selected areas
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Jan Feb Mar Apr May Jun Jul Aug Sept Oct Noy Dec
1 2 3 4 5 6 7 8 9 10 11 12
Nyeri 45 40.8 106.9 230 207 34.3 36.3 37.0 34. ; 109 149 117.2
Machakos 33 23.9 84.9 196 63 17.5 2.8 4.1 6.7 68 183 98.4
Kitui 34.7 34.9 91.8 155 45 3.4 1.7 2.3 4.1 47.4 247.5 114.1
Makueni 62 22 85 136.9 27 1.9 0.7 5.8 1 33.4 183.5 150.2
Kilifi 24 17.3 46 91.5 171.6 78.6 53 46.3 55.8 71.3 93.8 50
Mombasa 16 11 75 138 218 107 74 58 39 96 93 96.5
Kwale 61 51 104 110.4 139 76.8 65.6 64 34.6 59 68.4 82.6
Table 5.2 Mean monthly temperature (degrees centigrade) of selected areas
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Machakos 21.2 22 23.3 21.1 19.9 18.9 18.3 19.5 19.5 21 20.1 19.3
Makueni 24.6 25.8 25.8 24.4 23.9 22.2 21.5 21 21 23.9 23 22.7
Mombasa 23.3 23.4 24 23.6 22.1 21.2 20 20.2 20.7 21.7 22.8 23.4
Most of Machakos, Kitui, Mwingi and Makueni districts receive low rainfall. The climate is classified as hot and dry for most of the year, and regarded as arid and semi-arid with high temperatures, unreliable rainfall and high rates of evaporation. These are represented by semi- arid and arid zones in Figure 5.3. Historical data indicates that in four out of ten years, major droughts occur in the region usually resulting in devastating effects on crops, livestock and
people (Kaudia, 1996). On an average year, there are two rainy seasons with "long rains"
from March to June and "short rains" in November and December.
Total annual rainfall ranges from slightly over 1 OOOmm in some of the highlands to less than 500mm in the low lying range lands which represent over 60% of the land area. Mean temperature patterns are closely linked to altitude. The highest mean temperatures are recorded in the eastern parts of Kitui and vary from 26°C to 34 °C and the minimum mean figures from 14 °C to 18 °C (GOK, 1998).
The coastal region has a monsoon type of climate which is hot and dry from December - May.
The hottest months are December - February while the coolest months are June - August.
The mean minimum temperature recorded rarely fall below 25°C at Mombasa but varies slightly within the coastal districts (KIFCON, 1995). Tourism has benefited fr0m this type of climate as the coastal sunshine is a major attraction.
The region has a bi modal pattern of rainfall with its "long rains" during March - June and
"short rains" in November - December. The amount of rainfall in the long rains decreases from a strip of about 10km wide from the coastline into the hinterland at a rate of about 100mm per SkIn from over 1 200mm at the coastal bent /strip to 400mm in the hinterland (GOK and UN1CEF, 1990) (Figure 5.3). The coastal belt falls under semi-humid to semi-arid with pockets of humid areas.