The study area is located in the Ahafo-Ano South District, northwest of the capital city Kumasi of the Ashanti Region (Figure 3.1). The sites are upland sub-catchments of the Ofin River Basin, within the south western river system of Ghana. The district capital is Mankranso, 32 km away from Kumasi, on the Kumasi-Sunyani road. The district covers an area of 1,126 km2 and has a population of 133,874 (Ghana Districts 2006). Ahafo Ano is derived from the Akan word ‘ahayo’, which means hunting - as the district formed part of the hunting grounds for the Asantehene (the king of the Ashanti ethnic group).
Figure 3.1 Map of Ghana showing the three upland sub-catchments and general study area in the Ashanti Region.
50 3.1.1 Climate
The climate of Ghana, as in the rest of West Africa, is influenced by the position of the Inter Tropical Convergence Zone (ITCZ) (FAO 2004a). The ITCZ is the boundary between two air masses: (i) the dry tropical continental air mass which gives rise to the North East (NE) Trade Winds (‘harmattan’) and blows from the northeast across the Sahara during in the dry season, and (ii) the South Atlantic Maritime Air Mass referred to as the South Westerly (SW) Monsoon which brings moisture into the area from the sea during the rainy season.
The Ashanti Region has a bi-modal rainfall regime typical of a moist semi-deciduous forest zone. Two defined rainfall seasons occur annually: (i) a major season with heavier rains from mid March to the end of July with peak rainfall in June, and (ii) a minor season with lighter rains between September and November, with a relatively short dry period in July and August. The warm moist SW monsoon winds dominate during the rainy seasons, and the NE harmattan winds from December to March.
Rainfall
Rainfall in Ghana generally decreases from south to north. The wettest area is the extreme southwest, where annual rainfall is over 2000 mm, and the driest is a strip in the east (Takoradi) that extends up to 40 km inland, where rainfall is less than 750 mm (FAO 2004a). In the Ahafo-Ano district, the mean annual rainfall is 1200 mm (Ghana Districts 2006), although monthly and annual totals as well as seasonal distributions vary from year to year.
Relative humidity
Mean monthly humidity values range from 87-91% from 0900 hours and decrease to 62- 78% by 1500 hours. The relative humidity is usually lowest during the harmattan period from February to April (83-87% in the morning and 48-67% in the afternoon), and the highest during the wet season from June to October (Adu 1992).
51 Temperature
Temperatures are uniformly high throughout the year. Mean annual figures are around 26°C, with the hottest periods in February and March (28°C), and the coolest in July and August (24°C) (Meijerink et al. 2003). Variations between day and night temperatures are greater during the dry than wet seasons. The period July-August is markedly cool, dry and more cloudy than the rest of the year. The main dry season is less cloudy but more hazy.
The cloud cover is at its maximum in the early mornings and slowly decreases until 2100 hours (Adu 1992).
Wind
Wind speeds are generally low with an average speed of 8 km hr-1 (Adu 1992). Speeds are lowest at night and during the early morning, during which a high percentage of calm occurs, and highest in the middle of the afternoon when average values rise to 8-16 km hr-1.
3.1.2 Physical features Geology
Geomorphologically, the Kumasi region forms part of a dissected peneplain from which inselbergs or mountain ranges rise. Soils are developed over phyllites, granites, Tarkwaian and Voltaian sandstone rocks (Junner 1940; Moon 1962). Underneath, most of the Ahafo-Ano South District is made up of lower Birrimain rocks, which consist mostly of phyllites, greywackes, schists and gneisses. These contain clay deposits subsequently hardened and altered by heat and pressure, and due to few soil minerals present, are not rich. Two to three meters below the surface is weathered phyllite, which is soft and easily broken. During weathering, uneven distribution of veins and stringers of quartz in the phyllite break up and produce varying abundance of stones and gravel, which greatly reduce the capacity of the soil to store water and plant nutrients (Adu 1992).
Drainage
The Ahafo-Ano South District is part of the Ashanti Plateau and has an undulating landscape with a medium highland area between 150 to 600 m above sea level. Among the
52
hills with high elevation are Aya, Kwamisa and Tunte. The major rivers in the district are the Mankran, Abu and Aboabo. River Abu, the largest river, originates from the Kwamisa Hills and flows south-west (Ghana Districts 2006). Within the catchment area of the Ashanti Region, the Ofin River is fed by a number of secondary rivers and streams such as the Oda and the Jimi.
Soils
The soils in the study area form the Bewai-Nzima/Oda (Class 1) compound association (Orthi-ferric acrisol classification), and the Bekwai-Zongo/Oda (Class 4) complex association (Plinthosol classification) (Adu 1992). Associations are defined as larger groupings of soil series that have related parent material and similar profile morphology but are differentiated by relief and drainage. The Bewai-Nzima/Oda association occupies a large part (55%) of the Ashanti region, and the component soils occur in a definite topographical sequence: soils on the summits, upper and middle slopes are red, well drained (Bekwai series) and brown, moderately well drained (Nzima series) concretionary, silty clay loams; soils on the middle to lower slopes are brown to yellow brown imperfectly drained silty clays and silty clay loams (Kokofu series) developed from colluvium or hill wash material; and soils on the valley bottoms are grey, poorly drained alluvial loamy sands (Temang series) and clays (Oda series). The Bekwai-Zongo/Oda class is similar, but is characterized by large exposures of seepage ironpan soils (Zongo series), which consist of about 30 cm yellow brown imperfectly drained, clay loams containing ironpan boulders, which grade below into 90 cm of vesicular ironpan. This layer occurs above weathered phyllite and occurs extensively on lower slopes to flat land (0-2%) and about 6 m above the major streams traversing the tract.
3.1.3 Land use and agriculture
Based on the Bewai-Nzima/Oda compound association, the soils of the upland sub-catchments have been classified as Moderately Good Lands for agriculture (Adu 1992).
These lands are generally medium textured, highly or moderately gravely, or deep and non-gravely, and well, moderately well and imperfectly drained. They occur on gently
53
undulating topography (3-8% slopes), where exposure to erosion is slight to moderate under mechanical tillage and careful management. The upland and slope soils are recommended for all tree and arable crops, and the valley bottom soils for rice, sugarcane and vegetables.
Further downstream are the Bekwai-Zongo/Oda compound association soils, which have been classified as Fair Lands for agriculture. These soils are very shallow or shallow, very stony, very gravely or have ironpan subsoils, and range from well to imperfectly drained and occur along lower slopes to summits (3-12% slopes). Water holding capacity is fair to very low, and the soils are easily susceptible to drought. Hand cultivation only has been recommended with land-use being restricted to tree crops or woodland, and the valley bottom soils used for rice, sugarcane and vegetables.
3.1.4 Vegetation
The natural vegetation is typical of semi-deciduous rain forest and is characterized by plant species of the Celtis-Triplochiton association (Adu 1992). None of the original vegetation exists outside of the forest reserves, since most of the lands have been converted to agriculture. There is also a long history of intensive shifting cultivation such that the current vegetation is a variety of fallow farmlands consisting of secondary forests, thicket, forb re-growth and swamp vegetation. There are six forest reserves (Tinte, Opuro River, Kwamisa, Asufufu, Shelta Basin and Offin North) which cover a total area of 300 km2 (Ghana Districts 2006).
3.1.5 Social Setting
All land in the Ashanti Region is controlled by the Asantehene, with farms owned by residents through inheritance or leased by immigrants. Production of crop farms is based on shifting cultivation, and crops are usually grown in mixed stands especially during the first few years of cocoa farming. Cocoa is considered the main cash tree crop, although small citrus and oil palm plantations are scattered throughout the region. The major agricultural cash crops are plantain, cocoyam, cassava, and vegetables such as tomato, okra and garden egg (Asomaning unpublished in Meijerink et al. 2003).
54
The district is mainly populated by Ashantis with settler farmers from the Dagomba, Ewe Gonja and Basare ethnic groups. The population is 45% Christian, 30% Muslim and 25%
animists, with 80 primary schools, 30 junior secondary schools and one senior school in the district (Meijerink et al. 2003). There is no district hospital, although there are three health centres located at Mankranso, Mpasaso and Sabronum. There are also four private clinics managed by the Roman Catholic and Methodist churches and two village health posts. In many villages, there are herbalists and chemical sellers.