Manyame catchment is among the seven major river basins that constitute the Zimbabwean hydrological water management systems. This catchment sources in Marondera and drains into the Zambezi river downstream of the Kariba Dam and upstream of the Cabora Bassa dam to the northern part of the country. Geographically, the Manyame catchment lies between the 18.320S (latitude) and the 15.590S (latitude) and between the 28.740E longitude and the 32.040E longitude. The catchment also straddles over four administrative provinces namely Harare Metropolitan, Mashonaland East, Mashonaland West and Mashonaland Central covering the areas which all drain into the Manyame river system. Mazowe catchment borders it to the east, to the southeast by Save catchment and to the west, by Sanyati catchment. It has a total estimated catchment area of 40497 square kilometres and is characterised by Agro ecological region-II and low veld climatic conditions, with good red soils from Banket to below the Zambezi Escarpment and sand veld climatic conditions above the escarpment. Figure 2.1 shows the Manyame catchment and its sub-catchments. Table 2.1 shows Manyame catchment sub-catchments, hydrological sub zones and their respective catchment sizes.
Topographically, the Manyame slopes generally towards the northern direction as evidenced by the direction of the Manyame river, which flow into the Zambezi river basin. The whole catchment can be divided into roughly three mains plains based on the spatial distribution of the catchment terrain, that is, the upper part of the catchment which consists of Marondera, Chihota, Seke, Greater Harare, the middle part which consists of Chinhoyi, Banket, Mukwadzi, Mutorashanga, Raffingora, Mhangura, Guruve and lastly the low lying Dande communal areas below the Zambezi escarpment covering the Chirundu, Mushumbi, Mahuwe, Muzarabani and Mukumbura. The upper parts of the catchment is 1 800 m above sea level while the lower parts lies 300 m above sea level.
SURFACE WATER RESOUCES ASSESSMENT
Manyame catchment covers areas that are drained by three main rivers; Manyame, Musengezi and Angwa. The catchment constitutes of ten hydrological subzones of varying sizes, geology, climate, social and economic activities.
The level of surface water resources development varies throughout the catchment. The areas with the highest potentials being Upper Manyame composed in subzones CH4 and CH5, Middle Manyame’s CH3 subzone, Angwa-Rukomichi‘s CA2 subzone, Musengezi’s CUG2 subzone, and CH2 in Lower Manyame. These areas are in the central parts of Zimbabwe, which are characterised by relatively high rainfall and moderate temperatures.
The catchment is reasonably well endowed with water, having a mean annual rainfall of around 750mm although with considerable variation in space and time. The Mean Annual Runoff (MAR) of the catchment tends to also vary following a trend in the rainfall. That is it decreases in the northerly direction towards the escarpment and is lowest in the Zambezi basin, which has low laying areas such Muzarabani, Dande and Kanyemba.
The MAR is further reduced by evaporation, which is rather high in these areas due to high temperatures. The yields of the subzones almost follow the same pattern. Refer to table 3.1 below, which summarizes the surface water utilization and the hydrological conditions in the catchment. The coefficient of Variation (CV) increases in the northerly direction. This implies that the reliability of flow in the area would conversely vary as the MAR and the rainfall.
GROUNDWATER RESOURCES ASSESSMENT
Rock units that can store and transmit appreciable quantities of water are called aquifers. The nature of the aquifer depends on rock type, structure, present insitu stresses and extent of weathering. The geology of the Manyame catchment is shown in figure 3.2. The aquifer type determines the nature of occurrence of groundwater. The aquifer types found in Manyame catchment are crystalline (igneous/metamorphic rocks), consolidated and unconsolidated sedimentary aquifers. Groundwater occurrence in crystalline rocks is dependant on presence of deep weathered regoliths, width and stresses in fracture and fault zones. Groundwater potential is higher for unconsolidated sediments with greater thickness of the saturated layers.
HYDROGEOLOGICAL CONDITIONS OF THE CATCHMENT
Groundwater like surface water is part of the hydrological cycle. Groundwater occurrence is controlled by a number of factors such as amount of rainfall, topography, geological structures like fractures and faults developments, weathered zones, contact zones, dykes and quarts reefs intrusions. Groundwater flow direction in most cases mimics the topography, flowing from high ground to low ground. Successful well sites have to be located on or near the structures mentioned. The geology, particularly rock type geology structures play an important role in groundwater occurrence. Table 3.2 gives a description of rocks and their hydrogeological characteristics which are found in Manyame catchment.