A Study for rainwater (roof) harvesting with reference to Tanzania

Serious problem stand in the way of efforts to expand and sustain water supply and sanitation systems in the rural areas of the world. Fifty eight percent of rural residents have no access to improved water supplies and 84% have inadequate sanitation (WHO 1987). Rainwater harvesting from roof and small catchment runoff has been the most common practice in water resources development for rural communities in Tanzania, since it seems to be the most viable solution for providing acceptable quality drinking water. Accurate estimates of runoff from roof and small catchment are therefore important to the planning and efficient implementation of the rural community water supply. Roof runoff collection for drinking purpose has been commonly practiced among rural households. The parameters reguired for the analysis of the runoff harvesting system are rainfall amount, roof area, tank capacity (size) and household demand. A rainwater roof catchment system built with appropriate technology and introduced with cultural sensitivity will still fail without adequate rainfall. The literature available indicates that most rainwater roof catchment design gives minimal consideration to either the variability of rainfall or the probability of system failure. Some catchment system design simply employs a mean rainfall value. In such approximation, such systems will fail to provide the projected water half of the time. Although it is impossible to predict rainfall, it is possible to incorporate stochastic assessment into catchment system analysis. Stochastic behavior and the risks associated with alternative catchment designs. In this study daily rainfall, roof runoff and household water demand data have been analysed for 12 regions in Tanzania which among them experience serious dry season water supply problems. This was to determine whether sufficient amounts of rainwater could be harvested from corrugated metal roofs (G.I) to satisfy various demands either the whole year’s or the dry season household water demand. To determine reliability of the rainwater harvesting catchment systems with various roof catchment areas at various demands of 20, 25, 30, and 35 litres per day (1pd). The results indicate that the daily rainfall amounts in some regions are adequate to ensure that sufficient volumes of rainwater can be harvested from roof tops for household uses during the dry season. A part from few of the regions considered in this study most are adequate and reliable for continuous rainwater harvesting. The purpose of the analysis done in this study is to establish a storage capacity for rainwater for a certain region. The storage of collected rainwater from roof catchment is to service a water demand for domestic need for a given household. The design risk curves obtained can be used to determine storage volume required for all demands for any roof area and runoff coefficient.