Enhanced performance and design of rainwater harvesting based on rainfall regime - the case of ethiopia

This paper deals with an effective and enhanced performance of rainwater harvesting systems using rainfall regimes, so that economical and efficient sizing of storage is met. These days, there are a number of ways of rainwater harvesting techniques. One of the most effective option and technique is harvesting from roof catchments for household and communal /public domestic water services. Roof catchments gives very good run-off coefficient value and produce better amount and quality of water. Losses in the form of infiltration, interception or subsurface run off etc are in existent. However, and most often, rainwater harvesting is designed based on the annual rainfall and catchments characteristics. This approach (using the annual rainfall) results with higher storage value. On the other hand storage in rainwater harvesting scheme is one of the component which is very expensive. It wouldn't be affordable by most of the households. In the contrary, rainfall distribution pattern has its own features and variation (spatial and temporal). Depending on the geographic and other climatic variables, some areas have two rain seasons (bi-modal type) and others can have one rainy season (mono-modal type). On top of this local variation, a given rainfall can have its own intensity, duration and frequency. Rainfall with high intensities has short duration and rainfall with longer duration has low intensity in most of the cases. It is therefore, for those areas which have two rainy seasons followed by two subsequent dry periods (like most part of Ethiopia), the capacity of the storage (tank) can be designed to harness the rainwater from the first rainy season and to take care the subsequent dry season. The next dry season will be taken care by the second rainy season. By so doing the capacity of the storage tank or cistern can be reduced by half. For this reason, the knowledge and thorough study of the rainfall distribution (regimes), its intensity, duration and frequencies is very essential for an effective design and enhanced performance of rainwater schemes. It is therefore this paper is aimed and designed to provide technical design inputs for those practitioners involved in rainwater harvesting interventions.