Rainwater Harvesting For Domestic Use: A Case Study Of Machakos District

Abstract

This study investigated the characteristics of rainfall and quantified the probable volume of water that can be harvested from rainfall inputs. The region of study considered was Machakos district in Kenya. The area lies within (1.0oS -2.0oS) and (37.0oE-38.0DE). Daily rainfall data from fourteen stations was used covering a period from 1960 to 2004. The data was obtained from Kenya Meteorological Department headquarters, Dagoretti in Nairobi. Data on water consumption was obtained by using a questionnaire in an exploratory survey of the study area. The questionnaire also contained information on sizes of roofs of the houses in the region. Missing data were estimated using the linear interpolation formula. To test consistency of precipitation data in this project, a single mass curve was used. To determine variability of rainfall in the study area, time series analysis was used. Annual variation of rainfall was investigated using trend analysis. To determine the area of roof catchments, an exploratory survey was conducted in the area of study. Roof areas were determined by using a conversion factor multiplied by ground measurements of the houses. The minimum water demand in the area of study was investigated by obtaining the number of 20 litre jerry cans of water used daily for domestic purposes; drinking, cooking, washing, watering domestic animals etc. An appropriate volume of tank required to store excess harvested rainwater was estimated using the supply method. Satisfaction levels were inv~~tigated to assess the degree to which storages could supply enough volume of water i6 satisfy minimum demands. Reliability indices were investigated to estimate the fraction of time when the tanks contained water. Analysis results indicated that there was high variability of temporal distribution of rainfall over the years in all stations. All stations had less than 60 rainy days in a year. Amount of rainfall was higher during October-November-December seasons than March-April-May seasons in most stations. The per capita volume of water requirement per day was computed as 23.3 litres. In the study area, 96% of the houses were roofed with iron sheets. Harvestable volume of rainwater was below the minimum water demand of 4.26 m3 per month in all stations. Excess harvestable volume of water over its minimum demand was mostly experienced in April and November. The rest of the months experienced deficits of rainfall, with the driest months being July and August. Satisfaction of minimum demand by harvestable rainwater varied greatly over the months, ranging from 1% in July and August to ix over 100% in April and November. Reliability indices of storages varied from month to month, ranging from 4% to 36%.