Potential of roof rainwater harvesting for domestic water supply in upper Mikuyuni Sub-Catchment, Makueni County, Kenya
Mwendwa, Georgia Ndanu
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Water is one of the most vital and irreplaceable resources for human livelihood. However, it is limited and not homogeneously distributed leading to scarcity in many regions in the world including Kenya. Water scarcity is a major concern in the Upper Mikuyuni sub-catchment and rainwater harvesting has the potential to address the current shortfall in water supply. The main objective of the study was to assess the potential for roof rainwater harvesting and its contribution to domestic water supply in Upper Mikuyuni Sub-Catchment. Specific objectives are (i) to establish the rainfall characteristics influence on roof rainwater harvesting for the 30 years period ranging from 1983-2012 (ii) to determine the household's socio-economic characteristics influence on roof rainwater harvesting and (iii) to assess the roof rainwater harvesting potential in relation to rainfall, roof catchment extent and reliability to meet household water demand. The household socio-economic data was collected through a random survey of 100 households. The catchment and storage sizes were obtained through measurements and observation of 30 purposively selected households, respectively. The historical rainfall data for Kilome station which is within the study area was obtained by interpolation using inverse distance weighting method. The household socio-economic characteristics data was statistically analysed using descriptive statistics, stepwise regressions analysis and analysis of variance (ANOY A).The rainfall data was analysed using Microsoft excel while the potential for rainwater harvesting based on existing catchment sizes and storage was analysed using a rainwater harvesting model. The results obtained indicated that the household monthly income, means of storage of harvested water and household water demand were predictor variables to rainwater storage with R2 of 47.4%. The mean monthly rainfall time series demonstrated that the area received a bimodal rainfall pattern with four months of dry spell each year. It was also observed that both annually and interseasonally rainfall varies from 480mm/annum to 1180mm/annum. The empirical distribution curve obtained revealed that annual rainfall of 500mm/annum with occurrence of 93% offers good potential for rainwater harvesting. The potential for rainwater storage based on existing catchment sizes for design rainfall of 509 mm derived from the water supply reliability of 90-95% using rainwater harvesting model showed that with a roof catchment of 35-50m2 and storage <I m3 for monthly demand of 2.4 m', household experienced water shortages for 7 months. However with increased storage size up to 10m3 , the household experienced" water shortage in two months only. A household with catchment size of 50-100m2 and storage of 10 m3 showed no month of water shortage. Further a household with roof catchment of 100- 130m2 and storage of 15 m'and increased monthly water demand of 3 rn' showed no month of water shortage. Using the minimum water requirements for Arid and semi arid areas of 281/p/d for 5 persons (4.2 rn') there was one only month of water shortage. The study establ ished that a roof size of 122m2 and storage of storage 30m3 will meet monthly domestic water demand of 4.2 m3 which is adequate for household of five persons. Therefore, the study established that, Mikuyuni area has potential for roof rainwater harvesting. However it is limited by the storage capacity. Further, the inadequate income of the households' limits full utilisation of rainwater harvesting in Mikuyuni area. Therefore the study recommends financial and technical support to upscale rainwater storage and ensure sustainability to rainwater harvesting.