9th International Rainwater Catchment Systems Conference
"Rainwater Catchment: An Answer to the Water Scarcity of the Next Millennium."
Petrolina, Brazil - July 1999

Section 4: Technology of Rainwater Catchment Systems

Paper 4.10

Optimization of rainwater catchment systems design parameters in the arid and semiarid lands of Kenya

Stephen N. Ngigi
Department of Agricultural Engineering
University of Nairobi
P.O. Box 29053, Nairobi, Kenya
e-mail: sngigi@swiftkenya.com

It is evident from experiences in Kenya that rainwater could be the long awaited answer to water scarcity in the next millennium. However, some technical and policy hindrances need to be addressed. Lack of appropriate technical designs, among other factors, has led to low adoption of rainwater harvesting technology, especially in Arid and Semiarid Lands (ASAL), where rainwater is one of the most viable water supply. This calls for optimization of Rain Water Catchment Systems (RWCS) design parameters and formulation of comprehensive water policy. Therefore, to address this problem, the paper focus on the hydrological criteria for determining RWCS design parameters, especially storage capacity and catchment area, using historical rainfall records of Kibwezi rainfall station. Specifically, a design procedure for determining optimal design parameters and developing design curves is outlined. The mass curve analysis was adopted for the determination and optimization of the design parameters due to outlined inadequacies of most empirical formulae. The strength of the design procedure is the determination of optimal design parameters at various reliability levels of rainfall amount and distribution. The analysis of design parameters revealed that the catchment area and the storage capacity are affected by variations in rainfall amount and distribution respectively.

In addition, the paper proposes a procedure for incorporating rainfall distribution, which has been consistently ignored in the designs of RWCS. The proposed procedure involves adjustment of monthly rainfall by using rainfall distribution indices such that the monthly rainfall totals correspond to annual rainfall at a given rainfall reliability level. The adjusted monthly rainfall is subjected to mass curve analysis to determine the design parameters at various reliability levels. The selection of optimal design parameters is simplified by the development of design tables and curves from which the catchment area and storage capacity for a specific water demand can be easily obtained at various reliability levels. The paper concludes by proposing some recommendations to promote utilisation of rainwater, and adoption of RWCS technology in Kenya. Therefore, the developed procedure could enormously contribute to the adoption and implementation of optimal RWCS designs, and hence supplement government efforts towards meeting ever increasing water demand. The procedure could also be used to evaluate the reliability of existing RWCS.

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