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Chao-Hsien Liaw and Yao-Lung Tsai2

ABSTRACT: Major parameters and optimum storage volumes of rooftop rain water harvesting systems (RRWHSs) have not been investigated in detail in Taiwan. Accordingly, thefour major parameters of RRWHSs were herein identified and elucidated using a simulation method. Because the performance of the RRWHSs is sensitive to the runoff coefficient, a field experiment was conducted to determine the runoff coefficient more precisely for various types of roofs. A simulation model including production theory was developed and employed to estimate the most cost effectivecombination of the roof area and the storage capacity that best supplies a specific volume of water. Consequently, the expansion path of optimum solutions for different volumetric reliability of water supply can be determined. Additionally, the method based on the marginal rate of substitution can be used for determining the rational volumetric reliability. The procedures developed herein constitutean effective tool for preliminarily estimating the most satisfactory storage capacity of any specific roof area and for determining the rational reliability of a corresponding water supply. (KEY TERMS: rooftop rain water harvesting system; urban water management; cost function optimization; water supply; reliability analysis; runoff coefficient.) Liaw, Chao-Hsien and Yao-Lung Tsai, 2004. OptimumStorage Volume of Rooftop Rain Water Harvesting Systems for Domestic Use. Journal of the American Water Resources Association (JAWRA) 40(4):901-912.

INTRODUCTION Growing pressure on the earth’s limited resources and, in particular, the worsening water crisis in many regions have been widely discussed. Given the acute problem of water scarcity that many are likely to face in the near future, thedirect exploitation of the natural, simple, and most fundamental source of renewable fresh water – rain – should not be ignored (Postel, 1992). Perhaps the most important issue in

this regard is the growing number of potential catchment surfaces available globally due to the use of modern impervious roofing materials and paved surfaces. Roof water and rain water have been harvested in severalparts of the world where conventional water supply systems have failed to meet the needs of the people. A typical rooftop rain water harvesting system (RRWHS) comprises three basic subsystems – a catchment system (roof), a delivery system (filters and gutters), and a storage system. Rain water can be collected, but until recently, such collection has all too often been ignored. Comprehensive studiesof the parameters that influence RRWHS design are few. Most studies concentrate on methods of determining storage capacity and operating policy (Lo and Fok, 1981; Schiller and Latham, 1987; Waller, 1989; Chu and Fok, 1991, Dixon et al., 1999). However, determining the most cost effective dimensions of the roof and the tank is complicated by variations in rainfall and economic conditions. Duringrecent years, numerous metropolitan areas in Taiwan have experienced water shortages, partially caused by droughts, economic development, and rapid urbanization. The use of rain water to supplement the potable water supply in Taiwan has been demonstrated to be practical and effective where traditional ground water and/or surface water are limited (Liaw et al., 1997). To support the implementationof the rain water harvesting systems for domestic use, the government has published the “Rainfall Catchment and Dual Water Supply System Handbook” as a reference for engineers who are involved in the preliminary design stage of RRWHSs (WRB, 2000), but the manual lacks a comprehensive and detailed

1Paper No. 03014 of the Journal of the American Water Resources Association (JAWRA) (Copyright ©...
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