The 3 rd International Conference on Water Resources and Arid Environments (2008) and the 1 st Arab Water Forum Modeling and Reliability Assessment for Rainwater Harvesting System Thamer Ahmad Mohammad, Megat Johari Megat Mohd. Noor, Abdul Halim Ghazali Department of Civil Engineering,Faculty of Engineering Universiti Putra MalaysiaSelangor, Malaysia Abstract Rainwater harvesting is the technique of capturing the rainfall to meet some water needs in both urban and rural areas. The volume of rainwater collected from rainwater harvesting system varies from place to place and depends mainly on the climatic condition. Typically, the rainwater harvesting system is composed of the catchment (roof), gutter, rainwater pipe, and storage tank. Reliability of a rainwater harvesting system mainly depends on the collected volume in rainwater storage tank and it is also used to check whether the collected volume of rainwater can meet a specific water demand (either for potable or non-potable uses). In the present study, a rainwater harvesting system is installed at the Faculty of Engineering, University Putra Malaysia. The system is tested using data from 24 different rain events. The collected data includes rain depth and rainwater volume. It is found that the rainwater volume ranges form 0.027 m³ to 4.03 m³. The actual data is used to produce an empirical model for predicting the collected rainwater volume. Calibration and validation processes are conducted to the proposed model and T-test shows that the model prediction is within 95% level of confidence. Also, the water consumption for toilet flushing is monitored using water meter. Reliability of the installed rainwater harvesting system for toilet flushing is computed. It is found that the system reliability ranges from 26.61 % to 100 % depending on daily water demand and recorded rainwater depth. Keywords: Rainwater Harvesting, System Modeling, Assessment, Reliability Introduction Rainwater harvesting is the technique of collection and storage of rainwater from roofs during rain events for future use. This technique is appropriate in many countries such as United Kingdom, Germany, China, Japan, Thailand, Sri Lanka, India, Australia, Brazil and United States of America. The technique is