International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-3, Issue-2, May 2013 319 Published By: Blue Eyes Intelligence Engineering & Sciences Publication Retrieval Number: B1422053213/2013©BEIESP Abstract— One of the biggest environmental challenges that face Malaysia today is the scarcity of water. The increased need of water resources caused by population growth and rapid industrialization has created the need to manage water resources more efficiently and effectively. Many methods are developed to increase the source of water supply; one alternative source is rainwater harvesting. Rainfall harvesting from catchments has not received large attention in Malaysia. This method is one of the proactive action can be taken to avoid shortage of water resources in the future. The analysis results showed the maximum and minimum flow rate according to return year obtained using Fuzzy membership functions establishment. Retention times obtained from MATLAB are 7.27 days. Using a Rational method, the relationship between flow rate and intensity is created. The relationship yields an equation that is y = 0.0000317x and R2 = 0.9855. Then, the relationship between coefficient and intensity yields an equation that is y = -0.0031 x + 1.317 and R2 = 0.9238. All assumptions and calculations in this study are based on studies undertaken. In general, the findings can be summarized as good based on the results obtained. Index Terms— fuzzy membership function, MATLAB, overflow, rainwater harvesting and tank size. Abstrak - Cabaran terbesar alam sekitar di Malaysia hadapi hari ini adalah kekurangan air. Keperluan yang meningkat kepada sumber air bersih disebabkan oleh pertumbuhan penduduk dan perindustrian yang pesat telah mewujudkan keperluan untuk pengurusan sumber air yang lebih cekap dan berkesan. Banyak kaedah telah digunakan untuk meningkatkan sumber bekalan air; salah satu alternatif adalah penuaian air hujan. Namun, penuaian hujan dari kawasan tadahan tidak mendapat perhatian yang besar di Malaysia. Kaedah ini adalah salah satu tindakan proaktif yang boleh diambil untuk mengelakkan kekurangan sumber air pada masa akan datang. Keputusan analisa menunjukkan kadar aliran maksimum dan minimum mengikut tahun kembali diperolehi dari pembentukan fungsi keahlian fuzzy. Masa tahanan yang diperolehi dari MATLAB adalah 7.27 hari. Menggunakan kaedah Rasional, hubungan antara kadar aliran dan intensiti dicipta. Hubungan tersebut menghasilkan satu persamaan iaitu y = 0.0000317x dan R2 = 0.9855. Kemudian, hubungan antara pekali dan intensiti menghasilkan satu persamaan bahawa y = -0.0031 x + 1.317 dan R2 = 0.9238. Semua andaian dan pengiraan dalam kajian ini adalah berdasarkan kepada kajian yang dijalankan. Secara amnya, dapatan kajian boleh dirumuskan sebagai baik berasaskan kepada keputusan yang diperolehi. Kata kunci— fungsi keahlian fuzzy, limpahan, MATLAB, saiz tangki dan penuaian air hujan. Manuscript received on May, 2013. Supiah Bt Shamsudin. UTM Razak School of Engineering and Advances Technology, Universiti Teknologi Malaysia Kuala Lumpur, Jalan Semarak, 54100 Kuala Lumpur, Malaysia. Azmi Ab Rahman, Perdana School of Science Technology and Innovation (STI) Policy, Universiti Teknologi Malaysia, International Campus, 54100, Kuala Lumpur, Malaysia. Zaiton Bt Haron, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia. Salisu Danazumi, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia. Siti Hawa bt Jalil, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia. Mohd Azizi B. Zakaria,Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia. I. INTRODUCTION The rapid socio economic development in this country poses pressure to water supply and irrigation in the country. Furthermore, the limited water releases from the dam resulted in a reduction of normal production. Therefore, another alternative is to reduce the water needs of the country. Rainwater harvesting is one way to mitigating the problem of reduction water resources in addition to addressing the flooding problem that occurs in Malaysia. Water harvesting systems provide flexible solutions that can effectively meet the needs of new and existing, as well as of small and large sites, using a water harvesting system is an on going effort that can be developed over time. The greater attractions of a rainwater harvesting system are low cost, accessibility and easy maintenance at the household level [1] . Background Problems In Malaysia, population has increased and the demand for clean water has increased too. To cater for the demand, water treatment plants and dams have to be constructed to meet the needs of the people. As the demand increases and eventually exceed the water supply, the situation will create problem to the country. The people attitude also plays an important part towards creating a sustainable living. There are campaigns about how important the water is, but people still waste the water. If the water is not wasted, it not only will save the people’s water bill but will also reduce the stress on water demands as well as possess sufficient reserves for emergency uses such as droughts and dry spells. Since clean potable treated water is important, it is seen in this context as a waste for it to be used for flushing of toilets and for watering plants. Furthermore, rainwater can be used as a substitute for flushing and other purpose. Besides that, flash floods can be minimized as some proportion of the rainfall is retained and thus reducing the surface runoff. A. Study Objective The objectives of this study are: 1) To determine the optimum size of rainwater harvesting tank. 2) To determine the most likely range of flow rate using Fuzzy Membership Function. 3) To estimate optimum time to detain water to prevent overflows using Fuzzy Logic MATLAB. B. Scope of Study To accomplish the objective specified above, it is necessary to obtain pattern of rainfall in the study area which is M41, UTM Johor Bahru, Johor. Rainwater Harvesting Design Uncertainty Using Fuzzy Logic Supiah Bt Shamsudin, Azmi Ab Rahman, Zaiton Bt Haron, Salisu Danazumi, Siti Hawa Bt Jalil, Mohd Azizi B Zakaria