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Rooftop rainwater harvesting has received an increased attention as a potential alternative water supply source both in the coastal and arsenic affected areas of Bangladesh. Over the last several years, projects have been undertaken to promote and install both household and community based rainwater harvesting systems in order to mitigate drinking water problem both in the coastal and arsenic affected areas in the country. The annual average rainfall is the coastal area of Bangladesh is more than 2400 mm and rainwater harvesting has been practiced for a long time for drinking water supply. For household rainwater harvesting, the capacity of the storage tanks are 1000 to 5000 liters, provided under several projects of government and donor agencies. The optimum storage volume required by a household has not been investigated considering family size and demand, roof types and rainfall. As a result, most of the households can avail water from rainwater tanks for about 6 months and rest of the year, they have to depend on other unreliable and distance sources like pond or pond sand filter (PSF) water. With the aim of assessing the currently used household rainwater harvesting system in the coastal areas of Bangladesh, design curves for the storage volume were developed for three climate conditions (i.e. dry, average and wet years) considering available roof catchment area (10 to 50 m 2 ), generated runoff and household demand (2 to 12 lpcd) for a typical family of 6 members using mass curve. From the historical daily rainfall data over 10-years period from 1998 to 2007, three separate climate conditions were selected based on annual rainfall. Years corresponding to minimum and maximum annual rainfall were considered as dry and wet year, respectively. The average year was considered the year having annual rainfall closed to the average annual rainfall over 10 years' period. Moreover, spreadsheet based daily water balance model was developed considering daily rainfall, contributing roof area and generated runoff from the roof catchment area, storage (tank) volume, water uses and overflow from the tank to assess the reliability of the currently used water tanks (1000 to 5000 liters). The analysis showed that the currently used rainwater tanks in the coastal areas of Bangladesh are insufficient to meet the yearly drinking and cooking water demand. Under wet and dry climate conditions; the maximum achievable reliability is about 70%. The maximum reliability that can be achieved under average climate condition varies from 70 to 90% and the reliability does not increase significantly beyond 3000 L tank volume. A large quantity of water is lost as spilled water even with a tank size of 5000 L. This water can be used for other purposes if larger tanks are used to capture the excessive spilled water. This analysis also showed that although a huge amount of water is lost as spilled water, a significant amount of water would need to be collected from other sources like pond or PSF water during non-availability of water into the storage tanks. This water volume depends on tank size and reliability, an increase in tank size decreases the water volume collected from other sources. With increase of reliability, the water required to be collected from other sources also decreases.

Reliability analysis of household rainwater harvesting tanks in the coastal areas of Bangladesh using daily water balance model