Reliability and Storage analysis of rainwater Reservoirs: Comparison between North, Center, and West of Tehran

With increasing population and changing climate regime, water supply systems in many cities of the world are under stress. To tackle this problem, water authorities are adopting several measures including demand management and identifying alternative water sources such as stormwater harvesting, greywater and wastewater reuse and desalination. Among all the alternative water sources, stormwater harvesting perhaps has received the highest level of attention. Regarding to excessive water consumption in large cities, the problem of water shortage can be reduced to the extent desirable by using rainwater harvesting systems. “Reliability” is percentage of total days in a year when the water collected in the reservoirs is able to supply the demand of residents. “Rainwater storage potential” is the volume of runoff that is collected and used by a series of actions in the tank, before becoming a flood. The aim of this study was to estimate the reliability of rainwater storage tanks and to investigate the potential of rainwater storage. In the current study, the daily water balance between the rainfalls as input and domestic water-consumption per capita as output were used for the performance analysis and design the optimum of rainwater tanks, at three different regions of Tehran (center, north and west). These three different regions of Tehran are distinct due to notable different rainfall and topography characteristics. Charts of reliability of rainwater storage were produced for domestic rainwater tanks due to tank volume, roof area, number of people in a house (i.e. water demand) and percentage of total water demand to be satisfied by the harvested rainwater. In this study, the reliability of rainwater harvesting system was investigated for rainwater saving and non-potable water supply in three different regions of Tehran. It was found that for a relatively small roof area (100 m2), 100% reliability could not be achieved even with applying a very large tank (10,000 L). This roof area was very small to collect the appropriate amount of rainfall. In this case, the accumulated rain was used quickly and the reservoir remained empty until the next rain event occurred. For the larger reservoirs, the reliability remained constant; in which case, part of the volume of the reservoir or the entire volume remained in most of the time because the roof area for collecting rainwater was enough, or the demand for water was not high. For a large roof area (300 m2), about 40% reliability can be achieved with a tank size of 10,000 L. Reliability was independent of tank size for the sizes larger than 5000–8000 L; in these rang of size, the reliability depends on the region (location). Also, the average rainwater storage in reservoirs was determined annually for each of the three locations. The results showed that the amount of rainwater storage increases with increasing roof area. It was found that with increasing the daily water demand of building residents, the days of water supply by rainwater saving from roofs would decrease. According to the results, it can be stated that the effect of increasing the volume of the reservoir in storing rainwater for the roofs with larger areas were greater than the roofs with small areas because, the roofs with the small area do not fill the reservoir. The runoff overflow from the reservoir was often carried out on the large roofs, which can be reduced by increasing the volume of the reservoir (reducing the runoff from the reservoir and adding to the amount of rainwater stored). The maximum rainwater storage for a roof area of 300 m2 were 100 m3, annually in north of Tehran. The minimum storage for a roof area of 100 m2 were 17 m3, annually in west of Tehran. In general, the results showed that north of Tehran has more reliability and also more rainwater storage than the center and west of Tehran. The considered rainfall stations (which are located 10 to 20 km apart) produced significantly different results due to the difference in rainfall conditions and topographic variability. The results revealed that rainwater tank reliability can vary significantly within a large city like Tehran, which emphasizes the need to change the design pattern of considering a single annual rainfall value for the purpose of rainwater tank sizing.