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Assessing reliability of rainwater harvesting systems for meeting water demands in different climatic zones of Iran

Abstract

Assessing the performance of rainwater harvesting systems, which is one of the effective ways to cope with the water shortage crisis, leads to better management of these systems. In this paper, the reliability of rainwater harvesting systems and the overflow ratio of storage tanks were investigated for different climatic conditions, different volumes of tanks, and different roof areas. The results indicated that in the cities of Rasht, Sari, Tabriz, and Yazd, using a 10-m3 tank, non-potable water needs of a four-person family can be supplied from a 100-m2 roof area for 67.3, 42.98, 12.07, and 1.35% of the days during a year. Further, if a 1-m3 tank is used in Rasht, 50.67% of the total harvested water will be overflowed, which would decrease to 19.21% if the 10-m3 tank is replaced. For Tabriz, the ratio of overflow from the 5-m3 tank was zero, but for the city of Sari, even with use of a 10-m3 tank, 0.73% overflow occurred. In addition, for Yazd city, using a 1-m3 tank, an overflow of 48.4% occurred, but when the volume of the tank was increased to 2 m3, there was no overflow. For the city of Tabriz, the ratio of overflow from the 5-m3 tank was zero, but for the city of Sari, even using a 10-m3 tank, 0.73% overflow occurred. For a constant volume of storage, as long as the average rainfall of the area was high, the ratio of overflow was also elevated. The ratio of overflow, from the highest to the lowest, was related to the temperate climate of the Caspian Sea and the pseudo-Mediterranean climate, moderate and humid climate, mountainous climate, and desert hot and dry climate, respectively.

Introduction

Iran is considered to be a part of the arid and semi-arid regions of the world, facing a shortage of water resources. In addition, the distribution of precipitation is not the same in the country, and most precipitation occurs on the coast of the Caspian Sea and the west to the southwest of the country. Due to climate variability and inefficient water resource management, water supply has become an important issue in Iran. On the other hand, due to population growth and climate change, demand for water resources is increasing and water supply systems are under stress (Imteaz et al. 2011). One of the water-management strategies to reduce the pressure on water resources is the use of rainwater harvesting systems (RWHS). Therefore, the performance analysis of these systems plays a key role in convincing the users to adopt RWHS.

Rashidi Mehrabadi et al. (2013) showed that at least 75% of non-potable water needs could be provided in conventional buildings at roughly 70% at the time of rainwater storage. In a study in Bangladesh, Dakua et al. (2014) found that about 70–80% of the water demand could be supplied during June–September. Bashar et al. (2018) evaluated the reliabilities of RWHS for six cities in Bangladesh. They found that approximately 500–800 m3 of water can be stored per year if RWHS is used. Bailey et al. (2018) showed that increasing the level of roof area is an optimal strategy to increase the reliability of the rainwater harvesting system. Lani et al. (2018) showed that the reliability of RWHS can be achieved between 93 and 100%, depending on the size of the storage tank, for small and large commercial buildings, respectively. Zhang et al. (2018) investigated the effect of rainfall change on reliability (percentage of days in a year to supply residents’ water demand by rainwater) of rainwater harvesting systems at three cities in China. They found that the effect of rainfall change on RWHS was dependent not only on extent and trend of rainfall change, but also water demand and tank size scenarios.

This paper presents the performance and reliability of rainwater harvesting systems among the four cities of Iran (for residential buildings) under various climate zones (temperate climate of the Caspian Sea and the pseudo-Mediterranean, moderate and humid climate, mountainous climate, and desert hot and dry climate). Moreover, the ratio of overflow (OFR) for storage tanks with different volumes was investigated.

Materials and methods

Study areas

In this paper, four cities of Iran i.e. Rasht (temperate climate of the Caspian Sea and the pseudo-Mediterranean climate), Sari (moderate and humid climate), Tabriz (mountainous climate), and Yazd (desert hot and dry climate) were selected, since the climate and precipitation rates of these cities are representatives of other cities across the country. Figure 1 shows the locations of these cities.

Fig. 1
figure1

Locations of case study cities

Volume of harvested rainwater

This paper considered overflow ratio (OFR) and volumetric reliability which are described below. A daily water balance modeling approach was selected to analyze the RWHS, since this technique is easy to use and interpret. In this model, rainfall runoff is used as the primary source of water for meeting demands in residential buildings. This model has been used in previous researches by Rahman et al. (2012), Rashidi Mehrabadi et al. (2013), Imteaz et al. (2014), and Jing et al. (2017).

The volume of runoff from the surface of the catchment can be expressed by (Khastagir and Jayasuriya 2010):

$$ Q = I_{\text{eff}} \times C_{\text{R}} \times A $$
(1)

where Q is the runoff (L), Ieff is the daily rainfall (mm), A is the roof area connected to tank (m2), and CR is the runoff coefficient.

Reliability analysis

To estimate the reliability of rainwater harvesting systems, data on the daily rainfall were obtained by the Meteorological Organization for Tabriz, Sari and ,Yazd cities from December 24, 2000 to December 23, 2018, as well as Rasht city from June 1, 1993 to 23 December 2018. Table 1 shows the geographic location and the average annual rainfall of the stations. The RWHS reliability to meet the non-potable water needs of residents was calculated based on the following equation (Imteaz et al. 2012):

$$ \text{Re} = \frac{N - U}{N} \times 100 $$
(2)

where Re is the reliability of a rainwater harvesting system (%), U is the number of days in a year when the non-potable water needs of the residents are not provided to meet the water demand, and N indicates the total number of days in a year.

Table 1 General features of the selected cities

In this study, a range of tank sizes from 1–10 m3 were investigated under the scenarios providing 100 and 75% demand. Reliability analysis was performed for residential buildings with typical roof areas from 100–200 m2. Four climatic conditions were evaluated based on the analysis of rainfall data.

Overflow ratio

As stated in Bashar et al. (2018), overflow ratio is calculated as:

$$ {\text{OFR}} = \frac{{\sum\nolimits_{i = 1}^{n} {{\text{SW}}_{i} } }}{{\sum\nolimits_{i = 1}^{n} {V_{i} } }} \times 100 $$
(3)

where SWi is the volume of the water overflow (L) in the day i, Vi is the volume of collected water (L) in the day i and n is the number of days (during an evaluation period). The volume of overflowed water was calculated using the following equation (Rashidi Mehrabadi et al. 2013):

$$ {\text{SP}}_{t} = V_{t - 1} + I_{t} - O_{t} - V_{\rm{max} } $$
(4)

where SP is the volume of the tank overflow (L), Vmax is the volume of the tank (L), O is non-potable water demand (L), and I is the volume of daily collectable water (L).

The runoff coefficient was considered as 0.85 according to the material and slope of the roofs in the case study cities.

Results and discussion

Based on Fig. 2a (in Rasht), for roof area of 100 m2, the maximum percentage of daily supply (320 L) was equal to 67.3% and the minimum percentage was obtained for a 1-m3 tank (about 37%). The lowest reliability was obtained for Yazd city, such that in this city, the maximum percentage of days when rainwater harvested from a 100-m2 roof area could supply four-person daily water needs (320 L) which was equal to 1.35%. It was observed that with increase in tank volume, the percentage of days when rainwater could meet the daily non-potable water needs increased. Similar result was obtained by Rashidi Mehrabadi et al. (2013), Lani et al. (2018) and Bashar et al. (2018).

Fig. 2
figure2

The reliability curves for a residential building with a roof area of 100 m2 for a 100% demand (320 L) and b 75% demand (240 L)

In the cities of Sari and Tabriz, the highest percentages of days when rainwater could meet the daily non-potable water needs per tank volume of 10m3, were 43% and 12%, respectively, and per 1-m3 tank, were equal to 26.5 and 10.6%, respectively. According to Fig. 2b, it can be concluded that in Rasht, the maximum number of days when harvested rainwater from a 100-m2 roof area could supply at least 75% of the daily non-potable water needs of four residents (240 L), was equal to 79.4% of the total days per year. Further, the lowest percentage was related to the 1-m3 tank volume (45.6%). In Yazd city, the highest percentage of days when the rainwater harvested from a 100 m2 roof area could supply 75% of the daily non-potable water needs of four residents was related to a 10-m3 tank (2.33%), while the lowest percentage of days was obtained for 1-m3 tank volume, equal to 2.27% of the days during a year. In the cities of Sari and Tabriz, the maximum percentages of days when the rainwater was sufficient to meet 75% of daily non-potable water needs of four residents for 10-m3 tank volume were equal to 58.7 and 18.38%, respectively, while for tank volume of 1m3, they were 35 and 15.4%, respectively. Figure 3 shows the reliability of rainwater tanks for a residential building with a roof area of 200 m2 corresponding to 100% demand (640 L). Based on Fig. 3a, in Rasht city, the maximum percentage of days when collected rainwater from a roof area of 200 m2 could supply daily non-potable water need of eight residents (640 L) was 59.44%. The lowest percentage of days of daily non-potable water supply for the eight people was 27.75%, while the values for the city of Yazd were 1.35 and 1.23%.

Fig. 3
figure3

The reliability curves for a residential building with a roof area of 200 m2 for a 100% demand (640 L) and b 75% demand (480 L)

For the city of Tabriz, to meet the daily non-potable water needs of eight people (640 L) from a 200-m2 roof area, the maximum reliability of the rainwater tanks was related to a 10-m3 tank and equal to 12.07%, while the lowest reliability of the tanks was obtained for 1-m3 tank volume, equal to 8.55%. The maximum and minimum of these values for the Sari city were obtained for 10-m3 and 1-m3 tank volumes, which were 40.72 and 18.85%, respectively. According to the Fig. 3b, in Rasht city, the reliability of the system (for 75% of the daily non-potable water need of eight people) with a roof area of 200 m2, varied from 36.3-70.8%. This change in Sari’s city was from 26.85% for a tank of 1m3 to 54.15% for a 10-m3 tank. For Tabriz city, this change was from 18.36 to 12.95%, and for Yazd was from 2.33 to 2.07%.

According to Fig. 4a, it can be concluded that in all cities, with increase in water demand, the percentage of reliability of the tanks decreases, which is consistent with the results from the research by Rashidi Mehrabadi et al. (2013) and Bashar et al. (2018).

Fig. 4
figure4

The reliability curves for a residential building with a roof area of a 100 m2 and b 200 m2 under scenarios supplying 100 and 75% demand

The percentage of daily non-potable water supply rose as the volume of the tank increased, while the slope of this trend was constantly decreasing, such that reliability approached a constant value. According to Fig. 4b, it can be concluded that for all cities, for a fixed roof area, the percentage of days when 75% of daily non-potable water needs (for eight residents) could be supplied by rainwater, was more than that of days to supply 100% of daily non-potable water needs for the same number of residents.

Figure 5 shows the overflow ratio relation with different tanks sizes for case study cities for a roof area of 100 m2. The overflow ratio in the cities of Rasht and Sari was very significant as compared to the cities of Tabriz and Yazd. It can be concluded that for a fixed tank volume, as long as the average rainfall over the area is high, the ratio of overflow will also increase, such that the amount of overflows in Rasht was more than Sari, Tabriz, and Yazd. The least amount of overflow was observed in Yazd city. It can be concluded that as long as the volume of the tanks is larger, the overflow ratio will be downward, until there is no more overflow, which is consistent with the results from the research by Bashar et al. (2018). As shown in Fig. 5 for Rasht, for a tank size of 1m3, the ratio of overflow was 50.67% of the total harvested water. For a tank size of 10m3, this ratio was decreased to 19.21%. Based on Fig. 5, in Yazd city, for a tank size of over 2m3, the ratio of overflow was approximately equal to zero. With enlargement of the tank volume, there was a decrease in overflow. As the volume of the tanks used enlarged, the overflow ratio diminished as reported by Bashar et al. (2018).

Fig. 5
figure5

Overflow ratio relation with different tank sizes for a 100 m2 roof area

It can be concluded that the reliability of rainwater harvesting systems varies according to different climatic conditions. It is found that the efficiency and reliability of rainwater harvesting system increase with the increasing rainfall. In Yazd with dry weather conditions and low rainfall, the potential of RWH systems is far less. The reliability of rainwater tanks follows the trend of rainfall changes. In case of dry climate condition, RWH system in buildings is seemed to be financially non-beneficial and tank volume has no significant impact on RWH system reliability (Karim et al. 2015). Under the dry conditions, RWH system is not economically viable and feasible to install. However, it should be considered that rainwater is one of the most important alternative water sources under dry conditions as reported by Jing et al. (2017). For RWH systems with an increased storage capacity, the efficiency is limited by the rainwater demand and collectable rainfall scenarios (Jing et al. 2017). In general, for a sustainable and viable rainwater harvesting system, cost-effectiveness analysis should be evaluated to determine the optimum tank volume. Moreover, the water supply reliability of rainwater harvesting systems under various climatic conditions varies widely as reported by Rashidi Mehrabadi et al. (2013) and Bashar et al. (2018). Moreover, the building residents may be reluctant in adopting rainwater harvesting system. This reluctance may be attributed to lack of information about the economic benefit and reliability of using RWH system through comprehensive studies. Government should consider initiative to educate the households on rainwater conservation benefit associated with rainwater harvesting system installation and implementation in the cities (Bashar et al. 2018).

Conclusions

In this paper, the reliability of rainwater harvesting systems (RWHS) and the overflow ratio of storage tanks were investigated for different climatic conditions, different volumes of tanks, and various roof areas. By implementing rainwater harvesting systems with a catchment area of 100 m2 in Rasht, Sari, Tabriz, and Yazd, in 79.4, 58.72, 18.38, and 2.33% of the days in a year, 75% of non-potable water needs of a four-person family were supplied using a tank volume of 10m3. For a 1-m3 tank, the above quantities were equal to 45.65, 35.05, 15.43, and 2.27% of the days in a year, respectively. With enlargement of the tank volume, the reliability percentage showed an ascending trend, such that the reliability tended to reach a constant value. Usage of tank volumes larger than this constant volume will not be economical. For the same catchment area and constant water demand, as the rainfall grows, the reliability of rainwater tanks will be higher. On the other hand, with increase in water demand, the percentage of reliability of rainwater tanks decreases. As the volume of the tanks used enlarges, the overflow ratio diminishes, such that after a given tank volume, there will be no overflow and use of larger volumes will not be cost-effective. For a sustainable and viable rainwater harvesting system, economic benefit analysis should be performed to determine the suitable tank volume. Under the dry conditions, RWHS may not be economically viable to install. However, it should be considered that rainwater is one of the most important alternative water sources under dry conditions. The reliability of rainwater harvesting systems was different under various climatic conditions, such that the potential of these systems in Iran, from the highest to the lowest, was related to the temperate climate of the Caspian Sea and the pseudo- Mediterranean climate, the moderate and humid climate, Mountainous climate, and desert hot and dry climate, respectively. For a constant volume of storage, as long as the average rainfall of the area is high, the ratio of overflow will also increase.

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Correspondence to Fereshte Haghighi Fashi.

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Shokati, H., Kouchakzadeh, M. & Fashi, F.H. Assessing reliability of rainwater harvesting systems for meeting water demands in different climatic zones of Iran. Model. Earth Syst. Environ. 6, 109–114 (2020). https://doi.org/10.1007/s40808-019-00662-3

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Keywords

  • Climate condition
  • Rainwater harvesting
  • Reliability
  • Overflow ratio