Assessing efficiency and economic viability of rainwater harvesting systems for meeting non-potable water demands in four climatic zones of China

Resources, Conservation & Recycling 126 (2017) 74–85

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Full length article

Assessing efficiency and economic viability of rainwater harvesting systems for meeting non-potable water demands in four climatic zones of China

MARK

⁎

Xueer Jing, Shouhong Zhang , Jianjun Zhang, Yujie Wang, Yunqi Wang School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Rainwater harvesting Water balance equation Stormwater capture efficiency Time reliability Water saving efficiency Benefit-cost analysis

Rainwater harvesting is now increasingly used to manage urban flood and alleviate water scarcity crisis. In this study, a computational tool based on water balance equation is developed to assess stormwater capture and water saving efficiency and economic viability of rainwater harvesting systems (RHS) in eight cities across four climatic zones of China. It requires daily rainfall, contributing area, runoff losses, first flush volume, storage capacity, daily water demand and economic parameters as inputs. Three non-potable water demand scenarios (i.e., toilet flushing, lawn irrigation, and combination of them) are considered. The water demand for lawn irrigation is estimated using the Cropwat 8.0 and Climwat 2.0. Results indicate that higher water saving efficiency and water supply time reliability can be achieved for RHS with larger storage capacities, for lower water demand scenarios and located in more humid regions, while higher stormwater capture efficiency is associated with larger storage capacity, higher water demand scenarios and less rainfall. For instance, a 40 m3 RHS in Shanghai (humid climate) for lawn irrigation can capture 17% of stormwater, while its water saving efficiency and time reliability can reach 96% and 98%, respectively. The water saving efficiency and time reliability of a 20 m3 RHS in Xining (semi-arid climate) for toilet flushing are 19% and 16%, respectively, but it can capture 63% of stormwater. With the current values of economic parameters, economic viability of RHS can be achieved in humid and semi-humid regions for reasonably designed RHS; however, it is not financially viable to install RHS in arid regions as the benefit-cost ratio is much smaller than 1.0.

1. Introduction Urbanization is a worldwide process with well-known adverse hydrologic and environmental effects such as increasing flooding damage and decreasing water quality (Loganathan and Delleur, 1984; Zhang and Guo, 2013a). Along with urbanization, population in many cities of the world are quickly increasing and water supply systems in these cities are consequently under stress (Zhang et al., 2012). Moreover, uncertainties associated with climate change will intensify the pressure of future water supply and stormwater management systems in most urban areas (Hanson and Palmer, 2014). Rainwater harvesting systems (RHS) are operated to collect and store rainwater from contributing areas (e.g., building roofs and parking lots) during rainfall events, and store it in cisterns for use on dry days between rainfall events (Guo and Baetz, 2007; Khastagir and Jayasuriya, 2010). As RHS can alleviate urban water supply pressure and reinforce urban stormwater management system at the same time, it has been more and more widely used in many nations (Rahman et al., 2010; Mehrabadi et al., 2013; Jones and Hunt, 2010; Kim and Yoo, 2009; Ghisi, 2010; Vialle et al., 2015).

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Corresponding author. E-mail address: [email protected] (S. Zhang).

http://dx.doi.org/10.1016/j.resconrec.2017.07.027 Received 23 March 2017; Received in revised form 19 July 2017; Accepted 19 July 2017 0921-3449/ © 2017 Elsevier B.V. All rights reserved.

Harvested rainwater can be used to substitute tap water for potable (e.g., drinking and cooking) or non-potable (e.g., flushing toilets, washing clothes and irrigating lawns) purposes (USEPA, 2004) depending on its quality. As a result, rainwater harvesting can reduce urban water supply stress (Appan, 2000; Handia et al., 2003). Rainwater is usually one of the cleanest available water sources and rainwater harvesting is one of the best methods available for establishing sustainable water cycles in urban developments (Zhang and Hu, 2014). A study of RHS in 195 cities of Southeastern Brazil indicated that the tap water saving potential of RHS ranges from 12% to 79% (Ghisi et al., 2007). Zhang and Hu (2014) reported that 9.8 × 106 m3 rainwater can be harvested in one year from an industrial park (about 8 km2) located in a humid climatic zone of China. In several cities (dry climatic zone) of Saudi Arabia, the amounts of rainwater that can be harvested were estimated to be larger than 7.5 m3/100 m2 per year (Guizani, 2016). Karim et al. (2015) revealed that about 250 kL to 550 kL of rainwater can be harvested per year under catchment sizes varying from 140 m2 to 200 m2 in Dhaka, Bangladesh. RHS can be viewed as miniature multipurpose stormwater

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Fig. 1. Spatial distribution of mean annual rainfall and locations of the eight cities.

optimization, such as the design storm approach (Vaes and Berlamont, 2001), continuous simulations (Jenkins, 2007; Kim and Yoo, 2009; Campisano and Modica, 2012), the analytical probabilistic approach (Guo and Baetz, 2007), linear programming approach (Okoye et al., 2015), the nonlinear metaheuristic algorithm (Sample and Liu, 2014), and the dimensionless methodology (Campisano and Modica, 2012). Among these approaches, long-term continuous simulations based on water balance theory are most frequently applied to assess the stormwater management performance and economic benefit of RHS (Hajani and Rahman, 2014; Silva et al., 2015) and to optimize their storage unit size (Imteaz et al., 2011a; Hashim et al., 2013). China has been facing increasingly serious urban water scarcity crisis as the result of rapid industrialization and urbanization as well as extremely high density of population (Zuo et al., 2010). The application of RHS as multipurpose facilities has been widely promoted by the central and many local governments in China (Chen, 2013). Despite beneficial uses of RHS have been demonstrated by numerous researchers (Zuo et al., 2010; Liang and Dijk, 2011; Zhang and Hu, 2014) and many incentive programs (Li, 2009) have been provided by governments, there is still a reluctance in general communities to adopt them on a wider scale. This reluctance can be mainly attributed to lake of information about the environmental effectiveness and economic benefit of using RHS and easy-to-use tools to determine the size of them. The novelty of this study is the development of an easy-to-use computational tool which can be used in stormwater capture and water saving efficiency and economic viability assessment of RHS. Taking daily rainfall data, contributing roof area, rainfall loss factors, design storage capacity, and daily rainwater usage rate into consideration, a daily water balance model is developed and used to assess the stormwater management and water saving efficiency of RHS. Comparing the present value of benefits to the present value of costs of RHS, a benefitcost ratio is defined and calculated to evaluate their economic viability.

management facilities (Kim et al., 2015). The implementation of RHS diverts surface runoff away from stormwater collection systems and reduces the volume of runoff that needs to be managed during rainfall events (Guo and Baetz, 2007; Rostad et al., 2016). Steffen (2013) evaluated stormwater management performance of RHS located in 23 cities within 4 different climatic regions in the USA and found that higher stormwater capture efficiency could be achieved in semi-arid regions than in humid regions. As reported by Litofsky and Jennings (2014), the stormwater capture efficiency of rain barrel-urban garden systems for 70 selected locations across the USA over 2000–2009 time period ranged from 3.0% to 44.5%. Based on EPA SWMM simulations, Palla et al. (2017) reported that the average discharge peak and volume reduction rates of implementing domestic RHS in a residential urban block located in Genoa (Italy) were 33% and 26%, respectively (with maximum values of 65% for peak and 51% for volume). Implementation of RHS can also bring considerable economic and environmental benefits. Tam et al. (2010) evaluated the cost effectiveness of RHS in 7 cities of Australia and indicated that RHS are economically feasible in Gold Coast, Brisbane, and Sydney due to more rainfall and higher reliability. Zuo et al. (2010) developed an economic evaluation system to assess 267 RHS in Beijing and concluded that 66.7% of the systems could produce significant economic benefits. A financial comparison between using rainwater and using groundwater for agricultural irrigation in the rural areas of Beijing showed that using rainwater was economically feasible and had positive effects for society (Liang and Dijk, 2011). In Saudi Arabia, a study showed that harvested rainwater is cheaper than desalinated water produced from renewable energy-driven desalination plants but that is not the case for fossil fuelpowered desalination (Guizani, 2016). Even the scale of most RHS are small, sizing of rainwater storage units for them should be rigorously treated as a hydrologic engineering design similar to other stormwater management facilities (Guo and Baetz, 2007). Various approaches can be used in RHS size design and 75

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Table 1 Rainfall data of the eight cities.

Dt , Qt + St − Dt > 0 Yt = ⎧ St + Qt , Qt + St − Dt ≤ 0 ⎨ ⎩

City

Length of the data

Mean annual rainfall (mm)

Shanghai Chengdu Shenyang Beijing Xining Urumqi Yinchuan Kaxgar

1959–2013 1959–2013 1951–2013 1951–2013 1954–2013 1951–2013 1951–2013 1951–2013

1110 919 714 584 387 270 188 62

where Rt is the volume of rainwater retained by the storage unit on the t th day, in m3; Yt is the rainwater yield on the t th day, in m3; Qt is collectable stormwater runoff generated from the contributing areas (after bypassing first flush) on the t th day, in m3; Dt is the water demand on the t th day, in m3; St is the volume of rainwater remained in the storage unit at the beginning of the t th day, in m3; V is the designed storage capacity of the storage unit, in m3. The collectable stormwater runoff generated from the contributing areas (Qt) can be determined as

Eight cities across four climatic zones of China and three non-potable water demand scenarios are included in the application of the model and the benefit-cost ratio to reveal the impacts of climatic and water using conditions on the environmental effectiveness and economic viability of RHS.

Qt =

0, Ht ≤ δ ⎧ ψ (Ht − δ ) F ⎨ 1000 , Ht > δ ⎩

(3)

where Ht is the volume of rainfall on the t th day, in mm; ψ is the runoff coefficient of the contributing areas, dimensionless; F is the area of the contributing areas, in m2; δ is the volume of first flush over the area of the contributing area, in mm. As suggested by previous studies (Guo and Baetz, 2007; Kus et al., 2010), a first flush depth of 2 mm (i.e., δ = 2 mm) is usually diverted away from the rainwater storage units to improve quality of harvested rainwater. A 10% deduction is usually applied to account for losses of rainwater resulting from leakage, spillage and evaporation of a rooftop RHS (Imteaz et al., 2013; Sample and Liu, 2014; Okoye et al., 2015; Liuzzo et al., 2016). Thus, the runoff coefficient of rooftops is set as 0.9 in this study (i.e., ψ = 0.9). However, for real case studies, the values of both δ and ψ should be varying with specific conditions such as local climate, air quality, materials of the rooftops, and types of rainwater collecting systems and storage units. The rainwater remained in the storage units (St) can be calculated as

2. Study site and data As shown in Fig. 1, eight cities across four climatic zones of China are selected in this study, namely Shanghai and Chengdu (humid climate), Beijing and Shenyang (semi-humid climate), Xining and Urumqi (semi-arid climate) and Yinchuan and Kaxgar (arid climate). The climatic zones are classified by mean annual rainfall as humid zones with mean annual rainfall larger than 800 mm, semi-humid zones with mean annual rainfall between 400 and 800 mm, semi-arid zones with mean annual rainfall between 200 and 400 mm, and arid zones with mean annual rainfall less than 200 mm (Zheng et al., 2013). Daily rainfall data of the eight cities are obtained from the National Climatic Centre (NCC) of China Meteorological Data Service Center. The length of rainfall data ranges from 51 to 63 years and the mean annual rainfall volume varies from 62 mm in Kaxgar to 1110 mm in Shanghai (Table 1). Distributions of average daily rainfall of those cities are shown in Fig. 2. The most common building form in those cities is multi-storey buildings with relatively flat roofs. And most of the buildings are surrounded by green spaces planted with grasses or trees. The surrounding green-spaces in different cities are usually planted with different types of vegetation. Therefore, the study site in is assumed to be consisting of a multi-storey residential building and its surrounding lawns. In this study, the roof area of the building is 1000 m2 and it serves about 100 inhabitants. The area of the lawns is 2000 m2. Stormwater generated from the building roof, after bypassing first flush, can be collected and used for lawn irrigation or flushing toilets or urinals in the building.

0, Qt − 1 + St − 1 − Dt − 1 ≤ 0 ⎧ St = Qt − 1 + St − 1 − Dt − 1, 0 < Qt − 1 + St − 1 − Dt − 1 ≤ V ⎨ V, Qt − 1 + St − 1 − Dt − 1 > V ⎩

(4)

where St-1 is the volume of rainwater remained in the storage unit at the beginning of the (t-1) th day, in m3; Qt-1 is the collectable stormwater runoff generated from the contributing areas on the (t-1) th day, in m3; Dt-1 is the water demand on the (t-1) th day, in m3; t ≥1, Q0 = S0 = D0 = 0. Illustrative diagrams of the operating rule of RHS are given in Fig. 3. Fig. 3(a) illustrates the case when the total volume of the initial storage (St−1) and inflow (Qt−1) on the (t-1) th day is smaller than the water demand on that day (i.e., Qt−1 + St−1 − Dt−1 ≤ 0). In this case, all of the rainwater retained in the storage unit and the inflow will be consumed, the water demand on this day cannot be fully satisfied or can just be satisfied (i.e., Dt−1 ≥ Yt−1), and the storage unit will be empty at the beginning of the next coming day (i.e., St = 0). Fig. 3(b) illustrates the case when Qt−1 + St−1 − Dt−1 > 0 and the difference between the total volume and the water demand is smaller than the storage capacity of the storage unit (i.e., 0 < Qt−1 + St−1 − Dt−1 ≤ V). In this case, the water demand on this day can be fully satisfied (i.e., Yt−1 = Dt−1), and the storage unit will be partly occupied by the remaining water at the beginning of the next coming day (i.e., 0 < St ≤ V). Fig. 3(c) illustrates the case when the difference between the total volume and the water demand is larger than the storage capacity of the storage unit (i.e.,Qt−1 + St−1 − Dt−1 > V). In this case, spillage will occur, the water demand on this day can be fully satisfied (i.e., Yt−1 = Dt−1), and the storage unit will be completely full (i.e., St = V) at the beginning of the next coming day. RHS capture certain volume of stormwater generated from contributing areas and thus reduce the volume of surface runoff that needs to be managed by stormwater drainage systems. Stormwater management performance of a RHS can be partly evaluated by stormwater capture efficiency (Zhang and Guo, 2013b). Stormwater capture efficiency (η , %) of the RHS can be calculated using the following equation,

3. Methodology 3.1. Daily water balance model Applying the YBS (yield before spillage) operating rule, a daily water balance model is developed to simulate the inflow (runoff generated from contributing areas), outflow (desired rainwater for potable or non-potable uses), and overflow (first flush and excess rainwater directed into drainage systems) based on analysis of hydrologic operation of RHS. Details of the YBS operating rule can be found in Mitchell (2007) and Liaw and Tsai (2004). Stormwater capture efficiency, water saving efficiency, time reliability, as well as the benefitcost ratio of RHS are then determined on the basis of long-term continuous simulation results using this model. Ignoring leakage and evaporation losses from a closed RHS, the water balance equation of it can be expressed as

Qt , Qt + St − Dt ≤ V Rt = ⎧ V − St + Dt , Qt + St − Dt > V ⎨ ⎩

(2)

(1) 76

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Fig. 2. Average daily rainfall volume of the 8 cities.

η=

∑ Rt ψF ∑ Ht /1000

g=

× 100% (5)

∑ ∑

Yt Dt

(7)

Where U is the number of days in total the RHS is unable to meet the water demand, and N is the total number of days of the simulation period.

The efficiency of RHS for reducing urban water supply stress can be evaluated by water saving efficiency (w , %), which can be calculated as,

W=

N−U N

× 100%

3.2. Economic analysis

(6)

Economic benefit is one of the determinants of the feasibility of RHS

Time reliability of water supply of the RHS is expressed as follows, 77

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Fig. 3. Illustrative diagram of operating rule of rainwater harvesting systems [modified from (Mitchell, 2007)].

system is greater than its costs, which suggests that the system is economically feasible. The larger the benefit-cost ratio, the more economically feasible the system is. The benefit-cost ratio (RBC, dimensionless) can be calculated as

(Rahman et al., 2012). The economic benefit can be evaluated by benefit-cost ratio, which compares the present value of benefits (TB) to the present value of costs (PV) of RHS (Dallman et al., 2016). When the benefit-cost ratio of RHS is larger than 1.0, the benefits provided by the 78

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RBC =

TB PV

PV = I + C·

TB = B·

Table 3 Water rate and electricity charge in different cities.

(8)

(1 + i)n − 1 i (1 + i)n

(9)

i )n

(1 + −1 i (1 + i)n

(10)

where RBC is the ratio between the present values of benefits and costs of a project; I is the fixed investment of the project, in RMB; C is the operational cost of the project, in RMB; B is the annual average total benefit generated from the project, in RMB; i is the discount rate (according to the result from the National Development and Reform Commission of China of the People's Republic of China (2008), the discount rate is recommended as 8%); n is the design lifetime of the project. According to Li et al. (2001), the design lifetime of RHS could be approximately estimated as 30 years. As suggested by Zuo et al. (2010), the economic benefits of a RHS might involve tap water saving benefits (B1), national financial increase (B2), the decrease of social loss resulting from less pollution (B3), the decrease of municipal water discharge system operation and many other benefits (B4). The total economic benefit of a RHS can be estimated as 4

B=

∑

Bi = Ptap·Vr + PS ·Vr +

i=4

1 ·Pa·Vr + Pn·Vr α

In this study, three non-potable water demand scenarios were assessed. They are (1) lawn irrigation only, (2) toilet flushing only, and (3) combination of toilet flushing and lawn irrigation. According to the Code for Design of Stormwater Management and Harvest Engineering (DB11/685—2013), toilet flushing water demand for the multi-storey residential building is 32 L per person per day. The daily toilet flushing water demand for the building can be estimated as 100 person × 32 L

Total price (RMB)

Tank Pipe fittings and other equipment Pressured pump Electricity consumption

m3 m

V 150

1500 30

1500V 4500

Set RMB/ (kw h) m3/RMB m3/RMB

1 3653

2500 FEC

2500 3653 FEC

Vr Vr

0.1 0.65

0.1 Vr 0.65 Vr

Water treatment Maintenance and management

3.30 4.43 3.30 6.00 2.90 1.66 3.20 1.40

0.46 0.54 0.47 0.49 0.43 0.42 0.44 0.34

(12)

4. Results and analysis 4.1. Stormwater management performance of RHS Stormwater capture efficiency of RHS in the 8 cities under the 3 water demand scenarios is investigated using Eq. (5). Fig. 4 shows the changes of stormwater capture efficiency of RHS as a function of storage capacity. In general, higher stormwater capture efficiency can be obtained for RHS with larger storage capacity and for higher water demand scenarios. For RHS in different climate zones, stormwater capture efficiency sharply rises when storage capacity changes from 0 to about 50 m3. However, beyond a size of about 50 m3, further increases in storage capacity can only translate to marginal increases in stormwater capture efficiency. In cities such as Chengdu and Shenyang, stormwater capture efficiency becomes nearly constant when storage capacity increases beyond about 50 m3. The reason may be that when the storage capacity of a RHS is small, stormwater captured by the system during rainfall events can be completely consumed in dry periods, therefore, volume of stormwater captured by the system is determined by its storage

Table 2 Construction and maintenance costs of rainwater harvesting systems. Unit price (RMB)

Shanghai Chengdu Shenyang Beijing Xining Urumqi Yinchuan Kaxgar

where ET is the daily irrigation demand (i.e., the actual evapotranspiration) of a lawn, mm/day; Kc is the crop coefficient, which is affected by many parameters, such as types of grass, meteorological conditions, plant cover and management practices (Ding et al., 2015). As investigated in previous studies, the average value of Kc for warm season turfgrass in the southern cities of China (e.g., Shanghai and Chengdu) is around 0.83 (Pang et al., 2009). For cool season turfgrass in northern cities of China (e.g., Shenyang, Beijing, Xining, Yinchuan, Urumqi and Kaxgar), the average Kc value of 0.94 is applicable (Aronson et al., 1987; Zhao et al., 2003). Irrigation frequency is also an important factor for lawn maintenance, which should be determined by turfgrass species, climatic conditions and soil types (Fu and Peterh, 2009). Jordan et al. (2003) suggested that an irrigating frequency of every 4 days should hold enough moisture to ensure a lawn growing strongly in a hot, humid region. For extremely arid climates, a turfgrass lawn should be irrigated every day in summer and later spring, while the irrigation interval may be stretched to every five to seven days during early spring, fall and winter (ASIC, 2014). Tables 4–7 show the reference and actual daily evapotranspiration, the irrigation frequency and quota for turfgrass lawns located in each of the 8 cities.

3.3. Water demand

Number of Units

Electricity charge [RMB/(kw h)]

ET = ET0·K c

where Bi is the benefit from i item, in RMB; Ptap is the price of tap water, in RMB/m3; Vr is the volume of harvested rainwater, in m3; Ps is the decreases in national finance income due to water shortage, in RMB/ m3; Pα is the sewage treatment fee, in RMB/m3; α is the ratio of input and output of environment treatment, dimensionless; Pn is the operation cost of urban drainage system and other benefits, in RMB/m2. As presented in Zuo et al. (2010), Ps = 5.48 RMB/m3, α = 0.33, and Pa = 1.20 RMB/m3 are used in the following analysis. Ptap varies in different locations as shown in Table 3. The costs of a RHS may include fixed assets investment and annual operation cost. The annual operational cost (C) is consisted of electricity consumption (C1), water treatment (C2), maintenance and management (C3). The costs are shown in Table 2. In the table, FEC is the electricity charge, which varies in different locations as shown in Table 3.

Units

Water rate (RMB/m3)

per person per day, which is 3.2 m3/day. The frequency and quota of lawn irrigation depends on types of grass, soil properties and climatic conditions. In order to estimate the irrigation demand of the lawns, mean monthly reference evapotranspiration values of the eight locations were determined using the CROPWAT 8.0 software (Allen et al., 1998). The data required by the CROPWAT 8.0 is obtained from CLIMWAT 2.0 (Kiprop et al., 2015). Given the mean monthly reference evapotranspiration, irrigation demand of a lawn can be estimated as

(11)

Item

City

79

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Table 4 Irrigation frequency and quota of lawns in Shanghai and Chengdu (humid climate). Month

Shanghai

Chengdu

Evapotranspiration (mm/day)

January February March April May June July August September October November December

Reference

Actual

0 0 1.95 2.72 3.45 3.73 4.77 4.83 3.46 2.76 0 0

0 0 1.62 2.26 2.86 3.10 3.96 4.01 2.95 2.29 0 0

Irrigation Frequency

None None Monthly Fortnightly Fortnightly Every 5 days Every 5 days Every 5 days Fortnightly Monthly None None

Daily Average Irrigation Quota (m3/d)

0 0 0.10 0.36 0.37 1.24 1.53 1.55 0.40 0.15 0 0

Evapotranspiration (mm/day) Reference

Actual

0 0 1.64 2.30 2.95 3.12 3.31 3.35 2.38 1.63 0 0

0 0 1.36 1.91 2.45 2.59 2.75 2.78 1.98 1.35 0 0

Irrigation Frequency

Daily Average Irrigation Quota (m3/d)

None None Fortnightly Fortnightly Weekly Every 3 days Every 3 days Every 3 days Weekly Fortnightly None None

0 0 0.18 0.25 0.63 1.21 1.24 1.26 0.53 0.17 0 0

Chengdu. It should be noted that RHS in Kaxgar have the lowest stormwater capture efficiency when captured stormwater is used for toilet flushing and for the combination use of toilet flushing and lawn irrigation. This may be caused by that 2 mm of rainwater was bypassed as first flush in the model, which accounts for about 43% of the total volume of rainfall in Kaxgar. That is to say no matter how large the storage capacity and rainwater demand are, the stormwater capture efficiency of RHS in Kaxgar cannot exceed 53% when 2 mm of rainwater is bypassed as first flush. However, in humid regions such as Shanghai and Chengdu, bypassing 2 mm of rainwater as first flush only accounts for about 17% and 20% of the total volume of rainfall, respectively.

capacity. However, as the storage capacity increases and exceeds certain volume, though more stormwater can be captured by the system, the additionally captured stormwater beyond the certain volume cannot be completely consumed in dry periods. Therefore, the increased storage capacity beyond that certain volume is always occupied by stormwater and is not efficient for stormwater management. Water demand scenarios can significantly impact stormwater management performance of rainwater harvest systems, especially in humid regions. For instance, a RHS with a storage capacity of 200 m3 in Shanghai (humid climate) could capture 78% of stormwater when the captured rainwater is used for toilet flushing (Fig. 4b). However, when the captured rainwater is used for lawn irrigation, the stormwater capture efficiency drops to be 18% (Fig. 4a). This huge difference may be explained by that, in the condition of sufficient rainfall in Shanghai and a large enough RHS, irrigation water needs for 2000 m2 lawns (168 m3/a) is much lower than rainwater consumed by toilet flushing for 100 residents (1168 m3/a). Climate also plays an important role on the stormwater management performance of RHS, in particular that harvested rainwater is used for lawn irrigation. Generally, the stormwater capture efficiency of rainwater harvest systems with the same storage capacity in arid and semi-arid regions is higher than that in humid and semi-humid regions. For example, a 100 m3 RHS for lawn irrigation can capture only 21% of stormwater if it is located in Chengdu, while the stormwater capture efficiency of it can jump to 67% if it is located in Yinchuan (Fig. 4a). It’s because that the total volume of rainfall is much lower and the lawn irrigation water demands are much higher in Yinchuan than in

4.2. Water saving performance of RHS Water saving efficiency of RHS in the 8 cities under different water demand scenarios are calculated using Eq. (6) and the results are shown in Fig. 5. Generally, RHS with larger storage capacities, in more humid regions and for lower water demand scenario, have higher water saving efficiency. Depending on climatic conditions, different types of variations of water saving efficiency can be seen from Fig. 5. In arid regions (i.e., Kaxgar and Yinchuan), water saving efficiency increase quickly to different threshold values when storage capacity changes from 0 to about 20 m3, and then stay at the threshold values. The threshold values vary from 2% to about 20% depending on locations and water demand scenarios. However, in humid (Shanghai and Chengdu) regions, water

Table 5 Irrigation frequency and quota of lawns in Shengyang and Beijing (semi-humid climate). Month

Shenyang

Beijing

Evapotranspiration (mm/day)

January February March April May June July August September October November December

Reference

Actual

0 0 1.93 3.58 4.76 4.86 4.24 3.98 3.38 2.29 0 0

0 0 1.81 3.37 4.47 4.57 3.99 3.74 3.18 2.15 0 0

Irrigation Frequency

None None Fortnightly Weekly Every 3 days Every 3 days Every 3 days Every 5 days Weekly Fortnightly None None

Daily Average Irrigation Quota (m3/d)

0 0 0.12 0.90 2.02 2.13 1.80 1.45 0.85 0.28 0 0

80

Evapotranspiration (mm/day) Reference

Actual

0 0 2.58 4.19 5.20 5.48 4.47 3.93 3.57 2.51 0 0

0 0 2.43 3.94 4.89 5.15 4.20 3.69 3.36 2.36 0 0

Irrigation Frequency

Daily Average Irrigation Quota (m3/d)

None None Fortnightly Every 5 days Every 3 days Every 3 days Every 3 days Every 5 days Weekly Fortnightly None None

0 0 0.31 1.58 2.21 2.40 1.90 1.43 0.90 0.31 0 0

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Table 6 Irrigation frequency and quota of lawns in Xining and Urumqi (semi-arid climate). Month

Xining

Urumqi

Evapotranspiration (mm/day)

January February March April May June July August September October November December

Reference

Actual

0 0 2.43 3.94 4.89 5.15 4.20 3.69 3.36 2.36 0 0

0 0 2.28 3.70 4.60 4.84 3.95 3.47 3.16 2.22 0 0

Irrigation Frequency

None None Fortnightly Every 3 days Every 2 days Every 2 days Every 2 days Every 5 days Weekly Fortnightly None None

Daily Average Irrigation Quota (m3/d)

0 0 0.30 1.73 2.97 3.22 2.55 1.34 0.84 0.29 0 0

Evapotranspiration (mm/day) Reference

Actual

0 0 1.09 3.15 4.80 5.65 5.91 5.74 4.03 1.91 0 0

0 0 1.02 2.96 4.51 5.31 5.56 5.40 3.79 1.80 0 0

Irrigation Frequency

Daily Average Irrigation Quota (m3/d)

None None Monthly Fortnightly Every 3 days Every 2 days Every 2 days Every 2 days Every 5 days Monthly None None

0 0 0.07 0.40 2.04 3.54 3.59 3.48 1.52 0.17 0 0

than 0.1 when harvested rainwater is used for toilet flushing. Due to the extremely low reliability, it is not recommended to harvest rainwater for toilet flushing in arid regions.

saving efficiency can reach 100% for the lawn irrigation scenario when storage capacity is larger than about 60 m3 (Fig. 5a), and can get values higher than 50% for the other two water demand scenarios when storage capacity goes beyond 400 m3 (Fig. 5b and c). Lower threshold values of water saving efficiency in arid regions are resulted from very limited collectable rainwater caused by very limited rainfall. Take Kaxgar as an example, its mean annual rainfall volume is only 60 mm.

4.4. Economic benefit analysis of RHS Economic analysis is important for better planning, design, evaluation, and decision making for implementation of RHS. Using Eq. (8), the benefit-cost ratios of RHS are calculated and the results are presented in Tables 8–10. In humid and semi-humid regions (Shanghai, Chengdu, Beijing, and Shenyang), the benefit-cost ratios of RHS increase to peak values firstly and then drop as storage capacities rise from 5 to 100 m3. With relatively sufficient collectable rainwater in humid and semi-humid regions, RHS with very small storage capacity can only provide very limited water saving and environmental benefits, while RHS with very large storage capacity will be associated with very high fixed investment and operational costs. Therefore, RHS with too small or too large storage capacities cannot provide high benefit-cost ratios. The peak values of the benefit-cost ratios suggest the most economically feasible storage capacities of RHS for specific water demand scenarios at specific locations. For example, as shown in Table 10, the most economically feasible storage capacity of RHS collecting rainwater for combination use of toilet flushing and lawn irrigation is around 10 m3 as the benefit-cost ratio gets it peak value 2.52. In arid and semi-arid regions (Xining, Urumqi, Yinchuan, and Kaxgar), the benefit-cost ratios of RHS decrease with storage capacities

4.3. Time reliability of RHS Time reliability of RHS is calculated using Eq. (7), and the results can be found in Fig. 6. Similar to the trends of water saving efficiency, higher reliability can be achieved for RHS with larger storage capacities, for lower water demand scenarios, and in more humid regions. As shown in Fig. 6a, reliability of RHS for lawn irrigation in humid and semi-humid regions can get close to 1.0 when storage capacities are larger than 200 m3. In arid and semi-arid regions, its upper limit values vary from about 0.37–0.79, depending on local rainwater availabilities. Higher reliability can be achieved when harvested rainwater is used for lawn irrigation may be resulted by two reasons: (1) irrigation water needs for 2000 m2 lawns is much lower than rainwater consumed by toilet flushing for 100 residents; and (2) most of these cities are located in monsoon climate zones in which monthly water demand of lawns keeps pace with monthly rainfall as shown in Fig. 2 and Table 1. When the harvested rainwater is used for toilet flushing, the reliability of RHS is much lower, especially in arid and semi-arid regions. For instance, in the two arid cities (Yinchuan and Kaxgar), the reliability of RHS is less Table 7 Irrigation frequency and quota of lawns in Yinchuan and Kaxgar (arid climate). Month

Yinchuan

Kaxgar

Evapotranspiration (mm/day)

January February March April May June July August September October November December

Reference

Actual

0 0 2.21 3.76 4.84 5.22 5.01 4.36 3.31 2.23 0 0

0 0 2.08 3.53 4.55 4.91 4.71 4.10 3.11 2.10 0 0

Irrigation Frequency

None None Monthly Every 3 days Alternate days Alternate days Alternate days Every 3 days Weekly Monthly None None

Daily Average Irrigation Quota (m3/d)

0 0 0.13 1.65 4.40 4.91 4.55 1.85 0.83 0.14 0 0

81

Evapotranspiration (mm/day) Reference

Actual

0 0 2.42 4.33 5.58 6.62 6.57 5.56 4.14 2.59 0 0

0 0 2.27 4.07 5.25 6.22 6.18 5.23 3.89 2.43 0 0

Irrigation Frequency

Daily Average Irrigation Quota (m3/d)

None None Monthly Alternate days Alternate days Alternate days Alternate days Alternate days Every 3 days Monthly None None

0 0 0.15 4.07 5.25 6.02 6.18 5.06 1.82 0.16 0 0

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Fig. 4. Stormwater capture efficiency of rainfall harvesting systems for different water demand scenarios: (a) lawn irrigation use, (b) toilet flushing use, and (c) combination of toilet flushing and lawn irrigation use.

Fig. 5. Water saving efficiency of rainfall harvesting systems for different water demand scenarios: (a) lawn irrigation use, (b) toilet flushing use, and (c) combination of toilet flushing and lawn irrigation use.

as the latter change from 5 to 100 m3. Water saving and environmental benefits provided by the RHS are limited by the relative dry conditions, as a result, the benefit-cost ratios will inevitably decrease with storage capacities as the fixed investment and operational costs of RHS increases with storage capacities. In almost all of the cases (different water demand scenarios and storage capacities) in arid and semi-arid regions, the benefit-cost ratios are less than 1.0. It seemingly suggests that it is not economically feasible to apply RHS in these regions. However, it should be noted that rainwater is one of the most promising alternative water sources in arid and semi-arid areas and there is a long history of rainwater harvesting in these areas (Li et al., 2000; Hajani

and Rahman, 2014; Mahmoud et al., 2016). The less than 1.0 benefitcost ratios may be partly resulted from unreasonable valuation of water prices used in the calculation. The valuation of per cubic meter of water should be differentiated in arid and humid climatic zones given a reasonable market adjustment mechanism (Kim and Yoo, 2009; Hajani and Rahman, 2014). Water demand scenarios can significantly impact the economic performance of RHS, especially in humid and semi-humid regions. Comparing Table 8 with Tables 9 and 10, it can be seen that RHS with the same storage capacity and at the same location have lower benefit82

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5. Conclusions and discussions An easy-to-use computational tool based on daily water balance model is developed to evaluate the efficiency and economic viability of rainwater harvesting systems (RHS) across four climatic zones of China. Three non-potable water demand scenarios (i.e., toilet flushing, lawn irrigation, and combination of them) are consisdered in the evaluation. Stormwater capture efficiency, water saving efficiency and time reliability are defined and calculated as indicators for quantitaivly assessing the efficiency of RHS. The economic benefit is evaluated by benefit-cost ratio, which compares the present value of both direct and indirect benefits to the present value of costs of RHS. The computational tool requires daily rainfall, contributing roof area, runoff losses, volume of first flush, storage capacity of RHS, daily water demand and economic parameters as inputs. The Cropwat 8.0 and Climwat 2.0 will be used to support the application of this tool when there is a need to accurately determine the irrigation demand of green spaces on the basis of reference evapotranspiration. The efficiency of RHS is highly denpendent on spatial variability of rainfall, storage capacity and rainwater demand scenarios. In general, higher water saving efficiency and reliability can be achieved for RHS with larger storage capacities, for lower water demand scenarios and located in more humid regions, while higher stormwater capture efficiency is associated with larger storage capacity, higher water demand scenarios and less rainfall. For RHS with relatively small storage capacity, the efficiency is mainly limited by the storage capacity, but with an increase in storage capacity the collectable rainfall and rainwater demand scenarios become the limiting factors. These results are coherent with the results of Imteaz et al. (2011b) in Australia and Silva et al. (2015) in Portugal. Besides spatial variability of rainfall, storage capacity and rainwater demand scenarios, other factors such as contributing area and volume of first flush may also influence the efficiency of RHS. As reported in both Eroksuz and Rahman (2010) and Silva et al. (2015), the influence of the contributing area on water savings becomes more relevant as the storage capacity increases. The contributing area together with volume of local rainfall determine the collectable rainwater, as a result, various combinations of climate condition and roof area can lead to different efficiencies (Palla et al., 2012; Mehrabadi et al., 2013; Litofsky and Jennings, 2014). Seperation of the first flush is usually necessary to improve harvested rainwater quality, while the volume of first flush should be defined according to the contributing area (Gikas and Tsihrintzis, 2012). In this study, a constant value of 2 mm is applied in all of the analysis, as it is also used in Kus et al. (2010), and Zhang and Hu (2014). Fig. 7 shows the sensitivity of reliability of RHS (storage capacity of 50 m3 & rainwater used for toilet flushing) to the changes of first flush in Shanghai, Beijing, Xining and Kaxgar. It can be concluded that the reliability of RHS in arid areas is more sensitive to the volume of first flush than in humid areas. With the current values of economic parameters used in this study, economic viability of RHS in humid and semi-humid regions (i.e., Shanghai, Chengdu, Shenyang and Beijing) is demonstrated when harvested rainwater is used for toilet flushing or combined use of toilet flushing and lawn irrigation. The most economically feasible storage capacity of RHS for a specific water demand scenario can be determined by finding the specific peak value of the benefit-cost ratio. In arid regions (e.g., Yinchuan and Kaxgar), however, RHS is not financially viable as the benefit-cost ratio is much smaller than 1.0. Similar result was presented by Hajani and Rahman (2014) for arid regions in Australia. The economic viability of RHS depends on the balance between the fixed investment, operation and maintenance costs of the system and the public water supply cost savings and other indirect benefits (Fowler et al., 2007; Silva et al., 2015; Guizani, 2016). This balance is a nonlinear function of the collectable rainwater, the storage capacity, the actual rainwater demand for different purposes and the cost of the

Fig. 6. Water supply time reliability of rainfall harvesting systems for different water demand scenarios: (a) lawn irrigation use, (b) toilet flushing use, and (c) combination of toilet flushing and lawn irrigation use.

cost ratios when harvested rainwater is used for lawn irrigation. For example, the benefit-cost ratio of a 50 m3 RHS in Chengdu is 0.43 if the harvested rainwater is used for lawn irrigation (Table 8), while, it will rise to 1.0 if the harvested rainwater is used for toilet flushing (Table 9). The lower benefit-cost ratios of harvesting rainwater for lawn irrigation in Chengdu may be resulted from the relatively lower daily average irrigation quota in humid regions (Table 4), which results in lower rainwater consumption and therefore limits the benefits obtained by the RHS.

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Table 8 Benefit-cost ratios of rainfall harvesting systems (lawn irrigation). Storage capacity (m3)

Shanghai

Chengdu

Shenyang

Beijing

Xining

Urumqi

Yinchuan

Kaxgar

5 10 20 30 40 50 60 70 80 90 100

0.94 0.91 0.74 0.61 0.52 0.45 0.39 0.35 0.31 0.28 0.26

0.87 0.84 0.69 0.58 0.5 0.43 0.38 0.34 0.31 0.28 0.26

1.20 1.23 1.07 0.91 0.78 0.69 0.61 0.55 0.50 0.46 0.42

1.12 1.16 1.03 0.88 0.77 0.68 0.62 0.57 0.53 0.49 0.46

1.21 1.16 0.94 0.77 0.65 0.57 0.50 0.45 0.41 0.37 0.34

0.92 0.85 0.67 0.54 0.44 0.38 0.33 0.29 0.26 0.23 0.21

0.57 0.56 0.48 0.39 0.33 0.29 0.25 0.22 0.2 0.18 0.17

0.20 0.18 0.14 0.11 0.09 0.07 0.06 0.06 0.05 0.05 0.04

Table 9 Benefit-cost ratios of rainfall harvesting systems (toilet flushing). Storage capacity (m3)

Shanghai

Chengdu

Shenyang

Beijing

Xining

Urumqi

Yinchuan

Kaxgar

5 10 20 30 40 50 60 70 80 90 100

2.32 2.38 2.17 1.91 1.69 1.51 1.36 1.24 1.14 1.05 0.98

1.61 1.63 1.47 1.28 1.13 1.00 0.91 0.83 0.76 0.71 0.66

1.57 1.61 1.47 1.28 1.12 0.99 0.89 0.81 0.74 0.68 0.64

1.35 1.39 1.27 1.12 0.99 0.88 0.79 0.72 0.66 0.52 0.58

1.24 1.19 0.97 0.80 0.67 0.58 0.50 0.45 0.40 0.36 0.33

0.81 0.76 0.62 0.5 0.42 0.36 0.31 0.28 0.25 0.23 0.21

0.63 0.58 0.47 0.39 0.33 0.28 0.25 0.22 0.2 0.18 0.16

0.18 0.16 0.13 0.10 0.08 0.07 0.06 0.05 0.05 0.04 0.04

Table 10 Benefit-cost ratios of rainfall harvesting systems (toilet flushing + lawn irrigation). Storage capacity (m3)

Shanghai

Chengdu

Shenyang

Beijing

Xining

Urumqi

Yinchuan

Kaxgar

5 10 20 30 40 50 60 70 80 90 100

2.48 2.52 2.29 2.02 1.79 1.60 1.44 1.31 1.20 1.11 1.03

1.84 1.85 1.67 1.47 1.30 1.16 1.05 0.96 0.88 0.82 0.76

1.78 1.79 1.59 1.39 1.22 1.08 0.97 0.88 0.8 0.74 0.69

1.38 1.39 1.24 1.09 0.96 0.86 0.77 0.70 0.65 0.60 0.56

1.3 1.26 1.00 0.81 0.68 0.58 0.50 0.45 0.40 0.36 0.33

0.88 0.8 0.63 0.51 0.42 0.36 0.31 0.28 0.25 0.23 0.21

0.59 0.55 0.44 0.36 0.31 0.26 0.23 0.20 0.18 0.17 0.15

0.2 0.18 0.13 0.10 0.08 0.07 0.06 0.05 0.05 0.04 0.04

alternative water sources (Hashim et al., 2013; Sample and Liu, 2014; Bocanegra-Martínez et al., 2014; Silva et al., 2015). Evaluating the influence of these various economic factors on the benefit-cost ratio of RHS during their lifetime is essential for determining the optimal storage capacity. Similar to the benefit-cost ratio disscussed in this study, another index named payback period (i.e., the time period required to recover a project investment) can also be used to evaluate the economic viability of RHS (Silva et al., 2015). More practical results could be expected if the valuation of the alternative water resources is differentiated so that it can reflect the degree of water resources scarcity in metropolises such as Beijing or in arid regions such as Kaxgar.

Acknowledgements This work has been supported by the Fundamental Research Funds for the Central Universities (Nos. YX2015-18, 2015ZCQ-SB-01, and 2016ZCQ06) and the National Natural Science Foundation of China (No. 51609004).

Fig. 7. Sensitivity analysis of reliability of rainfall harvesting systems to volumes of first flush.

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