The Afghan government has planned the project of Kabul New City (KNC) to cope with the rapid growth of Kabul, an existing capital city. Due to climatic and topographical reasons, it is supposed that KNC suffers from a water scarcity problem. This study investigates the feasibility of a rooftop rainwater harvesting system in KNC to relieve the water scarcity problem. An applicability of the rooftop rainwater harvesting system was discussed for several types of residential houses and schools, using 11 years rainfall data. This study also examined the cost-effectiveness of the system by considering the service life of the system. Furthermore, an optimal size of the rainwater storage tank was discussed based on the balance among harvested rainwater volume, non-potable water demand, and cost-effectiveness.
Kabul is the capital city of Afghanistan, and the city is growing at 5th fastest speed in the world. The current population in Kabul Metropolitan Area is 4 million, and the number is twice as large as in 1999. This rapid population increase still continues in Kabul city, and the population is estimated to increase to 6.5 million in 2025 [
The Afghan government has planned the project of Kabul New City (KNC) to cope with the rapid population increase and its impacts. The area of KNC is about 740 km², 1.5 times as large as Kabul city, and 1.5 million people live in this new city. KNC was strategically placed between Bagram Airbase and Kabul International Airport, in the north east of Kabul as shown in
Due to climatic and topographic reasons, it is difficult to get permanent water sources around KNC. So, the project planned several new water sources far from KNC, and also planned the use of rainwater to reduce the consumption of water supplied from the new water sources.
This study investigates the feasibility of a rooftop rainwater harvesting system to cope with the water scarcity problem supposed in KNC. The system collects the rainwater fallen on rooftops of buildings, and it stores the water in a tank. The harvested rainwater is used as non-potable water, because Afghans hardly accept the rainwater for potable use due to the cost of water disinfection and filtration. This study uses 11 years daily rainfall data, from 2006 to 2016, which is newly opened for the research.
The rooftop rainwater harvesting system has already been proposed in some countries that have the water scarcity problem. Abdulla and Al-Shareef [
KNC project is divided into three phases. This study focuses on the area of Parcel-2.6, Block-1 in Phase-1, and the location is shown in
of Parcel-2.6, Block-1 is 227 ha, and about 3650 houses for 20,000 residents will be built in this area. The project of Parcel-2.6 will be completed by 2020. This study discusses the effectiveness of the rooftop rainwater harvesting system for several types of residential houses and schools in Parcel-2.6, Block-1. This study also examines the cost-effectiveness of the rooftop rainwater harvesting system by considering the service life of the system. Furthermore, this study discusses an optimal size of the rainwater storage tank based on the balance between harvested water volume, non-potable water demand, and cost-effectiveness.
The volume of water collected by the rooftop rainwater harvesting system is calculated by the following equation [
V R = ( R × A × C / 1000 ) . (1)
where VR (m3) is the volume of collected rainwater, R (mm) is a rainfall intensity, A (m2) is a rooftop area, and C is a runoff coefficient.
The runoff coefficient relates to many factors such as evaporation and infiltration. In this study, 0.9 is used as the runoff coefficient. This value is the same as that used in the design of drainage system in KNC project [
The performance of the system is evaluated by the following equation [
S W R ( % ) = 100 ∗ V R / P W D . (2)
where SWR means a saved water rate, and PWD (m3) means the volume of non-potable water demand.
The following equations are used to determine the ratio between benefit and cost of the system, BCR, and a payback period of the system, PBP [
BCR = Total Benefit/Total Cost. (3)
PBP = Total Cost/Annual Benefit. (4)
In above equations, the total cost means an initial investment amount. The initial investment cost consists of the market price of the system components, such as tank, pile, valve, and labor costs. The total benefit is equal to the saved water tariff by using the system for 20 years, where the period of 20 years is a service life of the system assumed in this study. The payback period is defined as a period for collecting the initial investment cost.
Ministry of Agriculture Irrigation and Livestock has been measuring the daily rainfall at 6 rain gauge stations in Kabul area after the end of Civil War in Afghanistan. The locations of each station are plotted in
| Year | Maximum of daily Rainfall (mm) | Annual rainfall (mm) | |||||
|---|---|---|---|---|---|---|---|
| Badam Bagh | Gul Khana | Darul Aman | Paghman | Qargha | Kariz Mir | ||
| 2006 | 37 | 43 | - | 38 | 50 | 29 | 240.6 |
| 2007 | 19 | 16 | 18 | 26 | 18.5 | 15 | 298.9 |
| 2008 | 25 | 34 | 24.3 | 45 | 20 | 38 | 228.3 |
| 2009 | 47 | 25 | 61 | 58 | 31 | 58 | 510.1 |
| 2010 | 20 | 19.2 | 23 | 45 | 18 | 27 | 293 |
| 2011 | 28 | 21 | 29 | 32 | 21 | 22 | 266.6 |
| 2012 | 25 | 21 | 23 | 42 | 35 | - | 356.2 |
| 2013 | 42 | 22 | 23 | 32 | 53.5 | - | 386.5 |
| 2014 | 32.5 | 34 | 31 | 37 | 3.5 | - | 366.6 |
| 2015 | 21 | 25 | 26 | 22 | - | - | 325.9 |
| 2016 | 27 | 28 | 48 | 36 | - | - | 330.7 |
| Average | 29.4 | 26.2 | 30.63 | 37.54 | 27.8 | 31.5 | 327.5 |
from 2006 to 2016. The maximum annual rainfall is 510.1 mm in 2009, and the highest daily rainfall is 61 mm at Darulaman in the same year. On the other hand, the minimum annual rainfall is 228.3 mm in 2008.
KNC provides water by water supply system, and the amount of water consumption for potable and non-potable use is supposed 120 LCD, Litter/Capita/Day [
In order to satisfy the water demand in KNC, the project sets several water resources developments. The construction of Panjshir Fan Aquifer will be completed by 2021, and Gulbahar or Salang Dam will be completed by 2025. The price of water in KNC was determined as 0.92US$/m3 by considering the redemption of the development expenses [
The potential of the rooftop rainwater harvesting system is examined based on the mean annual rainfall for 11 years, 327.59 mm.
| Total number of houses | 2017 |
|---|---|
| Total roof area of houses (m2) | 377669.4 |
| Mean annual rainfall (mm) | 327.59 |
| Total harvested rainwater, VR (m3/year) | 111348 |
| Non-potable water demand, PWD (m3/year) | 259733 |
| Saved Water Rate, SWR (%) | 42.9 |
of harvested water, VR, and the saved water rate, SWR, calculated from Equation (1) and Equation (2). This study assumes 6 residents in each house, and they consume the water 58.8LCD for non-potable use. The rooftop rainwater harvesting system has a potential to save 42.9% of the non-potable water demand on the mean annual rainfall. Similarly, the system saves 66.7% of the water demand on the maximum annual rainfall in 2009, and 29.8% on the minimum annual rainfall in 2008.
There are 6 types of residential houses planned in Parcel-2.6, Block-1 area. The site areas of each house are ranged from 750 m2 to 200 m2, and the roof area are also ranged from 417 m2 to 110 m2. There are 6 residents in each house, and they consume 58.8LCD of non-potable water.
| House type | Roof Area (m2) | VR (m3/year) | Saved water tariff (US$/year) | SWR (%) |
|---|---|---|---|---|
| Type A (750 m2) | 417 | 122.7 | 112.9 | 95.3 |
| Type B (500 m2) | 245.8 | 72.3 | 66.5 | 56.1 |
| Type C (300 m2) | 154 | 45.34 | 41.7 | 35.2 |
| Type D (200 m2) | 149.6 | 44.05 | 40.5 | 34.2 |
| Type E (375 m2) | 177 | 52.2 | 48 | 39.8 |
| Type F (200 m2) | 110 | 32.4 | 30 | 24.8 |
| Houses | Tank Price (US$) | Total Cost (US$) | Total Benefit (US$, for 20 years) | BCR | PBP (Year) |
|---|---|---|---|---|---|
| Type A | 2846 | 2992 | 2258 | 0.72 | 27.6 |
| Type B | 680 | 805 | 1330 | 1.65 | 12 |
| Type C | 300 | 425 | 834 | 1.95 | 10 |
| Type D | 295 | 420 | 810 | 1.92 | 10.1 |
| Type E | 294 | 419 | 960 | 2.28 | 8.7 |
| Type F | 136 | 261 | 600 | 2.22 | 9 |
In case of Type-A house, PBP becomes 27.6 years, and BCR is 0.72. Type-A house has a larger roof area, and the installation of a large tank, 44.2 m3, increases the initial investment cost. On the other hand, PBPs in Type-B, C, D, E and Type-F houses are shorter than the service life of the system, and BCRs are more than 1.0.
The cost-effectiveness of the system depends on the balance between the cost of initial investment and the benefit of water saving as shown in
There is a great non-potable water demand in schools, because many students and staff use a lot of non-potable water such as for flushes in toilets and watering. This study chooses a typical primary school and a secondary school, which are planned in Parcel-2.6, Block-1, to evaluate the effectiveness of the rooftop rainwater harvesting system.
Total number of the students and staff in the primary school is 1237, and the total roof area is 13,000 m2 [
| House | Tank Price (US$) | Total Cost (US$) | Total Benefit (US$, for 20 years) | BCR | PBP |
|---|---|---|---|---|---|
| Type A | 1423 | 1548 | 1626 | 1.05 | 19.01 |
and it has 9800 m2 roof area. KNC project assumed the non-potable water demand as 16.8 LCD in schools [
Based on the mean annual rainfall, VR is calculated as 3832.8 m3/year in the case of the primary school. The non-potable water demand is estimated 5590.2 m3/year, and SWR becomes 68.5%. Similarly in the case of the secondary school, VR is 2889.3 m3/year, the non-potable water demand is estimated 4370 m3/year, and SWR becomes 66.1%.
This study also calculated the maximum storage volume based on the mean daily rainfall. In the same manner as 3.1.3, the maximum storage volume is calculated as 317.5 m3 in the primary school and 180 m3 in the secondary school.
| Month | PWD (m3) | VR (m3) | C.PWD (m3) | C.VR (m3) | C.VR-C.PWD (m3) |
|---|---|---|---|---|---|
| Jan. | 519.5 | 455.8 | 519.5 | 455.7 | −63.8 |
| Feb. | 519.5 | 763.2 | 1039 | 1218.9 | 179.9 |
| March | 519.5 | 721.4 | 1558.6 | 1940.3 | 381.7 |
| April | 519.5 | 734.1 | 2078.1 | 2674.4 | 596.3 |
| May | 519.5 | 333 | 2597.7 | 3007.4 | 409.7 |
| June | 519.5 | 85.4 | 3117.2 | 3092.7 | −24.5 |
| July | 519.5 | 54.8 | 3636.7 | 3147.5 | −489.2 |
| Aug. | 519.5 | 94.6 | 4156.3 | 3242.1 | −914.2 |
| Sep. | 519.5 | 59.4 | 4675.8 | 3301.6 | −1374.3 |
| Oct. | 519.5 | 96.1 | 5195.4 | 3397.6 | −1797.7 |
| Nov. | 519.5 | 240.9 | 5714.9 | 3638.5 | −2076.4 |
| Dec. | 519.5 | 272.4 | 6234.4 | 3910.9 | −2323.6 |
Regarding the site area of each school, the maximum storage volume of 596m3 and 391 m3 are too large to build in each plot area. On the other hand, the maximum storage volumes calculated from the mean daily rainfall can be applicable in each plot area.
In the case of the primary school, the maximum storage volume is 317.5 m3, and 11 storage tanks with 29 m3 can be used to satisfy this volume. In this case, BCR is calculated as 2.92, and PBP becomes 6.8 years as shown in
| Educational Facility | Tank Price (US$) | Total Cost (US$) | Total Benefit (US$, for 20 years) | BCR | PBP |
|---|---|---|---|---|---|
| Primary School | 22,668 | 24,046 | 70,440 | 2.92 | 6.8 |
| Secondary School | 12,285 | 13,162 | 53,140 | 4.03 | 5 |
to satisfy 180 m3. In this case, BCR is 4.03 and PBP becomes 5 years.
This study investigated the feasibility of the rooftop rainwater harvesting system for 6 types of residential houses and 2 schools planned in Parcel-2.6, Block-1 in order to reduce the water consumption in KNC.
The residential houses have a potential for saving the amount of non-potable water by introducing the system. The saved water ratio, SWR, cost-effectiveness, BCR, and payback period, PBP, are different depending on the roof area on each house.
In the analysis of the maximum storage volume, BCR in Type-A house is less than 1.0, and its PBP becomes longer than the service life of the system. Type-A house has the largest roof area in Parcel-2.6, Block-1, and it causes the increase of the initial investment cost. This means that the balance between the harvested water volume, VR, the non-potable water demand, PWD, and the initial investment cost should be considered to decide an optimum size of the water storage tank.
This study also showed the potential for saving the amount of non-potable water in the primary school and the secondary school that are planned in Parcel-2.6, Block-1. The system saves around 69% of water tariff in the primary school, and 66% in the secondary school. Judging from VR, PWD, SWR, cost-effectiveness, and site area, the rooftop rainwater harvesting is a feasible system to cover the non-potable water demand in education facilities planned in Parcel-2.6, Block-1.
In order to fix the design method of the rooftop rainwater harvesting system in KNC, a prototype experiment should be done in next step. It is necessary to set an appropriate runoff coefficient through this field experiment to estimate VR more precisely. Based on the discussion about the cost-effectiveness in this study, an applicable cost-benefit model should also be established in further research.
The authors are grateful to Japan International Cooperation Agency (JICA) for giving research opportunity on PEACE Program.
Rahimi, O. and Murakami, K. (2018) Analysis of Rooftop Rainwater Harvesting in Kabul New City: A Case Study for Family Houses and Educational Facilities. Open Journal of Civil Engineering, 8, 155-165. https://doi.org/10.4236/ojce.2018.82013