Rooftop Rainwater Harvesting in Modern Cities: A Case Study for Sana’a City, Yemen

48 Journal of Science & Technology

Vol. (17) No. (2) 2012

ROOFTOP RAINWATER HARVESTING in MODERN CITIES: a CASE STUDY for SANA’A CITY, YEMEN

ROOFTOP RAINWATER HARVESTING in MODERN CITIES: a CASE STUDY for

SANA’A CITY, YEMEN Sharafaddin Abdullah Ahmed Salleh (1) and Taha Muhammed Taher (2)

Abstract Water resources in Yemen are limited, and water is becoming

scarce everyday due to ever-increasing demand due to the rapidly increasing population and to the drought climate the country is characterized with. Increasing overdrawn from groundwater causes a deficit of 900 Mm3 annually leaving the country to seek alternative resources. All major cities in Yemen facing water problems resulting in a mainly socio-economic change –beside other challenges- that produce unrest and unforeseen conflicts to acquire water when needed especially in the capital city of Sana’a where groundwater levels drop annually by an average of 6 m. Rainwater harvesting systems have been used since ancient times and evidence of roof systems date back to more than 4000 years ago in the middle east as the principal water source for drinking and domestic use. This paper summarizes the findings of a substantial work by the authors during the past three years in providing a reasonable, alternative solution to the water scarcity problem through dealing with water harvesting as an alternative resource. This paper estimated the amount of water that can be harvested annually from roof tops 11.31 Mm3 for urban areas using runoff coefficient of 0.75 and 0.172 Mm3 for rural areas using runoff coefficient of 0.6. This indicates that there will be an annual reduction in the usage of groundwater in urban and rural areas by 22% and 33% respectively. Simple and easy harvested water volume guide tables were developed for different run off coefficients of 0.6, 0.7, 0.75 and 0.8. It also presents a the main factors for the design of a complete Rooftop Rainwater Harvesting System for the city of Sana’a. Key word: Roof Tops, Water Harvesting, Design Tables, Guideline, Sana’a, Yemen

1-Assistant Professor of Hydraulics and Water resources , Civil Engineering Department,

faculty of Engineering, Water and Environment Center (WEC), Sana’a University e-mail: [email protected], [email protected].,

2- Associate Professor of Water resources, Civil Engineering Department, faculty of Engineering, Water and Environment Center (WEC), Sana’a University P.O. Box 14636, Sana'a Yemene-mail: [email protected], [email protected]

mailto:[email protected]





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1. Introduction 1.1 History of Water Harvesting

Rainwater harvesting systems have been used since ancient times and evidence of roof catchments systems date back to early Roman times. Roman villas and even whole cities were designed to take advantage of rainwater as the principal water source for drinking and domestic purposes.

Rainwater harvesting is an ancient technique enjoying a revival in popularity due to the inherent quality of rainwater and interest in reducing consumption of treated water. Archeological evidence attests to the capture of rainwater as far back as 4,000 years ago, and the concept of rainwater harvesting in China may date back 6,000 years. Ruins of cisterns built as early as 2000 B.C. for storing runoff from hillsides for agricultural and domestic purposes ]1[ .

2000 B.C. In the Negev desert in Philistine, tanks for storing runoff from hillsides for both domestic and agricultural purposes have allowed habitation and cultivation in areas with as little as 100mm of rain per year. The earliest known evidence of the use of the technology in Africa comes from northern Egypt, where tanks ranging from 200-2000 m3 have been used for at least 2000 years – many are still operational today. The technology also has a long history in Asia, where rainwater collection practices have been traced back almost 2000 years in Thailand. The small-scale collection of rainwater from the eaves of roofs or via simple gutters into traditional jars and pots has been practiced in Africa and Asia for thousands of years. In many remote rural areas, this is still the method used today. The world's largest rainwater tank is probably the Yerebatan Sarayi in Istanbul, Turkey. This was constructed during the rule of Caesar Justinian (A.D. 527-565). It measures 140m by 70m and has a capacity of 80,000 cubic meters.

According to UNESCO, arid regions are defined as areas where potential evapo-transpiration is much greater than precipitation. Table (1) shows the extent of aridity in the Medial East and North Africa region (MENA) as reflected in rainfall data. It also shows that arid and semi-arid areas amount to about 96% of the North African part and 95% of the Asian part of the MENA region.


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Table1: Extent of aridity in the Arab region ] 2[

Amount of Rainfall Less than 100 mm (A)

Arid Areas 100-400mm (B) Semi-arid Areas Sub-region Total Area

(1000 km²) Area (1000

km²) % Area (1000 km²) %

A+ B as % of Total

North Africa 5751 4864 85 653 11 96 Near East 3705 3033 79 589 16 95

Total MENA 9456 7897 84 1242 13 97

Many countries, therefore, in the Middle East increasingly suffer from water shortages due to the unavailability of renewable water resources and to the rabid increase in population (see table 2).

Table2: Indicates water deficiencies in some Arab countries ] 3[ .

No Country

Annual renewable resources 106 M3

Annual consumption

106 M3

Deficiencies 106 M3

Deficiencies%

Level of deficiencies

1 Iraq 42560 47330 - 4770 10 Limited 2 Kuwait 508 640 - 132 21 Medium 3 Qatar 259 334 - 75 22 Medium 4 Libya 3980 5580 - 1600 29 Medium 5 Jordan 880 1280 - 400 31 Medium 6 Bahrain 157 250 - 93 37 Medium 9 UAE 1050 2230 - 1280 57 Critical 8 Yemen 1500 3600 - 2100 58 Critical 9 Oman 345 1417 - 1072 76 V. Critical

10 Saudi 2900 23100 - 20200 87 Dangerous

Therefore, rainwater harvesting in some rural areas seen as the main source for water supply but in other communities is the only feasible water supply. In both cases, rainwater harvesting is an option for improving the living conditions of many communities facing serious water supply shortages by providing an improved water source qualitatively and quantitatively.

Rainwater Harvesting in Yemen is a traditional practice, and in many areas Cisterns are used to conserve rain water. The cisterns of Tawaila (rain flood harvesting), or the Tawaila Tanks are Aden’s best historic sites. Mareb dam is an example of a water harvesting technology started 2000 years B.C in Yemen to provide agricultural and domestic waters to the left


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and right paradise as stated in the Holy Qura’an.

1.2 Water Scarcity in Yemen

Generally speaking, the total supply of water in aquifers is non-expandable. The central challenge facing the country in general and Sana'a Basin in particular today and in the foreseeable future is therefore how to produce more food and enhance farmer income besides meeting the other demands like drinking water and industrial needs. With a rapid increase of population, it is expected that by the year 2025, the basin population will reach 5.85 M people (recently the population is 1.75 M people ]4[ . Between now and then, a significant amount of the additional food supply needed to serve the growing requirement will have to be produced on land served by irrigation. There are four profound effects of the population growth and the drought climate as a result of global climatic changes:

Rising competition by different sectors for scarce water; Rising pressures to use water much more efficiently; Rising socio-economic pressures to define water rights more clearly and Look for alternative water resources such as water harvesting

1.3 Sana’a Climate and Water Characteristics

Sana’a Basin is experiencing a serious depletion of groundwater resources with associated water quality degradation. The water resources situation in Sana’a Basin is extremely serious as abstraction exceeds recharge by more than five folds. Consequently, the piezometric level declines about 4-8 meters annually. Groundwater is mainly used for agricultural activities, which have expanded several times since 1980's, and consume about 90% of water. Mismanagement of water resources is mainly caused by lack of data, policy and institutional framework for groundwater abstraction and use, and inefficient irrigation practices. In addition, rainfall is becoming much less each year due to climatic changes. There are two rainy seasons, separated by a distinct dry interval (May-mid July). The annual rainfall generally varies between 150 and 350 mm, with some years having, higher rainfall amounts above 350 mm. The first rainy period starts in mid-March-beginning of April, the second rainy period begins mid-July-beginning of August and stops abruptly end of August. The months September through February are generally dry, although occasional


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thunderstorms may bring some rain during these months. Sixty-five to seventy-five percent of the rain falls during the months January-June. The number of rain days with rainfall amounts above 5 mm/day varies between 5-15 days. The average amount of rainfall per rain day is about 16-17 mm.

The potential evapotranspiration (PET) for an average year varies depending on altitude, wind exposure and latitude. The PET varies between 3-3.5 mm/day during the dry, cold period and 5-6 mm/day during the months May-June. The average total amount of evapotranspiration per year is about 1700 mm.

2. RAINFALL AND RUNOFF ANALYSIS 2.1 Rainfall rate validation

For water harvesting purpose we will use the average year rainfall from ten years data in the Sana’a city (NWRA, 2010). The average rainfall for the years (1993-2001) is 243 mm/year (see table 3).

Table3: Rainfall data of Sana’a City (1990-2003) for 10 years

Months Year

1 2 3 4 5 6 7 8 9 10 11 12 Annual

1990 0 2.5 40.5 19 3.5 0 31.5 2 25 0 0 0 124

Mini Year 1991 0 5.5 45 11 11.5 0 2.5 35 0.5 0 0 0.5 111.5

Max Year 1992 2.5 0.5 20 20 64.5 3 10 140 24.5 26 0 39.5 350

1993 2.5 9 13.5 83 79.5 6 3 25 30.5 1 45 19 316.5

1997 5.5 1.5 14.5 29.5 7.5 2 12.5 33.5 0 60.5 34 1 201.5

1998 0 0.5 8 19 68.5 0 63 176 0 0 6.5 341

2000 0.5 8 30 57.5 9 58.5 2.5 16 2.5 146 330 Medi

an Year

2001 29 108 31 13 1 0 49 21.5 21 22.5 7 1 303

2002 0 0.5 8 1 1 0 49 21.5 21 22.5 0 0 124.5

2003 0 0 10.5 52.5 12.5 0.5 0 0 0 3 2 146 227 Ave year 4.33 12.80 19.90 27.80 30.70 1.28 22.95 51.20 12.50 15.15 9.60 39.17 243

The selection of the 2001 year to be used for the calculation of the maximum storage requirement is based on the following:

1.It is one of ten years data which is the minimum requirement for the numbers of years of data.


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2.The year value it has is more than the average year. 3.It has 12 months of reading Arranging the rainfall in descending order: 350, 341, 330, 316.5, 303, 227, 201.5, 124.5, 124, 111.5

The 350 mm/year is equaled or exceeded only once in ten years, and the average 243 mm/year is equaled or exceeded in five years. Use 243 mm/year for the design. More accurate estimation was done through analysis of rainfall data from additional source i.e. NASA Tropical Rainfall Measuring Mission (TRMM) in order to validate the above selection of rainfall rate. TRMM is a joint mission between the National Aeronautics and Space Administration (NASA) (disc2.nascom.nasa.gov) of the United States and the Japan Aerospace Exploration Agency (JAXA). Using the TRM model, the authors have obtained table (4) for 10 years (1999-2009)

Table 4: Sana’a rainfall for the range from 1999 to 2009

latitude Longitude 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Average rainfall

15.0000 44.000 244 144 398 306 244 263 347 464 316 241 158 284 15.0000 44.2500 184 80 368 173 288 141 347 395 200 139 106 220 15.2500 44.0000 232 101 280 192 275 182 275 426 210 168 130 225 Average rainfall 220 109 349 224 269 195 323 428 242 183 131 243

Selected parameter: 3-hourly TRMM 3B42(V6) Accumulated Rainfall Selected area: lat=[15N,16N], long=[44E,45E], (44°13'E, 15°28'N, Elevation: 2190m) Selected time period: (21Z31Jan1999-21Z31Dec2009) Unit: (mm)

The average of 243 mm/yr. coincides with the previous one obtained from NWRA data. Therefore, calculations of the harvested water volume are based on an annual average rainfall of the year’s period of 243 mm (see table 5 above).

2.2 Water Harvested Estimation The Rational method is probably the most popular method and

preferable in storm design systems in urban areas. It has been applied all over the world and many refinements of the method have been produced. It has the following simple form:

Harvested water = C x I x A Where:

The harvested water is the quantity of the water harvested from the roofs (m3) C : The runoff coefficient (dimensionless)

I : used here annual average rainfall (mm/yr.) and A : the roof area (m2)

The total harvested water volume is calculated based on: • Average rainfall • Size of the catchments area (rooftop)


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• Runoff coefficient (see table 5) • For flat slopes or impermeable soils use higher values • For flat slopes or permeable soils use lower values, • For steep slopes or impermeable soils use the higher values.

Table5: Runoff coefficients

Area Description Runoff Coefficient C Business

Downtown 0.70-0.95 Neighbourhood 0.50-0.70

Residential Single-Family 0.30-0.50

Multi-units, detached 0.40-0.60 Multi-units, attached 0.60-0.75

Residential (suburban) 0.25-0.40 Apartment 0.50-0.70 Industrial

Light 0.50-0.80 Heavy 0.60-0.90

Parks, cemeteries 0.10-0.25 Playgrounds 0.20-0.35 Railroad yard 0.20-0.35 Unimproved 0.10-0.30

Character of surface Runoff Coefficient C Pavement

Asphaltic and concrete 0.70-0.95 Brick 0.70-0.85 Roofs 0.75-0.95

Simple design tables where developed applying the above simple equation as basic guidance to estimate the water harvested volume based on several run off coefficients of 0.6, 0.7, 0.75 and 0.8, the rainfall and the surface area. Tables 6, 7, 8 and 9 illustrating the water volume harvested from roof tops using rainfall average of 243 mm/year with different roof surface areas.


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Table6: Harvested water volume guide table using run off coefficient (C=0.6)

Rainfall (mm)

100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas (m2)

Harvested Water Volume from Roof Top (m3), C= 0.6

20 1 2 2 3 4 4 5 5 6 7 8 10 11 12 18

30 2 3 4 5 5 6 7 8 9 11 13 14 16 18 27

40 2 4 5 6 7 8 10 11 12 14 17 19 22 24 36

50 3 5 6 8 9 11 12 14 15 18 21 24 27 30 45

60 4 5 7 9 11 13 14 16 18 22 25 29 32 36 54

70 4 6 8 11 13 15 17 19 21 25 29 34 38 42 63

80 5 7 10 12 14 17 19 22 24 29 34 38 43 48 72

90 5 8 11 14 16 19 22 24 27 32 38 43 49 54 81

100 6 9 12 15 18 21 24 27 30 36 42 48 54 60 90

150 9 14 18 23 27 32 36 41 45 54 63 72 81 90 135

200 12 18 24 30 36 42 48 54 60 72 84 96 108 120 180

250 15 23 30 38 45 53 60 68 75 90 105 120 135 150 225

300 18 27 36 45 54 63 72 81 90 108 126 144 162 180 270

350 21 32 42 53 63 74 84 95 105 126 147 168 189 210 315

400 24 36 48 60 72 84 96 108 120 144 168 192 216 240 360

450 27 41 54 68 81 95 108 122 135 162 189 216 243 270 405

500 30 45 60 75 90 105 120 135 150 180 210 240 270 300 450

600 36 54 72 90 108 126 144 162 180 216 252 288 324 360 540

700 42 63 84 105 126 147 168 189 210 252 294 336 378 420 630

800 48 72 96 120 144 168 192 216 240 288 336 384 432 480 720

900 54 81 108 135 162 189 216 243 270 324 378 432 486 540 810

1000 60 90 120 150 180 210 240 270 300 360 420 480 540 600 900

1500 90 135 180 225 270 315 360 405 450 540 630 720 810 900 1350

2000 120 180 240 300 360 420 480 540 600 720 840 960 1080 1200 1800

2500 150 225 300 375 450 525 600 675 750 900 1050 1200 1350 1500 2250

3000 180 270 360 450 540 630 720 810 900 1080 1260 1440 1620 1800 2700

3500 210 315 420 525 630 735 840 945 1050

1260 1470 1680 1890 2100 3150

4000 240 360 480 600 720 840 960 1080 1200

1440 1680 1920 2160 2400 3600

4500 270 405 540 675 810 945 1080 1215 1350

1620 1890 2160 2430 2700 4050

5000 300 450 600 750 900 1050 1200 1350 1500

1800 2100 2400 2700 3000 4500


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Table7: Harvested water volume guide table using run off coefficient (C=0.7)

Rainfall (mm) 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas (m2) Harvested Water Volume from Roof Top (m3), C= 0.7

20 1 2 3 4 4 5 6 6 7 8 10 11 13 14 21

30 2 3 4 5 6 7 8 9 11 13 15 17 19 21 32

40 3 4 6 7 8 10 11 13 14 17 20 22 25 28 42

50 4 5 7 9 11 12 14 16 18 21 25 28 32 35 53

60 4 6 8 11 13 15 17 19 21 25 29 34 38 42 63

70 5 7 10 12 15 17 20 22 25 29 34 39 44 49 74

80 6 8 11 14 17 20 22 25 28 34 39 45 50 56 84

90 6 9 13 16 19 22 25 28 32 38 44 50 57 63 95

100 7 11 14 18 21 25 28 32 35 42 49 56 63 70 105

150 11 16 21 26 32 37 42 47 53 63 74 84 95 105 158

200 14 21 28 35 42 49 56 63 70 84 98 112 126 140 210

250 18 26 35 44 53 61 70 79 88 105 123 140 158 175 263

300 21 32 42 53 63 74 84 95 105 126 147 168 189 210 315

350 25 37 49 61 74 86 98 110 123 147 172 196 221 245 368

400 28 42 56 70 84 98 112 126 140 168 196 224 252 280 420

450 32 47 63 79 95 110 126 142 158 189 221 252 284 315 473

500 35 53 70 88 105 123 140 158 175 210 245 280 315 350 525

600 42 63 84 105 126 147 168 189 210 252 294 336 378 420 630

700 49 74 98 123 147 172 196 221 245 294 343 392 441 490 735

800 56 84 112 140 168 196 224 252 280 336 392 448 504 560 840

900 63 95 126 158 189 221 252 284 315 378 441 504 567 630 945

1000 70 105 140 175 210 245 280 315 350 420 490 560 630 700 1050

1500 105 158 210 263 315 368 420 473 525 630 735 840 945 1050 1575

2000 140 210 280 350 420 490 560 630 700 840 980 1120 1260 1400 2100

2500 175 263 350 438 525 613 700 788 875 1050 1225 1400 1575 1750 2625

3000 210 315 420 525 630 735 840 945 1050

1260 1470 1680 1890 2100 3150

3500 245 368 490 613 735 858 980 1103

1225

1470 1715 1960 2205 2450 3675

4000 280 420 560 700 840 980 1120

1260

1400

1680 1960 2240 2520 2800 4200

4500 315 473 630 788 945 1103

1260

1418

1575

1890 2205 2520 2835 3150 4725

5000 350 525 700 875 1050

1225

1400

1575

1750

2100 2450 2800 3150 3500 5250


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Table 8: Harvested water volume guide table using run off coefficient (C=0.75)

Rainfall (mm) 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas

(m2) Harvested Water Volume from Roof Top (m3), C= 0.75

20 2 2 3 4 5 5 6 7 8 9 11 12 14 15 23

30 2 3 5 6 7 8 9 10 11 14 16 18 20 23 34

40 3 5 6 8 9 11 12 14 15 18 21 24 27 30 45

50 4 6 8 9 11 13 15 17 19 23 26 30 34 38 56

60 5 7 9 11 14 16 18 20 23 27 32 36 41 45 68

70 5 8 11 13 16 18 21 24 26 32 37 42 47 53 79

80 6 9 12 15 18 21 24 27 30 36 42 48 54 60 90

90 7 10 14 17 20 24 27 30 34 41 47 54 61 68 101

100 8 11 15 19 23 26 30 34 38 45 53 60 68 75 113

150 11 17 23 28 34 39 45 51 56 68 79 90 101 113 169

200 15 23 30 38 45 53 60 68 75 90 105 120 135 150 225

250 19 28 38 47 56 66 75 84 94 113 131 150 169 188 281

300 23 34 45 56 68 79 90 101 113 135 158 180 203 225 338

350 26 39 53 66 79 92 105 118 131 158 184 210 236 263 394

400 30 45 60 75 90 105 120 135 150 180 210 240 270 300 450

450 34 51 68 84 101 118 135 152 169 203 236 270 304 338 506

500 38 56 75 94 113 131 150 169 188 225 263 300 338 375 563

600 45 68 90 113 135 158 180 203 225 270 315 360 405 450 675

700 53 79 105 131 158 184 210 236 263 315 368 420 473 525 788

800 60 90 120 150 180 210 240 270 300 360 420 480 540 600 900

900 68 101 135 169 203 236 270 304 338 405 473 540 608 675 1013

1000 75 113 150 188 225 263 300 338 375 450 525 600 675 750 1125

1500 113 169 225 281 338 394 450 506 563 675 788 900 1013 1125 1688

2000 150 225 300 375 450 525 600 675 750 900 1050 1200 1350 1500 2250

2500 188 281 375 469 563 656 750 844 938 1125 1313 1500 1688 1875 2813

3000 225 338 450 563 675 788 900 1013 1125 1350 1575 1800 2025 2250 3375

3500 263 394 525 656 788 919 1050 1181 1313 1575 1838 2100 2363 2625 3938

4000 300 450 600 750 900 1050 1200 1350 1500 1800 2100 2400 2700 3000 4500

4500 338 506 675 844 1013 1181 1350 1519 1688 2025 2363 2700 3038 3375 5063

5000 375 563 750 938 1125 1313 1500 1688 1875 2250 2625 3000 3375 3750 5625


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Table 9: Harvested water volume guide table using run off coefficient (C=0.8)

Rainfall

(mm) 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas

(m2) Harvested Water Volume from Roof Top (m3), C= 0.8

20 2 2 3 4 5 6 6 7 8 10 11 13 14 16 24

30 2 4 5 6 7 8 10 11 12 14 17 19 22 24 36

40 3 5 6 8 10 11 13 14 16 19 22 26 29 32 48

50 4 6 8 10 12 14 16 18 20 24 28 32 36 40 60

60 5 7 10 12 14 17 19 22 24 29 34 38 43 48 72

70 6 8 11 14 17 20 22 25 28 34 39 45 50 56 84

80 6 10 13 16 19 22 26 29 32 38 45 51 58 64 96

90 7 11 14 18 22 25 29 32 36 43 50 58 65 72 108

100 8 12 16 20 24 28 32 36 40 48 56 64 72 80 120

150 12 18 24 30 36 42 48 54 60 72 84 96 108 120 180

200 16 24 32 40 48 56 64 72 80 96 112 128 144 160 240

250 20 30 40 50 60 70 80 90 100 120 140 160 180 200 300

300 24 36 48 60 72 84 96 108 120 144 168 192 216 240 360

350 28 42 56 70 84 98 112 126 140 168 196 224 252 280 420

400 32 48 64 80 96 112 128 144 160 192 224 256 288 320 480

450 36 54 72 90 108 126 144 162 180 216 252 288 324 360 540

500 40 60 80 100 120 140 160 180 200 240 280 320 360 400 600

600 48 72 96 120 144 168 192 216 240 288 336 384 432 480 720

700 56 84 112 140 168 196 224 252 280 336 392 448 504 560 840

800 64 96 128 160 192 224 256 288 320 384 448 512 576 640 960

900 72 108 144 180 216 252 288 324 360 432 504 576 648 720 1080

1000 80 120 160 200 240 280 320 360 400 480 560 640 720 800 1200

1500 120 180 240 300 360 420 480 540 600 720 840 960 1080 1200 1800

2000 160 240 320 400 480 560 640 720 800 960 1120 1280 1440 1600 2400

2500 200 300 400 500 600 700 800 900 1000 1200 1400 1600 1800 2000 3000

3000 240 360 480 600 720 840 960 1080 1200 1440 1680 1920 2160 2400 3600

3500 280 420 560 700 840 980 1120 1260 1400 1680 1960 2240 2520 2800 4200

4000 320 480 640 800 960 1120 1280 1440 1600 1920 2240 2560 2880 3200 4800

4500 360 540 720 900 1080 1260 1440 1620 1800 2160 2520 2880 3240 3600 5400

5000 400 600 800 1000 1200 1400 1600 1800 2000 2400 2800 3200 3600 4000 6000


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3. ATER VOLUME ESTIMATION Average household at Sana’a city uses about 30-70 liters per person

per day ] 5[ . According to WHO guidelines ] 6[ a minimum value of 25 l/day is acceptable for hygiene and health care in dry regions. In this paper the estimated consumption from harvested water is 30 liter/capita/day for rural areas and 70 liters/capita/day for urban areas. Households served previously by a water utility can prepare simple management plan to use both the harvested rainwater and the utility water supply efficiently. Households solely dependent upon rainwater should adopt efficient water use practices both indoors and outdoors.

Household water demand is largely affected by changes in weather, although changes in household occupancy rates depending upon seasons and ages of household members, more water use during the hot summer months, and very minor changes in consumption of water due to increases in temperature may be worth factoring in some instances.

In this paper we will deal with the computational method of the Supply Side Approach (SSA) and try to develop a systematic process for the design of storage tanks according to the volume of water harvested.

3.1 Supply Side Approach (SSA)

In low rainfall areas or areas where the rainfall is of uneven distribution, more care has to be taken to size the storage properly. During some months of the year there may be an excess of water, while at other times there will be a deficit. If there is sufficient water throughout the year to meet the demand, then sufficient storage will be required to bridge the periods of scarcity. As storage is expensive, this should be done carefully to avoid unnecessary expense.

3.2 Computational Method

According to the background of the study, several types of buildings categories with different roof surface areas have been selected to calculate the RWH quantity. The buildings categories and areas are:

1. Hospital educational building 2. Commercial building


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3. Public building with an average roof surface area of 200 m2 and 4. School building

The examples of the sample calculations of the above surface areas of the aforementioned buildings can be applied to similar buildings with different roof surface areas. Accordingly, the only variables in the calculations are roof surface area and the average rainfall of that specific location, however, rainfall can be estimated generally as an average for the whole city leaving the surface roof area the only variable. The following example clarifies the steps of calculations for the RWH quantity and storage tank capacity.

Example

Site: Public building, Sana’a Yemen Given data: Roof area: 200 m2 Annual average rainfall: 243 mm per year Runoff coefficient: 0.75 (concrete roof) Required parameter to be found:

1. Harvested volume/ month 2. Harvested volume/day 3. Storage capacity

Solution: Annual available water (assuming all is collected and using Rational Method) = 45.3675.0243.0200 =×× m3 /yr. or from table (7) you can get directly the same value.

1. Monthly water requirement = 038.312

45.36= m3/ month

2. Daily available water = 0.101330038.3

= m3/ day

3. The calculation of the storage tank is listed in table 10 below


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Table10: Estimation tank capacity for a public building (200 m2)

(1) Month

(2) Median Rainfall for the years 1993- 2003 (mm)

(3) Rainfall

harvested (m³)

(4) Cumulative

rainfall harvested (m³)

(5) Demand

(based on total requirement

(m³)

(6) Cumulative

demand (m³)

(7) Difference between

column (4) and (6)

Jan 29 4.35 4.35 3.04 3.04 1.31 Feb 108 16.20 20.55 3.04 6.08 14.47 Mar 31 4.65 25.20 3.04 9.12 16.08

Apr 13 1.95 27.15 3.04 12.16 14.99 May 1 0.15 27.30 3.04 15.20 12.10 Jun 0 0.00 27.30 3.04 18.24 9.06 Jul 49 7.35 34.65 3.04 21.28 13.37

Aug 22 3.30 37.95 3.04 24.32 13.63 Sep 21 3.15 41.10 3.04 27.36 13.74 Oct 23 3.45 44.55 3.04 30.40 14.15 Nov 7 1.05 45.60 3.04 33.44 12.16 Dec 1 0.15 45.75 3.04 36.48 9.27

Totals 45.75 36.48

Column (2): The median year rainfall is used (refer to table 3) Column(3): Rainfall Harvested (m³) = (C × Average Rainfall×Roof

Area)/1000 Column (4): Cumulative rainfall harvested (m³) Column (5): Demand ( Calculated from the step 2 of the example above) Column (6): Cumulative demand based on column (5) Column (7): The tank storage capacity [select the max value]

Table 10 explains the process taken to calculate the storage tank capacity by taking into consideration the incoming and the outgoing cumulative water quantity. The storage tank capacity is taken as maximum value in column (7) as the difference between the water harvested (incoming) column 4 and the water requirement for the building (outgoing) in column 6 in any month. This value is shown in the month of March to be 16.08 m3. According to this value the tank size can then be designed with an extra of 25% of the water volume in to accommodate any higher rainfall might occur.

Graphically we can calculate the storage capacity from the rainfall data graphically by comparing the water harvested and the amount that can


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be supplied to the building using the harvested water. It can be noted that there are two rainy seasons with dry periods (see figure 1). The month of January yields some quantity after the dry months of November and December. If we therefore assume that the tank is empty at the end of December, we can form a graph of cumulative harvested water and cumulative demand and calculate the maximum storage requirement (figure 2) which occurs in March. All this water will have to be stored to cover the shortfall during the dry period.

Figure1: Comparison of the harvestable water and the demand water for each month ]7[


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Figure2: Showing the predicted cumulative inflow and outflow from the tank ] 7[

4. Estimation of benefits of harvested water in Sana’a

It is apparent that using harvested water is essentially going to reduce the stress on groundwater. Rainwater harvesting utilization strategies and policies must be drawn towards the benefits of minimizing the use of fossil groundwater. Several uses of rainwater harvesting is seen such as drinking, livestock, complementary irrigation and gardening. The following paragraphs estimate the amounts of rainwater that can be harvested and used rather than groundwater for Sana’a city.

The annual utilization of harvested water in Sana’a will result in reducing pressure on groundwater. This means that there is substantial amount of water in the deep aquifers is being saved as the same amount was being consumed from water harvesting. Such amount is represented in table 11 with a value of 11,305,952 m3/yr, and 171,519 m3/yr, as a reduction in the usage of groundwater in urban and rural areas respectively. These values are calculated with an average roof top area of 200 m2 with a total number of buildings according to ]8[ is 310,177 and 5,882 for urban and rural areas respectively. The percentages savings therefore are 22 % for urban areas and 33% for rural areas and the benefits of using rainwater harvesting is about (26,204,808 US$/year); refer to table 11 below.


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In addition to the cost and saving advantages motioned above there are some other advantages of RTRWH in Sana’a city Include:

• Roof top rainwater harvesting can co‐exist with and provide a good supplement to other water sources and utility systems, thus relieving pressure ground water as the unique water source in Sana’a city.

• Rainwater harvesting provides a water supply for the city areas which are not cover by water supply network especially during rainy seasons.

• Rainwater harvesting provides a water supply for use in times of emergency or breakdown of the public water supply systems, particularly during natural disasters.

• Water received is free of costs, so the use of this water significantly reduces water bills for purchased water from municipal supply.

• Harvesting rainwater is not only water conserving, it is also energy conserving since the energy input required to operate a centralized water system designed to treat and pump water over a vast service area is by-passed.

• Rainwater harvesting can reduce storm drainage load and flooding in streets, so it reduce local soil erosion and flooding caused by the rapid runoff of water from impervious cover such as pavements areas and roofs. Also, the RWH reduced level of storm water requires smaller sized storm water drainage systems and helps in reducing soil erosion into the waterways.

• Rainwater Collected From Roof and Stored Underground or in Storage tanks – Scarcity Period to meet Increasing Demand for Water in Urban Areas.

• Rainwater Collected From Roof can be used for groundwater recharge through the shallow dry wells which was installed inside the house or near of it, which will help in control decline of water levels (Recharge the aquifers)

• Rainwater Collection in ponds through the water ways inside the city will contribute in recharging groundwater as well as for gardening and street trees irrigation by Tankers water for these ponds instead of watering them by groundwater.


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Table 11. Water harvesting and consumption estimation for Sana’a city

Description Unit Quantity No. Urban Houses No. 310,177 No. rural Houses No. 5,882 No. of total Houses No. 316,059 Roof area (average) m2 200 Average rainfall mm 243 Urban runoff coefficient (C) Unit less 0.75 Rural runoff coefficient (C). Unit less 0.6 Estimated quantity of harvested water in urban areas m3 11,305,952 Estimated quantity of harvested water in rural areas m3 171,519 Total Quantity of water harvested (Urban +Rural) m3 11,477,471 Estimated consumption in urban areas per capita l/capita/day 70 Estimated consumption in rural areas per capita l/capita /day 30 Estimated consumption in urban areas from GW m3/year 51,621,041 Estimated consumption in rural areas from GW m3/year 521,450 Estimated consumption in total areas from GW m3/year 52,142,491 Groundwater saving in urban areas % 22 Groundwater saving in rural areas % 33 Water value YR/m3 130 Benefits of using Harvested water (urban) YR 5,240,961,635 Benefits of using Harvested water (urban) US$ 26,204,808

5. MAIN FACTORS OF THE RTRWH SYSTEM The main factor affects the harvested water is the rainfall availability, and the costs that could be incurred in the construction process. Other parameters such as water quality, hygiene and maintenance are also important issues. The following points should be considered when thinking to use Roof Top Rain Water Harvested (RTRWH) system:

1- Find out how much is the annual average rainfall in the city

2- Calculate the rooftop area

3- Select the type of storage tank

4- Locate the storage tank in an area away from pollution or depending on the space of the house compound.

a. In many cases of unavailable space install a readymade steel or


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Ferro-cement above ground tanks close to the house [ ]7 .

b. In some cases it is advisable to construct a single storage tank serving several houses.

5- Design the inflow pipes system

6- Design and install first flush pipe system to flush out the first few minutes of rains which is usually contains debris, dust leaves.. etc.

7- Design the storage tank according to the maximum storage requirement adding 25% with the necessary openings for maintenance. Typical tanks (above and below ground are available with complete details for common sizes including costs). Figure 3 shows a residential building in a village utilizing the rooftop rainwater harvesting which is stored in a tank made of masonry and concrete.

Figure3: An existing rooftop harvesting tank used for more than 40 years (home

village of the second author)

8 Use Overflow of water from tank or from first flush for gardening, livestock or recharge.

9 Test water quality at regular basis especially at beginning season of rain either taking samples to the lab or on site. On site water quality tests can be done simply by:


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H2S strip test bottle check: Wash your hands thoroughly with soap. With clean hands the sealed bottle should be opened. From the tap in the rainwater storage tank fill the bottle to the mark line. Close the cap tightly. Bring the bottle back to a safe place in a room. Observe for 24 to 48 hours. If the water turns black in the bottle then it is micro-biologically contaminated and requires treatment before being used for drinking. If the water color stays brown, then the water is fit for drinking.

10 Chlorinating the water at least once during the rainy system and when necessary

11 Awareness campaigns about importance of harvesting water from roofs and about water quality with brochures could be prepared for this purpose.

12 More hints are summarized below: a. construct rainwater tanks far away from existing cesspits b. regular cleaning of the storage tank from sediments and before

the beginning of the rainy season c. Keep the roofs catchment area clean d. Boil water or use filter systems when using harvested water for

drinking or Solar disinfection (SODIS): In this method, rainwater is kept in a

glass bottle under the sun for 6 hours. One side of the bottle is painted black. The black surface is kept on the ground. With a combination of UV disinfection and infra-red heat, water is sterilized and then becomes fit for consumption. In cloudy weather the bottles need to be kept in the sun longer

]8 [ . Several bottles can be used with this method.

6. CONCLUSIONS AND RECOMMENDATIONS Rainwater harvesting is a potential parameter to be used both in

saving the costs of utilizing groundwater from the water supply utility and saving the precious non-renewable fossil groundwater. It is estimated that an annual volume of 11,305,952 m3/yr, and 171,519 m3/yr. can be harvested in urban and rural areas respectively resulting in an annual savings of the groundwater by 22% and 33%. Simple calculation of the costs saved when using rainwater harvesting is 26,204,808 US$/year. The development of guide tables present an easy and direct method to select the amount of the water harvested according to the roof area, the run off coefficient and the annual average rainfall. Several runoff coefficients have been used according to the type of roof surfaces such as 0.6, 0.7, 0.75 and 0.8 that corresponds to the present roof surfaces available in Sana’a. Such guide tables can be easily modified to be used in any country by modifying the necessary parameters applicable to that specific country. Municipality


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and local city government should develop appropriate legislation to allow residents to employ rainwater harvesting in their households. They should provide certain technical support and financial aid if needed to residents. Awareness and educational campaigns should be conducted to encourage people to use the rainwater harvesting systems. It is recommended that local government starts the initiative using rooftop harvesting in their buildings.

7. REFERENCES ]1[ Centre for Science and Environment, “Rainwater Harvesting and

Utilization, An Introductory Guide for Decision-Makers”. Tughlakabad Institutional area, New Delhi - 110062, India2005..

] 2[ Noman, A., Taher, T., “ Water Harvesting and Spate Irrigation in Wadis: Yemen Case”. Wadi Hydrology Conference, Amman, Jordan, 2004.

"االستخدام األمثل للموارد والطاقة"محمد بن عبد الكریم الصوفي، ]3[ مؤتمر الخلیج الرابع ..م1999، 122- 111للمیاه، المجلد العربي، ص

]4[ (MoPIC) Ministry of Planning and International Cooperation, “Statistical Year-Book”. Sana’a, republic of Yemen , 2004.

]5[ Ward, C., Beddies, S., Taher, T., Sahooly, A., Gerhager, B., Al Harethi, NEquity and Efficiency in Yemen’s Water Reform- A sector Study and Poverty and Social Impact Analysis, Ministry of Water and Environment, Sana’a, Yemen . 2009.

]6[ WHO, “Guidelines for Drinking-water Quality”. third edition, Volume 1 Recommendations, Geneva, 2008.

]7[ Harteng, H., Karuki, I., Sharafaddin A. Saleh, “Design and Construction of Ferrocement Tanks Using Rooftop Water Harvesting”. Social Fund for Development, Sana’a Yemen, 2008.

] 8[ Ministry of Water Resources, “A Guide on Artificial Recharge to Groundwater”. Central Ground water Board, Ministry of Water Resources, New Delhi, India, 2000.

]9[ (NWRA) National Water Resources Authority, “Rainfall data of Sana’a, Taiz and Ibb”. Yemen, 2010

]10[ http://disc2.nascom.nasa.gov/Giovanni/tovas/TRMM_V6.3B42.2.shtml ] 11[ http://www.ems-

i.com/wmshelp/Hydrologic_Models/Models/Rational/Equation/Runoff_Coefficient_Table.htm .

http://disc2.nascom.nasa.gov/Giovanni/tovas/TRMM_V6.3B42.2.shtml

http://www.ems

Rooftop Rainwater Harvesting in Modern Cities: A Case Study for Sana’a City, Yemen

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