ROOFTOP RAIN WATER HARVESTING – AN ALTERNATIVE TECHNOLOGY FOR
FRESH WATER AUGMENTATION IN CHRONICALLY DEFICIENT URBAN
AGGLOMERATES OF INDIA
P.G.Dhar Chakrabarti 
ABSTRACT
In the context of the burgeoning water crisis in urban India, the technology of Rooftop Rain
Water Harvesting holds great promise in some of the water deficit cities of India. The
technology is simple, cost effective and sustainable. An integrated system of rainwater
harvesting can be designed for a city, in which RRWH at the domestic and neighbourhood
level can be combined with other rain water harvesting techniques at the city level for
recharging the aquifer which will augment the net availability of fresh water for consumption.
This paper draws from an experimental Artificial Recharge Study of Delhi to prove how
simple check dams and recharge wells based on roof top rainwater harvesting has helped to
raise the ground water level by 3 m in each successive monsoons. The paper also suggests that
an appropriate regulatory and incentive mechanism can be developed by the city governments
to operationalise the technology.
URBAN WATER CRISIS IN INDIA
India is one of the less urbanised among the developing countries, with less than 30% of its
population living in urban areas, but in absolute terms it has more than 300 million people
living in towns and cities. This is almost twice the combined urban population of France,
Germany and United Kingdom. The Urban India is growing more than 3.5% per annum, and
it is projected that by the year 2041, urban population shall swell to 800 million, which is
larger than the total population of the whole of Europe.
India has 23 metropolitan (million plus) and 3 mega (ten million plus) cities and it is
estimated that by the year 2021 the number of metropolis shall go up to 75 and that of mega
cities to 6, when India will have probably the largest concentration of mega cities anywhere in
the world (Singh, K and Steinberg, F, 1996).
Already many of the Indian cities are facing acute shortage of potable water. Exploitation of
surface water resources have reached a saturation point and excessive extraction of ground
water and limited open area for recharge in some of the cities have resulted in sharp decline in
the ground water table, which is manifested in failure in of tube wells, deterioration in ground
water quality, saline water ingress etc (TERI, 2000). The problem has become acute in some
of the cities, especially during summer, when potable water has to be carried in trucks or
trains from distant sources at a heavy cost and harsh water ration has to be introduced. Water
strife and riots have become regular features in some of the towns of western and southern
India during the summer months. In some of the towns even the police is asked to supervise
the distribution of water supply to avoid violence and clashes amongst the residents.
 The author is Director in the Ministry of Urban Development, Government of India, New Delhi
Many people are forced to access water from unsafe sources, leading to widespread
waterborne diseases like diarrhoea, hepatitis, roundworm, with a telling effect on public
health and hygiene. A recent study has indicated that about 30.5 million Disability Adjusted
Life Years (DALY) are lost each year due to the poor quality of drinking water and absence
of sanitation facilities. The financial loss in terms of productivity has been quantified at Rs.
360 billion (US $ 9 billion) annually (MUD, 2000).
Water crisis in urban India is assuming a cause of very serious concern of the planners,
engineers, scientist, administrators and policy makers (Dhar Chakrabarti, 2000).
STRATEGY OF RAIN WATER HARVESTING
Conventional methods of tapping surface water sources or of exploiting natural ground water
reserves may not be the only solution to India’s burgeoning urban water crisis. As an alternate
method, which can supplement the existing sources on a sustainable and low cost basis, the
strategy of rainwater harvesting is assuming increasing significance.
India receives on an average 1100 mm of annual precipitation, which is the highest in any part
of the world. Although its distribution in space and time is highly variable, most of it is
allowed to go waste as run off water into the sea through the river system. In the urban areas
there is very little recharge to the sub-surface since most of the surface is either occupied by
buildings or roads. Therefore, even if a part of the rainwater can be harvested, this may
recharge the depleting ground water level and significantly contribute to the net availability of
water for drinking and other purposes in the urban areas, particularly in the deficient areas
(CGWB 1999).
Roof- water or rainwater harvesting techniques had traditionally been practised by the urban
communities in different parts of the country. Ranging from a purely domestic based system
such as collection of falling water in containers or storage tanks to a community or even a
town level systems such as ponds, percolation tanks, dams or dykes had been in existence in
many towns. Over the years, the introduction of pipe water supply on the one hand and
gradual replacement of the community by the Government and Municipalities for the
management of water supply in the cities on the other, has led to the abandonment of the
‘ancient wisdoms’(Agarwal, A. and Narain, S. 1997). The deepening urban water crisis has
off late revived the interest on water harvesting structures. Various research organisations and
hydrological institutes have been working on different techniques of water harvesting in urban
areas, such as injection or recharge wells, bore whole flooding, lateral recharge shaft, sub
surface dukes etc. A number of cities are adopting regulatory regimes on extraction of ground
water resources and construction of sub surface tanks not only to conserve ground water but
also to prevent it from pollution.
DOMESTIC ROOFTOP RAIN WATER HARVESTING
Of all the techniques of rainwater harvesting, Domestic Rooftop Rain Water Harvesting
(DRRWH) is very simple and inexpensive and can be adopted on a decentralised scale at the
domestic level without much of additional investment. What are required are simply a
catchment surface, an inflow conduit, a storage structure and a filtration system, if the
collected water is to be used for drinking purposes (CGWB 2000).
The catchment surface of rainwater harvest is the roof. Rainwater can be collected from any
roof, but if the water is to be used for drinking purposes, roof should not be covered with
asphalt and lead flashings or lead based paints or damaged asbestos, which may contaminate
the collected water.
The inflow structures consist of gutters and inflow pipes. Gutters collect rainwater from the
roof and transport it to the inflow pipe. These could be made of aluminium or galvanised iron
sheet, or half cut PVC pipe or even half cut large dia bamboo trunk. The size and shape of
gutters would obviously depend on the surface area of the roof and the intensity of the
rainfall. To keep leaves and other debris from entering the system, the gutters should be
covered with wire mesh along their entire length. The inflow pipe is the pipe, which connects
the gutter to the filter and then to the tank reservoir. It could be made from PVC, cast iron or
cement pipes. To avoid the water from the first shower, which usually contains dirt and other
impurities, a by-pass arrangement is made by a valve to drain the first showers from the filter.
Filters must be used when the water is to be stored in tanks for direct consumption. Filter can
be divided into three parts: a container which can be made from either galvanised iron sheet
or Ferro cement, a perforated plate of non-corroding metal or PVC with 1 cm dia holes and
the filtering media which may be composed of three layers - sand, gravel and pebble bed. If
the water is not to be stored in a container and fed into the ground water reservoir, filter in the
inflow structure is not required, but it should pass through a desilting pit before entering the
aquifer.
There is wide variety of options for storing water. The storage tank could be either an
underground or over-ground structure or it can even be an abandoned dug well or an
abandoned/running hand pump. An underground structure is generally cheaper, requires little
or no space above ground and is more difficult to empty by leaving the tap on, but water
extraction is more problematic, often requiring a pump, leaks or failures are more difficult to
detect and contamination of the tank from groundwater is more common. An aboveground
structure, on the other hand, can be manufactured from a wide variety of materials, can be
easily purchased ‘off-the-shelf’, can be extracted by the gravity, and be easily inspected for
cracks or leakages. Much work has been carried out on the development of the ideal tank for
DRRWH (IRCSC 1995, 1997, 1999)
It is also possible to recharge the rooftop rainwater into the ground water reservoir through the
abandoned dug well or an abandoned/running hand pump. Water to be recharged is guided
through a pipe to the bottom of the dry/unused dug well or below the water level to avoid
scouring of bottom and entrapment of air bubbles in the aquifer. Bottom of the dug well
should be cleaned and all fine deposits should be removed before its use for recharge. Period
chlorination is required in order to control bacteriological contamination. This method is
particularly suitable for large buildings having the roof area of more than 1000 sqm.
WATER HARVESTING SYSTEM IN A BUILDING
For a smaller building having the roof area up to 150 sqm. water can be diverted from the
rooftop to the hand pump through the pipe of 50 to 100 mm diameter. To avoid entry of air in
suction pipe, a closing valve is fitted in the conveyance system near the hand pump. However,
during the period the water is being recharged, water extracted from the hand pump should be
used after proper chlorination.
Various studies on DRRWH, based on different types of roofing materials and storage
systems have established that generally a loss up to 20% may take place due to evaporation
and inefficiencies in collection processes. Thus only 80% of rainfall can be harnessed through
rooftop. Therefore the following formula can be adopted to determine the total quantity of
rainwater that can be reused:
Total quantity of water to be collected (cum.) =
Rooftop Area (Sqm.) x Average Monson Rainfall (m) x 0.8
Based on this formula, the following table can be worked out regarding the quantum of
rainwater that can be harvested from area of various sizes.
Rainfall (mm)
Rooftop
Area (sqm)
20
30
40
50
60
70
80
90
100
150
200
250
300
400
500
1000
2000
3000
100
200
300
400
500
600
800
1000
1200
1400
1600
1800
2000
28.8
43.2
57.6
72
86.4
100.8
115.2
129.6
144
216
288
360
432
576
720
1440
2880
4320
32
48
64
80
96
112
128
144
160
240
320
400
480
640
800
1600
3200
4800
Harvested Water from Rooftop Area (cum)
1.6
2.4
3.2
4.0
4.8
5.6
6.4
7.2
8
12
16
20
24
32
40
80
160
240
3.2
4.8
6.4
8
9.6
11.2
12.8
14.4
16
24
32
40
48
64
80
160
320
480
4.8
7.2
9.6
12
14.4
16.8
19.2
21.6
24
36
48
60
72
96
120
240
480
720
6.4
9.6
12.8
16
19.2
22.4
25.6
28.8
32
48
64
80
96
128
160
320
640
960
8
12
16
20
24
28
32
36
40
60
80
100
120
160
200
400
800
1200
9.6
14.4
19.2
24
28.8
33.6
38.4
43.2
48
72
96
120
144
192
240
480
960
1440
12.8
19.2
25.6
32
38.4
44.8
51.2
57.6
64
96
128
160
192
256
320
640
1280
1920
16
24
32
40
48
56
64
72
80
120
160
200
240
320
400
800
1600
2400
19.2
28.8
38.4
48
57.6
67.2
76.8
86.4
96
144
192
240
288
384
480
960
1920
2880
22.4
33.6
44.8
56
67.2
78.4
89.6
100.8
112
168
224
280
336
448
560
1120
2240
3360
25.6
38.4
51.2
64
76.8
89.6
102.4
115.2
128
192
256
320
384
512
640
1280
2560
3840
CONSTRAINTS OF DOMESTIC ROOFTOP RAIN WATER HARVESTING SYSTEM
Judging by this formula, four-fifth of the annual rainfall should be harvested or recycled for
domestic use, but in reality that does not happen. No organised and authentic data regarding
domestic rooftop rain water harvesting in any Indian city is available, but as things stand
today, not even a small fraction of the total rainfall is harvested in the urban areas, barring
possibly one or two cities, although the possibilities are enormous. The reasons are the
following:
First, all the urban areas are already covered by a centralised pipe water system, which is
highly subsidised. Although the system is deficient and does not cater to the full requirement
of the residents, there is not much incentive for the residents to invest on an additional system
at the domestic level. Only those who own an independent and large roof area and have
sufficient open space for the installation of storage tanks can afford to set up such a system,
but such categories of people are usually well off to purchase water from alternate sources
rather than invest on RRWH. Even if they invest on such a system, it is usually for garden or
lawn irrigation rather than for drinking purposes.
Secondly, the occupancy pattern and ownership system of urban property is such that a very
small part of the total roof area is owned individually and often the density of population
living under a roof is so high that the per capita availability of recycled rainwater to the
occupants of the building is very insignificant and therefore the economics of rainwater
harvesting at the domestic level may not work out very favourably. Joint management of roof
water not only for the purpose of collection and conservation but also for their extraction and
reuse shall also become a difficult proposition because of the complexities of the issues
involved, unless these have been planned at the time of construction of houses.
Thirdly, a large part of the urban landscape is not roofed, although it may be built up, for
example the road space, the lanes and bye lanes etc. Similarly the set backs of the buildings,
the open spaces, the parks, gardens, play grounds, city forests etc have no roofs and therefore
the run off from such areas can not be pooled under a DRRWH system.
Fourthly, domestic conservation and extraction of ground water through dug well or tube well
shall also depend on the depth of the local aquifer. If it is too deep it may not be extracted
economically and if it is too shallow it can either pollute ground water or be polluted by other
discharges.
Finally, the buildings of urban India have not been designed with built in provisions for
DRRWH system. Although the concept is old and antiquated, its application in the context of
modern city life is a very recent phenomenon. Neither the master and zonal plans of the cities
nor the building bye laws had any stipulation regarding rooftop rainwater harvesting and
therefore gutter, inflow system and storage tanks were not conceived when the buildings were
designed and no incentive structure has yet been developed for adding such facilities on the
existing buildings.
These are formidable constraints, which have hindered any large-scale use of DRRWH
system in urban areas. These also underline the inherent limitations on the use and application
of the system at a domestic level in modern city life. Possibly a purely domestic based and
decentralised rain water harvesting in urban areas do not stand a very great chance of success
unless this technique is used as part of an integrated and holistic framework of rain water
harvesting in urban areas and unless a proper regulatory and incentive structure is developed
to make the system popular and attractive.
INTEGRATED SYSTEM OF URBAN RAIN WATER HARVESTING – DELHI STUDY
In an integrated system of urban rain water harvesting, not only the domestic but also the
institutional and commercial roof area as also the non roof area are properly mapped and a
combined and holistic framework of rain water harvesting is drawn for the city as a whole
which integrate the system with the water supply, sewage and drainage system of the city.
Such an integrated system is yet to be put on place in any of the cities in India or elsewhere in
the world, but many research and management efforts are being made in this direction.
This paper focuses on the Experimental Artificial Recharge Studies (EARS) in some selected
areas of the national capital of Delhi conducted by the Central Ground Water Board, in
collaboration with the Indian Institute of Technology, Delhi. This study combined the
techniques of artificial recharge through Check Dams and Rooftop Rain Water Harvesting
(CGWB and IIT 2000).
PHYSICAL FEATURES OF THE STUDY AREA
The study area is spread over a geographical area of 10 sq. km., falling between 28 31’ and
28 33’ North latitudes and 77 09’ and 77 11’ East longitudes. The area is underlain by Delhi
quartzite in South and alluvium in North. Principal rock types are siliceous quartzite,
ferruginous quartzite, silicate-mica schist, sand and clay. The ground elevation ranges from
226 to 270 m aMSL and ground slope ranges from 0.15 to 1.43 % towards North East. The
depth to bedrock beneath alluvium varies from 60 m to 119 m in IIT Campus and increases
towards North East. There are five streamlets originating in the area and these finally join the
Yamuna River. Ground water in the area occurs in the semi-confined condition in fractures of
hard rocks. The average depth of water level during pre monsoon varied from 15 to 20 m.
Normal annual rainfall of Delhi is 611.8 mm, out of which 533 mm (87 %) occurs during
South West monsoon period (July to September). Average annual evaporation loss in a year is
2.2 m, out of which 0.85 m (38.6&) occurs during the South West monsoon months.
Five stream-gauging stations were stations were established and average run-of coefficients
were calculated to vary between 11.7 to 14.5 % for different watersheds. In a normal rainfall
year, 0.46 mcm stream water goes out of the area as surface run-off. Infiltration tests
conducted using the double ring infiltrometer, at fourteen sites, indicated that infiltration rates
varying from 1.02 mm/hr to 11.08 mm/hr in fractured quartzite and 2.74 to 42.30 mm/hr in
alluvium. Resistivity surveys, using Schlumberger method, at nine locations were conducted.
Resistivity values ranged for different rock types have been established viz. top soil 15-170
ohm.m, weathered/fractured quartzite 100-200 ohm.m, less fractured quartzite 300-600,
compact quartzite more than 1000 ohm.m. At three sites, less fractured quartzite underlain by
fractured quartzite was indicated, while at remaining sites predominantly clay formation
underlain by less fractured quartzite were inferred.
CHECK DAMS AND PIEZOMETERS
Based on these scientific studies, four check dams and one gravity head recharge well were
constructed. The height of the dams is 4.0. 3.6, 2.0 and 1.5 m and their storage capacities are
0.01533, 0.02218, 0.006587 and 0.0046 MCM respectively. To study the impact of water
storage due to check dam reservoir, 12 piezometers ranging in depth from 48 to 119 m near
dam sites were constructed. In addition, 28 observation wells were established. All the
piezometers were monitored on a daily and observation wells on a monthly basis.
A. Pre-recharge Scenario:
The ground water behaviour during pre-recharge scenario was:
Pre-monsoon water level (1997)
20 - 22 mbgl
Post-monsoon water level (1997)
Pre-monsoon water level (1998)
17 - 21 mbgl
17 - 19 mbgl
Water level fluctuated between 1 to 3 m due to the monsoon. One tube well, supplying water
to the local residents was operative for 4 hours during May 1997. Pump was not lifting water
due to heavy draw down.
Volume of water generated due to monsoon rainfall
over 0.45 sq km of catchment area
Total Run-off water available for recharge to aquifers
Volume of water spilled over
0.370 MCM
0.058 MCM
0.010 MCM
Hence, effectively 0.048 MCM water was stored during monsoon period due to filling of dam
reservoir of which 0.0465 MCM water was added to ground water repository.
B. Post-Recharge Scenario:
Post- monsoon water level (1998)
Pre- monsoon water level (1999)
Post- monsoon water level (1999)
Pre-monsoon water level (2000)
Post-monsoon water level (2000)
13 - 16 mbgl
14 - 18 mbgl
10 - 13 mbgl
11 - 16 mbgl
07 - 10 mbgl
Actual impact of artificial recharge can be judged from the water levels as below:
Pre- monsoon water level
Pre- monsoon water level
Pre- monsoon water level
Pre- monsoon water level
(1997)
(1998)
(1999)
(2000)
20 – 22 mbgl
17 – 19 mbgl
14 – 18 mbgl
11 – 16 mbgl
Therefore each successive year water level during pre-monsoon days rose consistently by 3m
due to the rainwater harvesting. The tube well which was able to lift water only for 4 hours in
1997 is now capable of lifting water for 24 hours daily during the pre-monsoon days.
ROOFTOP RAIN WATER HARVESTING
In IIT Campus, the roof of Block No. 6 having the roof area of about 1666 sqm was selected
for conducting the roof top rainwater harvesting experimental studies. The aim was to inject
the storm runoff directly from roof top to two recharge wells of 203 mm dia of 95 and 42 m
depths respectively and provision was also made for excess water to go to nearby dug well
through conveyance system. The conveyance system was designed considering the rainfall
intensity of 10 cm/hr, which may generate about 158 cum runoff in one hour. Three water
meters were installed in the conveyance system for measuring the quantity of water given to
different recharge structures.
During a single monsoon, 830 cum of rainwater was recharged into the aquifer resulting into
the rise of water level of 2.29 to 2.87 m. Similar rise in water level were observed in the next
two seasons as well. The experiment was replicated on a number of buildings in the project
area with similar results.
The combined effects of the of the check dams and the rooftop water harvesting on major
institutional buildings in the study area have been a general rise in the water table in the study
area. Even the area outside the study has been benefited by these measures. Some of the
residential colonies which had a chronic shortage of drinking water due to failure of tube
wells in the pre-monsoon period are now having uninterrupted supply of water during summer
months.
A few other interesting changes have been observed in the ecology of the area. A large part of
the down stream area, which was earlier denuded of any vegetation, is turning green with
grasses and shrubs. This is due to the presence of more soil moistures in the upper layer than
the previous years. Secondly, migratory birds are settling on the reservoir. All these have
taken place during the course of three years of the study on which a total amount of Rs10
million only (US $ 0.20 million) has been spent. The scope of the study is now being widened
to construct a few more check dams and to divert the run off from the storm water drains into
the dams. A further experiment to inject the treated sewage into the ground water is also under
way.
The Delhi study has proved that it is possible to integrate the various types of rainwater
harvesting in an urban area to recharge the aquifer for supplementing the net availability of
drinking water for the city water supply system. In the capital territory of Delhi, the total roof
top area has been worked out to be about 138 sq. km, which would yield about 67.9 mcm
water. It has been estimated that even if rainwater of 10% of the area were harvested about 6
mcm of rainwater would be recharged into the ground water.
REGULATORY FRAMEWORK AND INCENTIVE STRUCTURE
Encouraged by this and similar other experiments conducted in different parts of the country,
the city governments have started working on various innovative policy interventions to
encourage the increasing use of rain water harvesting at the domestic, neighbourhood and
institutional levels. Madras Metropolitan Development Authority has notified a Regulation
making it mandatory for all new houses to have in built facilities for roof top water harvesting
system (MMDA 1993). The Municipal Building Bye Laws have also been amended to that
effect. No Building Plan of a new construction is approved by the Municipality unless it has
provisions for DRRWH system. This has also been made mandatory for all commercial and
institutional complexes. Incentives have been provided by way of rebate in property taxes for
installation of such facilities in the existing residential houses, which do not have any system
for rooftop water harvesting. Group Housing Societies are also being encouraged to set up
injection well at the neighbourhood level for conserving the rainwater. The result achieved
has been very encouraging – the city has been able to recharge its ground water to some
extent.
More and more city governments are now replicating the Madras model to operationalise the
technology of Rooftop Rain Water Harvesting. The technology is simple, affordable and cost
effective and holds a great promise for water deficit cities in the years to come.
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