DRINKING WATER HARVESTING – ITS SIGNIFICANCE AND SCOPE IN
DESERT ENVIRONMENT
N.S.VANGANI
Introduction
Water harvesting practices in the form of nadis, talabs, lakes and tankas ( cistern ) had
always remained the centre of main activity and struggle of all the prudent rulers of
the Rajasthan state. These structures have been serving the human, livestock and wild
life population besides very precisely supporting the system of irrigation.
Unfortunately, due to ignorance and un towards attitude, these typical systems today
stand defunct. The main reason is; planners and engineers are living in maniac
paradise of ultra sophisticated technology ignoring the hard reality that these systems
are best suited in the arid belt, where in most of the areas the ground water is saline
and unsuitable for drinking , and rainfall is erratic and unpredictable. Water havesting
systems are efficient, easy operative and cheap in the long run in many ways. Among
the water harvesting systems narrated above, the tanka can be the answer for reliable
and safe drinking water source. Its structural important features, optimize capacity,
efficient catchment area characteristics, size required and after care are discussed
here.
1. Structural Features:
Tanka is a local term for the covered underground cistern, generally of masonry (
stone or brick ) or concrete, for collection and storage of surface runoff. The
provision of tanka near religious centres and in village for community use is an
ancient practice since 1607 A.D. However, now a days it is preferred to construct it at
each hamlet on the individual ownership basis. It is mainly because in case of
community tanka, the dominant group get individual supply of water to their homes
which poor do not receive. Moreover, an access to water supply might be
monopolized. Generally, community structures failed through poor workmanship and
after care.
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Capacity:
It is simple to calculate the annual drinking water need of the family. For an example,
a family consists of six persons; a bare minimum per capita per day requirement of 7
litres of water only for drinking is usually considered by the villagers that one has to
have to live without distress. Clearly, a family needs 15.624 cum or say 16 cum of
minimum storage to survive throughout the year solely using tanka water for
drinking. However, in the case of unusual consumption of drinking water say 14 liters
per capita per day during the summer period of 4 months, an additional provision of 5
cum. can be provided in the storage capacity of tanka. Therefore, an optimal capacity
of can be fixed at 20 to 21 cum.
Design Components:
( a ) Rainfall probability:
Annual rainfall, distribution, amount, and intensity plays an important
role in designing of catchment area and structure. Most of the times the structures
designed on the basis of average rainfall do not serve the purpose because of frequent
droughts in the arid region. In general, the water supply structures are either over
designed or under designed. Since the peculiar rainfall conditions are existing in the
arid and semi arid regions, it may be worthwhile to predict probability of dependable
rainfall of that particular zone where the structure is being proposed. Experience
indicates that the dependable annual rainfall occurring at 60 % probability worked out
through the Log Pearson Type III distribution meets the criteria for suitably designing
the catchment area for the tanka. By considering this probability of rainfall and
catchment area needed, the tanka meets the water requirement throughout the year.
( b ) Design of Tanka:
The circular cross section is the most economical form of the tanka. It
has also been found to be more stable as the pressure exerted by the water is uniform
radial pressure in all directions in the diametrical plane at right angle to the curved
surface.
The successful installation of tanka depends on the selection of site,
particularly the catchment characteristics such as size, shape, topography, soil type,
vegetation etc in case of natural catchment. The size of the catchment artificially
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prepared should produce adequate runoff to meet the storage requirement of the tanka
and at the same time it should cause minimum soil loss. The catchment area could be
worked out by the relation ;
Ac x Pd x C = m x d x Ao
Where Ac = Catchment area
Pd = Dependable rainfall
C = Runoff coefficient ( found to be 0.3 )
m = Storage coefficient taken as unity for the tanka
d = Depth of tanka below G.L. ( 3.0 m )
Ao = Cross sectional area of tanka
The catchment area required for 21 cum capacity tanka is worked out to be 330 m2
After selection of the site and size of the catchment, in the case of
natural catchment, it is imperative to carry out some minor treatment for augmenting
the runoff. These treatments may be such as construction of earthen diversion bund,
removal of bushes, increasing the land slope, compacting soil etc. However, in case
of artificial catchment, it should be in the round in shape around the tanka, giving
proper slope of 2.5- 3.0 % towards the tanka with zero slope at inlet. After forming
the shape of the cachment, it should be rammed and spreading 15 cm thick layer of
calcium carbonate nodules ( murrum ) and again ramming. In case calcium carbonate
nodules are not available, a layer of silt clay can be spread and rammed properly.
Both the treatments on the artificial catchments will result in reduction in the intake
rate of the soil.
In both the cases of the treatments to the catchment, the maintenance is required
every year. Moreover, during runoff, the treated surface is worn out and the materials
are carried out as suspended load and deposited in the tanka. This deteriorates the
quality of the stored water. To avoid these, a courtyard or open area of the hamlet is
treated with cement plaster or cement tiles laid on the bare soil. In this case, the
additional water is collected of better quality from the treated area. The coefficient of
runoff becomes 0.9 instead of 0.3 due to pucca floor. Hence, maximum water is
harvested from small area for storage. The runoff water is transferred through a pipe
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net work to the tanka. The catchment requirement and the cost estimates will be
presented later on.
( c ) Structural components of tanka :
( i )Catchment – It should be impermeable and smooth to obtain improved
hydraulic efficiency.
( ii ) Silt collecting pit – It is provided to arrest the sediment load before the
inlet point.
( iii ) Cemented apron – This is one of the main structural component
provided to control seepage and also to prevent caving from sides. Suitable
cement concrete 1: 3: 6 is being used for construction.
( iv ) Catch pit – This is made in the form of depression in the foundation slab
of cement concrete 1 : 3 : 6 to facilitate drawing minimum available water
from the tanka.
( v ) Top cover – It is required to control evaporative losses and to prevent
pollution of water stored by foreign materials. An opening at top with G.I.
cover to facilitates withdrawal of water.
( vi ) Inlets – There should be three inlets to collect the runoff , and will also
prevent floating debris from entering into the tanka. Moreover, by providing
these, there will be free access of air in the tanka which will prevent the bad
odour of stored water. A suitable mesh with iron bars in the angle iron frame
is to be provided in the inlets.
( vii ) Outlets – There is one outlet for letting out the extra water entering in to
the tanka.
(viii ) Guide walls – These are provided upstream of outlet for arresting all the
available runoff from the catchment.
( ix ) Withdrawal arrangements – A stand with pulley at opening is provided
for withdrawal of water with the help of rope and bucket.
( x ) Fencing of catchment area – Fencing of the catchment area is required to
restricted animal entry.
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( d ) Constructional details :
The construction materials used are, all locally available viz. stones or bricks,
cement, iron sheet, etc. After selection of site and carrying out desired earth work in
excavation; 3.25 m depth and 3.0 m diameter, cement concrete foundation 1 : 3 : 6
with 0.25 m thick should be laid. Entire construction of stone masonry in 1 : 6 cement
mortar should be done with special care to ensure against any crack and leakage from
floor and walls of the structure. This is achieved by providing cement concrete
flooring and at least 1- 1.5 cm thick cement plaster on walls, incorporated with water
proofing compounds or with 1 % soap solution in water (soap solution forms
insoluble fillers on reaction with cement). Proper cementing of joints in the super
structure should also be ensured, especially when further construction on a half
finished work is resumed after the lapse of a few days. The tanka is covered with
stone slab roofing with a suitable opening on the top for withdrawing the water.
The inlets and outlet in the tanka should be placed 3.0 cm below the ground
level that is the point where the artificial catchment ends with tanka wall. All the
inlets and outlet should be made of angle iron frame with vertical iron bars spaced at
5 to 7 cm apart and expanded metal mesh to prevent entry of floating debris, birds
and reptiles, etc. which attempt to approach the water, usually drop inside the tanka
and spoil the water.
The inward sloping cement concrete apron around the tanka with about 3 %
slope helps in collection of even meager runoff. Moreover, this will also prevent the
entry of runoff water along the outer face of tanka wall. and will reduce the possible
damage to the structure during high runoff rate of heavy rainfall event. The masonry
check wall nearer to the outlet ( both the sides ) of about 50 cm. high constructed
across the apron will divert the water into the tanka. Thorn fencing may be provided
around the catchment area to minimize pollution of the stored water with livestock
wastes and other farm wastes.
(e)Maintenance and after care:
At least once in a year cleaning of tanka, ramming of its catchment, and
painting of the inlets and the outlet is must for keeping it in good working condition.
Also de silting of the tanka is necessary, which may be taken up before the onset of
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the monsoon. Periodically use of oxidizing agents like potassium permanganate will
prevent the growth of microscopic organisms, and development of bad taste, odour
and colour of the water. Alum helps in settling the dirt in suspension. At least a few
cm of water column should always be maintained in the tanka to avoid development
of cracks etc.
(f) Expected life – It is expected that this tanka will last at least for 30 years
Tankas Constructed By WfI:
General Condition:
It is unpleasant truth that water availability in the arid region of Rajasthan is limited
and inadequate, even after a hypothetical consideration that all utilizable water
resources are developed. Further with increasing population and modernization, the
scarcity of water will go on increasing and per capita water availability will be
decreasing. Moreover, in less than normal rainfall years the situation is much more
critical. Nonetheless, the importance of the old tradition of water conservation /
harvesting was reflected in the seventeenths century when the structures were
developed. So far, particularly in the rural areas, the water harvesting structures have
served as the best solution in mitigating the drinking water problem. But the fact
remains these water harvesting systems should be revived and encouraged at
Government level.
Water Demand and Consumption:
Estimating the demand of drinking water and water needed for cooking is the prime
important for designing any runoff storage structure. The field investigations
indicates that an average daily water consumption which includes animal
consumption is 10.75 litres / day and 23.54 litres / day in Pabupura and Bhalu Rajwan
cluster villages respectively. It is about two times the consumption at Bhalu Rajwan
than at Pabupura cluster of villages. In reality it is very difficult to separate out the
water consumption by human and livestock. It may be only approximation. However,
as per the WHO, minimum water quantity standard is 20 liters per day for human
survival. (WHO, 2000).
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Basically, the runoff water stored in the tanka is meant for human drinking and
cooking only. Animals are not supposed to be provided water from this structure. If
the water is used by both, than the size of the tanka will be large or tanka is to be
refilled by transported water which may be uneconomical to the poor rural
population. The size of tanka, for a single family, of capacity of 21000 liters for
drinking only is sufficient. As per the report, the Wells for India has considered only
3 liters of water required by animals per head per day. It is too less. In case the
provision cooking and animals drinking water is required then the requirement can be
calculated as follows;
Table Livestock water consumption:
Type of animal
Age
Water consumption
Liters per day
1. Heifer
1- year
10
2. Heifer
1-3 years
15
3. Cow, bullock
_
35
4. Buffalo
1- year
20
5. Buffalo
1-3 years
30
6. Buffalo
more than 3 years
49.5
7. Sheep
1-year
4
8. Sheep
more than 1 year
4.5
9. Goat
1-year
3
10.Goat
more than 1 year
4.5
11.Horse
young stock
20
12.Horse
more than 3 years
48
13. Donkey
-
20
14.Camel
-
49.5
The requirement of drinking water for different type of animals can be assed by the
above table and for human consumption and for cooking, 20 liters per capita per day
could be considered ( WHO ). Based on these data, at initial stage approximately the
capacity of the tanka can be decided for each family. However, by collecting data on
actual consumption of water per capita per day and by animals on average basis , it
7
will be possible to standardized the construction size of tanka for given number of
family members and animals along with extra allowance. The estimation of
consumption by human could also be done by the following method :
Estimation of consumption :
The maximum yield possible from rain water catchment system is directly
proportional to the catchment area, its runoff coefficient ( i.e. the ratio of harvestable
rainfall to total rainfall ) and rainfall probability. Thus, for a full hydrological year,
the main daily yield is given by the equation :
Y = fi x Ri
-------365
Where
Y = mean daily catchment yield, litres / m2
fi = runoff coefficient
Ri = probable rainfall , mm
The runoff coefficient depends on the catchment treatment material and age of the
catchment surface. Theoretically, the consumption can be set at constant level of Y
liters / m2 /day if adequate storage is available for dry season requirement. This is
based on the assumption that evaporation and other losses from storage are negligible.
Per capita consumption from catchment is calculated by :
Y
C = --N
Where
C = per capita consumption , liters / m2 /day
N = number of persons served by the catchment
Y = mean daily catchment yield , liters / m2
Example :
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Six persons are to be served by tanka stored water, having treated catchment area of
500 sq.m ( f = 0.2 ). Rainfall probability is 212 mm
Mean yield---
Y = 212 x 0.2
----------- = 0.116 liters / m2 / day
365
Per capita consumption
C=Y/ N
0..116 x 500 / 6 = 9.66 liters per day
Therefore for six member family the annual requirement will be = 21.155 cu.m
Size and Runoff Efficiency of Catchment Areas :
Due to the high variability of hydro meteorological phenomena it is not possible to
harness the available rainfall to optimum level, unless feasible measures are taken.
For generating reliable runoff, a water harvesting catchment needs to be impermeable
and smooth. The perfect sealing of the soil surface is little bit expensive, therefore,
some infiltration losses must be accepted. In many cases, ground forming can
increase the runoff yield by improving the hydraulic efficiency of the catchment and
thus reduce the time available for infiltration to take place.
The observations on rainfall and runoff from artificial prepared catchments around
tankas were recorded from the year 2001 to 2004 ( WfI ). The runoff observations
were recorded at 21 tankas of Pabupura cluster of villages and at 6 tankas in Bhalu
Rajwan cluster of villages. However the rain gauge station was only one for all the
observation sites. The rainfall data used for interpretation ( rainfall- runoff
relationship ) were largely erroneous due to spatial variability in rainfall distribution.
Even in the same period of the year the results were highly variable at different sites.
Moreover, the year 2001 and 2002 were drought years. There after, under monitoring
programme in 2004, a net work of seven and four rain gauging stations was
established in Pabupura and Bhalu Rajwan cluster of villages respectively.
Compacted murrum is the universal choice for catchment surfaces which was adopted
in Pabupura and Bhalu Rajwan villages but the quality of material varied
considerably. Along with rainfall observations, other data are being collected on ( i )
depth of runoff water collected in the tanka of each rainfall event ( ii ) quantity of
water withdrawal for human and livestock consumption ( iii ) runoff efficiency of the
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catchment area. The ultimate aim is to find out the catchment requirement for every
cubic metre of water collection and self sufficiency of drinking water availability of
each household .
The details of tankas selected for monitoring at Pabupura cluster of villages are as
follows :
Details of selected tankas in Pabupura cluster of villages : ( rainfall at 60%
probability = 212 mm )
Tanka Name of village
capacity
Available
Required catchment
No.
( cu.m )
Catchment
area ( m2 ), fi = 0.3
Area ( m2 )
1.
Mayakhor
46.061
263
724
2.
Mayakhor
40.457
358
636
3.
Mayakhor
37.910
467
596
4.
Pabupura
23.857
358
375*
5.
Pabupura
24.007
358
377*
6.
Pabupura
24.007
265
377
7.
Narayanpura
23.707
338
373
8.
Narayanpura
32.860
358
517
9.
Narayanpura
26.108
358
410*
10.
Loona
25.183
358
396*
11.
Loona
25.323
358
398*
12.
Loona
31.627
358
497
13.
Lumbasar
30.963
358
487
14.
Lumbasar
32.973
358
518
15.
Lumbasar
28.173
358
443
16.
Mithadia
22.414
358
352*
17.
Mithadia
20.175
358
317*
18.
Mithadia
27.022
358
425
19.
Chakoo
39.363
358
619
20.
Chakoo
44.350
358
697
21.
Chakoo
44.350
358
697
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* The variation in the catchment area is about + !0 % of the required for generating
the runoff to the capacity of tanka. These can serve the purpose. However, at all other
locations either the catchment treatment is to be altered as cementing surface or spray
of polymer solution or to be enlarged as per the requirement.
The details of the tanka selected for monitoring in Bhalu Rajwan cluster of villages
are presented in the following table :
Tanka Name of beneficiary Capacity
Available
Required catchment
No.
catchment
area ( m2 ), fi = 0.3
( cu.m )
area ( m2 )
1.
Sugani
49.664
141
781
2.
Chandani
27.610
129
434
3.
Sugan
30.490
141
479
4.
Shanti
23.933
141
377
5.
Loon
30.685
212
482
6.
Palriya ka
49.652
234
781
In all the above cases, the alteration in the catchment area is must for generating the
runoff to the capacity of the tanka. In case the same catchment area and same treated
surface is to be kept, then nearly the runoff storage will be around 18.06 % to 43.93
% of the tanka capacity ( 56.07 to 81.94 % storage will remain empty ).
Catchment treatment
Normally the poor rural population can not afford the expenses on the treatment of
catchment surface for generating the runoff. But, they can be advised to either go for
enlargement of the existing catchment with same type of murrum treatment so as to
harvest the runoff up to the capacity of the tanka constructed by WfI. or use tanka
water for human drinking and cooking only. As usual they must take their animals
either to the other water point such as village tank, or near by water supply scheme. In
case the size of the existing catchment can not be altered then the surface of the
catchment should made smooth which may have 0.8 or 0.9 runoff coefficient. Several
experiments on use and efficiency of different materials has been carried out in
different countries. Such treatment materials are; spray able asphalted compounds,
plastic and metal films bounded to the soil, soil compaction and dispersion field
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fabricated asphalt, fiberglass membrane, sodium salts, gravel covered plastic
membranes and melted wax, bentonite and few chemical treatments. It can be
cemented surface also. The life of continue cemented surface in open area is less
durable and cracks develop on the top of it. To avoid this problem, cemented tiles of
30 x 30 cm on bare sandy soil can be provided without cement joints. Such surface
will improve efficiency of the catchment area in runoff generation to the extent up to
90 %. The CAZRI, Jodhpur, on field experiment bases recommended the spray of
sodium carbonate at the rate of 1 kg. / 10 sq.m as efficient and cheap treatment
material which generated, on an average of four years, 66.32 % of rainfall as runoff (
37.32 % more runoff than the untreated soil ).
Catchment area needed for one cum. of runoff from cement tile surface :
Assuming ;
Capacity of tanka = 40.0 cum.
Ri = Rainfall at 60 % probability = 212 mm ( Phalodi Tehsil )
fi = runoff coefficient = 0.9
Ca = Catchment area ( m2 )
Capacity ( runoff ) = Ri x fi x catchment area
40 .0 = 0.212 x 0.9 x Ca
Ca = 209.64 m2
Therefore, for 40 cum of runoff , 209.64 m2 area is needed
Hence for one cubic metre of runoff , 5.24 m2 area is needed.
The cement tiles can be used in open area as well in court yard of the dhanis’. Before
the on set of monsoon, the surface is to be cleaned properly . In case of court yard,
run off is carried to tanka through cement pipe. No one should be allowed to walk on
cemented tile surface.
Cost estimates of paved cement tiles surface:
By adopting paved cement tiles surface, the requirement of the catchment area will be
reduced. The runoff coefficient will be around 0.9. Considering the rainfall pattern
already stated as 212mm and the existing storage capacity, the catchment requirement
at observation sites to be paved with cement tiles at Pabupura and Bhalu Rajwan
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cluster of villages is presented in the following tables. As per the market rates The
cost of cement tiles is about Rs. 45.00 per sqm.
Pabupura cluster of villages
Tanka No.
Beneficiary
Catchment area ( m2 )
Cost estimates ( Rs. )
1
Meera
241.41
10,863
2.
Tulsi
212.04
9542
3.
Kistri
198.69
8941
4.
Mangi
125.03
5626
5.
Bhoori
125.82
5662
6
Mohini
125.82
5662
7
Gawari
124.25
5591
8.
Jethi
172.22
7750
9.
Kamala
136.83
6157
10.
Shanti
131.98
5939
11.
Lichmo
132.72
5972
12.
Kamala
165.76
7459
13.
Chagani
162.28
7302
14.
Kama
172.81
7776
15.
Lichmo
147.65
6644
16.
Dhapoo
117.47
5286
17.
Seeta
105.74
4758
18.
Nainu
141.62
6373
19.
Paru
206.30
9283
20.
Surjee
232.44
10460
21.
Tulsi
232.44
10460
Catchment area ( m2 )
Cost estimates ( Rs. )
Bhalu Rajwan cluster of villages
Tanka No.
Beneficiary
1.
Sugani
260.29
11713
2.
Chandani
144.70
6512
3.
Sugan
159.80
7191
4.
Shanti
125.43
5644
13
5.
Loon
160.82
7237
6.
Palriya ka
260.23
11710
The above two tables revealed that the estimated cost of paved cement tiles is bit
high. On an average it is Rs. 7310 and Rs. 8334 per tanka in Pabupura and Bhalu
Rajwan cluster villages. This could be the additional amount towards the cost of tanka
construction.
Polymer treatment
The National Geophysical Research Institute and the Indian Institute of Chemical
Technology have discovered that polymers are a cost effective way to increase runoff
water, based non toxic polymer solution that permeate in the high porous sandy soils .
These polymers acts as binders and reduce permeability and infiltration rate on the
sandy soil. Use of water repelling chemicals, combination with these binding agents,
results in even better runoff. The cost of this polymer treatment is approximately one
quarter of the traditional catchment construction cost. (Bisht, 2001). The rate of
polymer spray is about Rs. 3 /- per sq m. The polymer spray could be done on the
murrum formed surface of the catchment. The efficiency of the catchment on average
would be 60 % for the first 4 years. In the case of Pabupura cluster of villages the
average catchment size is 353 sq m. Hence the additional expenditure will be Rs.1059
in every 4 years at each site. Similarly, For Bhalu Rajwas cluster of villages the
additional expenditure will be 166 x 3 = Rs. 498 for each site. The life of polymer
spray is about four years. Therefore, it must be reapplied after this period. Although,
the initial construction cost is little bit higher, in case of treated murrum surface with
a polymer spray but will increase the runoff yield which in turn will be cost effective.
Conclusion:
The concept of water harvesting is not new in arid region of Rajasthan, where acute
problem of drinking water exist. It has been a traditional practice since last four
centuries. Runoff is being harvested from natural catchments, in the form of dugout
ponds or impoundage reservoirs and from roof tops or artificial catchments in the
form of cisterns ( tankas ). However, no sizable work on water harvesting from
treated catchments have been initiated in the region. The limitation to use of treated
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catchments has been the cost per unit of water produced. Nevertheless, there is
importance of the treating the catchments so as to render the soil surface
impermeable, there by, eliminating infiltration and generate more runoff for water
harvesting structures. The research investigations have revealed that water harvesting
efficiency up to 60 % can be achieved at affordable cost. On murrum surface layer,
polymers may be spread to augment the surface runoff. Second, alternative could be;
the use of cement tiles in the catchment area (Coefficient 0.9). Though, the initial cost
will increase on runoff generation component but there will be assured availability of
water in the tanka.
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