4. India – Village – Methodologies - Runoff
4.3.
Assessing impact of water harvesting structures
This section explains a simple methodology that was developed to assess the impact
of existing water harvesting structures on downstream tanks and other water users,
and the potential impact of new structures. It can be used as a planning tool to
maximise the benefits of water harvesting, and to help and identify potential negative
impacts.
4.3.4. Background
Different forms of water harvesting have been used successfully in semi-arid areas of
India for millennia as a means of protecting domestic water supplies and increasing or
stabilising agricultural production. In recent years, water harvesting both in field (e.g.
contour bunding) and along drainage lines (e.g. check dams) has been promoted and
funded on a massive scale as part of different government and non-government
programmes.
Accepted wisdom is that rainfall should as far as possible be harvested where it falls
and that these technologies are totally benign. However, evidence is emerging that
water harvesting in semi-arid areas, if used inappropriately, can lead to inequitable
access to water resources and, in the extreme, to unreliable drinking water supplies
(often these are recharged by tanks located close to villages). Methodologies are
needed to identify when the identify the possible downstream impacts of new water
harvesting structures, and to answer questions like how many check dams are suitable
for a catchment? Structures of which size? Where should they be sited? And, how
may new water harvesting structures affect the performance of tanks and other
existing structures?
4.3.5. Survey of structures
In the WHiRL study, all water-harvesting structures within the catchment area of the
main village tanks in each study village were identified and mapped (see Figure 4.3.3
for example). The structures included tanks, nala bunds, check dams, and others, for
example, farm ponds. Measurements were made of the upstream crest height and
breadth of every check dam and nala bund so that the potential storage of these
structures could be estimated. Example results are shown in Table 4.3.1.
Figure
4.3.3
Structures
mapped for the catchment of
the main tank in Battuvani
Palli
Other useful data collected
included the number of wells
(often wells are located close
to structures because they
improve
groundwater
recharge),
areas
under
irrigation, numbers of irrigator
farmers and the physical status of the tank and command area structures.
Table 4.3.1 Results of the survey of tank and structures
Tank Name
Tank details
CatchStorage
ment area
capacity
(ha)
(ham)
No. of structures in tank catchment area
Tanks
Nalabunds
Checkdams
Others
Total
Total
“structure”
storage
capacity
(ham)
Battuvani Palli
1145
61.0
0
11
3
1
15
15.3
Manirevu
1192
55.6
0
5
1
4
10
8.1
Pathcheruvu
996
16.8
0
4
7
2
13
7.3
Obulapuram
3014
20.0
2
16
14
8
40
95.0
4.3.6. Modelling impact of structures on tanks
The impact of intensive water harvesting along drainage lines on tank inflows was
estimated using a simple “bucket-type” water balance model (see Figure 4.3.4). The
main aim of this model being to assess the affects of the additional storage created by
new water harvesting structures, including the impacts in years with different patterns
of rainfall. As it is commonly assumed that structures such as check dams help to
drought-proof villages, drier years were studied in particular detail.
Figure 4.3.4 The
water balance model
Runoff analysis was
carried out using data
from the surveys of the
catchment areas of the
tanks located within
the four focus villages.
Runoff estimates were
made using a version
of the SCS method that
has been modified for
Indian conditions (see Box 4.3.1). These calculations used an Excel-based spreadsheet
model with a daily time step, using 20 years of daily rainfall data.
Box 4.3.1 Runoff estimation using the SCS method
The SCS method for estimating runoff can be applied to small agricultural catchments. Although it was
developed originally using data obtained in the US, the method has been modified to suit Indian
conditions (e.g. Singh et al., 1990). The equation governing the relations between rainfall and runoff
used in the study reported here was: Q = (P – Ia)2 / (P – Ia) + S where, Q = actual runoff (mm); P = runoff generating rainfall (mm); S = maximum potential rainfall retention (mm); and I a = Initial abstraction
(0.1S and 0.2S for black and red soils respectively)
The maximum potential rainfall retention was calculated by the equation: S = (25400 / CN) -254
where, CN is the curve number taken from Singh et al (1990). (CN falls in the range 1-100. When CN
= 100 all the rain runs off and S = 0 and P = Q).
Runoff generating rainfall was calculated using the following equation: P = 0.7R – 21.2 where, R =
daily rainfall (mm).
The model made the assumption that water harvesting structures were uniformly
distributed around the tank catchment area and that all runoff that was not impounded
flowed into to the tank. Evaporation losses from structures were calculated using
potential evaporation data for open water and on the basis that water remains ponded
behind nala bunds and check dams for 21 and 7 days respectively. These figures
were based on visual observation and discussions with local people, NGOs and
relevant specialists. It was assumed that water that did not evaporate from behind
structures went to groundwater recharge. Evaporation and percolation losses for the
tank were estimated as 15% of the net inflow to the tank.
The main findings of this analysis were crosschecked against the perceptions and
knowledge of villagers and NGO staff living and working in the area. This model was
then used to analyse the affects of water harvesting on patterns and availability of
runoff in the catchment areas. Scenarios included simulations with and without the
existing water-harvesting structures, and scenarios with more structures.
4.3.7. Example results
Example modelling results are shown in Table 4.3.2. For Battuvani Palli,
Pathacheruvu and Obulapuram, the analysis shows that the presence of waterharvesting structures does significantly reduce average inflows to the tanks located in
these villages (by between 30 and 38%). There is less impact in Manirevu, where the
storage volume of the structures is a relatively smaller proportion of the total tank
storage volume than in the other villages. As well as reducing estimated average
inflows, we can see that water-harvesting structures may be expected to reduce the
number of times when a tank is full and spills, providing surface water to downstream
users.
Table 4.3.2 Results of modelling for scenarios ‘with’ and ‘without’ waterharvesting structures
Average annual volume of water
retained (ham)
Frequency of tank spillage during
a twenty year period
Frequency tank fills to >20%
capacity during a 20 year period
Without
structures
With
structures
%
change
Without
structures
With
structures
%
Change
Without
structures
With
structures
%
Change
Battuvani Palli
52
32
-38
4
2
-50
18
13
-28
Manirevu
63
60
-5
8
8
0
19
18
-5
Pathacheruvu
25
17
-32
13
11
-15
19
17
-11
Obulapuram
27
19
-30
13
11
-15
19
17
-11
Tank name
An alternative measure of impact is to identify in how many years the tank fills to a
level which provides significant water for recharge of village water supplies and/or
other activities. Using the Battuvani Palli tank as an example and an arbitrary level of
20% of capacity, we see that the presence of water-harvesting structures in this tank
catchment reduces the frequency with which this level is reached by between 5 and
28%. In other words, there are 1-5 more years within a 20 year spell when the 20%
storage level will not be reached, with potentially major impacts on tank utility in
these years.
Figure 4.3.5 illustrates the major differences in the impact of water harvesting
structures that can be expected between wet and dry years. In wet years, we see that,
in this example, the main tank and the structures within the catchment collect a similar
amount of water. The effect of the structures is certainly to spread the benefits of
harvested water more widely across the catchment in these wetter years. However, in
the dry year, the water harvesting structures in the catchment collect much more water
than the tank with potentially important impacts on the tank water users in dry years,
who may face increased vulnerability to drought.
Figure 4.3.5 Water harvested by a tank and smaller structures within the same
catchment during wet and dry years, Battuvani Palli
4.3.8. Lessons learnt
The assessments have shown that water harvesting programmes impact significantly
on patterns of water use and that this can result in distinct winners and losers.
Winners include people who have improved access to water for productive purposes
(e.g. irrigated agriculture) and losers include people whose access to water for
domestic, productive and other purposes is reduced. It is also clear that livelihood
gains experienced by some “winners” can dissipate as competition for water resources
increases and traditional drought coping strategies become less viable and/or
increasingly expensive.
The recommendation from the analysis presented here is that water harvesting should
be encouraged but within an integrated or adaptive water resources management
framework using procedures that weigh up the benefits and tradeoffs associated with
altered patterns of water use. The aim being identify potential unintended impacts so
that, if at all possible they are avoided altogether, but if these do occur, they are
recognised at an early stage and steps are taken to mitigate their affects.
4.3.9. Using the model
If you are interested in using the spreadsheet model and need further information,
please contact Charles Batchelor (see contributors section for contact details).
Read more
Batchelor, C., Singh, A., Rama Mohan Rao, M. S. and Butterworth, J. 2002.
Mitigating the Potential Unintended Impacts of Water Harvesting, in
International Regional Symposium on Water for Human Survival, Proceedings
of a conference held in New Delhi, 26-29 November, 2002.
WHiRL. undated. Using watershed development to protect and improve domestic
water supplies. Briefing note, India No. 3. Accion Fraterna, Anantapur.
Rama Mohan Rao, M. S., Batchelor, C. H., James, A. J., R., N., Seeley, J. and
Butterworth, J. A. 2003. APRLP Water Audit: Phase I Report. Andhra Pradesh
Rural Livelihoods Project, Hyderabad.
Other references and links
Batchelor, C. H., Rama Mohan Rao, M. S. and James, A. J. 2000. Karnataka
Watershed Development Project: Water Resources Audit. KAWAD Report 17.
KAWAD Society, Bangalore.
Singh, G., Venkataramanan, C., Sastry, G. and Joshi, B. P. 1990. Manual of soil and
water conservation practices. Oxford/ IBH Publishers, New Delhi.