We had
good rainfall (1000mm plus on an average) in 2015. Yet, this summer was harsh
for everyone. Would it have helped if we had harvested rainwater? The primary analysis
from the project area (Yamalur watershed) observes that there is enough water
available for everyone if we do rainwater harvesting leaving excess water to go
to the lakes and/or groundwater recharge.
A watershed is
the area of land where all of the water that falls in it and drains off of it
goes to a common outlet. Biome Trust has been involved in an action research
project called Particpatory Groundwater Mapping in a Yamalur watershed of an
area 33.81 sq.km.
Now if we
overlap the watershed area over a Google map and the administrative boundary
map i.e. BBMP ward map; we would see overlaps with the existing BBMP wards.
This helped us in understanding that this watershed consists of 6 BBMP wards-
HAL Airport, Bellandur, Marathahalli, HSR layout, Singasandra, Begur, Mangmanapalya
and some non-BBMP region still under village Panchayats viz. 4 villages in Halanayakanahalli Gram Panchayat
(Halanayakanahalli, Chikkanalli, Chikkanayakanahalli, Hadosiddapura),
Rayasandra, Choodasandra, Kodathi.
Rainfall data
is very critical for it gives an idea of water falling in an area (inflow), how
much is percolating, evaporation percentage, etc. The rainfall related data
collection involved contacting Karnataka State Natural Disaster Monitoring
Center (KSNDMC). KSNDMC has installed raingauges in 89 BBMP wards so far and
almost all village panchayats have these raingauges. We managed to get 2015
rainfall data (daily and monthly) from KSNDMC except Mangmanapalya (where there
is no raingauge and so have used closest ward Bommanahalli for the analysis)
and villages Rayasandra, Choodasandra, Kodathi. We’ll keep collecting this
information. However, some primary analysis throws some interesting results:
Raw data from
KSNDMC:
Month
|
HAL Airport
|
Bellandur
|
Marathahalli
|
HSR Layout
|
Bommanahalli
|
Singasandra
|
Begur
|
Halanayakanahalli
GP
|
Jan
|
3
|
0
|
3
|
2.5
|
3
|
2
|
5.5
|
24
|
Feb
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
Mar
|
42.5
|
43.5
|
44
|
36.5
|
45
|
77.5
|
16.5
|
0
|
Apr
|
185
|
118.5
|
128.5
|
139.5
|
129
|
127.5
|
102.5
|
111.5
|
May
|
100
|
64.5
|
75
|
102
|
92
|
78
|
100.5
|
73.5
|
June
|
105
|
132.5
|
103.5
|
77
|
83.5
|
96.5
|
100
|
120.0
|
July
|
68.5
|
55.5
|
59
|
58
|
58
|
49.5
|
47.5
|
55.5
|
Aug
|
131.5
|
187
|
96.5
|
169.5
|
186.5
|
184.5
|
138
|
186.5
|
Sep
|
175.5
|
210.5
|
164.5
|
163.5
|
137.5
|
175
|
175
|
171.5
|
Oct
|
144
|
148
|
170.5
|
121
|
80
|
117.5
|
105
|
160.0
|
Nov
|
269.5
|
200.5
|
186.5
|
215.5
|
229
|
182.5
|
171
|
201.5
|
Dec
|
7.5
|
7.5
|
3.5
|
3
|
3
|
7
|
9.62
|
9.5
|
Total
|
1232
|
1168
|
1034.5
|
1088
|
1046.5
|
1097.5
|
971.12
|
1113.5
|
Some quick
observations:
- On an
average it rained 1093.89 mm across the watershed with highest annual rainfall recorded
at HAL airport station (1232 mm)
- The
lowest was recorded in Begur ward station @971.12 mm
- November
being the rainiest month with 200 mm rainfall on average, followed by
September with 171 mm and April and October tied at 130 mm
Total
area of the watershed
|
33.81
|
sq
km
|
33810000
|
sq
m
|
Total
annual rainfall in 2015
|
1093.89
|
mm
|
1.093
|
m
|
Total rainfall endowment
|
36954330
|
cum
|
36954330000
|
L
|
Per
capita requirement
|
150
|
LPCD
|
|
|
Total
residential population
|
123780
|
|
|
|
Total
residential water demand
|
18567000
|
LPD
|
6776.955
|
ML/Yr
|
Demand
as rainfall
|
200.4423248
|
mm
|
|
|
Sustainability/Difference
between actual rainfall and annual water demand
|
893.4476752
|
mm
|
This
much rainfall is available/in excess for the population in the
watershed. So if everyone does rainwater harvesting there is enough rainfall for
a year for this much residential population
|
|
Assumption
is that all the rainfall falling is harvested. Rainfall
distribution is uniform in the ward.
|
The
per capita requirement in urban area is always found to be more than 150LPCD
from our experience.
|
Commercial
establishments are not considered in this calculation
|
Based on this
data, we want to understand some more details:
·
Runoff available- Based on the land use understanding
this information would be easy to obtain
·
Recharge- how much of the runoff can be used for
recharge, how many wells would be essential
·
Recharge and borewell yields- to determine the
efficiency of shallow groundwater recharge and correlation with borewell yields
(if any). So we are collecting Static water level (SWL) data from many
borewells in the watershed. We use
manual
and
automatic
sensors to facilitate the data collection
·
Spatial distribution of rainfall- Bangalore
rainfall is varying at the spatial scale so understanding which areas are the
high rainfall/low rainfall would help
·
Microwatershed level rainfall analysis. This
yamalur watershed consists of 8 micro-watersheds
We are still
learning to analyze this data and would like to hear from you on how best to
put this data to use. We would also like to use better visualization. This is what the
Participatory Groundwater mapping action
research project is attempting to do.
