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Recharge
Computation
Recharge to the
ground water body occurs through various sources like rainfall, canals,
surface water bodies, irrigated fields, water conservation structures
and other sources.
Recharge from rainfall The recharge from rainfall was computed
separately for monsoon period and non-monsoon period.
The recharge during the monsoon period was
computed based on water level fluctuation method and infiltration
method. Afterwards this was normalized using normalization method.
The recharge using water level fluctuation method
was computed based on the water level data collected by Central Ground
Water Board (CGWB) through 625 NHS (all open wells tapping phreatic
aquifer) spread over the State. The April measurement considered as the
pre-monsoon water level and the subsequent measurement was taken as the
post-monsoon measurement. The water level data of the last five years
were used (1994-1998). The average water level fluctuation of each NHS
was calculated separately. The area occupied by each formation was
calculated and the average fluctuation of the NHS wells tapping the
formation is taken. The specific yield values were taken as per the
recommended norms of the Committee. The specific yield values used for
calculation are given below in Table 3.1
Table 3.1 Specific
yield values used for recharge calculation
|
Sl.No. |
Formation |
Sp.Yield value (as
fraction) |
|
1. |
Sand |
0.16 |
|
2 |
Laterite |
0.025 |
|
3. |
Weathered crystallines
with less clay content |
0.03 |
|
4. |
Weathered crystallines
with significant clay content |
0.0 |
The recharge was computed
using the following formula as per 1984 methodology
R = h * Sy * A
where
R = Recharge of
ground water during monsoon in MCM
h = Rise in water
level in m in monsoon
Sy = Specific
yield value of the formation
A = Area occupied
by the formation in sq.km.
While computing the area (A),
the areas where the slope is more than 20% were excluded. Then the
recharge figures were obtained for each block
The recharge was computed in
this report using the following formula as per 1987 methodology
R = h * Sy * A + Dg
where
R = Recharge of
ground water during monsoon in MCM
h = Rise in water
level in m in monsoon
Sy = Specific
yield value of the formation
A = Area occupied
by the formation in sq.km.
Dg = gross ground
water draft during monsoon
Subsequently the
ground water recharge by rainfall infiltration method also was
calculated for the monsoon period. This was also done based on the
aerial extent of each formation in the block. Rainfall infiltration
factor recommended by the Committee was mostly used. However, the
experience gained in the field was used to refine the figures. The
values of rainfall infiltration used are given below in Table 3.2.
Table 3.2 Rainfall
infiltration values used
|
Sl.No. |
Formation |
Rainfall factor |
|
1. |
Alluvium |
0.10 |
|
2. |
Weathered rock |
0.08 to 0.05 |
|
3. |
Laterite |
0.07 |
The normal monsoon rainfall
figures were used for computing the recharge so that the variation of
rainfall is minimum.
Recharge assessment based on
rainfall infiltration factor is calculated using the formula
R = f * A*
Normal rainfall in monsoon season
Where
f = rainfall
infiltration factor
A = Area occupied
by the formation in sq.km.
Recharge figures computed
using the two methodologies were compared as below
((A-B)/B) * 100 = P.D.
where
P.D. is the percentage
deviation
A is the recharge by
water level fluctuation method
B is the recharge by
rainfall infiltration method
Depending upon the value of
P.D. the recharge was finalised as given below ;
If P.D. is between +20% and
-20% --- A is taken as the recharge
If P.D. is less than
-20% ---- 0.8 * B is taken as the recharge
If P.D. is greater than +20%
---- 1.2 * B is taken as recharge
Based on this formula the
monsoon recharge for each block was computed
Recharge from non-monsoon rainfall
The recharge by non-monsoon rainfall
was also computed by rainfall infiltration method. Here the field
information regarding the soil condition and the type of formations
underlined were used to compute the infiltration value and the recharge
was calculated to the best realistic side.
The total rainfall recharge was
obtained by adding the recharge figures of monsoon period and that of
non-monsoon period.
Recharge from canals
For computing the recharge from the canals of major irrigation projects,
lot of data such as the canal length, wetted perimeter, the details of
the water let out, period of flow etc. were required. Even though the
Irrigation Department was approached several times the data could not be
obtained. However, based on the data collected during 1992, an attempt
was made to compute the recharge for the Projects other than Neyyar,
Peechi and Chalakudi. The values were computed from the Ayacut, which
was distributed to each block according to the aerial extent.
Recharge from applied irrigation
The recharges
from the applied irrigation were computed for the area under paddy
crop. Other garden crops like coconut, were irrigated once or twice in
a week and the studies indicated that there may not be much water to go
as return seepage to ground water.
For computing the recharge
flow the area under paddy during summer and winter months were taken
from the block level statistics. The paddy grown during monsoon period
was not taken since the return flow will be negligible and more over the
aquifer itself is fully saturated due to rainfall recharge.
For computing the recharge flow the figures
obtained from the Kerala Agriculture University were used. However,
this value is tallying with the recommended figure given in the GEC
report. Then figures were computed block-wise.
Recharge through tanks and ponds
The recharge through tanks and ponds will be found low in Kerala
conditions since almost all the ponds and tanks are situated in the
topographic lows and their command area is very much restricted. Even
though an attempt was made to compute the figures, it could not be done
since the details on the tanks and ponds were not made available.
Recharge by Artificial Recharge structures and other sources
It is also felt that this will be negligible in the state of Kerala and
will not be of so much influence. However, as per the data available in
the Region about the AR structures, recharge has been computed.
Total annual recharge
The total annual recharge is obtained as the sum of recharge in the
monsoon and non-monsoon season, where in each season, the recharge
comprises recharge from rainfall and recharge from other sources.
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