chapter 8 subsurface water conditions -...

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Chapter 8

Subsurface Water Conditions

8.1 Justification of the Study

Seasonality of rainfall is the major character of Indian monsoon. So, it is considered

as the barrier against multi cropping all through the years. But to meet the need of the

growing demand of the crops, multi cropping practices in the limited land resources is

necessary. Analysis of subsurface water condition may create proper database

regarding ground water availability, water yield capacity, trend of ground water

status, spatial pattern, potentiality for ground water use etc. In the extreme upper and

lower catchments of Kuya river basin, where, there is no provision of canal water

irrigation, for non monsoon agriculture, people have to depend on the ground water.

In this context, this chapter is relevant.

8.2 Ground Water Table Lowering Dynamics

8.2.1 Long Term Dynamics

Satyakam Sen in his article “The Importance of Drainage in Agriculture of

West Bengal” reportedly mapped the water table as per the survey of May, 1965 and

shown that average water table depth was 6.09 m. at Kandi block which is at the

confluence stream of Kuya river. But, on May, 2011, average water table depth is

about 14 m. So, the rate of water level lowering is about 18cm. per year which is

Major Themes:

8.1 Justification of the Study 8.2 Ground Water Table Lowering Dynamics 8.3 Spatio-temporal Water Yield Dynamics 8.4 Relation Between Ground Water Table (GWT) and Ground Water Yield (GWY) in the Lower Catchment

8.5 Methods for Ground Water Estimation

8.6 Topographical Control on Ground Water Level

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threateningly high. Similarly, water level was very near to surface (within 2 m.)

during monsoon, but at present water level has lowered down to about 6.5 m. on the

said period (As per the records of SWID and Pal, 2010).

8.2.2 Seasonal Dynamics

The table (Table 8.1) describes the block wise variation of ground water level during

monsoon and pre-monsoon periods for 1990 and 2011.

Water table depth has rapidly declined since 1990 to 2011. Growing water lifting

from underground storage is largely responsible for such steady water lowering rate.

Table 8.1: Average Ground Water Level in Different Periods (Below Ground

Level or bgl.); value in m. Block 1990 2011 Change of GWL

Since 1990-2011 Yearly Rate of GWL

Lowering Ground Water Level (bgl, m.)

Ground Water Level (bgl, m.)

Sept May Sept May Sept May Sept May Kundhit 2.05 3.2 4.8 6.2 -2.75 -3 -0.14474 -0.15789 Suri II 2.17 3.3 5.1 6.37 -2.93 -3.07 -0.15421 -0.16158 Rajnagar 3.15 7.11 4.5 7.35 -1.35 -0.24 -0.07105 -0.01263 Khayrasole

1.23 5.28 2 7.6 -0.77 -2.32 -0.04053 -0.12211

Dubrajpur 3.62 7.86 3.9 8.43 -0.28 -0.57 -0.01474 -0.03 Illumbazar

1.56 4.54 4.83 7.19 -3.27 -2.65 -0.17211 -0.13947

Bolpur 1.55 5.9 2.4 6.18 -0.85 -0.28 -0.04474 -0.01474 Nanoor 4.76 7.13 5.24 9.47 -0.48 -2.34 -0.02526 -0.12316 Labhpur 3.21 5.84 4.6 6.2 -1.39 -0.36 -0.07316 -0.01895 Kandi 3.5 7.5 6.5 14 -3 -6.5 -0.15789 -0.34211 Burwan 2.2 8.1 10 15 -7.8 -6.9 -0.41053 -0.36316 Bharatpur 1

3.5 7.5 11.23 14.4 -7.73 -6.9 -0.40684 -0.36316

Ketugram 3.4 6.8 8.3 11.78 -4.9 -4.98 -0.25789 -0.26211 Sainthia 2.2 4.4 5.6 6.9 -3.4 -2.5 -0.17895 -0.13158

Source: SWID, West Bengal and calculated by the researcher.

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Fig. 8.1

Source: Information collected from SWID, Govt. of West Bengal

Fig. 8.2


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Fig. 8.3


Fig. 8.4


201

Fig. 8.5


Water lowering rate is abnormally high in the downstream catchment of the basin

(Table 8.1). Intensive ground water based agriculture has created this situation of fast

rate ground water level lowering (see fig. 8.2 to 8.5). It is expectable that in the

downstream catchment, the water table will be very nearer to the surface and

potentiality is very high. Expectedly, the ground water potentiality is very high in the

lower catchment but unexpectedly the lifting rate is also too high. Therefore,

juxtaposition of these two conflicting situations is observed in this portion.

Interestingly, even during monsoon season (September), the ground water lowering

rate is excessively high in between 1990 to 2009. It means amount of water lifting

from subsurface aquifer during non monsoon season is not replenished in every

monsoon season.

8.3 Spatio-temporal Water Yield Dynamics

As per the report of Centre Ground Water Board (2001) average ground water

potentiality of downstream catchment ranges from 45 to 55 liters per second (lps.). In

Hizole wetland the water potentiality is relatively high.

As per field survey report average yield of the mini deep tube well is 4.75lts/sec. to

5.25lts./sec. in the downstream catchment but yielding capacity is relatively less in the

middle and upper catchments. Depth of bore well for most of the deep tube wells are

in between 40m. to 50m. Ground water level change is so significant in the lower

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segment of the river, therefore more attention has been paid to this section during

study.

8.4 Relation Between Ground Water Table (GWT) and Ground Water Yield

(GWY) in the Lower Catchment

Study of this relation is highly needed for the analysis of the agricultural status of the

river basin, impact of these will be discussed in detail in chapter 9.

There is a negative relation between GWT depth and water yield both during

monsoon and pre-monsoon periods. The values of co-relation co-efficient are –0.54

and –0.465 respectively during monsoon and pre-monsoon periods. A relationship

also being drawn between bore well depth and pre-monsoon yield where ‘r’ value

indicates 0.64 means if depth of bore well can be increased water yield will obviously

be increased or otherwise.

In regard to pre-monsoon water table depth and water yield it has found that about

38.72% mini deep tube well has yielding rate between 4.5 to 4.75 lps. another 38.71%

of deeps have water yielding capacity between 4.75 to 5.0 lps. and water yielding

volume of the rest portion of the mini deep tube-wells range between the range of

5.00 to 5.25 lps.

However, in spite of having very good feasibility to have a potential ground water,

there is a lack of seasonal water storage consistencies. Such a large scale water table

lowering from monsoon to pre-monsoon is threatened enough. So, to protect such

tendency, estimation and planning of ground water resource is necessary.

8.5 Methods for Ground Water Estimation

Ground water balance can be estimated following Chaturvedi and modified and

remodified Chaturvedi’s equations (Chaturvedi 1973, Report of GWEC,1997, Kumar &

Seetapathi 2002, Pal, 2011). Similarly, it can be calculated from ground water

balance equation (Ri+Rc+Rr+Rt+Si+Ig = Et+Tp+Se+Og+∆s).

Where,

Ri = recharge from rainfall. Rc = recharge from canal seepage

Rr = recharge from field irrigation Rt = recharge from tank

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Si = influent seepage from rivers Ig = inflow from other basin

Et = evapotranspitation Se= effluent seepage to rivers

Tp = draft from ground water Og=outflow to the other basin ∆s = change in ground water storage

A part of the rain water that falls on the ground, is infiltrated into soil. This infiltrated

water partially used to fill up the soil moisture deficiency and part of it is percolated

down reaching the ground water table. The water reaching the aquifer is known as the

ground water recharge from rainfall. Recharge due to rainfall depends on various

hydrological, topographical, geological, pedological, anthropogenic characteristics

and depth of water table or like.

The following empirical relationship (similar to Chaturvedi’s formula) has derived by

fitting the estimation values of rainfall recharge and the corresponding values of

rainfall in the monsoon season through the non linear regression technique.

R= 1.2(p – 13)0.5 ------------- (4)

Where, R= ground water recharge from rainfall during monsoon

p= mean rainfall in monsoon

Table 8.2: Ground Water Recharge Status of Different Years.

Year Rainfall in monsoon season (cm.)

Ground water recharge (water balance) Recharge co-efficient

1999 145.80 21.34 0.1463

2000 130.07 20.508 0.1576

2001 82.24 13.414 0.1631

2002 108.17 16.161 0.1494

2003 139.22 18.869 0.1355

2004 98.84 13.72 0.1388

2005 106.58 16.53 0.1550

2006 159.02 21.104 0.1327

2007 157.71 20.93 0.132

2008 132.7 19.25 0.145

2009 135.8 20.31 0.149

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2010 116.6 14.26 0.122

2011 152.1 20.58 0.135

Source: Rainfall from Kandi and Santiniketan and recharge and recharge coefficient calculated by researcher

Table 8.3: Rainfall Recharge Calculation Using Ground Water Balance and Chaturvedi’s Formula (Recharge value in cm.)

Year

Rai

nfal

l rec

harg

e fro

m

grou

nd b

alan

ce st

udy

(cm

) Chaturvedi formula Modified

Chaturvedi formula Remodified Chaturvedi

formula

Rai

nfal

l rec

harg

e

R=2

.0 (P

–15)

0.4

Rel

ativ

e Er

ror (

%)

Rai

nfal

l rec

harg

e

R=0

.83

(P–1

5.28

)0.76

Rel

ativ

e Er

ror (

%)

Rai

nfal

l rec

harg

e

R=1

.2 (P

–13)

0.5

Rel

ativ

e Er

ror (

%)

1999 21.34 22.74 6.560 27.46 29.05 20.30 4.87

2000 20.508 21.34 5.074 24.33 18.636 18.83 8.18

2001 13.414 15.914 18.638 13.837 3.153 13.414 0

2002 16.161 19.147 18.476 19.75 22.207 16.577 2.58

2003 18.869 22.175 17.519 26.172 38.703 19.708 4.44

2004 13.72 18.084 31.807 17.70 29.008 15.514 13.08

2005 16.531 18.973 14.776 19.412 17.427 16.40 0.00

2006 21.104 23.818 12.860 30.006 42.181 21.466 1.715

2007 20.93 23.715 13.30 29.759 42.187 21.35 2.033

2008 19.25 23.94 24.36 31.86 20.62 7.11

2009 20.31 24.29 20.08 32.49 20.97 3.24

2010 14.26 21.97 54.06 26.79 18.68 30.99

2011 20.58 26.03 26.48 37.10 22.73 10.44

Source: Rainfall from Kandi and Santiniketan and recharge and recharge coefficient calculated by researcher

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Fig. 8.6

8.5.1 Rain Fall Recharge Condition during Monsoon Period

Being a Monsoon dominated country; seasonality of rainfall is evident feature. Most

of the concentration of rainfall occurs during five months (June to October) of

monsoon spell and rest periods almost remain rainless. That is why most of the

recharge happened during monsoon span. Since 1999 to 2011 monsoon rainfall ranges

from 82 cm. to 159 cm. So, fluctuation level is high enough. In response to monsoon

rainfall, recharge volume is not very high which is ranging from 13.41 cm. to 21.46

cm. which is calculated by re-modified Chaturvedi’s equation. Recharge value is only

13% to 16% of total monsoon rainfall. The recharge value dynamics is also very high

as indicated by recharge co-efficient. Whatever the volume of recharge, it is directly

related with rain fall amount in this basin as signified by r value (0.94) and this value

is significant at 0.01 confidence level.

Although favourable ponding time of water recharging, mild ground slope, thick

alluvium beds are present in the lower part of the basin but so many other constricted

situation like highly sticky soil with less degree of permeability, lack of vegetation

coverage, presence of impermeable hard pan layer below this zone etc. resist full

fledge water penetration.

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Table 8.4: Recharge Co-efficient During Different Years.

Year

Rainfall in monsoon season (cm.)

Ground water recharge (water balance)

Re-modified Chaturvedi’s equation

Recharge co-efficient in regard to water balance

Recharge co-efficient in regard to re-modied Chaturvedi’s equation

1999 145.80 21.34 20.30 0.1463 0.1392

2000 130.07 20.508 18.83 0.1576 0.1447

2001 82.24 13.414 13.41 0.1631 0.1619

2002 108.17 16.161 16.57 0.1494 0.1351

2003 139.22 18.869 19.70 0.1355 0.1415

2004 98.84 13.72 15.51 0.1388 0.1532

2005 106.58 16.53 16.40 0.1550 0.1538

2006 159.02 21.104 21.46 0.1327 0.1349

2007 157.71 20.93 21.35 0.132 0.1354

2008 132.7 19.25 23.94 0.145 0.180

2009 135.8 20.31 24.29 0.178 0.178

2010 116.6 14.26 21.97 0.188 0.188

2011 152.1 20.58 26.03 0.135 0.135

Source: Rainfall data is collected from Kandi Meteorological Station and others are

compiled by the researcher.

8.5.2 Recharge Condition During Non Monsoon Period

During non monsoon period, most of the years show complete absence of recharge

because rain fall in these periods never exceeds 13 inch or 42.64 cm. threshold limit

as per the requirement of the equation. Moreover, rainfall is so infrequent over

November to May, evaporation always exceeds rain fall intensity. Only during 2000,

2002 & 2011 recharge were taken place which is only ranging from 4 cm. to 10 cm.

due to greater concentration of rainfall.

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Table 8.5: Ground Water Recharge During Non monsoon Period of Different

Years.

Year Non monsoon rain Water recharge as per ground water balance equation

Recharge co-efficient

2000 37.46 cm. 4.02 cm. 0.1073

2002 60.58 cm. 10.03 cm. 0.1655

2011 61.45cm. 10.12 0.1646

8.6 Topographical Control on Ground Water Level

Drainage frequency negatively controls ground water level in all periods but the

degree is very marginal and not significant.

Table 8.6: Correlation Matrix between Morphometric Variables and Ground

Water Level

Variable 1 2 3 4 5 6 7 8 9 1 1 2 .606(*) 1 3 .355 .428 1 4 .395 .712(**) .831(**) 1 5 -.037 -.240 -.167 -.079 1 6 .117 -.151 -.111 .061 .900(**) 1 7 -.263 -.380 -.571(*) -.547(*) .392 .151 1 8 -.308 -.379 -.734(**) -.671(**) .281 .034 .863(**) 1 9 -.129 -.427 -.808(**) -.868(**) -.122 -.204 .542(*) .737(**) 1

* Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.01 level (2-tailed). 1.GWL,Sept,, 1990; 2. GWL,May, 1990 3. GWL,Sept,, 2009; 4. GWL,May, 2009; 5. Drainage

Frequency; 6. Drainage Frequency; 7. Drainage Texture; 8. Relative Relief; 9. Dissection Index

The control of relative relief and dissection index on ground water level is more and

significant. Relative relief, dissection index are more in the upper part of the basin

where ground water level is less and vice-versa.

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8.7 Major Findings

As per the survey of May, 1965, average water table depth was 6.09 m.

at Kandi block which is at the confluence Stream of Kuya River. But, on

May, 2011, average water table depth is about 14 m. So, the rate of water

level lowering is about 18cm. per year which is threateningly high.

There is a negative relation between GWT depth and water yield both

during monsoon and pre-monsoon periods. The values of co-relation co-

efficient are –0.54 and –0.465 respectively during monsoon and pre-

monsoon periods.

As per field survey report average yield of the mini deep tube well is

4.75lts/sec. to 5.25lts./sec. in the downstream catchment but yielding

capacity is relatively less in the middle and upper catchments.

In response to monsoon rainfall, recharge volume is not very high which

is ranging from 13.41 cm. to 21.46 cm. using re-modified Chaturvedi’s

equation. Recharge value is only 13% to 16% of total monsoon rainfall.

The recharge value dynamics is also very high as indicated by recharge

co-efficient. Whatever the volume of recharge, it is directly related with

rain fall amount in this basin as signified by r value (0.94) and this value

is significant at 0.01 confidence level.

chapter 8 subsurface water conditions -...

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