INTRODUCTION
Selection
of sites for irrigation wells in micro watersheds in an undulated terrain is
some times difficult due to want of proper understanding of the hydrological
environments of the area. In a watershed the correct location of ground water
harvesting structure is the area where the main hydrological phenomenon is
natural exfiltration of ground water into the surface. The delineation of the
zone of infiltration and zone of exfiltration in a watershed is the fundamental
activity to be considered before site selection of any groundwater structure.
Naturally in a watershed, a line called the spring line, which is defined by
the natural spring points, separates these two zones. But due to current land
use practices in a well-managed watershed the spring points sometimes are not
very apparent in the field. In such it becomes very difficult to locate the
spring line as well as to demarcate the recharge and discharge zone.
The
field area has been selected in a micro watershed named Chagalkuta watershed
(2309’49” to 23014’30” N and 860 51’ to 86 55’
E) in the Chhatna and Indpur Block of Bankura district. The area of the
watershed is about 68 Sq.Km. and covers
about 29 revenue villages.
The area is basically a
monocrop area depending upon rain fed irrigation. Groundwater is exploited
through small diameter (> 1 m ) open wells for domestic purpose only. The
existing wells often go dry during the peak summer. Due to population growth
crop production has to be increased by application of irrigation from
groundwater in the lower reaches. To have efficient irrigation suitable site
selection is necessary.
In this paper firstly an
attempt has been made to define the spring line as well as the recharge and
discharge zone in a watershed with the help of simple G.I.S. technique of
contour crossing or by overlay of different contour maps. The fundamental
concept is that a spring occurs at a point where the groundwater contour
intersects the surface contour.
Secondly,
for pinpointing the location of prospective water harvesting structures like
irrigation dug wells Surfer Vector map of the watershed has been analysed.
And, thirdly, SWAT model is
applied to estimate the impact of groundwater development for irrigation on the
hydrological environment of the area. It is assumed that the watershed boundary
is a hydrological entity.
The
altitude of the watershed is from 94 m to 172 m AMSL. The terrain is adulatory and the general
slope is from south to north. A small stream named Chagalkuta flows through it
from south to north and meets Arkosa River , a tributary of Darakeswar River
GEOLOGY
Geologically the area is
granite terrain with intrusive
basic rocks and quartzite. Geomorphologically the watershed can be divided into
three regions viz. ridge, toe-slope and valley. The ridge is comprised of exposed basement rock with often a
thin cover of lateritic soil over the pediment. The toe-slope region has steep
slope and up to 1 m pediment depth. The valley is filled with moderately thick
sediment comprising of sand and silt. The pediment depth here is not above
three metres. Groundwater in this region occurs in fractures and fissures in
the hard rock and in the weathered mantle covering the hard rock. The total
thickness of this unconfined aquifer is 10 to 12 metres.
AQUIFER
Groundwater occurs in pore spaces in the weathered
mantle of the base rock and in the valley fill deposits. The thickness of this
unconfined aquifer is 7 to 10 metre. Soil depth is 15 cm to 1m.
DATA BASE-
Two types of data have been generated in this
area
A. Temporal data:
i)
Pre and Post monsoon depth to water level data from 56 selected
monitoring wells.
ii)
Meteorological data
B. Spatial data:
i) The reduced level data and
location of each well plotted in a map.
ii) Sufficient number of surface
altitude data required to generate a 1 mt. contour map of the watershed.
DETERMINATION OF RCHARGE AND DISCHARGE ZONE
METHODOLOGY
The
data have been processed in computer with the help of software “Surfer 7”.
At first the
following surfer files are created.
1.
Surfer boundary file (.bln) containing boundary of watershed and mouja
boundary.
2.
Spread sheets (.xls):
(a) “well.xls” containing the location of wells
(x-y coordinates), depth to water level and RL of water level (Z coordinates).
(b) “RL.xls” containing the reduced level data
of surface points.
In the second stage the
following grid files are created using Krigging method.
1.
“Pre-monsoon.grd “containing the grid of pre monsoon water table
contour data.
2.
“Post-monsoon.grd” containing the grid of post
monsoon water table contour data.
3.
“RL. grd containing” the grid of surface contour.
The grid files are blanked with “watershed.bln” so that
the contours remain within the watershed boundary. With the help of the grid
files the following contour maps are created.
1.
Pre monsoon water table contour map with 1mt. contour interval (Figure
II)
2.
Post monsoon water table contour map with 1 m contour interval
3.
Surface contour map with one meter contour
interval. (Figure I)
Now the pre monsoon contour map is
superimposed (overlayed) on the surface contour map. After superimposition, the
points of intersection of same value of surface contour with the same value of
the water table contour are marked. These points are the possible spring points
of the watershed during the pre-monsoon period. A line joining the points will
generate the spring line. Area lying at lower altitude of the spring line is
the discharging zone and the area at higher attitude is the recharging zone
Using SURFER/ GRID /MATHEMATICS this line can be
generated digitally.
The equation is: - Map A – Map B = Map C.
Where Map A =
Surface Contour map (RL.grd)
Map B = Water table contour map (premonsoon.grd/ postmonsoon.grd)
Map C = Recharge discharge zone map (Rechargedischarge.grd)
In this operation grid node of both maps
having same value will assign zero value.
In map C a isopach is generated where there is a
line with value ‘0’ (Zero) and some lines with (-Ve) and some (+V) value..
The isopach line with ‘0’
value is the spring line because along this line the altitude of land surface
and the water table is the same and the area with all +Ve value isopaches
indicates that the water level is above surface which means that the aquifer is
saturated and the area with all “- ve” value isopaches indicates that the water
level is below surface and here the aquifer is ready to accept recharging. So
the –ve area is the recharging zone and the +ve area is the discharging zone.
Thus the pre monsoon recharge /discharge zone map is generated (Figure IV).
Similarly a post monsoon recharge/ discharge map has also
been generated by overlay of the post
monsoon water table contour map and the surface contour map. In this map the
discharge zone is more extensive because the groundwater level has gone up
after the monsoon (Figure V)
HYDROLOGY OF DIFFERENT ZONES
Recharge Zone
In this zone rainwater directly percolates to the ground
and recharges the aquifer. The water table slope is towards the valley. The
water table intersects the land surface along the spring line and water comes
out as surface runoff.
Discharge Zone
Here groundwater comes out to the surface and adds
to the surface flow. In this region water is lost from the system by way of
surface flow, base flow and evapo-transpiration.
Intermediate Zone
This is the region between the pre and post monsoon
spring line. Here surface outflow is maximum during the rainy season and
minimum during the pre monsoon period.
CONCLUSION
Since the recharge /
discharge map indicates the spring zone, the zone of recharge and the zone of
discharge the map can be utilized for locating sites for water harvesting
structures.
The main phenomenon of the discharging zone is surface exfiltration loss
of groundwater or transmission of water back to the catchment..
If groundwater structures were located in the pre-monsoon discharging
region the structures would yield water throughout the year.
For further pin pointing the ground water harvesting structures, especially
for dug wells, some specific criteria are to be observed. Using SURFER a
surface vector map can be generated. This map shows the magnitude and direction
of slope. From this map it can be observed that the surface gradient is
divergent in some places and convergent in some other places. In the valley
region the slope is convergent. So location of wells can be pin pointed at the
points where the convergent gradient is found. In plate VII pin pointed well
locations are shown on a vector map. Similarly for surface water harvesting
structures like bunds and ponds should be located in the valley region of the intermediate zone
ACKNOWLEDGEMENT
Acknowledgement is due to the
School of Fundamental Research
REFERENCE
Beasley R. C. (1976). Contribution of
Subsurface Flow From the Upper Slopes of Forested Watersheds to Channel Flow,
Soil Sci. Soc. Am., Proc. 40: 955-957
Bonnel M., Cassells D.S. and Gilamour D. A.
(1982). Vertcal and lateral Soil Water Movement in a Tropical Rainforest
Catchment. First National Symposium on Forest
Hydrology. 30-38 Melbourne ,
Australia
Dominico P. A. Concepts and Models in
Groundwater Hydrology. MacGraw-Hill Inc. 1972
