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 PHYSIOGRAPHICAL SETTING OF THE WATERSHED
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 .
The 3D model of the watershed is shown in Fig1. Darakeswar River
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.
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.
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
The data have been processed in computer with the help of software “Surfer 7”.
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 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
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.
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.
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.
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 is due to the
, Kolkata for giving
permission to use their data in preparation of this paper. School of Fundamental Research
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