DEM for locating areas for groundwater recharge

DEM for locating areas for groundwater recharge

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Nitin Bhardwaj
Department of Geology,
University of Delhi, India
[email protected]

The National Capital Territory of Delhi, India is plagued with the problem of dwindling groundwater resources. This paper shows the usefulness of Digital Elevation Models in the identification and location of suitable areas for artificial water recharge

The area selected for the present study encompasses western and southwestern parts of NCT – Delhi, and comes under the catchment of Sahibi River (Fig 1). In general, the terrain has undulating topography with regional slope towards northeast. This region is beset with typical hydrogeological problems of semi-arid region, such as rapid depletion of fresh groundwater resources and increasing salinity in shallow aquifer zones. These problems further become more complicated due to heterogeneity in the aquifer material. Fresh groundwater resources in the area are scarce and are available only in the vicinity of Najafgarh drain (stretch between Dhansa and Kakraula, Fig 1) and that to a limited depth of around 20m. Sustainable fresh groundwater availability in the area can only be expected if the shallow fresh water aquifers are regularly recharged over a vast region. In order to find the remedial measures it has become essential to identify suitable areas, where large-scale artificial recharge can be done. Also, as the amount of recharge is sensitive to geomorphic situation, it is imperative to relate the same with the areas of recharge. The present study is an attempt to map such areas using geospatial information provided by Digital Elevation Models (DEM) in conjunction with subsurface data of the region.

Methodology
The methodology adopted in the study is as follows: First, a DEM (Fig 2) of the area was generated with the help of MAPINFO V.6.0 GIS software using elevation data provided in Survey of India’s (SOI) toposheets. Drainage network of the area was also digitized using the same SOI toposheets and was superimposed on the DEM (Fig 3) for further analysis. Secondly, hydrogeological logs (Fig 4) from several sites situated within different geomorphic zones are prepared to correlate the changes in topography and geomorphology with that of subsurface material. Then from the analysis of DEM, drainage network and hydrogeological logs, suitable sites for recharge have been located.

Generation of Digital Elevation Model
For generating the DEM, topographic contours were digitized using 1:50,000 scale toposheets (53H/2, 53H/1, 53H/3, 53D/13, 53D/14 and 53D/15) having contour interval of 10m. Though the 10m contour interval is enough for southeastern part of the study area where topography is hilly (structural ridges), it is however insufficient for the rest of the area where it is subdued (alluvial plain occurring mostly between 210m to 220m above MSL). Hence in order to enhance minor topographic variations 1:25,000 scale toposheets (53H/2-SW, 53H/2-SE, 53H/2-NW and 53H/2-NE) having 5m contour interval were also used to digitize the intermediate contours. All these contours represent the elevation ranging from 205m to 300m above MSL. The contours were then overlain by a grid of 0.005-degree interval. Contour values are then correspondingly assigned to the overlapping grid cells with the IDW (Inverse Distance Weightage) interpolation method. Based on this grid, a colour coded DEM was generated (Fig 2), which classifies the study area into seven main topographic zones, i.e. areas occurring below 205m (dark blue zone), between 205m to 210m (medium blue zone), 210m to 212m (light blue zone), 212m to 215m (green zone), 215m to 230m (yellow zone), 230m to 260m (orange zone) and 260m to 300m (red zone) respectively.

Analysis of Digital Elevation Model
The DEM (Fig 2) provides a detailed topographic picture of the area. In general, the southeastern part of the area shows