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GIS application in hydrogeological studies
Data base and structure : Since most of the groundwater related data would be available from pumping wells/bore hole logs they would be point information, which could be interpolated to get spatial data (i.e.) each gridcell would have valid data without any gaps. Point data could be water level, weathered zone thickness, saturated zone thickness, yield in the wells, rainfall at various rain gauge stations porosity of aquifer material, Transmissivity (T) and Storage co-efficient (S) depending on the type of porous media from the pumping test data etc. Bore hole and geophysical sounding data can be interpolated to prepare aquifer basement map. In order to assess to groundwater prospect by qualitative modelling, data on geomorphology, geological structures, lineaments would also be required in addition to weathered zone thickness, saturated zone thickness and yield in the wells. Lineaments being line data has to be converted into a spatial data by finding the lineament density on a coarse grid say, 1 km x 1 km. In order to assess the suitability of groundwater quality for irrigation, drinking or industrial purpose, Electrical Conductivity (EC), pH, Sodium Absorbtion Ratio (SAR) data etc has to be colected from the monitoring wells and interpolated to get spatial data (Jacob et. al. 99) Groundwater Assessment
 Supply - Demand analysis Hydrogeologic studies : Aquifer recharge occurs in nature by rainfall, seepage from canals and reservoirs and return flow from irrigation. The geomorphic features like alluvial fans buried pediments, old stream channels and the deepseated interconnected fractures are the indicators of subsurface water accumulation (Mukherjee and Das, 1989). These features are the natural recharge sites due to their high permieability and water holding capacity, moreover it is clear that higher the permiability lower the drainage density and higher the drainage density higher the surface run off. It has been observed that the terrain transmissibility is inversely proportionate to the square of drainage density (Omar, 1990). Das and Kader (1996) observed that combined effect of drainage density (01.15-14.76 km/sq km), stream frequency (0.95-12.11), bifurcation ration (2-10) and granitic lithology favours high surface runoff and low infiltration. Satellite imageries of Visible (0.38 um-0.72 um) to near infrared (0.2um-0.2um) region of electromagnetic spectrum are very much useful in extracting information on aerial aspects of drainage basin and various hydrogeomorphic features. The National Remote Sensing Agency, Govt. of India (1989-1990) under the auspices of the National Technology Mission for Drinking Water and with the active collaboration of State Departments has prepared hydrogeomorphological maps (scale 1:2, 50,000) for the whole of India, utilizing landsat TM/IRS satellite imageries. The identification of lineaments has immense importance in hard rock hydrogeology as they can identify rock fractures that localize groundwater (Das, 1990). The hydrogeolosist usually infer subsurface hydrological condition through surface indicators, such as aerial geological features, linear structures. Most of the geological linear features are assumeed to be the zone of fractured bed rocks and the position of porous and permeable state where enhanced well yields can be expected (Das, 1997). Scientists observed that yields of wells on lineaments are about 14 times than that of wells away from lineaments in the case of Gondwanas, Warangal district, A. P. India, Parizek (1976) and Lattmen and Parizek (1964) have shown that wells located on fracture traces (lineaments) in the lower Palaeozoic Carbonic rocks of Pennsylvania yielded about 10-100 times morewater than wells located in similar condition but away from fracture traces (Shankar Narayana et al., op. cit). Image rectification and preparation of a GIS file through visual interpretation of standard FCC data is performed to extract surfacial expression of subsurface water accumulation. Edge enhancement is done mainly for structural interpretation, Vegetal cover, landuse, lithology and structural lineaments influence infiltration of water into subsurface condition (Perry et al., 1988). Hydrogeoloical landuse classification through supervised classification technique provides very good thematic information regarding different types of hydrogeomorphic features including vegetation pattern, burried channels, flood plain areas, shallow to deep surface water bodies, surface drainage and areas with moisture content, dry sandy soil or surface material with higher reflectance (Das, 1991). Band rationing gives vegetation index map (Jensen, 1986). Spectral band (0.6-0.7um) gives valuable information regarding fracture pattern in the rock anddrainage pattern in the study area (Das, op. cit). Observations from the satellite data must be complemented by field checks, and existing geologic maps, topotgraphic sheets are very much useful as supplementary data sources. Geophysical survey and test drilling are also helpful for determining transmissivity and storativity in relation to artificial recharge of subsurface water (Mukherjee and Das, 1996). Data integration and composite map generation may be performed through GIS technique (1:50,000 scale). Delineation of pertinent area (such as open deepseated fractures, weathered residuum, alluvial fans, old channel courses etc.) in the composite map is one of the most desired task for groundwater development, for construction of artificial recharge structure and for surface water storage augmentation (Geomorphic depression with impermeable layer) by water impounding structure. This sort of integrated study is usually undertaken watershedwise (Das, 98). |