Department of Environmental Science, University of Kalyani
W.B. – 741235, India
GIS is a tool for storing, manipulating, retrieving and presenting both spatial and non-spatial data in a quick, efficient and organised way. Since most land information elelments have a geographic connotation, geographically referenced data with GIS techniques come to the fore in such and application. The term ‘geographic’ in GIS refers to the locational attributes which define the spatial positioning of the piece of information on the face of the earth. Preparation and maintenance of data in the form of maps and referenced tabular files itself can be considered as a primitive form of GIS. However, with the advent of digital computers, with high data processing speed and the development of analytical tools thereon to handle geographically referenced data with ease and flexibility, computer aided GIS has become a reality of late. Such systems generally deal with data classified/segregated into the spatial type (locationally referenced), attribute type (without locational connotation) and the time variant or repetitive types of data. The three components-location, attributes and time-represent the content of most GIS. Using of GIS in hydrogeology is only at its beginning, but there have been successful applications that started to develop. As an example there is the Sali river basin, Bankura district, West Bengal, where sites for groundwater exploration and artificial recharge have been demarkated applying GIS technique using ARC/INFO and ILWIS 2.1 Software.
Key Words: GIS, Hydrogeology, ARC/INFO.
Groundwater resources are dynamic in nature as they grow with the expansion of irrigation activities, industrialization, urbanization etc. As it is the largest available source of fresh water lying beneath the ground it has become crucial not only for targeting of groundwater potential zones, but also monitoring and conserving this importanat resource. The expenditure and labour incurred in developing surface water is much more compared to groundwater, hence more emphasis is placed on the utilization of groundwater which can be developed within a short time. Besides targeting groundwater potential zones it is also important to identify suitable sites for artificial recharge usage cycle. When the recharge rate cannot meer the demand for water, the balance is disturbed and hence calls for artificial recharge on a country wise basis (Sameena et. al. 2000).
Remote sensing with its advantages of spatial, spectral and temporal availability of data covering large and inaccessible areas within short time has become a very handy tool in assessing, monitoring and conserving groundwater resources. Satellite data provides quick and useful baseline information on the parameters controllig the occurrence and movement of groundwater like geology, lithology/structural, geomorphology, soils, landuse/cover, lineaments etc. However all the controlling parameters have rarely been studied together because of non-availability of data, integrating tools and modeling techniques. Hence a systematic study of these factors leads to better delineation of prospective zones in an area which is then followed up on the ground through detailed hydrogeological and geophysical investigations. Visual interpretation has been the main tool for evaluation of grounwater prospective zones for over two decades. It has also been found that remote sensing besides helping in targeting potential zones for groundwater exploration provides inputs towards estimation of the total groundwater resources in an area, the selection of appropriate sites for artificial recharge and the depth of the weathering area. By combining the remote sensing information with adequate field data, particularly well inventory and yield data, it is possible to arrive at prognostic models to predict the ranges of depth, the yield, the success rate and the types of wells suited to various terrain under different hydrogeological domains. Based on the status of groundwater development and groundwater irrigated areas (though remote sensing), artificial recharge structures such as percolation tanks, check dams and subsurface dykes can be recommended upstream of groundwater irrigated areas to recharge the wells in the downstream areas so as to augment groundwater resources.
Apart from visual interpretation, digital techniques are used by many researchers for deriving geological, structural and geomorphological details. The various thematic layers generated using remote sensing data like lithology/structural, geomorphology, landuse/cover, lineaments etc, can be integrated with slope, drainage density and other collateral data in a Geographic Information system (GIS) framework and analysed using a model developed with logical conditions to derive at groundwater zones as well as artificial recharge sites. Digital enhancement techniques are found to be suitable since they improve the feature sharpness and contrast for simple interpretation. This paper addresses the strategies for an integrated approach of remote sensing and GIS for groundwater targeting, management and conservation of groundwater resources that ensures optimum and judicious use of groundwater and in identification of artificial recharge sites. 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)
- Qualitative model — Groundwater prospects
- Quantitative model — Groundwater quantity – estimation of safe yield
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). GIS Analysis :
Themetic maps on Geomorphology lineament, drainage (3rd order), land cover have been prepared using IRS data (1,50,000 Scale) aided by field checks. Data sets like
(i) depth of Weathered zone (well inventory method)
(ii) drainage design
(iii) lineament density have been generated as collecteal data. Digitization and overlay analysis have been performed to show sites
for groundwater development a constitution of artificial rechange structure cross analysis among linearment density, hydrogeomorphology drainage density (high, medium), land cover raster maps show pertinent results revealing sites for groundwater exploration and artificial recharge sites. DEM has been generated to show the general terrain condition.
Apart from direct benefits, space technoloy has clearly demonstrated its usefulness in understanding the factors responsible for maintaining the hydrological cycle, mainly the vegetel cover, surface water bodies, lithotypes and landform. Supply of groundwater even in the dry month is very much essential for sustainable development. Evolution and implementation of well thought, long term national policies and creation of a promotive international atmosphere alone can lead to such development across the world, enabling entire humankind to share the benefits of satelite remote sensing. It is true that satellite alone cannot proude information regarding confined aquifers, then geophysical and drilling data have to be consulted for acquiring subsurface information and decisions file shall be created trough overlay by GIS technique. Geoinformatics have made attempts to incorporate spatial data and non spatial attributes to solve problems on hydrogeology.
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