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Abstract
The city of Guwahati with a total municipal area of more than 313 sq. km, has witnessed a rapid growth in population particularly during the last one and half decades. The population jumped from 1,23,783 in 1971 to 5,77,791 in 1991 and as per census 2001, the figure stands at 8,14,575. As a result, there is tremendous pressure on the natural resources like groundwater. Although the mighty river Brahmaputra flows through the northern periphery of the city, water supply depends heavily on the dugwells and deepwells. However, it is found that due to the geological factors, aquifers with adequate yield are not well developed and are mostly laterally discontinuous. The problem of depleted groundwater table is acutely felt in the new growth centres like Beltola, Panjabari, Motoria etc. The high growth of built up areas and filling up of natural channel ways have adversely affected recharge of the ground water regime. In this backdrop, it is of vital importance to identify the groundwater potential zones of the city area taking into account the geological and anthropogenic factors. Geologically the city is characterised by mostly Precambrian granite gneisses, quartzites forming residual hills and occupying a major part of the landscape. Small elongated intermontane valleys with various thickness of sediment fill and alluvium form the rest of the areas. Study of satellite data supplemented by field check shows that a characteristic feature of the terrain is the presence of a number of paleochannels which are perceived to be old channel ways linked to the river Brahmaputra towards north. Using Analytical Hierarchy Principle (Saaty, 1980), a paired comparison matrix was prepared for the five criteria (geomorphology, slope, soil, geology and landuse) for Guwahati city. The computed values shows acceptable level of consistencies. The consistency ratio (CR) for all the five thematic layers is found to be 0.013, while for the individual criterion the CR values are : geomorphology (0.0039), soil (0.00528), slope (0.0176), geology (0.) and landuse (0.0053), which are within the acceptable limit of 0.10. The weights were fed into the SPANS 7.2 for multicriteria analysis to determine the groundwater potential zones for the city.

Introduction
Multi-criteria evaluation (MCE) techniques are numerical algorithms that define the suitability of a particular solution on the basis of the input criteria and weights together with some mathematical or logical means of determining trade-offs when conflict arise (Heydon et. al., 2003). In this technique, ‘weight’ is assigned to the data layers to reflect their relative importance. Ground water exploration in a geologically complex terrain requires consideration of a number of factors – both natural and anthropogenic. It is important to understand the control of these factors on the groundwater regime of any area for optimal exploitation and aquifer management particularly in and around urban growth centers. As such, to arrive at a clear picture of the situation, the controlling factors have to be treated and integrated giving weight that is specific to a particular area. In this backdrop, the present study is aimed at testing the efficacy of MCE technique used in GIS environment as framework for decision making problems pertaining to ground water exploration.

Study area
The present study area, measuring about 229.94 sq. km., encompasses southern part of the Greater Guwahati Municipality in Kamrup (Urban) district of Assam (Fig. 1). It is bounded by graticule lines 91°34¢-91°52¢N and 26°5¢-26°13¢E. The city is extended more in an E-W trend occupying the position between the Brahmaputra river towards north and Precambrian hills of Shillong Plateau towards south. It is home to a population of over 8,14,575 (Census 2001) occupying mostly the narrow tracts of alluvium and sediment filled low lands interspersed with Precambrian residual hills and inselbergs. Intensified anthropogenic activities particularly in and around the hills has led to high rate of aggradation in the low lying areas clogging the city drainage system. The city enjoys a subtropical humid climate with an average rainfall of 1600mm. April to September are the months that receive abundant rainfall occasionally causing flashflood in many areas of the city. Southwest monsoon reaches this area by mid of May and recedes back by mid of October. The temperature fluctuate between 17°C to 38°C with maximum summer temperature between 30°C -38°C and minimum winter temperature between 17°C to 10°C.


Fig 1. Location map of the study area
Database
SoI topomaps (78N/12 and 78N/16) in the scale of 1:50,000 were used to prepare the reference map for the study area with supplementary information from satellite images (IRS IA, Geocoded FCC, B2 3 4; IRS ID, LISS III PAN sharpened). Two vector layers were generated for spot height and elevation contours at 20m interval to work out the DEM and DEM derivatives. The database for geology and geomorphology were prepared from satellite images in combination with field work and input from available literature on the subject. Small scale published maps of NBSS/LUP and a few reference borehole data were used to generate the thematic layer on soil.

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