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Generation of curve number using Remote Sensing and Geographic Information System
Ashish Pandey and A. K. Sahu Department of Agricultural Engineering North Eastern Regional Institute of Science and Technology Nirjuli, Arunachal Pradesh India-791 109 apandey@agni.nerist.ac.in
India supports 16 percent of world population on 2.42 percent of global land area (Manorama Year book 2000). An estimated 175 Million Hectares (M ha) of land constituting about 66 percent ( Motsara, 1998) of total geographical area suffers from deleterious effect of soil erosion and land degradation. Active erosion caused by water and wind alone accounts for 150 M ha of land, which accounts to a soil loss of about 5300 million tones of top soil. In addition 25 M ha have degraded due to ravine and gullies, shifting cultivation, salinity / alkalinity, water logging etc. In the wet tropical and subtropical climate, which prevails in most parts of Arunachal Pradesh, there is a usual phenomenon of erosion by degradation by the action of water. It is estimated that in Arunachal Pradesh about 669.35 million tones of soil is eroded annually with the rate of 90.7 ton/ha/yr (Singh, 1999).
Watershed, a geographically dynamic unit area that contributes runoff to a common point has been accepted as a basic unit for planning and implementation of the protective, curative and amelioration programmes. An accurate understanding of the hydrological behaviour of a watershed is important for effective management. RS and GIS technique can be used effectively to generate the land use/land cover and change detection map for evaluating the changes in an area. Land cover / Land use is perhaps the category in which remote sensing has made its largest impact and comes closest to maximizing the capabilities of remote sensing. For hilly and inaccessible terrain such as Arunachal Pradesh, it is often impractical to conduct even a limited ground survey, this has become very effective tools. GIS, which has been designed to restore, manipulate, retrieve and display spatial and non-spatial data, is an important tool in analysis of parameters such as land use/ land cover, soils, topographical and hydrological conditions. To carry out resource monitoring and assessment of area of interest, information derived through remote sensing data has to be merged or integrated with database in GIS. Thus the remote sensing along with GIS application aid to collect, analyze and interpret the data rapidly on large scale intermittently and is very much helpful for watershed planning. For ungauged watersheds accurate prediction of the quantity of runoff from land surface into rivers and streams requires much effort and time. But this information is essential in dealing with watershed development and management problems. Conventional methods of runoff measurements are not easy for inaccessible terrain of Arunachal Pradesh. Remote sensing technology can augment the conventional method to a great extent in rainfall-runoff studies. Many researchers (Ragan and Jackson, 1980; Slack and Welch, 1980, Tiwari et al., 1991) have been utilized the satellite data to estimate the USDA soil conservation Services (SCS) Runoff Curve Number (CN). In this study SCS Curve Number technique modified for Indian condition has been used for generation of CN for Remi Watershed. Materials and Methods Study Area The area selected for the present study is the REMI watershed, which is located in the East Siang district of Arunachal Pradesh under Pasighat circle. The area of watershed is 210.00 Km2. The watershed area lies in the Survey of India (SOI) topo-sheet No. 82 P/4, 82P/8, 83M/1 and 83M/5 .It is located between 27o 50to 28o 05 N latitude and 950 05 to 950 25 E longitude. The topo-sheet of the watershed in the scale of 1:50000, soil texture, soil series map and soil resources data were collected from State Remote Sensing and Application Centre, Itanagar. The topography of the watershed is undulating and the drainage is medium to excessive. The average slope of the watershed is very gentle to moderate ranging from 1- 20 % .The elevation of the watershed ranges from 140 to 1500 m above mean sea level. The climate of the area is humid sub-tropic. The region receives a high rainfall during monsoon and it turns sporadic during pre and post monsoon. The annual average rainfall is about 2000 mm and a mean annual temperature of 20o C. The surface texture of the soil is Clay Loam. The problem of erosion hazard is moderate to severe. 
Satellite Data The IRS-1B geocoded imagery was collected from State Remote Sensing and Application Center, Itanagar. The path 13 and 14, row 48 scenes of IRS-1B (LISS II) satellite with date of pass December 16, 1996 were used to prepare the land use/ land cover maps of the Remi watershed. Hardware and software used for data analysis Workstations available at State Remote Sensing and Application Centre, Itanagar equipped with GIS facilities were used. The ARC INFO software packages were used for generation of vector layers. Then it was transferred to Arc-view for taking the output. The visual interpretation of the satellite imagery was done and the study area has been classified into seven classes, namely
The soil map (Fig:2), was also collected from State Remote Sensing and Application Centre, Itanagar and it was digitized in polygon mode. For different soils (i.e., polygons) different values were assigned. Different values of soils (Polygons) were latter used for determination of SCS curve numbers. 
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