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1. Introduction
Advances in computational power and growing availability of spatial data have made watershed-based analysis more systematic and meaningful. Watersheds are appropriate spatial units for managing environmental problems and reflect the hydrological responses of the delineated spatial unit. The key property of watershed delineation is the creation of a watershed boundary. The watershed boundary uniquely defines the land area from which the surface water drains to the watershed outlet. The advent of spatial data in the form of Digital Elevation Models (DEM), Triangulated Irregular Network (TIN) and Digital Line Graphs (DLG) has led to the use of improved tools for watershed management and hydrologic modeling. Geographic Information System (GIS) software has made the task of spatial data management much easier, interactive and informative. In this paper, a user interface in GIS has been developed for watershed delineation, natural drainage network generation and drainage density estimation from the available DEM. The DEM is a digital representation of land elevation and can be stored in one of the three data structures such as grid, TIN, and contour based. Grid structure usually consists of a square grid matrix with an elevation value specified at each grid square, called a cell. Most of the DEMs available now are in grid structure format (Garbrecht et al., 2001)

Watershed management requires physiographic information such as watershed slope, configuration of channel network, location of drainage divide, channel length and geomorphologic parameters viz. relative relief, shape factor, circulatory ratio, bifurcation ratio, drainage density and Hypsometric Integral (HI) for watershed prioritization and implementation of soil and water conservation measures. Traditionally, these parameters are obtained from topographic maps or field surveys. Over the past two decades this information has been increasingly derived from digital representation of topography, generally called the DEM (Moore et al., 1991; Martz and Garbrecht, 1992). The automated derivation of topographic watershed data from DEMs is faster, less subjective and provides more reproducible measurements than traditional manual techniques applied to topographic maps (Tribe, 1992).

1.1 Background information on User Interfaces (UI) in GIS.
In order to facilitate the implementation of the concepts for watershed delineation and drainage network generation, User Interfaces (UI) in GIS have been developed, which provide communication between the user and the GIS software through a set of methods or procedures. The interfaces developed perform integration of multiple databases, simulation models and flow logic to arrive at the desired result with minimal time and without expert GIS knowledge by the user. Initially, the interfaces were developed in Arc View 3.x using Avenue script language, in ArcInfo workstations using AML (Arc Macro Languages) and now it is done through VBA (Visual Basic for Applications) programming language with ArcGIS, a recent GIS software by ESRI (Environmental Systems Research Institute). The VBA is COM (Component Object Model) compliant and is supported by the ArcObjects technology of ESRI, which are embedded in Arc View 8.1 and higher software versions of ArcGIS [1] (at http://www.esri.com).

In a review of the interfaces developed for watershed generation and stream network generation, Olivera and Maidment (2000) developed CRWR-PrePro, an interface with ArcView 3.2 of ESRI with spatial analyst extension for accomplishing watershed delineation and natural drainage network generation from DEMs. This interface was developed at the Centre for Research in Water Resources (CRWR) of the University of Texas at Austin as a Pre-Processor (Pre-Pro) for establishing the topology of hydrologic elements and prepare a watershed parameter input file for use in running HEC-HMS (Hydrologic Engineering Centre-Hydrologic Modelling System) hydrologic model. The interface is written in Avenue script, the in-built macro language embedded with ArcView 3.2 (ESRI) GIS software. The process of watershed delineation from DEM is covered through a number of steps such as filling the sinks and preparing a depression less DEM, flow direction and flow accumulation using D-8, watershed delineation based on user specified threshold on contributing number of grid cells making up the drainage basin or a branch of drainage network. Moreover, there is also the option for interactive selection of watershed outlets. The sub-basins are thus generated for the DEM along with the drainage network. Finally, the raster coverages of sub-basins and stream network are converted to vector files for further analysis.

The Arcview Interface for SWAT (Soil and Water Assessment Tool) i.e. AVSWAT 2000 (DiLuzio et al., 2002) [2] (at http://www.brc.tamus.edu/swat/avswat), is a complete preprocessor, interface and post processor of the hydrological model SWAT. AVSWAT 2000 has been developed as an extension of ArcView 3.x GIS entirely in avenue script and dependent by spatial analyst and dialog designer extensions. The Spatial Analyst functions are used within the Watershed Delineation and Landuse and Soil Definition components while the Dialog Designer controls have been used to build up all the user interface tools. Its interface is different from CRWR-PrePro, but it follows the same procedure as described for watershed delineation and stream network generation.

Valenzuela and Olivera (2002) developed an interface WSDT (Watershed and Stream Delineation Tool),[3]( http://ceprofs.tamu.edu/folivera/GISTools/wsdt/home.htm) for watershed delineation and stream network generation from DEMs. The concept of this is similar to the basics of watershed delineation with the steps viz. filling the DEM, flow direction grid formulation, flow accumulation, stream grid generation, Links grid generation, watershed grid formation, topology building and vectorizing the stream and watershed grids. This is developed in VB programming language and shows a simpler UI in which all the steps for watershed demarcation and stream network generation is achieved by clicking a single command button on the interface VB form within ArcView 8.1 and higher version GIS.

Tarboton (2002) developed a set of tools for terrain analysis viz. TAUDEM (Terrain Analysis Using Digital Elevation Models)[4], which uses a DEM and operate within ArcGIS 8.1 and higher version. The interface performs DEM preprocessing and multiple methods for the delineation of channel networks including curvature-based methods sensitive to spatially variable drainage density and objective methods for determination of the channel network delineation threshold based on stream drops and stream ordering. It also performs delineation of watersheds and sub-watersheds draining to each stream segment and association between watershed and segment attributes for setting up hydrologic models. Moreover, it also incorporates some specialized functions for advanced terrain analysis.

It is evident from the literature that there is a need for development of an User Interface in ArcGIS for estimating the geomorphological parameters to understand the watershed morphology and interpret their impact on watershed hydrology.

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