Application of Geographical Information System (GIS) tools in watershed analysis

Transcription

1 Application of Geographical Information System (GIS) tools in watershed analysis Paritosh Gupta 1, Damanjit S Minhas 2, Rajendra M Tamhane 1, A K Mookerjee 2 1.ESRI India New Delhi 2. LEA Associates South Asia Pvt. Ltd. Abstract Geographic Information Systems (GIS) has emerged as a significant support tool for managing and analysing water resources using digital elevation models (DEMs) of land surface terrain. GIS is now accepted as a useful tool for assembling water resources information. Water management agencies are building GIS data sets to support their operations. In the present work, GIS provides a consistent method for watershed delineation for the proposed and existing irrigation schemes for some districts of the state of Jharkhand, India. The types of schemes are Checkdams, Ahar, Ponds etc. Around 2500 irrigation Schemes are marked on or near the drainage network of the study area as point feature classes, which are stored in the geodatabase (GDB) format of ArcGIS. A Digital Elevation data set has been created for the area to get basic elevation data of the area for use in watershed analysis. A Digital Elevation Model (DEM) is a grid or raster of square cells whose cell value is the land surface elevation in the centre of cell. As digital elevation data sets for land surface terrain become more refined, their cell size decreases and the number of cells increases to the point where studying a large region, like a district, with a single DEM data set is cumbersome and impractical. This is the reason why the 4 meter DEMs were created block wise, after dividing the districts in various blocks. With 4 m cell size, the information from the DEMs for watershed analysis becomes more precise and reliable. Furthermore, 1 m contours are interpolated from the DEMs using ArcGIS. The NRSA Satellite Image of the area of concern was acquired and processed for land use classification in five broad categories namely Agriculture, Forest, Waterbody/Wetland, Settlement, Wasteland (Wasteland with scrub, Wasteland without scrub and sand). In order to carry out watershed analysis of various irrigation schemes, ArcHydro tools were used. ArcHydro is a geo spatial and temporal data model for water resources that operates within ArcGIS. ArcHydro has an associated set of tools that populate the attributes of the features in the data framework, interconnect features in different data layers, and support watershed analysis. Watershed areas of all the irrigation schemes were calculated and saved as polygon feature classes using ArcHydro tools. Introduction Water, one of the most essential material in day-to-day life is becoming scarce due to various reasons including reduction in infiltration rates, higher rates of runoff, uneconomical use, overexploitation of the surface resources etc; as a result of change in land use patterns, degradation of forest cover and public apathy towards its importance. An understanding of the complex inter sectoral dynamics would be crucial for developing a holistic approach to the utilization of water resources. For managing the data at basin level and analysing the data correlation between the various sectors in the basin, GIS has been found to be an effective tool. People have varying goals and values relative to use of local land and water resources, which need to be properly managed. Watershed Management is an iterative process of integrated decision-making regarding uses and modifications of lands and waters within a watershed. This process provides a chance for stakeholders to balance diverse goals and uses for environmental resources, and to consider how their cumulative actions may affect long-term sustainability of these resources. Watershed management requires use of the social, ecological, and economic sciences. Common goals for Page 1 of 8

2 land and water resources must be developed among people of diverse social backgrounds and values. The decision process must also weigh the economic benefits and costs of alternative actions, and blend current market dynamics with considerations of long-term sustainability of the ecosystem. Methodology and Objectives of the Study The basic requirement for watershed analysis is the DEM (Digital Elevation Model). For accuracy and reliability purposes, it is decided that DEMs will be created with the cell size of 4 m. The instant problem, which popped up, was the heavy size of the DEM files if created for each district. Therefore, it was decided to build the DEMs for the blocks, subdivisions of the district. The 4 m DEMs are successfully created for all the blocks using Topogridtools of ArcInfo workstation. For watershed analysis Arc Hydro tools are used. The work has been carried out in the following manner: District level resource mapping creation was done using 1: 50,000 Survey of India sheets keeping in view the project objectives. IRS 1D, LISS-III digital satellite data of 23.5 meter resolution were procured from NRSA, Hyderabad for two seasons (i.e. Rabi and Kharif cropping seasons) for land cover mapping and updating the information/data gathered from the base maps generated from 1:50,000 Survey of India sheets. Digital Image processing of satellite data using standard software packages was done for data merging, enhancement of relevant features, digital classification and conversion to thematic maps bringing the processed data into GIS environment for water resource mapping from satellite imagery. By combining the remote sensing information with adequate field data, based on the status of water resources development and irrigated areas (through remote sensing), artificial recharge structures such as check dams, nala bunds etc were recommended upstream of irrigated areas to recharge downstream areas so as to augment groundwater resources. Page 2 of 8

3 Data inputs and its accuracy levels Imagery Analysis The satellite images were geo-referenced. This mapping procedure removes geometric distortions of the image and changes the co-ordinate system of the image to spatial database coordinate system. Maximum possible Ground Control Points [GCPs] were selected, uniformly over the total area for better interpolation. Geo-referencing of satellite image, image processing and generation of classified information from (False Color Composite) FCC. (LISS-III) Digital data integration, warping, edge matching updating and sketching of data base system modification and access capability. Image Classification using various techniques including Visual Interpretation Digital enhancement techniques were used as they help to improve the feature sharpness and contrast for simple interpretation along with visual interpretation techniques. The training sites were selected based on limited field checks and visual interpretation of imagery for areas that represent known features. Based on these identified sites classification techniques were applied to generate the required thematic maps. Certain mathematical models and filters were used as well to enhance certain features like water resources to help in easy interpretation and mapping the same. Various thematic layers generated using remote sensing data like, land use/cover etc, was integrated with slope, drainage density and other collateral data in a Geographic Information system (GIS) framework and analysed. Software components Arc Hydro has been designed to represent data from hydrography and hydrology, thereby creating a basis for obtaining a deeper understanding of surface water systems. The description, study and charting of bodies of water, such as rivers, lakes and seas, is called hydrography. The natural partner of the hydrographic description of water bodies is the study of water movement through them. This is the domain of hydrology, the science dealing with the properties, distribution and circulation of water on the surface of the land, in the soil and underlying rocks, and in the atmosphere. The Arc Hydro tools are a set of utilities developed on top of the Arc Hydro data model. They operate in the ArcGIS environment. Some of the functions require the Spatial and 3D Analyst extensions. Two major functions, which have been used in the creation of database for watershed analysis purpose, are Terrain Pre-processing and Watershed Processing. Terrain Pre-processing The purpose of terrain pre-processing is to perform an initial analysis of the terrain and to prepare the dataset for further processing. A Digital Elevation Model (DEM) of the study area is used as input for terrain pre-processing. The DEM must be in ESRI GRID format. During the processing, potential problems with the terrain representation can be identified, thus preventing the DEM errors from propagating to the later stages of the analysis. A successful pre- Page 3 of 8

4 processing is an indication that the underlying DEM does not contain major problems that will prevent further analyses. The following functions, in order, are involved in terrain pre-processing:- The DEM Reconditioning function modifies Digital Elevation Models (DEMs) by imposing linear features onto them (burning/fencing). The function needs two inputs: 1. Raw DEM Grid 2. Agree Stream Grid. The output one gets is Agree DEM Grid. The Fill Sinks function fills sinks in a grid. If higher elevation cells surround a cell, the water is trapped in that cell and cannot flow. The Fill Sinks function modifies the elevation value to eliminate these problems. The function takes as input a DEM grid, which can be either an unprocessed DEM or a reconditioned DEM created with the DEM Reconditioning function i.e. Agree DEM Grid. The function produces as output a grid Hydro DEM, where no sinks exist. The Flow Direction function takes Hydro DEM as input, and computes the corresponding flow direction grid. The values in the cells of the flow direction grid indicate the direction of the steepest descent from that cell. The Flow Accumulation function takes a flow direction grid as input and computes the associated flow accumulation grid that contains the accumulated number of cells upstream of a cell, for each cell in the input grid. The Stream Definition function takes a flow accumulation grid as input and creates a Stream Grid for a user-defined threshold. The stream grid contains a value of "1" for all the cells in the input grid that have a value greater than the given threshold. All other cells in the Stream Grid contain no data. The Stream Segmentation function creates a grid of stream segments that have a unique identification. Either a segment may be a head segment, or it may be defined as a segment between two segment junctions. All the cells in a particular segment have the same grid code that is specific to that segment. Page 4 of 8

5 The Catchment Grid Delineation function creates a grid in which each cell carries a value (grid code) indicating to which catchment the cell belongs. The value corresponds to the value carried by the stream segment that drains that area, defined in the stream segment link grid. The Catchment Polygon Processing function takes as input a catchment grid and converts it into a catchment polygon feature class. The adjacent cells in the grid that have the same grid code are combined into a single area, whose boundary is vectorized. The single cell polygons and the "orphan" polygons generated as the artefacts of the vectorization process are dissolved automatically, so that at the end of the process there is just one polygon per catchment. The Drainage Line Processing function converts the input Stream Link grid into a Drainage Line feature class. Each line in the feature class carries the identifier of the catchment in which it resides. The Adjoint Catchment Processing function generates the aggregated upstream catchments from the "Catchment" feature class. For each catchment that is not a head catchment, a polygon representing the whole upstream area draining to its inlet point is constructed. This feature class is used to speed up the point delineation process. Page 5 of 8

6 Location identification of minor irrigation projects Diversion Structures/ Check Dams: These structures of low height have to be built where the river/nallahs are in plain country. A deep gorge is strictly not suitable for such structures. An ideal location would be where the stream emerges from a gorge into the plain so that it will have plain area under its command and will also have some storage behind it in the gorge. Depending on the command area available the height of the structure could be varied up to a limit when the backwater does not submerge land upstream so as to avoid problems of rehabilitation and resettlement. A location where exposed rock is visible and the reach is narrow should be preferred to reduce construction costs. From such structures irrigation could be done on both sides subject to availability of land. Command of plain land has to be preferred over that of sloping land from considerations of retention of soil moisture from rains, which will reduce the need for irrigation water. Ahars Ahars are generally built on fairly sloping ground at a level higher than that of the fields to be irrigated. Preferably, the location should be as close to the command area as possible to reduce the cost of water conveyance system. In addition ground having cross slope from both sides as for a river valley for a short distance will be most suitable so that the Ahar in the form of a low height bund can be constructed in the shape of an arc of a circle. Because of the longitudinal slope down stream and depending on the general topography, a contour or a ridge channel can be made to draw water for irrigation. The height of the bund has to be adequate to store the water coming from above the slope and if necessary some rough channels can also be dug above the Ahar from different directions to lead the water to the pond formed by the Ahar. The extent of irrigation possible from these will be small and large areas will not be covered. The project location becomes viable when i. Streams flowing through plain areas with particular reference to gorge upstream for diversion structures. ii. Sloping country with availability of cross slope for short lengths for Ahars iii. Natural depressions or artificial excavated areas for ponds iv. All the above locations should have plain area downstream for irrigation. Selection of feasible schemes will keep in view the following i. The type and location of the structures given above. ii. Adequacy of catchment area to ensure water availability for the cultivated areas located downstream of the structures. iii. Optimum area downstream for irrigated cultivation. iv. Suitability of land to establish an economical water conveyance system and drainage system. Page 6 of 8

7 Watershed processing Arc Hydro manages the input/output to the tools by using tags that are automatically assigned by the functions to the selected inputs and outputs. A tag may be used as input by one function and as output by another one. For example, the "Flow Direction Grid" tag is an input from Flow Direction, and an input to Batch Watershed Delineation. The Watershed Processing functions allow fast watershed delineation and topographic characteristics extraction. The Data Management function provides a global view of the tags assignments for that menu in the active Map/Data Frame. The function also allows assigning, reassigning or resetting the tags. When a reset tag is used as output, the function presents the user with default layer name associated to the tag. The Batch Watershed Delineation function allows delineating watersheds in batch for points defined in the BatchPoint feature class. Points are added to the BatchPoint feature class using the tool Batch. Page 7 of 8

8 Outputs generated The outputs were generated in the form of hard copy maps depicting the location of the projects and its catchment areas on the drainage pattern. For easy handing of the hard copy maps these were converted into an atlas format. Conclusions In this work an attempt has been made to bring out the extensive tools available in Arc Hydro for analysis of water resources. However, there are more utilities, which still need to be explored for realising the full potential of Arc Hydro. This study is used as a base for carrying out detailed topographic surveys for schemes, which appear feasible as per the laid out criteria under the study. For each Dam/Pond/Ahar location, catchment areas and submergence areas are being computed. From the catchment area annual rainfall data available, water yield is being estimated and feasibility of the scheme being evaluated. The maps thus produced also help in planning and design of structures, canal systems and command areas. Reference David, R. Maidment, Arc Hydro GIS for Water Resources, ESRI, 380 New York Street, Redlands, California Page 8 of 8