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Application of Geographical Information System (GIS) tools in watershed analysis
Paritosh Gupta and Rajendra M Tamhane
ESRI India New Delhi
Damanjit S Minhas and A K Mookerjee
LEA Associates South Asia Pvt. Ltd.
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 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:
Imagery Analysis
The satellite images were geo-referenced. This mapping procedure removes geometric distortions of the image and changes the c o-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.
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-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:
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.
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.
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.
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’.
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
Paritosh Gupta and Rajendra M Tamhane
ESRI India New Delhi
Damanjit S Minhas and A K Mookerjee
LEA Associates South Asia Pvt. Ltd.
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 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.
Imagery Analysis
The satellite images were geo-referenced. This mapping procedure removes geometric distortions of the image and changes the c o-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
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-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:
- Raw DEM Grid
- Agree Stream Grid. The output one gets is ‘Agree DEM Grid’.
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.
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.
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…
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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’.
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, 2002. Arc Hydro – GIS for Water Resources, ESRI, 380 New York Street, Redlands, California 92373-8100.