Steps Involved in Delineation of Watershed: DEM, Fill, Flow Direction and Accumulation, Network, Links, and Watersheds (Especially for GATE-Geospatial 2022)

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Area or Point Based Delineation of Watersheds

Delineation of watersheds can be area-based or point-based. An area-based method divides a study area into a series of watersheds, one for each stream section. A point-based method derives a watershed for each select point. When delineating watersheds or defining stream networks, we proceed through a series of steps, following are the steps:

Some Steps Are Required, While Others Are Optional Depending …

1. DEM Acquisition

The first input required for watershed analysis is DEM. Digital Elevation Models (DEMs) are a type of raster GIS layer. DEM data files contain the elevation of the terrain over a specified area, usually a fixed grid interval over the “Bare Earth” .

Picture Shows DEM Acquisition Watershed - Courtesy: ESRI

2. Fill

Fills sinks in a surface raster to remove small imperfections in the data. A filled DEM or elevation raster is void of depressions. A depression is a cell or cells in an elevation raster that are surrounded by higher elevation values, and thus represents an area of internal drainage. Therefore, depression must be removed.

Picture Shows Fill DEM in Watershed - Fill DEM, Courtesy: ES …

3. Flow Direction

A flow direction raster shows the direction water will flow out of each cell of a filled elevation raster. A widely used method for deriving flow direction is the D8 method, used by Arc GIS. In D8 method, assigns a cell՚s flow direction to the one of its eight surrounding cells that has the steepest distance-weighted gradient.

Picture Shows Flow Direction, Courtesy: ESRI

4. Flow Accumulation

A flow accumulation raster tabulates for each cell the number of cells that will flow to it. The tabulation is based on the flow direction raster.

Picture Shows Flow Accumulation, Courtesy: ESRI

A flow accumulation raster can be interpreted in two ways.

1. Cells having accumulation values generally correspond to stream channels, whereas cells having an accumulation value of zero generally correspond to ridgelines.

2. If multiplied by cell size, the accumulation value equals to drainage area.

5. Stream Network

Stream network can be derived from a flow accumulation raster. The deviation is based on a threshold accumulation value. A threshold value of 500, for example, means that each cell of the drainage network has a maximum of 500 contributing cells.

Picture Shows Stream Netowrk and Stream Order

A higher threshold value will result in a less dense stream network and fewer internal watersheds than lower threshold value.

6. Stream Links

A unique value and associating with flow direction to each section of stream network is a step or procedure to derive stream links.

A stream link raster therefore a topology-based stream layer: the intersections or junctions are like arcs of reaches.

Picture Shows Stream Links in Watershed

7. Area-Based Watersheds

This is final step to delineate watershed for each stream section. Inputs required for area-wide watershed delineation are flow direction raster and stream link raster. A denser stream network will have more but smaller watersheds.

Picture Shows Area-Based Watersheds

8. Point Based Watersheds

Delineation of individual watersheds based on point of intersection follows the same procedure as for delineation of area-wide watersheds.

Point based watersheds delineation is based on the point of interest. This point of interest may be stream gauge stations or dams. They may also correspond to surface drinking water system intake points of interest are called pour point or outlets.

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