Prepared By
Md. Ismail Firoz
Dept. Of Geography & Environmental Studies(RU)
OUTLINES
• Introduction
• Delineation of Watershed
• Area-wide Watersheds
• Point-Based Watersheds
• Application
Introduction
What is watershed?
A watershed refers to an area, defined by topographic
divides, that drains surface water to a common outlet.
There are two types of watershed
Area-based:
➢ Divides a study area into a series of watersheds, one
for each stream section.
➢ Follows a series of steps, starting with a filled DEM
Point-based:
➢ Derives a watershed for each select point.
➢ The select point may be an outlet, a gauge station,
or a dam.
Area-based watersheds
Point-based watersheds
(shaded area).
Delineation of Watershed
In order to complete the watershed analysis just you have to follow the below steps1. DEM Acquisition
2. Filled DEM
3. Flow Direction
4. Flow Accumulation
6. Stream Links
7. Area-wide Watersheds or point
based watershed.
point based
5. Stream Network
Area based
1. DEM Acquisition
The first input required for
watershed analysis is DEM.
❑ Digital Elevation Models
(DEMs) are a type of raster GIS
layer.
❑ In a DEM, each cell of raster
GIS layer has a value
corresponding to its elevation.
❑ DEM data files contain the
elevation of the terrain over a
specified area, usually at a
fixed grid interval over the
“Bare Earth”.
1. DEM Acquisition(cont..)
Zoom in
Extract DEM data of Chattogram
Raster layer(DEM)
Raster layer(DEM)
2. Filled DEM
A filled DEM means void of depressions. Fills sinks
in a surface raster to remove small imperfections in the
data.
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.
Although some depressions are real, such as quarries
or glaciated potholes, may be imperfection in the
DEM. Therefore, depression must be removed.
How to remove depression ?
A common method to remove depression is to increase its cell value to the lowest
overflow point out of the sink. This results in flat surface.
3. Flow Direction
A flow direction raster shows the direction water will flow out of each cell of a filed
elevation raster.
There are two ways to determine flow direction:
➢ D8 method (single flow direction)
➢ Multiple flow direction method.
D8 method
Used by ArcGIS, the D8 method assigns a cell’s flow direction to the one of its eight
surrounding cells that has the steepest distance-weighted gradient.
D8 method (single flow direction)
The flow direction of the center cell
(a) is determined by first calculating the distance-weighted
gradient to each of its eight neighbors.
(b) For the four immediate neighbors, the gradient is
calculated by dividing the elevation difference between
the center cell and the neighbor by 1.
(c) For the four corner neighbors, the gradient is
calculated by dividing the elevation difference by 1.414.
The results show that the steepest gradient, and
therefore the flow direction, is from the center cell to its
right.
Multiple flow direction method
An example :
The D∞ (D Infinity) method
which partitions flow from a cell into two adjacent cells.
❑ The D∞ method first forms eight triangles by
connecting the centers of the cell and its eight
surrounding cells.
❑ It selects the triangle with the maximum downhill
slope as the flow direction.
❑ The two neighboring cells that the triangle intersects
receive the flow to the aspect of the triangle in
proportion to their closeness
❑ Using the flow direction raster, the next step is to
calculate a flow accumulation raster.
4. Flow Accumulation
A flow accumulation raster calculates for each cell the number
of cells that flow to it.
The flow accumulation raster can be interpreted in two ways
Firstly
Cells having high accumulation values generally correspond to
stream channels, whereas cells having an accumulation value
of zero generally correspond to ridge lines.
Secondly
If multiplied by the cell size, the accumulation value equals the
drainage area. The flow accumulation raster can then be used
for deriving a stream network.
Flow Accumulation(cont..)
This illustration shows
➢ A filled elevation raster (a),
➢ A flow direction raster (b),
➢ and a flow accumulation raster (c).
Both shaded cells in (c) have the same flow accumulation value of 2.
The top cell receives its flow from its left and lower-left cells. The
bottom cell receives its flow from its lower-left cell, which already has
a flow accumulation value of 1.
With the appearance of a spanning tree the
flow accumulation raster records how many
upstream cells contribute drainage to each
cell (the cell itself is not counted).
5. Stream Network
Stream network can be derived from a flow
accumulation raster. The derivation is based on a
threshold accumulation value.
The threshold (cells) value is the number of upstream grid
nodes that must flow into a particular cell before a stream line is
created.
For example:
A threshold value of 500,, means that each cell of the
drainage network has a maximum of 500 contributing cells.
➢ A higher threshold value will result in a less
dense stream network and lower threshold value
result high dense stream network.
➢ Threshold values between 100 to 500 cells
seem to best capture the stream network in
the area.
6. Stream Links
A stream link raster includes reaches, junctions,
flow directions, and an outlet.
Assigning a unique value and associating with flow
direction to each section of stream network is a
step or procedure to derive stream links.
7. Area-wide Watersheds
This is final step to delineate watershed for each
stream section.
Inputs required:
➢ flow direction raster
➢ stream link raster.
Delineated Watershed, Courtesy: ESRI
Area-based watersheds
A stream link raster therefore resembles a
topology-based stream layer. the intersections or
junctions are like arcs or reaches.
Figure does not cover the entire area of the
original DEM.
The missing areas around the rectangular
border are areas that do not have flow
accumulation values higher than the specified
threshold value.
8.Point-Based Watersheds
Point based watershed is a area based on pour point . The pour point must be
located over a cell that falls on a stream link.
If the pour point is not located directly over a stream link, it will result in a small,
incomplete watershed for the outlet.
These points of interest may be pour point:
❑ Gauge stations
❑ Dams or
❑ Water quality monitoring stations.
Point-Based Watersheds(cont..)
If a pour point (black circle) is not snapped to
a cell with a high flow accumulation value it
usually has a small number of cells identified
as its watershed.
When the pour point in is snapped to
a cell with a high flow accumulation
value its watershed extends to the
border.
Applications
➢ Used for the management and planning of natural resources.
➢ Watershed area management.
➢ The organization and planning of human activities
➢ To provide the necessary inputs for hydrologic modeling (Flood prediction models
snowmelt runoff Models)