GIS Paper

INTRODUCTION TO GIS CATCHMENT DELINEATION

PRATAMA RIZQI ARIAWANStudent Number: 45794

Locker Number: 269

DECEMBER 8, 2014

Lecturer: Suryadi, PhD

Introduction to GIS

PRATAMA RIZQI ARIAWAN 1

Catchment Delineation

A Description

Catchment delineation is creation of a boundary that represents the contributing area for a particular control point or outlet. It is also used to define boundaries of the study area, and/or to

divide the study area into sub-areas.

In this particular assignment, Arc Map 10.1 is used to perform the catchment delineation.

Afterwards, Arc Scene 10.1 is used to visualize the catchment in three dimensional view. The Dem

data is obtained from Lecturers file as well as some other files which have greatly assisted in the

completion of the work.

By using Arc Map, it is possible to perform terrain process and catchment delineation by using

extensions functions. For analysing the catchment, spatial analyst extensions is used, therefore,

it is important to make sure that it is loaded by checking the extensions.

The hydrologic modelling tools in the ArcGIS Spatial Analyst extension toolbox provide methods

for describing the physical components of a surface. The hydrologic tools allow you to identify

sinks, determine flow direction, calculate flow accumulation, delineate watersheds, and create

stream networks.

B The Stages

B.1 Preparing the data and software set up

In this stage, raster image which contain DEM information namely dem_raw_Clip.img is used

as initial DEM file.

Figure 1 Image of dem_raw_Clip.img

Introduction to GIS


B.2 Producing flow direction raster using the unprocessed DEM

Flow direction stage is one of the key to deriving hydrologic characteristics of a surface is the

ability to determine the direction of flow from every cell in the raster.

To do so, flow direction toolbox which is located under the Spatial Analyst Tools Hydrology

Flow Direction is chosen, hereafter, the result will be shown as follows:

Figure 2 Flow Direction image of the map namely FlowDir

B.3 Determining Sinks

With the Sink tool, any sinks in the original DEM are identified. A sink is usually an incorrect

value lower than the values of its surroundings. The depressions shown in the graphic above

(the scattered coloured points) are problematic because any water that flows into them

cannot flow out. To ensure proper drainage mapping, these depressions can be filled using

the Fill tool later.

Figure 3 Sinks are shown as dots in the map namely sinks

Introduction to GIS


B.4 Filling in the Sinks

In this stages, we try to fill in the sinks from the data of unprocessed dem.

Figure 4 The result of Fill in the sinks extracted from dem_raw_Clip.img namely demfill

B.5 Determining Flow Direction by using the sink-free DEM

By repeating the step B2, with changing the input to sink-free DEM namely demfill, we get

the similar result with as shown in B2.

B.6 Creating Flow Accumulation

The Flow Accumulation tool is used to create a stream network. Moreover, it can also be

used for calculating the number of upslope cells flowing to a location. The output flow

direction raster created in a previous step is used as input. This is the result.

Figure 5 Flow accumulation image namely fillflowacc

Introduction to GIS


By using raster calculator, a threshold can be specified on the raster derived from the Flow

Accumulation tool; the initial stage is defining the stream network system. This task can be

accomplished with the Con tool or using Map Algebra. An example of general syntax to use

in Con is Stream1 = con(fillflowacc > 20000, 1). All cells with more than 20.000 cells flowing

into them will be part of the stream network.

B.7 Creating Streamlinks

Streamlinks assign unique values to sections of a raster of a linear network between

intersections. Links are the sections of a stream channel connecting two successive

junctions, a junction and the outlet, or a junction and the drainage divide.

Figure 6 Image of streamlink

B.8 Getting the Stream Order

Stream ordering is a method of assigning a numeric order to links in a stream network. This

order is a method for identifying and classifying types of streams based on their numbers of

tributaries. Some characteristics of streams can be inferred by simply knowing their order.

There are two optional methods including Strahler method and Shreve method.

The result is similar to the image above, but now the stream has some order numbers

B.9 Converting Stream Network into vector format

The algorithm used by the Stream to Feature tool is designed primarily for vectorization of

stream networks or any other raster representing a raster linear network for which

directionality is known.

This feature also convert the raster images into

simple lines

Introduction to GIS


B.10 Creating Catchments

The catchments are created by locating the pour points at the edges of the analysis window

(where water would pour out of the raster), as well as sinks, then identifying the contributing

area above each pour point.

Figure 7 Image of basin namely catchments

B.11 Adding pourpoints (outlets)

In many cases, we need to have the catchment above an outlet. To do so, we need to specify

the locations of those outlets. Apparently they need to be located on the stream. However,

the outlet location getting from other sources is not necessarily on the stream (could be

quite near indeed.) So we need to snap them on to the stream.

B.12 Snapping the outlets to the drainage lines

Snap Pour Point will search within a snap distance around the specified pour points for the

cell of highest accumulated flow and move the pour point to that location.

Figure 8 Image of snapping pourpoints namely outlet

Introduction to GIS


B.13 Creating the Watershed for the outlet

A watershed is the upslope area that contributes flowgenerally waterto a common

outlet as concentrated drainage. It can be part of a larger watershed and can also contain

smaller watersheds, called subbasins. The boundaries between watersheds are termed

drainage divides. The outlet, or pour point, is the point on the surface at which water flows

out of an area. It is the lowest point along the boundary of a watershed.

Figure 9 Final image of main watershed (watsub) and sub watershed (watsub_sub)

B.14 Converting raster to polygon

By converting raster into polygon, we will obtain simple look and much smaller in data size,

so that it will be ideal for further use.

Figure 10 Image of raster to polygon conversion

Introduction to GIS


B.15 3D visualization of the watershed

By using Arc Scene, three dimensional view of the project can be seen as follow

Figure 11 Image of three dimensional view of the watershed

C Conclusion

Delineating watershed in arc map can be done by using several items in spatial analyst tools,

however, the process must be done in sequence otherwise the program will not respond to the

command.

Arc scene is used to display the watershed in three dimensional view. It is important to set the

reference from surface to show the 3D view of the object, meanwhile the height of the object

from reference surface is set by changing the value of base height.

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