DEM Based Hydro - Tutorials - Part 3

7/30/2019 DEM Based Hydro - Tutorials - Part 3

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Part 3: DEM Hydro-processing with corrected DEM

The DEM-hydro-processing module has a

sequential menu. Apart from the Flowmodification sub menu, all the others have to be

run in the sequence given; output map produced

in the current step will be used as an input map

during the next step. In the Help function

additional info is presented on the functionality

of the routines.

The data set that is going to be used is a

preprocessed DEM and an outlet location. Copy

the data provided to an appropriate directory on

your hard disk, start ILWIS, select the directorycreated and display the data sets provided.

In the next steps you are going to work with the

DEM called DEMLangat. This is a modified

DEM, so you will not use the routines as given

under Flow Modification, as these options have

already been incorporated.

Run the routines Fill Sink, Flow Direction and

Flow Accumulation and use the DEMLangat as

input for the Fill Sink, the output of the Fill Sink

as input for the Flow Direction (according to

steepest slope) and the output of the Flow

Direction as input in the Flow Accumulation.

Display each of the maps and evaluate by

moving the cursor over the map the results of theoperation. The Help function is providing further

details.

Move to the Variable Threshold Computation.

The DEM is going to be used to calculate the

internal relief that is reclassified into 5 flow

accumulation threshold classes. Some

generalization is applied using a majority filter of

5 by 5. Use the thresholds as specified in the left

hand figure.


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Display the results obtained and compare them with the satellite image. Also display the internal

relief map created, use the Inverse Representation (dark is high internal relief and white is low

internal relief).

Move to the Drainage Network Extraction

operation. Specify the appropriate FlowAccumulation map and use the stream threshold

map created in the previous step. Now also

specify the appropriate Flow Direction map and

an output map name. In this step you create a

raster map indicating the drainage lines. This

map is going to be vectorized in the next step

and a linked topological data base is created as

well. Display the results and move to the next

operation: Drainage Network Ordering.

Specify the input as requested to run the

operation. An example is provided in the figure.

Here the original DEM is used as input for the

variables that are extracted for the drainage

network. Display the output and use the option

Pixel Information to see which attributes are

created.

Display the satellite image. In the image window, use Layer, Add Layer and select the drainage

vector file created. Use as Attribute StrahlerClass and the default Representation indicated. In the

Legend the colour and line thickness can be modified according to your preferences. Zoom in,

open Pixel Information, move the cursor over the drainage lines and check the results and

evaluate the topology created. If you want to modify the density of your drainage network then

you have to repeat the procedure starting with other variable drainage thresholds.

Under the operation Catchment Extraction, for

each drainage segment created the

corresponding catchment area is computed.

This is again a raster map. Also here an

attribute table is computed giving a number of

relevant variables. Display the catchment map

and the associated catchment table. Note that

the drainage and catchment tables are linked as

they have the same Identifier number. Thesesingle catchments have to be merged as there


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are far too many. In order to do so a merging can be done using Strahler or Shreve order, but also

using one or multiple user defined outlet locations.

Proceed to the operation Catchment Merge.

Specify the input parameters as indicated inthe figure. Within this operation also the

drainage can be extracted for the selected sub

catchment area and the longest flow path

segment can be computed.

Display the results, using the satellite image as

background and add the drainage segments as

another layer. Also display the polygon file of

the extracted catchment, using only the

boundary within the display options. Visuallyinspect your results and use Pixel Information

to see the attributes, also those of the merged

catchment polygon. Also display using another

colour the longest flow path.

You have now obtained a lot of information describing your drainage network. This information

can be used to parameterize your hydrological model. Other information, relevant for more

generic type of catchment management related studies can be obtained when computing the

compound indices. Within the module Compound Parameter Extraction, four routines aredeveloped to facilitate this module; they are Overland Flow Length, Flow Length to Outlet, Flow

Path Longitudinal Profile, and Compound Index Calculation. An explanation is given in the

ILWIS Help.

There is another module which provides the user with additional information about the catchment

in relation to the drainage network as well as with regard to other parameter maps. Open the

module Statistical Parameter Extraction and select the Horton Statistics. In hydrology, the

geomorphology of the watershed, or quantitative study of the surface landform, is used to arrive

at measures of geometric similarity among watersheds, especially among their stream network.

The quantitative study of stream networks was originated by Horton. He developed a system for

ordering stream networks and derived laws relating the number and length of streams of different

order. Hortons original stream ordering was slightly modified by Strahler and Schumm added

the law of stream areas. Number of streams of successive order, the average stream length of

successive order and the average catchment area of successive order is found to be relatively

constant from one order to another. Graphically this can be visualized by construction of a Horton

plot.


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Specify the requested input as given in the left

hand figure. Two tables will be calculated, the

table with the output file name is containing for

the extracted catchment the number of streams,

the length (km) and the catchment area (km2)

per Strahler order. This table will be used to

visualize the regularity of your stream network

extracted. It can also serve as a quality control

indicator as during the whole DEM

modification and network extraction process a

lot of decisions have been taken and these

should result in a relative constant increase or

decrease from one order to the next.

The other table, with the default extension _Ratio is containing the Bifurcation (Rb), Length (Rl)

and Area (Ra) Ratios. These are obtained using a least square fit through the (logarithmic

transformed) points of e.g. the number of streams per order. The ratio value represents the

increase or decrease in number, length and area from one order to the next.

If not displayed already, open the table Hortonplot_1. It should be similar to the figure below.

Columns C1_N, C1_L and C1_A show the number of streams, average length and average

catchment area per Strahler order. The last three columns show the results of the least square fit

that has been applied. These columns will be used to construct a graphical presentation, Horton

Plot.

Open the table Hortonplot_1_Ratio too. There are two columns in this table each giving the Rb,

Rl and Ra. The _a column is the ratio calculated using all stream orders, the _b column excludes

the lowest and highest Strahler order from the computation and these ratios might therefore be

slightly more representative (depending on the size of the catchment). Close this table and

activate the Horton_plot table.


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Now construct the graphical representation for the Horton statistics. Use the option display graph

to make the final Horton plot. Display the columns C1_N, C1_L and C1_A as points, use the left

Y-axis for C1_N and the right Y-axis for C1_L and C1_A and the order on the X-axis. Transform

both the left and right Y-axis to a logarithmic scale, make sure that the data range limits are set

appropriate. Display the columns C1_N_Lsq (using left Y-axis), C1_L_LSq and C1_A_LSq

(using right Y-axis) as lines (you can select a different line type representation). Make sure the

points and the lines for N, L and A have the same colour. The results should look like the graph

below.

The Horton plot shows the regularity from one order to the next. If you would select a smaller or

larger catchment area there should be a consistency of stream numbers, length and area given

their geometric similarity. If this is the case you can relate characteristics of flood hydrographs tostream network parameters.


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You can define a smaller sub catchment within the catchment selected for which the previous

Horton Plot is developed to check if there is a geometric similarity between the two.

Besides using the Stream Order method, you can choose to use the option Outlet Location to

merge catchment areas. To define a new outlet, first display the segment map with the drainage

network. Then, add in the flow direction map. Zoom in to the area where you want to insert yournew outlet location and make sure that in your zoom window you can see the individual raster

pixels of the flow direction map. Open in the map window, File, Create and PointMap and

specify an appropriate point map output name, the map extent can be left default. Use the Insert

mode and add a new outlet location. Make sure that the location is slightly downstream of the

node of the junction of the drainage network you want to define as your sub catchment (see the

yellow circle in the example provided in the figure below).

This outlet location is now going to be used in

the Network and Catchment Extraction Module

for the Catchment Merge routine. Enter the

appropriate input maps, use the outlet locationjust defined. Specify an appropriate output

raster map, activate the option Extract Stream

Segments and Attributes. Open in a new map

window the polygon map of the newly created

sub catchment and display the extracted

drainage network as well. Use Pixel Information

to look at the attribute information.

Additional statistical information.

Aggregate statistics is adding aggregated statistical information to the merged catchment table

based on the information from value maps. The value map, e.g. an elevation model is crossed

with the catchment map and statistics like average, minimum, maximum, standard deviation,

median elevation is added to the table per catchment. Run the script and study the catchment table

once more to see what has been added.

Cumulative hypsometric curve is another option to calculate the area versus elevation curve for a

selected catchment. A plot can be made using the cumulative area or cumulative percentage as X-

axis and the elevation as Y-axis. Open the script and compute the area-elevation curve for your

catchment. Enter the appropriate variables, you might need to check the name of your catchment

by displaying the catchment map if you have extracted multiple catchments. In the case where

you have only one catchment, when the table is displayed, select the graph option and display for

the X-axis the cumulative area and from the Y-axis the elevation (given by the column name

identical to the input DEM). Change the symbol from a point symbol to a line symbol.

You can also overlap a selected catchment with another layer to determine for example the

coverage of a certain feature in a catchment, for example: the area of forest. In this case you can

use the threshold map that was already generated. This option is very useful if area related

statistics per sub-catchment have to be produced.

Not all functionality has been addressed here. Additional exercises have been developed dealing

into more detail with advanced functions, such as the topological and DEM optimization options.


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These are described in Part 4 and demonstrate additional tools to be able to extract a proper

drainage network in complex terrain, either by lowering the DEM values along drainage lines, or

by indicating the flow direction through flat areas or lakes, ensuring proper topological

relationship.

Documents

DEM Based Hydro - Tutorials - Part 3