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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.
Recommended