Catchment merge

Functionality

The Catchment merge operation is able to merge adjacent catchments, as found by the Catchment extraction operation. In fact, new catchments will be created on the basis of the Drainage network ordering map and its attribute table.

As input is required:

the output map and table of a previous Drainage network ordering operation,
the output map of a previous Flow direction operation,
the output map of a previous Flow accumulation operation.

You can merge catchments in two manners:

by specifying a point map that contains locations of stream outlets within a catchment; all adjacent catchments that drain into such outlets will be merged,
by simply specifying a Strahler or Shreve ordering value: all adjacent catchments that have this Strahler or Shreve order value (or a lower value) and which drain into a common catchment will be merged.

As output a new catchment raster map, polygon map and attribute table are produced. These all use a new ID domain.

The attribute table contains information on the new catchments, similar to the output attribute table of the Catchment Extraction operation, but you will also find information on:

total drainage length, total upstream area,
drainage density,
longest flow path length and longest drainage length.

Optionally, you can also obtain:

a segment map with the longest flow path per catchment and a linked attribute table,
a segment ordering map and attribute table, that only contain the segment streams within the new catchments; other streams will not appear anymore; output is similar to the segment map and attribute table of the Drainage Network Ordering operation.

The last option is only available when you use an input point map with outlets.

Finally, this operation also has an option to include undefined pixels (from the Flow direction map) into a catchment. Then, a point map that only contains a single point is required.

Tips:

A point map can be created in a map window, using for instance the input flow direction map and the input drainage network ordering segment map as background.
Choose File, Create Point Map in the map window and the Point editor will be opened. For more information, refer to Point editor : functionality, section Working with the mouse.

1. Example using a point map with multiple points:

In this example, a user first performed the Drainage network extraction, the Drainage network ordering and the Catchment extraction operations as usual. The catchments obtained from Catchment extraction are shown in the third picture in green.

Then, a point map is created containing the locations of the outlets of the desired catchments. These points are also shown in the third picture (6 points in gray).

When the Catchment merge operation is now performed using the point map with the outlets, only 6 catchments will be created (see the fourth and fifth picture). An output attribute table will give additional information.

Drainage network ordering map: displayed by attribute Strahler	Drainage network ordering map: displayed by attribute Shreve


Input catchments (in green) from catchment extraction and input point map with 6 outlet points (in gray):


Output merged catchments (bounds only, in red) other layers for reference	Output merged catchments (polygons and new IDs)


Output merged catchments (bounds only, in red) and additional output extracted segments (light blue); outlet points for reference	Output merged catchments (bounds only, in red) and additional output longest flow paths (light blue); outlet points for reference

2. Example using Strahler or Shreve ordering values:

For your information, a point map with labels representing Shreve ordering values for the drainages was created from the attribute table of the Drainage network ordering operation. This was done just for reference purposes and is not a required step to be able to perform Catchment merge; the Shreve labels are also shown in the second picture.

Then, the Catchment merge operation was performed. All contiguous catchments which drainages had Shreve order values up to 15 were merged. The new catchments are shown in the third picture in red. The fourth picture also shows the IDs of the new catchments.

Similarly, catchments can be merged for connected drainages according to the Strahler method; such a merge up to Strahler order 3 is shown in the fifth and sixth picture.

Drainage network ordering map: displayed by attribute Shreve	Input catchments (in green) from catchment extraction with Shreve label points:


When merging catchments up to Shreve order 15: Output catchment merge map (bounds only, in red), other layers for reference	Output merged Shreve catchments (polygons bounds and new IDs)


When merging catchments up to Strahler order 3: Output catchment merge map (bounds only, in red), other layers for reference	Output merged Strahler catchments (polygons bounds and new IDs)

3. Example using the "Include Undefined Pixels" option:

In the last example, a user had a DEM in which lakes were excluded. After a Flow direction operation, you see that there are no flow direction values for the lake area(s) (first picture). When subsequently Drainage network extraction and Drainage network ordering are performed, no drainages are found in the lake area(s) (second picture).

To remove these white spots, some segment maps were created, and the Topological optimization operation was performed several times. For more information, refer to Topological optimization : functionality.

The output Flow direction map from the Topological optimization operation, then shows flow direction values also in the lake area(s) (third picture). The Flow accumulation operation should not be performed again now.

Then, the Drainage network extraction and the Drainage network ordering operations were performed again (fourth picture). You now see a continuous drainage network.

Subsequently, a point map was created, containing a single point. This point represents the outlet of a single catchment in the lake area. The point is shown in the fifth picture.

When now the Catchment merge operation is performed, using the latest Drainage network ordering map and the latest Flow direction map, and when choosing the Use Outlet Locations and the Include Undefined Pixels options, a single new catchment will be created for the lake area. The new catchment is shown in the fifth picture.

Initial flow direction map	Initial drainage network ordering map


Updated flow direction map from Topological optimization	Improved drainage network order map from a subsequent Drainage network ordering operation


Output of Catchment merge when using the Include Undefined Pixels option: output polygon (in red); other layers for reference

Other general options for the Catchment merge operation:

Longest flow path segment map:
	Optionally, you can obtain an additional segment map (and attribute table) containing the longest possible flow path within each new catchment, based on the flow direction and flow accumulation input maps. The attribute table will contain information like Length and StraightLength and Sinuosity for each longest flow path. You can specify a name for this segment map yourself. The attribute table will obtain the same name.

Extract stream segments and attributes:
	Optionally, you can obtain an additional segment map (and attribute table) that only contains those segment streams that fall within the new catchments; other streams will not appear anymore. The attribute table for this segment map will contain information like the drainage network ordering attribute table. Compared to the previous drainage network ordering attribute table: stream IDs are kept the same, records of streams that no longer fall within a new catchment are simply deleted. This segment map will obtain the same name as the output catchment merge map. The attribute table (and the domain of this segment map and attribute table) will generally obtain the same name, followed by __1. This option can only be used when you selected the option Use Outlet Locations.

Determination of whether an outlet point in a point map is close enough to a stream:

When using one or more outlet points: any outlet point should be within a 5x5 pixel window near an existing drainage line, otherwise the outlet point will be ignored. You can check whether a point is close enough to a stream in a map window; you can adjust the position of points in the Point editor. For more information, refer to Point editor : Move Points Mode.

Input map requirements:

the output map and attribute table of a previous Drainage network ordering operation,
the output map of a previous Flow direction operation,
the output map of a previous Flow accumulation operation.

Domain of output maps and attribute tables:

Standard output of the Catchment merge operation:

an output raster map and an output polygon map will be created containing one or more catchments; the maps will use a newly created ID domain;
the new ID domain will obtain the same name as the output maps;
an output attribute table will be created, it also uses this ID domain;
the output raster map and the output polygon map are both linked to the output attribute table;
for more information on the contents of the output attribute table, see below.

When the option Longest Flow Path Segment Map is used:

an additional segment map is created containing the longest possible flow path within each new catchment, based on the flow direction and flow accumulation input maps;
an additional attribute table is created containing information like Length, StraightLength, and Sinuosity for these longest flow paths;
the name of this segment map is specified by the user; the attribute table will obtain the same name;
the additional segment map and the table use the same domain as the maps and table that are standard output of Catchment merge.

When the option Extract Stream Segments and Attributes is used:

an additional segment map is created containing only those segment streams that are located within the new catchments; other segments will not appear anymore;
these segments will keep their 'original' (input) IDs from Drainage network ordering;
an additional attribute table is created containing the same information as the Drainage network ordering output attribute table;
as the segments keep their 'original' (input) IDs, there are no records in the table for segments that do not fall within the new catchments;
the segment map will use the same name as the standard output of Catchment merge;
the attribute table will use a similar name as the segment map, generally followed by __1;
the segment map and the attribute table will use a new ID domain;
this new ID domain has a similar name as the segment map, generally followed by __1.

Columns in the Catchment merge output attribute table:

domain	The IDs of the table's domain, every record (ID) represents a new catchment.
DrainageID	A column listing the IDs of all streams located within a new catchment.
UpstreamLinkCatchment	The ID(s) of the new catchments that directly contribute to this new catchment, e.g. when catchments 1, 2, 3, and 4 flow together into catchment 5, then the UpstreamLinkCatchment column will read for the record with ID 5: {1, 2, 3, 4}
DownstreamLinkCatchment	The ID of the new catchment into which a current new catchment will flow (down-flow), e.g. when catchment 5 flows into catchment 6, then the DownstreamLinkCatchment column will read for the record with ID 5: 6. This column is a value column.
Perimeter	The perimeter of each new catchment.
CatchmentArea	The area (m²) of each new catchment.
TotalUpstreamArea	The total area (m²) of the catchments that directly contribute to a current catchment, i.e. the sum of the areas of the catchments listed in column UpstreamLinkCatchment.
TotalDrainageLength	The sum of the lengths of all drainages in a catchment.
DrainageDensity(m/km2)	The drainage density within a catchment as TotalDrainageLength / CatchmentArea
LongestFlowPathLength	The length of the longest flow path found in a catchment, from the catchment's outlet to the most distant source on the catchment boundary, according to the Flow direction and Flow accumulation input maps.
LongestDrainageLength	The length of the longest actual stream within this catchment.
CenterDrainage	The XY-coordinate in the middle of a longest flow path. This column is a coordinate column.
CenterCatchment	The XY-coordinate at the center of a catchment. This column is a coordinate column.