Catchment Statistics

Catchment Statistics

RivEX is capable of analysing a river network on a catchment by catchment basis. A catchment is defined as all polylines flowing to a mouth node. Therefore a river network can potentially have many catchments, especially if it covers a coastal region.

To compute the catchment statistics you must complete the parameters section of the main interface and press build to construct the dictionaries used to analyse the river network.

RivEX Interface

Select the Analyse Network tab, tick the Catchment statistics checkbox and start the process by clicking on Go!

RivEX assumes you may have changed the default field names so you will always be asked to identify the following fields: Catchment ID, Strahler Order, Strahler Segment, Shreve Order, Link Type and the Main Channel ID field.

Field selector dialog

These fields are all generated by RivEX so you must run your river network through RivEX first to create these fields.

Once the process has finished, you will be asked where to direct the output as shown below.

Output option dialog

Select YES sends the output to MS Excel. The Excel application will minimise when data transfer has completed, do not click on the window until this has happened!

Select NO sends the output to a dbase file called Catchment_Statistics.dbf stored in C:\RivEX\Output. If this file exists already you will be asked to overwrite it.

The sequence of output fields which RivEX creates in both the dbase and Excel is dictated by the sequence of the highest stream order found. This is not an error and can have the effect of bunching columns together. It is simply a 'quirk' in how it stores the data.

The statistics calculated for each catchment are:

Statistic	Field Name
The sum length of polylines	SumLength
The length of the main course as defined by RivEX	MainLength
The maximum Strahler order value	MxStrahler
The maximum Shreve order value	MxShreve
Number of Interior links	I_Links
Number of Exterior links	E_Links
Number of Outlet links	O_Links
The number of river reaches for Strahler order X *	N_Strah_X
The sum length of polylines for Strahler order X *	Strah_X
The bifurcation ratio for Strahler order X *	BR_X

* = This row will expand to the largest Strahler order value achieved.

The bifurcation ratio (BR) is calculated as the number of stream segments of order X divided by the number of stream segments of order X+1. A single coastal stream polyline which has a stream order of 1 has no BR as there is no 2nd order stream to divide with. The image and table below demonstrate the BR for a simple catchment.

Bifurcation example

Stream order ( X )	# of segments	Bifurcation ratio (X /X +1)
1	10	2
2	5	1.25
3	4	0

The Strahler ordering algorithm labels both sides of a braid the same segment number if the polylines it consists of flow in a downstream direction. The bifurcation ratio is calculated from the number of segments, therefore a braid (which may physically consist of 2 or more polylines) will be considered as part of a single segment when calculating the ratio.

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