[GRASS-user] Longest flow path

[Rich Shepard]

On Tue, 3 Aug 2010, Jarek Jasiewicz wrote:

> r.watershed elevation=elev drainage=dirs streams=stream treshold=<some value>
> r.stream.order streams=streams dirs=dirs hack=longest
>
> the stream with order 1 will be the longest streams of the catchment

Jarek,

   Please correct me if I'm wrong, but I don't believe this answers David's
question. Using both the Horton and Strahler systems of calculating stream
order, order 1 is the headwater reach. However, a drainage basin does not
have each stream length from origin to outlet the same length.

   I believe that what David asked is how to calculate the longest path from
the top of first order streams to the basin outlet.

   Thinking about this now, I can see that looping across all streams from
headwaters to outlet, and presenting those in a table (with some indication
of which path has which length) would be highly useful. One could calculate
average stream length and other descriptive statistics that could be
cleverly applied to questions of runoff, flooding, and habitats
(particularly if incorporated with gradient measures and a few other basin
attributes).

   You research guys could work on this; us applied folks need to focus on
billable work. :-)

Rich