We use stream power in our landscape evolution script r.landscape.evol. One version is already in the GRASS addons repository. We are continuing to enhance and refine this.
Michael ______________________________ C. Michael Barton Director, Center for Social Dynamics & Complexity Professor of Anthropology, School of Human Evolution & Social Change Head, Graduate Faculty in Complex Adaptive Systems Science Arizona State University Tempe, AZ 85287-2402 USA voice: 480-965-6262 (SHESC), 480-965-8130/727-9746 (CSDC) fax: 480-965-7671(SHESC), 480-727-0709 (CSDC) www: http://csdc.asu.edu, http://shesc.asu.edu http://www.public.asu.edu/~cmbarton On Mar 4, 2015, at 2:50 AM, [email protected]<mailto:[email protected]> wrote: From: Markus Metz <[email protected]<mailto:[email protected]>> Cc: grass-user <[email protected]<mailto:[email protected]>> To: Helmut Kudrnovsky <[email protected]<mailto:[email protected]>> Date: March 4, 2015 at 2:50:51 AM MST Subject: Re: [GRASS-user] Stream power Index calculation? On Wed, Mar 4, 2015 at 9:46 AM, Helmut Kudrnovsky <[email protected]<mailto:[email protected]>> wrote: in the book SPI is defined as: SPI = A x tan(beta) where A is the specific catchment area and beta is the local slope angle. in link [1] there is the SAGA GIS formula. sometimes SPI is also defined as: SPI = ln(A x tan(beta)) This is also known as Topographic Convergence Index (TCI) or Topographic Wetness Index (TWI). You can calculate this index with r.watershed and r.terraflow. If you need the non-logarithmic form, you can use r.mapcalc "SPI = exp(TCI)". AFAIU from the manual: http://grass.osgeo.org/grass71/manuals/r.watershed.html TCI is definded as: Topographic index ln(a / tan(b)) divison vs. multiplication OK. You can get SPI with a little math: exp(TCI) = a / tan(b) tan(b) = a / exp(TCI) a = flow accumulation / res or a bit more precise, for flow direction to direct neighbors: a = flow accumulation / ((ns_res + ew_res) / 2) and diagonal neighbors: a = flow accumulation / sqrt(ns_res * ns_res + ew_res * ew_res) now you have a, can calculate tan(b) and SPI note that slope from r.slope.aspect could be used but does not necessarily point to the predominant flow direction. r.slope.aspect calculates slope for a given cell from its 3x3 neighborhood whereas TCI uses the slope from the current cell to the next downstream cell. Thus for TCI and SPI the upstream slope does not matter so much, it is important how it continues immediately downstream (e.g. waterfall). Since a and tan(b) are already calculated by r.watershed for TCI, implementing SPI (as yet another option to r.watershed) would be relatively easy. Markus M ----- best regards Helmut -- View this message in context: http://osgeo-org.1560.x6.nabble.com/Stream-power-Index-calculation-tp5190756p5191377.html Sent from the Grass - Users mailing list archive at Nabble.com<http://nabble.com/>. _______________________________________________ grass-user mailing list [email protected]<mailto:[email protected]> http://lists.osgeo.org/mailman/listinfo/grass-user
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