[GRASS-user] Watershed divides from DEM

[Michael Barton]

On Dec 31, 2009, at 5:50 PM, grass-user-request at lists.osgeo.org wrote:

> Date: Thu, 31 Dec 2009 13:40:54 -0500
> From: "Lyle E. Browning" <lebrowning at att.net>
> Subject: [GRASS-user] Watershed divides from DEM
> To: grass-user <grass-user at lists.osgeo.org>
> Message-ID: <D52248EB-C3DE-450F-9D32-1F36770CAC18 at att.net>
> Content-Type: text/plain; charset=us-ascii
> 
> In my archaeological work, I will survey across 4 identical spurs of hilly land, of which 1 will have a large Native American component and the others zilch, despite all having equally level spur tips, access to water and the like. What makes the 4th one advantageous is that it sits astride a watershed divide for two typically small (5-200cfs flow) streams. The deer still use these to cross into the next watershed as it is the least exerting means of doing so. The hunters follow the deer and archaeologists follow the lithic scatters from the hunters.
> 
> Is there a way in GRASS to have it do the grunt work of finding these watersheds and overlaying a line or something on them such that they can then be accentuated and displayed?

It depends on whether or not you can define the watersheds you want to find in some kind of geographic/topographic terms and can distinguish the streams of interest in a region-wide, systematic manner. If you have observed water flows in some streams and deer trails in the field, GRASS cannot replicate these on-the-ground observations. It can systematically analyze the data you give it.

As Rich Shepard says, several modules (r.watershed being a very good one) can create maps of watersheds. You need to tell GRASS what the minimum size of watersheds you desire. You can then convert the watershed map to vectors, whose lines will trace all the drainage divides. You can obtain ridges in various ways. One of the easiest is to use r.param.scale. The intersection of watershed boundaries and ridges will be drainage divides. Are these the divides you want? It depends on whether you can give GRASS information to distinguish the desired drainage divides from the undesired ones. 

Keep in mind, too, that the land today is not the land of the past. What is the age of the sites you seek? In much of North America, current forests are regrowth--possibly for the 2nd or 3rd time. Soils that once trapped and infiltrated surface water may be gone or greatly degraded, with consequences for springs and once influent streams. Drainages that once held permanent streams may now be dry. The route that deer follow today will map onto modern vegetation, water, property lines, and hunting regulations. In these conditions, GRASS might do a better job of finding locales--based on coarse scale topography that has not changed much in the past millennia or so--that were important environmentally once, even if they do not appear to be so attractive today.

Michael

____________________
C. Michael Barton
Director, Center for Social Dynamics & Complexity 
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University

Phone: 480-965-6262
Fax: 480-965-7671
www: www.public.asu.edu/~cmbarton, http://csdc.asu.edu