U can calculate slope from DEM for your area (full watershed) and than use sub-watershed map along with slope and DEM, flow direction map to calculate sub-watershed wise slope, longest flow length and centroidal flow length etc. These calculated flow length can be taken as slope length for these sub-watershed..

all these steps can be done in GeoHMS-Terrain processing Tool in ArcGIS..

else if want slope length at pixel level than refer to page 5 of doc at:

http://onlinegeographer.com/slope/hickey_slope_length.pdf

Thank you very much Praveen Thakur , I will check GEOHMS, Can you give some more instructions on how to use those maps to calculate it in a subwatershed ?do i cut my raster DEM using my subcatchment polygons ? i 've read someplace that its better to not calculate the slope length on subcatchments but on the whole catchment.. but , ok i do not want that detail, I think i ve read this article already, it seemed better than others but didnt have an example and i think i need it .......

It will depend on level of detail u are interested in..!! if your catchment has large area and relief, than u might use sub-catchment based slope and slope legth, if u r working on small catchment with uniform slope, than u might take single catchment only..and GEO-HMS has online good user and tech manual...u will be able to all GIS operation easily.

regards..

Hello Praveen , i installed GEO-HMS its very usefull and its easy to use.I calculated the average slope of my subasins ! I cannot Calculate the longest flow path on Basin Characteristics first it says some error, then another about hydroids , when i assigned hydro ids it tryed to work but then it said something about a field called discription , and so i checked out the shape file of the catchments and it had created the 5 fields (that longest flow path creates ) but discription was named discriptio withouth the last "n" , so i think thats the problem for not puting the values on the fileds .. the name its too long, i calculated the longest flow path after all with Arc hydro but this is the same with my rivers as someone would expect . I want the slope length from the higher points just until before it reaches the rivers .

Dear George, in my Ph.D. Thesis I had to calculate the slope length from a DEM. I used the methodology established in USLE. In fact there is an equation for the slope lenght; however it is not clear how to applied in DEMs. If you want to have a complete look at my thesis, you can find it at the link below. The part regarding what I explained is in the attached file.

http://biomath.ugent.be/publications/download/matamorosdavid_phd.pdf

regards

David

Dear All,

you you want to dedicate sometimes to learn something new, uDig (http://uDig.refractions.net), the free and open source GIS offers a Spatial Toolbox where tools for doing all of this stuff are included. These tools are called the Horton Machine and you can find a brief description here (http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&sqi=2&ved=0CCIQFjAA&url=http%3A%2F%2Fwww.ing.unitn.it%2Fdica%2Ftools%2Fdownload%2FQuaderni%2FHorton%2520manual.pdf&ei=Ol9kUOy6LY7T4QTY04HoAg&usg=AFQjCNFNmBXL0AyjmBuQTiPuDvFXU1mgag&sig2=3VmizZkI4nX17BJ7XKpCUw). These tools were used for the calculation you can find in the papers referenced at http://abouthydrology.blogspot.it/2012/09/my-past-research-on-hydro-geomorfology.html and for practically any of my catchments related work.

Investing time to learn open source tools is never lost.

riccardo

Thanks a lot Dr Metamoros , i calcualted the flow length for my flow direction grid but i didnt use any other layer to clip it or limit the calculation to the catchment areas , so i think i did something wrong there . and i do not know what this i created depicts.. its the flow lenght downstream of the selected pixel?And i want the Average slope length pf my basins . I think i ll just count the length of my streams leading to the half of the length main rivers ... very scientific i know... !PS : YOUR THESIS is excellent !!!! HUGE WORK !!

Dear George, sorry for not answering before. I just checked today the portal. When you calculate the slope, the GIS software usually calculate the difference in elevation between adjacent raster cells, then to calculate the flow direction, GIS use the maximum slope in this calculations. For example, an internal cell (within the raster matrix) has 8 potential directions of flow, one of them has a maximum slope. GIS software selects that direction to calculate the flow direction. For the flow lenght, I remember that there is a possibility to calculate accumulated flow length in two possible ways (upstream or downstream). When I did my thesis, I used ArcView 3.3. I do not know if the new ArcGis calculates that in the same way.

Hi Hussein karaman sir, Two of you people have suggested WMS software for slope calculation. Can u plz be more specific what is WMS software.?

hi hussein Sir,

The WMS software is proprietary and there is no free educational version.

Average slope calculation can be easily done with gis and also a little easyer with hec-geohms aspraveen said. Slope length on the other hand for sheet flow i havent found a way yet

In udig spatial toolbox there is a tool called H2C (hillslope to channel distance) that does it.

That sounds nice !!

You may also try ILWIS. It works out these things...

I used Udig a little.. it seems nice and interesting , with some nice attributes, like some Evapotranspiration calculations , or some other basin characteristics in the Horton machine( the toolbox ) but it also lucks some attributes , it doesnt have the ability to create raster from polylines only from polygons , and i also found some problems in adding the .asc grid files.I had the decimal separator as a comma and i have to change it in dot manualy in notepad, plus i dont know how to use the coordinate systems there , and maybe thats why i had some problems on creating the slope lengh map with the H2C tool ( its called something like that.. hillslope to river-channel )witch by the way is in an experimental version( beta.. the tool not something else..)and it moved the river a little to left and down but this might be causes from my files . And it also crashes easily .But its a very good software in general (and FREE) and it has a lot of potential.Anyway Slope length doesnt play a very big role in the simulated runoff at least not in changes of 100-400 meters .so i ll do it by hand.

I suggest to use pcraster to do this (see http://www.pcraster.eu/). It is free and will do what you want using only a couple of statements. Comes with python bindings also.

Thanks a lot . I be finished my thesis , you can view it in my profile but it is in Greek only yet!

Try Saga GIS software, it's free!

http://www.saga-gis.org/en/index.html

http://www.saga-gis.org/saga_module_doc/2.1.3/tin_tools_6.html

Hi， I came across the same proble as you did, could you tell me how to calculate the slope length, I tried PCRaster, however I cannot generate point map thus I cannot go on calculating the slope length, what was your method   ?I am in great need of your help , thank you.

Hello, it has been a long time since i did that Thesis . I dont remember very well what i did then but i think i ended up doing it manualy .. measuring the length manualy and i might have had some help by exporting the lenght of the smallest stream (smallriver) brunches and doing some averages.. i am not so sure. I will try to remember. At that time i found that Udig GIS had a formula for calculating slope lenght but it didnt work very well for me, i coundlnt import my data very well in udig, i think it took only ASCII files and it was tough , i could see my map and the slope length moved to the left and tilted. Maybe after theese years they made it easier to use ! try it ! or Do it Manualy ! If you find another way i d like to know too : . thanks 

Hello!

The information used here is very useful, but how to get the slope height? I refer to the "Rise" part, could be extracted in some way from the slope length and slope angle?

Many thank, Margarit!