hy,

here`s a video about how to do a atershed delineation using QGIS (GRASS GIS )

http://www.youtube.com/watch?v=6BQltTwsU7A

I hope it will help you. Also, on youtube, if you searchyou`ll find more videos.

that video didn't give me the clear cut idea about it,,, would you please suggest another... thank you..

hy, I do not know what to say ! I have worked from this video. But I send you a picture qith what to do !

You'll have to use GRASS GIS (e.g. http://landmap.mimas.ac.uk/index.php/Learning-Materials/Image-Processing-for-GRASS/11.2-Watershed-Delineation). I've tried different QGIS plugins without success. Just remember to follow the basic steps:

1. Fill voids in DEM [http://grass.osgeo.org/grass64/manuals/r.patch.html]

2. Calculate flow direction

3. Calculate flow accumulation

4. Define your outlet

5. Delineate watershed

There is a very nice tutorial of catchment delineation with QGIS. Please check this out. https://www.youtube.com/watch?v=uSscn6ImRxU

Hope this help.

Sorry for the delayed response, but I was trying to this myself and found an elegant tutorial which worked perfectly for me.

http://www.digital-geography.com/beautiful-watershed-polygons-in-qgis/

Just posting it here for anyone who happens to stumble across this question.

The only drawback I see from r.watershed is that you don't manually define your outlet (pour point).

also you can check: https://geogeek.xyz/catchment-delineation-with-qgis.html

Good answer. One point that geogeek skipped over is that your outlet point needs to fall on a pixel with high flow accumulation. Rather than guess, zoom into your flow accumulation raster and find the coordinates closest to your outlet that falls on a pixel with high accumulation.

Watershed from GRASS