I'm new user of this software, please can somebody guide me, how to calculate stream order in QGIS or GRASS GIS, step by step.... mean commands and how to calculate watershed in this.
I have calculated stream order with the SAGA GIS module. You can read about the modules here: http://www.saga-gis.org/saga_module_doc/2.1.3/ta_hydrology.html All this assumes you have an elevation model in GeoTIFF.
In order to get that working you need to install QGIS with the OSGeo4W installer and make sure SAGA GIS is checked for install:
Then you have an assortment of hydrology tools ready in the processing toolbox. In order to calculate a drainage route network on overland flow you need to fill in all the small holes of the DEM (so the stream simulation doesn't get stuck.) I used Wang & Liu's method for this:
Now you will have rasters for a filled elevation model, as well as watershed basins and flow directions. Then you can perform a Sink Drainage Route detection on the filled DEM:
This will give you a shapefile with the drainage routes, with an attribute containing their Strahler order. Very useful. Good luck!