As title, I want to find the watershed area, but using QGIS.
I have following files to work with：
- stream network shapefile(polyline)
- point shapefile clipped from the polyline shapefile
- a whole island(the stream is inside the island) DEM(I got 2 types:.lan & .tiff)
I want to find the total watershed area located upstream of every point in the network, here's the schematic diagram：
the green, red, and yellow line enclose the watershed of each point(here I only take 3 points as the example), and I want to calculate their area.
I already got the DEM file, and I must cut the region that contains the stream off, should I polygonize it first？Can I directly use this form(.lan or .tiff) to work with GRASS？