This is my first time writing in this forum and I hope this question offers some quality of information easing a solution. Here goes.
I am new in analyzing hydrology with DEMs and I have followed several steps to identify watersheds and drainage basins. (I am using QGIS for all my operations)
- Obtained the DEMs and combined them using "Build Virtual Raster"
- Filled the holes using "Fill sinks (wang & liu)" providing a filled DEM and Flow Directions
- With the filled DEM I've calculated the Strahler order (which I'm not familiar with) and identified rivers with this tool.
- A new raster was calculated with the Strahler order's values >= 8 resulting in an acceptable river raster layer (validated through a basemap).
- Then I've done a Channel network and drainage basins with a threshold 8 (to have the same acceptable result as my Strahler order) and saved the channels and drainage basins. (I suspect my error might be in this step)
- Then the result that I get gives me a drainage basin going straight through a lake, which makes no sense I find. Shouldn't the lake it'self be a receiver of a whole network of watershed?
The drainage basins are the red limits. I figured the limits of the drainage basins would either be where I inserted the arrows 1 or 2 or both, as we can clearly see the lake/reservoir in the DEM. I figure I might have missed a step to tell my procedure the limit of every water plan in this process as it doesn't seem to take them into account.