Watershed GIS Part 1

Jul 8, 2024 | GIS, GRASS, Lidar, QGIS

I typically spend June wondering about what I’m doing, have done, and need to do work wise. One thing I keep thinking about is “the old days”. By old days I mean the start of my career which was doing a lot of Watershed Mapping. Back in the mid 90’s we would be approached with a project area in some random state. We would lay out a project area. Fly photography. Map buildings and roads and anything that could cause a problem in a watershed (septic tanks). We did a lot of these projects. Funniest part? Part of the delivery was printed maps overlaying a 7.5 minute topographic sheet (this was 30 years ago) and it took two people to carry a finished atlas to the client. A project containing 10 sheets? That’s 10 trips for two people to move the final deliverable.

I sat down over the long holiday weekend and decided to recreate the process as quickly as possible for one small watershed near town. Partly for nostalgia. Partly I’m still fascinated by watershed/environmental work.

I’m going to start this process in GRASS although I could have done it all in QGIS using the GRASS Extensions. Variety is the spice of life (for me).

Tennessee has a lot of free LIDAR data available. I grabbed the data from the state for my area of interest some time back. My intention is to not lay out the watershed manually. I linked the DEM into GRASS to save processing time.

One of the weird things with GRASS (which isn’t that weird really) is setting up a computational region. I set my region to about the area of interest. You set a boundary and a cell size and that is what constrains your tools and your work. I’m 95% sure my watershed is contained in the area below.

The tool I’m going to use (that you can get to through QGIS) is r.watershed.

The only thing that took a bit of time was figuring out the minimum size of a basin. It’s not a large watershed so I ran this a couple of times and my last test was about a 20 acres minimum or a million square feet. I don’t know what the magic number is BUT – that one worked for me. My projection is EPSG:6576 (feet).

At the end I have this Blue Basin.

Why not grab is from the USGS Hydro Data? This watershed was much smaller than what they currently have.

How much time did I kill so far? About an hour BUT I now have a study area to work with. Moving forward I’ll be jumping back and forth between GRASS and QGIS quite a bit. Anyway – I’ll do something fun with this by the end.

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