I want to be able to calculate the catchment area that would feed rainwater into proposed swales.
I can do this easily enough, and with sufficient accuracy--not much accuracy is really needed, on a case by case basis by hand simply by tracing perpendiculars to the contour lines backwards uphill from the opposite ends of the proposed swale.
However, I'd like to be able to use software to speed this up, and to make it easier for helpers who don't know how to do this. I've been considering creating this software (not trivial for sure), but is there such a program out there that's free?
Watersheds basins algorithm
. If you search for "QGIS" and any combination of "catchment" "basin" or "watershed" you'll find additional tutorials.