QGIS
The free, open-source GIS that has become the default spatial platform for water engineers worldwide.
The QGIS interface for SWAT+ — delineate, discretise and build a watershed model without leaving the GIS.
by Texas A&M AgriLife / USDA-ARS
Install
QGIS → Plugins → Manage and Install Plugins → "QSWAT+"QSWAT+ is the official QGIS plugin for building SWAT+ models: watershed delineation from a DEM (via TauDEM), landscape-unit and HRU discretisation from soils and land use, channel and reservoir setup, and visualisation of model outputs back on the map.
It replaced the old ArcSWAT/ArcGIS dependency with an entirely free chain — QGIS + QSWAT+ + SWAT+ — which matters for the global user base in agencies and universities that never had Esri licences. If you are starting with SWAT+ today, this is the intended front door.
Other geospatial & hydro-gis tools covering similar workflow stages.
The free, open-source GIS that has become the default spatial platform for water engineers worldwide.

500+ fast geospatial analysis tools with the best DEM hydrology toolkit in open source.
JAXA's 30m global DSM from PRISM stereo imagery — a strong third opinion alongside SRTM and Copernicus DEM.
A 30m global DEM reaching to 83° latitude — the go-to where SRTM/NASADEM have no coverage at all.
Esri's free hydrology framework for ArcGIS Pro — terrain preprocessing and the data model behind many agency workflows.
ESA's TanDEM-X-derived global DEM — the most accurate free global surface model available today.