GRASS GIS

GRASS GIS has been in development since 1982, and its hydrology modules carry that maturity: r.watershed delineates basins and streams with an AT-least-cost algorithm that handles depressions without explicit filling, r.stream.* tools build and analyse networks, and r.sim.water runs overland-flow simulation — all scriptable from Python or shell.

Like SAGA, GRASS is available inside QGIS as a Processing provider, which is how most water engineers consume it today. For large rasters and long processing chains, native GRASS with its region-based workflow is exceptionally robust — the kind of software that simply does not break.