SWAT+

SWAT+ is the restructured successor to the Soil and Water Assessment Tool, the USDA-ARS/Texas A&M model that has become the world's most cited watershed simulator. It runs continuous simulation of hydrology, sediment, nutrients, pesticides and land-management practice at catchment to basin scale, with the landscape split into hydrologic response units.

SWAT+ is the right tool when land use and water quality questions meet: how conversion, conservation practice or climate scenarios change flows and loads. The QSWAT+ QGIS plugin handles delineation and model building, and SWAT+ Toolbox and swatplus-python support calibration and automation. Expect a real learning curve — and a vast literature to lean on.