USGS Hydrologic Toolbox

The Hydrologic Toolbox consolidates two long-serving USGS programs — the Surface-Water Toolbox and Groundwater Toolbox — into one Windows application for streamflow-record analysis: baseflow separation by the standard methods (BFI, HYSEP, PART), flow-duration curves, n-day low-flow and high-flow frequency statistics, trend tests and recession analysis, with direct retrieval of USGS gauge data built in.

These are the statistics that water-supply assessments, low-flow permitting and baseflow studies actually report, computed by the agency's own reference implementations. For scripted equivalents, R and Python packages cover much of the same ground; the Toolbox remains the citable point-and-click standard.