National Water Model (NWM)

Conceptual overview of the National Water Model framework, formulations, and related hydro-infrastructure.
Author

NOAA

Published

August 9, 2026

Flood Hazard Outlook

Executive Summary

Prior to 2016, the Nation relied on a piecemeal approach to forecasting the operational condition of our waterways, leaning heavily on the local expertise of regional River Forecast Centers (RFCs) and the National Weather Service (NWS).

With the release of the National Water Model (NWM)—run by the Office of Water Prediction (OWP) in Tuscaloosa, Alabama—the nation gained a single, unified, continental-scale picture of its waterways at an unprecedented resolution. It is a hydrologic modeling framework designed to take atmospheric forcings and landscape states and produce a streamflow forecast for every river reach in the continental United States. It accomplishes this in operational time intervals (hourly predictions using 15-minute timestep inputs).

The NWM is not just one single model, but a framework and particular implementation of the BMI standard. It is currently composed of models like SCHISM and T-Route. These outputs are what drive continental-scale Flood Inundation Mapping (FIM) and impact workflows.

Key Concepts & Architecture

NextGen Framework

NextGen is a means of coupling modeling efforts and allowing for different representations of the hydrologic cycle to interact across various spatial extents. This framework allows different types of model formulations to communicate, hiding the complexity associated with making them interface explicitly. This enables “the right model, in the right geography, for the right reasons,” greatly increasing explainability.

Reference Time

Within the NWM, Reference Time is the specific time that the model is forecasting from (e.g., “At 12Z, the rivers all had this initialized value”).

Hydrofabric & Domains

The underlying spatial network that the NWM routes water through. It dictates the connectivity of the stream reaches, catchments, and waterbodies. (Note: See domains created in coastaalfim)

Flood Inundation Mapping (FIM)

Since its inception, FIM has generated 100% CONUS coverage. However, to allow the public to adjust to this forecasting revolution and to iron out technical hiccups, it has traditionally been automatically generated only for areas receiving a discharge prediction higher than a specific return-period threshold. These values (disseminated by HUC8, or runoff efficiency clusters) dictate when FIM is visually triggered. See: High Water Thresholds for reparameterization details.