Information About the National Water Model

The National Water Model

View the NOAA publication on the National Water Model here.

The National Water Model (NWM) is a hydrologic model that simulates observed and forecast streamflow over the entire continental United States (CONUS). The NWM simulates the water cycle with mathematical representations of the different processes and how they fit together. This complex representation of physical processes such as snowmelt and infiltration and movement of water through the soil layers varies significantly with changing elevations, soils, vegetation types and a host of other variables. Additionally, extreme variability in precipitation over short distances and times can cause the response on rivers and streams to change very quickly. Overall, the process is so complex that to simulate it with a mathematical model means that it needs a very high powered computer or super computer in order to run in the time frame needed to support decision makers when flooding is threatened.

The NWM complements current hydrologic modeling which is done in a simplified manner for approximately 4000 locations across the CONUS by providing information at a very fine spatial and temporal scale at those locations, as well as for locations that don’t have a traditional river forecast.

Diagram of Water Cycle

NWM Operational Configuration:

The model runs an hourly uncoupled analysis (simulation of current conditions). Short-range forecasts are executed hourly while medium-range forecasts out to 10 days are produced four times per day. A daily ensemble long range forecast to 30-days is also produced. All model configurations provide streamflow for 2.7 million river reaches and other hydrologic information on 1km and 250m grids. The NWM will provide complementary hydrologic guidance at current National Weather Service (NWS) river forecast locations and significantly expand guidance coverage and type in underserved locations.

WRF-Hydro Plus NHDPlusV2 Data

The core of this system is the National Center for Atmospheric Research (NCAR)-supported community Weather Research and Forecasting Hydrologic model (WRF-Hydro) . It ingests forcing from a variety of sources including Multi-Radar/Muti-Sensor System (MRMS) radar-gauge observed precipitation data, and High Resolution Rapid Refresh (HRRR) , Rapid Refresh (RAP) , Global Forecasting System (GFS) and Climate Forecast System (CFS) Numerical Weather Prediction (NWP) forecast data. WRF-Hydro is configured to use the Noah-MP Land Surface Model (LSM) to simulate land surface processes. Separate water routing modules perform diffusive wave surface routing and saturated subsurface flow routing on a 250m grid, and Muskingum-Cunge channel routing down National Hydrography Dataset (NHDPlusV2) stream reaches. River analyses and forecasts are provided across a domain encompassing the CONUS and hydrologically contributing areas, while land surface output is available on a larger domain that extends beyond the CONUS into Canada and Mexico (roughly from latitude 19N to 58N). The system includes an analysis and assimilation configuration along with three forecast configurations. United States Geological Survey (USGS) streamflow observations are assimilated into the analysis and assimilation configuration and all four configurations benefit from the inclusion of 1,506 reservoirs.

Model Details

Operational Model Configuration

General Framework:

The NWM is run in four configurations:
  1. Analysis and assimilation: snapshot of current hydrologic conditions
  2. Short-Range: 18-hour deterministic (single value) forecast
  3. Medium-Range: 10-day deterministic (single value) forecast
  4. Long-Range: 30-day ensemble forecast

Analysis and Assimilation:

The Analysis and Assimilation configuration cycles hourly and produces a real-time analysis of the current streamflow and other surface and near-surface hydrologic states across the contiguous United States (CONUS). This configuration also produces a single model restart file which is used to initialize the 18 hour, 10 day and 30 day forecast simulations. Meteorological forcing data are drawn from the MRMS Gauge-adjusted and Radar-only observed precipitation products along with short-range RAP and HRRR.

Forecast Ranges

Short Range

Forced with meteorological data from the HRRR and RAP models, the Short Range Forecast configuration cycles hourly and produces hourly deterministic forecasts of streamflow and hydrologic states out to 18 hours. The model is initialized with a restart file from the Analysis and Assimilation configuration and does not cycle on its own states.

Medium Range

The Medium Range Forecast configuration is executed four times per day, is forced with GFS model output and extends out to 10 days. It produces 3-hourly deterministic output and is initialized with the restart file from the Analysis and Assimilation configuration.

Long Range

The Long Range Forecast cycles four times per day (i.e. every 6 hours) and produces a daily 16-member 30-day ensemble forecast. There are 4 ensemble members in each cycle of this forecast configuration, each forced with a different CFS forecast member. It produces 6-hourly streamflow and daily land surface output, and, as with the other forecast configurations, is initialized with a common restart file from the Analysis and Assimilation configuration.


All NWM output will be stored in NetCDF format in one of three file types:
  1. 1km gridded NetCDF (land surface variables and forcing)
  2. 250m gridded NetCDF (ponded water depth and depth to soil saturation)
  3. Point-type NetCDF (stream routing and reservoir variables)

The two gridded files cover a rectangular domain stretching beyond the CONUS roughly from 19N to 58N, while the point NetCDF files contain model output from the CONUS and hydrologically contributing areas.

Output File Contents:

A full breakdown of output file contents can be found here

Viewing Output:

Output from the National Water Model can currently be viewed using the experimental interactive National Water Model image viewer. Output is also available for display on the interactive zoomable mapping interface mapping interface.

Downloading Output:

The full set of NWM output and a subset of forcing files is available on the NOAA Operational Model Archive and Distribution System (NOMADS) and the National Centers for Environmental Prediction (NCEP) FTP server at the following URLs:

Most NWM NetCDF output files are directly viewable using standard NetCDF visualization utilities. The exception are the channel output files containing streamflow and other variables representing processes along river reach segments. To support visualization of these variables, the latitude and longitude coordinates of the centroid of each reach are stored within a netCDF template file, outside of the NWM outputs. This file and scripts to append the coordinates are available here: here

NCEP encourages all users to ensure their decoders are flexible and are able to adequately handle changes in content order and also any volume changes which may be forthcoming. These elements may change with future NCEP model implementations. NCEP will make every attempt to alert users to these changes prior to any implementations.

To download the National Water Model product description document, click here.

Parameter Information

A subset of the model parameter files used by the operational implementation of the NWM is available. For a description of the files available, click here. To download a tar file of the available parameter files, click here.

Contact Information

Model implementation questions
Brian Cosgrove
OWP/Analysis and Prediction Division in Silver Spring, MD

Phone: (301) 427-9513
Data flow questions
Carissa Klemmer
NCEP/NCO Dataflow Team in College Park, MD

Phone: (301) 683-0567