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 modelling framework 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 produces hydrologic guidance at a very fine spatial and temporal scale. It complements official NWS river forecasts at approximately 4000 locations across the CONUS and produces guidance at millions of other locations that do not have a traditional river forecast.

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NWM Operational Configuration:

The NWM runs four uncoupled analyses (simulations of current conditions) with look-back periods ranging from 28 hours to 3 hours. These analyses are used to provide initial conditions for the model’s forecast runs. Over the CONUS, short-range forecasts are executed hourly, while over Hawaii they are executed four times per day. Additionally, the CONUS features medium-range and long-range forecasts which are each produced four times per day. All model configurations provide streamflow for 2.7 million river reaches and other hydrologic information on 1km and 250m grids. The NWM provides complementary hydrologic guidance at current National Weather Service (NWS) river forecast locations and significantly expands guidance coverage and type in underserved locations.

The core of the NWM 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) and Stage IV Multisensor Precipitation Estimator (MPE) radar-gauge observed precipitation data, and High Resolution Rapid Refresh (HRRR) , Rapid Refresh (RAP) , North American Mesoscale Nest (NAM-Nest) , 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, Hawaii and additional hydrologically-contributing areas. Land surface output is available on a larger CONUS+ domain that extends beyond the CONUS into Canada and Mexico (roughly from latitude 19N to 58N), and covers Hawaii as well. United States Geological Survey (USGS) streamflow observations are assimilated into each of the four analysis and assimilation configuration and all analysis and forecast configurations benefit from the inclusion of over 5,000 reservoirs.

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Model Details

General Framework:

The NWM is run in six CONUS configurations and two Hawaii configurations:

• CONUS
  • Standard Analysis (self-cycling with 3-hour look-back, used to initialize CONUS short- and medium-range forecasts)
  • Extended Analysis (28-hour lookback used to initialize Standard Analysis once per day)
  • Long-Range Analysis (12-hour lookback, used to initialize Long-Range forecasts)
  • Short-Range Forecast: 18-hour deterministic (single value) forecast
  • Medium-Range Forecast: Seven-member ensemble forecasts out to 10 days (member 1) and 8.5 days (members 2-7)
  • Long-Range Forecast: 30-day four-member ensemble forecast
• Hawaii
  • Hawaii Analysis (self-cycling with 3-hour look-back, used to initialize Hawaii short-range forecasts)
  • Hawaii Short-Range Forecast: 60-hour deterministic (single value) forecast for Hawaii domain

Analysis and Assimilation Configurations:

Standard Analysis and Assimilation

The Standard 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 is internally cycling, with each subsequent Standard Analysis starting from the previous hour’s run. The exception is the 19Z Standard Analysis cycle which ingests initial conditions from the Extended Analysis below. The Standard Analysis also produces restart files each hour which are used to initialize the short-, medium-, and long-range 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, while stream-gauge observations are assimilated from the USGS.

Extended Analysis and Assimilation

The Extended Analysis and Assimilation configuration cycles once per day and produces an analysis of the current streamflow and other surface and near-surface hydrologic states across the contiguous United States (CONUS). This configuration is internally cycling, with each subsequent Extended Analysis starting from the previous day’s run. This configuration also produces restart files which are used to initialize the 19Z Standard Analysis simulation. Meteorological forcing data are drawn from the Stage IV national mosaic of RFC MPE precipitation data along with short-range RAP and HRRR data.

Long-Range Analysis and Assimilation

The Long-Range 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), using higher quality precipitation data than is available to the Standard Analysis and Assimilation. This configuration is internally cycling, with each subsequent Long-Range Analysis Standard Analysis starting from the previous hour’s run. The Long-Range Analysis also produces restart files which are used to initialize the long-range forecast simulations. Like the Long-Range configuration, it uses a simplified set of physics in comparison to the Standard and Extended Analyses. Meteorological forcing data are drawn from the MRMS Gauge-adjusted and Radar-only observed precipitation products along with short-range RAP and HRRR, while stream-gauge observations are assimilated from the USGS.

Hawaii Analysis and Assimilation

The Hawaii 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 Hawaii domain. This configuration is internally cycling, with each subsequent Hawaii Analysis starting from the previous hour’s run. The Hawaii Analysis also produces restart files each hour which are used to initialize the Hawaii Short-Range forecast simulation. Meteorological forcing data are drawn from the NAM-NEST NWP Model, while stream-gauge observations are assimilated from the USGS.

Forecast Configurations:

Output:

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 file types are used for two separate rectangular domains, 1) the CONUS+ (roughly from 19N to 58N) and 2) the Hawaiian Islands, while the point NetCDF file type is used for the CONUS (including adjacent hydrologically contributing areas) and the Hawaiian Islands.

Viewing Output:

Output from the National Water Model is currently visualized on this website using the experimental interactive map, and the experimental image viewer.

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Downloading Output:

The full set of NWM output data 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 links:

• NCEP NOMADS HTTP service.
• NCEP FTP service.

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 made available in a separate ESRI file geo-database (gdb), outside of the NWM outputs. The file gdb can be used in many common GIS software utilities to attach coordinates to the data. The file gdb is available 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.

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