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NSS example

WMS includes an interface to the National Streamflow Statistics Program (NSS). The NSS program is one of the simplest models to set up and is the only statistical (rather than deterministic) model in WMS. It is a graphical interface to all the current statewide and metropolitan area regression equations for the United States, and will output peak outflow and a rough hydrograph for ungauged watersheds. Each US state has different subregions with different equations. The NSS interface in WMS version 8.1 and later uses the same database as the Windows version of the NSS program (released in 2006) which supersedes all previous versions of the NFF program including the Windows version of NFF (released in 2003) and the 1993 derivative used in previous versions of WMS.

It may be necessary to install the latest version of the NSS program from this web site to get the program to work:

The regression equations are a result of years of effort by the United States Geological Survey (USGS) to develop regional regression equations for estimating flood magnitude and frequency of ungauged watersheds. The USGS, in cooperation with the Federal Highway Administration and the Federal Emergency Management Agency compiled all the regression equations into a single database file. This database file is the basis of the NFF program, which can be used to guide through the input required to compute peak flows for different frequencies using the database of state by state regression equations. The accuracy of results will be based on how close in properties the ungauged watershed is to a gauged one used to determine the regional regression equation.

The NSS interface in WMS provides a Windows-based, graphical user interface to the national database of regression equations. The entire program is run from a single dialog. Further, if a digital terrain model is available for the study area, all of the geometric parameters required for the regression equations are automatically supplied as the individual equations are specified. These parameters include area, slope, elevation, distances, and others. The GIS overlay command can be used to compute other variables such as forest cover, lake cover, etc.

The NSS equations are useful for estimating a peak flood discharge and typical flood hydrograph for a given recurrence interval of an unregulated rural or urban watershed. These techniques should be useful to engineers and hydrologists for planning and design purposes.

WMS uses the DLL's that are part of the USGS Windows based version of the NSS program. The USGS program does not include any kind of GIS component, but if wanting to run the NSS outside of WMS it is still possible to use WMS to determine the necessary input. Complete documentation on the USGS NSS program, including specific information for each state can be found on line at the following USGS website.

The descriptions for each state are found at the bottom of the above web page.

The NSS interface is included with all paid editions of WMS.

NSS Menu

The NSS model menu has the following commands:

  • Read Simulation – Restorea the tree and parameters to continue with a particular model at a later time.
  • Save Simulation – Used to save topologic tree structures with any state, regions and parameters which have been defined.
  • Run Simulation – Brings up the National Streamflow Statistics Method dialog.

NSS State and National Urban Equations

The regional equations for each state correspond to analysis for rural watersheds. However each state includes the national urban equation in its list of regions. The national urban equation contains additional parameter values and must be used in conjunction with the appropriate regional rural equation. In addition, some states include regional urban equations which are used separately from the rural equations.

The list of available regression equations changes according to which equations have already been selected. For example if a regional urban equation is selected all other equations disappear, since it must be used by itself. Further, if a regional rural equation is selected any regional urban equations are removed since they cannot be combined.

Model Troubleshooting

If unable to get NSS to run from within WMS, try the following:

There may not be write access to the C:\Program Files directory on the computer. To bypass this, perform the following steps:

  1. Navigate to the C:\Program Files\WMS82 directory
  2. Copy the "wmsnss.exe" file and the "NSSv4.mdb" file from this directory and place them in a directory where there is write access, such as My Documents
  3. Launch WMS and Select Edit | Preferences
  4. Scroll down to the NSS heading and click the Browse... button.
  5. Navigate to the wmsnss.exe file in the directory to where there is write privileges and click Open

NSS can now be run.

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