The purpose of the NFF mailing list is to provide information on the U.S. Geological Survey (USGS) National Flood Frequency (NFF) program. NFF is a computer application that is used to estimate peak discharges for unregulated streams. Values (discharges) derived by the program often serve as input for other applications (such as hydraulic computer models) that are used to technically support end-products (map revisions) of the National Flood Insurance Program (NFIP).

Discussion on USGS Regression Equations and the NFF Program

The USGS developed a computer program titled "National Flood Frequency" or "NFF" that estimates the flood frequency and magnitude for ungaged sites through the application of the appropriate regional regression equations. NFF was released in 1993 and does not incorporate any revisions to regional regression equations that occurred after September 30, 1993. Since 1993 a significant number of the regression equations have been revised. The USGS is in the process of revising the NFF computer program to incorporate the updated regression equations. The revised version of NFF will be released soon.

The regional regression equations are currently being used for National Flood Insurance Program (NFIP) purposes. Therefore, FEMA would like to continue with this listserv and discuss issues of interest in the application of the regional regression equations for NFIP purposes. Upon the release of the revised NFF program, the focus of this listserv will shift to assist users in becoming familiar with the revised NFF program and its application for NFIP purposes.

Southwestern United States Equations

In addition to equations for all the individual States, NFF also contains equations from a regional flood-frequency study for the southwestern U. S., which includes all of Arizona, Utah, most of Nevada, and parts of California, Colorado, Idaho, New Mexico, Oregon, Texas, and Wyoming. This study area was divided into 16 hydrologic regions as shown in Figure 1.

Figure 1. Southwestern U. S. Hydrologic Regions

The purpose of the study was to develop more reliable methods for estimating flows on a truly regional basis. Some of the individual State equations have large standard errors and large differences in flood frequency relationships at State boundaries. In addition, the data set available for the regional study provided over 1,300 gaged sites with about 40,000 station years of annual maximum peaks. The focus of the study was on watersheds with drainage areas of less than 200 square miles and arid areas with less than 20 inches of mean annual precipitation.

In the 16 sets of equations for the regions there are six explanatory variables: 1. Drainage area in square miles (AREA); 2. Mean basin elevation in feet NGVD divided by 1,000 (ELEV/1000); 3. Mean annual precipitation in inches (PREC); 4. Mean annual free water-surface evaporation in inches (EVAP); 5. Latitude of the site in decimal degrees minus 28 divided by 10 [(LAT-28)/10]; and 6. Longitude of the site in decimal degrees minus 99 divided by 10 [(LONG-99)/10]. When using the software for calculations, the user should enter the actual values of ELEV, LAT, and LONG, as the software will perform the needed adjustments.

The values of AREA, ELEV, LAT, and LONG can be measured from the appropriate USGS topo map(s). The NFF publication (WRIR 94-4002) contains figures for mean annual free water-surface evaporation in regions 11 and 16 on pages 184 and 185, respectively. Mean annual precipitation values in inches can be determined from U.S. Weather Bureau publications (Climatography of the United States, no. 60, 1959-1961 [section for each state], and Normal Annual Precipitation [1931-1960] for the States of Arizona, Colorado, New Mexico, Utah, 1:500,000 scale maps).

To illustrate the differences between the regional and State equations, we will use as an example a portion of Tombstone Gulch in southern Arizona. Figure 2 shows the portion of the gulch with the drainage basin delineated on the Tombstone, AZ, 7.5-minute USGS quad map.

Figure 2. Tombstone Gulch Selected Drainage Basin

We will calculate the discharges using both the Southwestern U.S. equations and the Arizona equations. For both, the only variable is the drainage area. We measured the drainage area as 130 acres, or 0.203 mi2.

Figure 3 shows the results from the Southwestern U.S. equations and Figure 4 shows the results from the Arizona equations.

Figure 3. Results from Southwestern U.S. Equations
Figure 4. Results from Arizona Equations

From this example, we see that the results can vary significantly between the two sets of equations. The Southwestern U.S. standard error varies from 37% to 57% with equivalent years of record from 2 to 16 years. The Arizona standard error varies from 58% to 102% with equivalent years of record from 3 to 10 years. Obviously, the Southwestern U.S. result provides better estimates for this example. Since no guidance is provided in the NFF publication as to the preferability of one set of equations over the other for a particular region, we advise that you contact the USGS State representative for the area of interest. For assistance visit the USGS Water Resources District Chiefs listing at: http://water.usgs.gov/district_chief.html to contact any State office.

Previous Bulletin Topics

  • Introduction to the NFF Program and USGS regression equations, the applicability of the regression equations, and the advantages and limitations of the regression equations
  • Use of USGS regression equations in the NFIP and criteria for using USGS regression equations in the NFIP
  • Revisions to the USGS regression equations since the NFF software was released
  • Part 1. Unusual parameters of USGS regression equations and how to obtain them
  • Part 2. Unusual parameters of USGS regression equations and how to obtain them
  • Part 3. Unusual parameters of USGS regression equations and how to obtain them
  • Examples in which USGS regression equations are used for NFIP purposes
  • How to treat State Line faults (basins lying in more than one state)
  • Estimating drainage area and cross sections from USGS topo maps
  • Measures of accuracy in NFF
  • Weighting NFF results with observed data
  • Estimation of extreme floods
  • Flood hydrograph estimation using NFF
  • Urban Flood Hydrograph Estimation Using NFF

Upcoming Bulletin Topics

    NFF Version 4.0

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Last Modified: Monday, 25-Jun-2007 11:57:20 EDT