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Burned Area Emergency Response Tools
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Curve Number Methods ->
Details of Curve Number Methods
The curve number methods consider rainfall, soils, cover type, treatment/conservation practices, hydrologic conditions, and topography (slope steepness). Users have to choose a curve number (CN) based on cover type, treatment, hydrologic conditions, and Hydrologic Soil Group to estimate runoff and peak flow; therefore, the curve number is a single most important parameter in this method.
Tehre are two CN methods that BAER teams frequently use- WILDCAT4 (Hawkins and Greenberg 1990) is an MS DOS program, and FIRE HYDRO (Cerrelli 2005) is an EXCEL spreadsheet.
The WILDCAT4 (Hawkins and Greenberg, 1990) is a storm runoff/hydrograph model that uses triangular unit hydrographs. The WILDCAT4 model requires the following information:
WILDCAT4 tends to have long times of concentration (Tc). If a shorter Tc is preferred, the user may put a substitute in the Tc formula below (Dunne and Leopold, 1978), which will generate a higher peak flow due to a quicker watershed response to the storm events.
WILDCAT4 is recommended for watersheds of 5 mi2 or less. WILDCAT4 is easy to use. However, the user has to specify the CN of pre- and post-fire conditions and the program runs in DOS. WILDCAT5, a Windows version of the WILDCAT program, is in development and will be released in the near future (pers. comm. Hawkins 2008).
Cerrelli (2005) developed a spreadsheet, called FIRE HYDRO, to assist NRCS and Forest Service personnel in estimating design peak flows for the burned areas of Montana. FIRE HYDRO is a peak flow analysis tool for 2-, 5-, 10-, 25-, 50-, and 100-year, 24-hour rainfall runoff events for the pre- and post-fire conditions. The required input data includes the following: drainage area (acre); average watershed slope (%); CN; and 2- to 100-year, 6- and 24-hour rainfall depths that are available from NOAA (2008). The 6- and 24-hour rainfall depths are required to determine the SCS rainfall distribution type (Type I, IA, II, or III). Most of Region 1, including Montana, has Type II, whcih produces the highest peak flow among the SCS rainfall distribution types. Cerrelli (2005) assumed that the runoff Curve Numbers of bare soil cover type or poor hydrologic condition were used for post-fire conditions. However, there is no clear guideline to choose post-fire runoff Curve Numbers. FIRE HYDRO is applicable for 24-hour rainfall events only, and not applicable for short duration rainfall events such as a 1-hour storm or less.
Hydrologic Soil Groups (USDA SCS, 1991)
There are limited numbers of studies that provide post-fire runoff Curve Numbers. Springer and Hawkins (2005) attempted to provide a guideline to choosing post-fire runoff Curve Numbers based on the 2000 Cerro Grande Fire in New Mexico and concluded that "the post-fire trends in CN and peak flows are not readily explained and will be a topic of future research."
Cerrelli (2005) provided a guideline to select post-fire CN based on burn severity and hydrologic soil grouping specific to the Bitterroot National Forest wildfires. He did not find appropriate CNs in his initial search of the literature for CN values for burned areas in southwestern Montana. Consequently, Montana NRCS engineers created a guideline based on the existing NRCS CN/land use table. However, no gaging or calibrating took place to verify or improve this guideline. New protection practices (e.g. road treatments) were implemented using these newer NRCS guidelines. In the spring and summer following the fires, the region experienced its 2- and 5-year, 24-hour storm events and the new protection practices were not adversely affected.
Story (2003) circulated an e-mail to suggest the following post-fire CNs for Region 1.
Stuart (2000) used FIRE HYDRO (Cerrelli 2005) to estimate storm event peak flow on the 2000 Maudlow Fire, Montana. Based on observations of unburned conditions, land type/cover type, burn intensity, and water repellency conditions, the following CNs were selected.
Livingston and others (2005) provided a guideline to choose the post-fire CNs with a range of values. They used computed CNs and compared pre-and post-fire CNs for 31 small (0.12 to 2.5 mi2) subbasins in the Los Alamos area in New Mexico, and 24 small (0.11 to 2.3 mi2) subbasins affected by the 2002 Long Mesa Fire at Mesa Verde National Park in Colorado. Their study results are applicable to the Los Alamos area and other areas in the southwest with similar pre-fire CN values and hydrology; however, they are not applicable to areas with different pre-fire rainfall and runoff characteristics.
To classify the soil burn severity of the whole watershed/basin, they used Wildfire Hydrologic Impact (WHI), based on the percentage of high and moderate soil burn severity, and a general relation between pre- and post-fire CN ratio. The WHI can be determined using the table or figure below.
Post-fire runoff curve number can be estimated using the figure below if pre-fire CN is known.
U.S. Forest Service Coronado National Forest (2003) used WILDCAT4 (Hawkins and Greenberg 1990) to estimate peak flow runoff in key watersheds under pre-and post-fire conditions on the 2003 Aspen Fire, Arizona. Limited sampling of water repellency conditions indicated moderate water repellency occurred on severely burned soils.
Since there are very limited studies and guidelines for choosing CNs for post-fire conditions, BAER team members often use simple rules of their own. Details on these rules are found in the NRCS CN Methods, section 3.6.2. For example, in the Salt Creek BAER Hydrology Speical Report, href="#Higginson2007">Higginson and Jarnecke (2007) used WILDCAT4 (Hawkins and Greenberg 1990) to estimate pre- and post-fire runoff on the 2007 Salt Creek Fire, Utah. They used the following rules to determine post-fire CNs.
Solt and Muir (2006) used WILDCAT4 (Hawkins and Greenberg 1990) to estimate pre- and post-fire runoff on the 2006 Warm Fire, Utah. The limestone derived soils of the burned area were determined to be in Hydrologic Soil Group D (low infiltration) and in the ponderosa pine/juniper vegetation type. The following CNs were selected.
The user has to determine pre-fire and post-fire CN, which is a sensitive parameter; therefore, the estimated peak flow is subjective to users.
Once the user has determined CNs for each HRU within a watershed, the problem arises of how to combine them. CNs and runoff depth are not linearly related, but curvilinearly related (Grove and others 1998). A weighted average of all CNs in a watershed is commonly used to reduce the number of calculations. The underestimation of runoff using weighted average CNs is most severe for wide CN ranges, as would occur in watersheds containing low and high severity burns. Low CN values and low precipitation depths, as would occur in unburned southwestern watersheds, would result in underestimation of runoff. Therefore, care should be exercised when applying weighted average CNs.
Another approach is to use distributed CNs in a GIS application. However, White (1988) and Stuebe and Johnston (1990) reported that using distributed CNs resulted in as much as 100 percent higher runoff than using weighted average CNs.
The preferred method to estimate runoff from watersheds with different
CNs is to combine runoff amounts from each HRU.
Modified October 28, 2010
USDA Forest Service - RMRS - Moscow Forestry Sciences