Burned Area Emergency Response Tools

Curve Number methods

The NRCS Curve Number methods are the second most commonly used post-fire runoff estimation method by BAER team members (30%) (Foltz and others 2008).

The Curve Number method was developed by the U.S. Department of Agriculture Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service (SCS), to estimate runoff depth. It considers 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; therfore, the Curve Number is the single most important parameter in this method.

To use NRCS Curve Number methods, you will need WILDCAT4 or FIRE HYDRO.

Input Requirement (USDA SCS 1991)

To use the NRCS Curve Number methods, the following information is required (USDA SCS 1991):

• drainage area (A) in ft², mi², or acres;
• rainfall amount (P) for a storm duration of 24 hours, with a given recurrence interval;
• Hydrologic Soil Groups (A, B, C, and D) in which the watershed soil is classified;
• average watershed slope (Y) in percent;
• flow length (l), the longest flow path, from the watershed divide to the outlet, in feet; and
• pre-fire and post-fire runoff Curve Numbers (CNs).

Use of Post-Fire CNs by BAER Specialists

• Cerrelli (2005) for Region 1
• Story (2003) for Region 1
• Stuart (2000) for Region 1
• Kuyumjian for Region 3
• Livingston and others (2005) for Region 3
• U.S. Forest Service Coronado National Forest (2003) for Region 3
• Higginson and Jarnecke (2007) for Region 4
• Solt and Muir (2006) for Region 4

Advantages

The following were advantages to applying the NRCS CN methods for post-fire runoff and erosion estimation.

• NRCS CN methods are applicable for input to methods that calculate peak flow.
• Two CN methods and models (WILDCAT4 and FIRE HYDRO) are available for post-fire application.
• WILDCAT4 considers shorter-duration storm (e.g., 15-min) to 24-hour storm duration, which is adequate for the regions where the damaging storm is short duration, such as 15 or 30 minutes.

Disadvantages

The following were disadvantages to applying the NRCS CN methods for post-fire runoff and erosion estimation.

• NRCS CN methods do not estimate erosion.
• NRCS CN methods do not consider post-fire debris flow/torrent.
• NRCS CN methods are applicable to smaller watersheds (<5 mi²).
• The FIRE HYDRO method only considers 24-hour storm duration.
• The user must determine pre-fire and post-fire CNs, so the estimated peak flow is subjective to users because the CN is a sensitive parameter.
• There are no guidelines to determine post-fire CN except in Regions 1 and 3.
• There is difficulty in combining runoff from areas of different CNs within a watershed.
• The NRCS CN methods will likely underestimate runoff when applying weighted averages of CNs for high burn severity areas in arid weighter conditions.
• The NRCS CN Methods use English units only.

Example Results

The 2005 Blackerby Fire in the Nez Perce National Forest, Idaho

REFERENCES

Foltz, Randy B.; Robichaud, Peter R.; Rhee, Hakjun. 2008. A synthesis of post-fire road treatments for BAER teams: methods, treatment effectiveness, and decision-making tools for rehabilitation. Gen. Tech. Rep. RMRS-GTR. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station (in preparation).

U.S. Department of Agriculture, Soil Conservation Service. 1991. Engineering field handbook: chapter 2--estimating runoff. In: National engineering handbook. Washington, D.C.: U.S. Department of Agriculture, Soil Conservation Service: part 650.

 

USDA Forest Service - RMRS - Moscow Forestry Sciences Laboratory
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