Post-fire Soil Erosion Potential, California

This data represents FRAP's best estimate of the Revised Universal Soil Loss Equation (RUSLE) in a post-wildfire environment. The RUSLE is an empirical equation designed for the computation of average soil loss in agricultural fields. This equation was developed for detachment capacity limited erosion in fields with negligible curvature and no deposition. It represents soil loss averaged over time and total area. The equation has the following form (Wischmeier and Smith 1978, Renard et al. 1991) A = R * K * L * S * C * P where, A [ton/acre/year] is estimate average soil loss, R [Erosion Index units/year] is rainfall-runoff erosivity factor, K [tons/acre/unit R] is soil erodibility factor (source data STATSGO) LS [dimensionless] is topographic (length-slope) factor (Moore and Burch 1986, Van Remortel et al. 2001), C [dimensionless] is coverage factor, and P [dimensionless] is prevention practices factor. FRAP estimated the coverage factor (C) for post fire conditions by examining the relationship of major land cover types (WHR10NAME) and WHR density classes (WHRDENSITY) from the FRAP Multi-source Land Cover data set (v02_2) crossed with wildfire fuel rank (FUEL_RANK) from the FRAP Fire Threat (v04_1) data set. For example, areas with low vegetative cover and high fuel rank (high fuel rank indicates intense fires that can remove a large proportion of the cover) receive higher values. Areas of high cover and low fuel rank receive lower values, because after fire, a higher proportion of their original cover will remain. FRAP did not include prevention practices (P) in the calculation of post-fire erosion potential. Because the RUSLE was developed for use in agricultural fields it has not been as extensively tested in natural land cover landscapes. FRAP found that the coverage factor (C) had the greatest influence on the total erosion value so slight variations in C can produce very different results. FRAP researched the literature to find the best empirical measures of C in undisturbed and disturbed landscapes yet the values of C used in this model are still no better than rough estimates (Lopez et al. 1998, Dissmeyer and Foster 1981, 1980) Rainfall intensity data (R) is the 2-year, 6-hour amount and comes from the NOAA Atlas 14, Volume 1, Version 3 estimates for Southeastern California (sa2yr06ha) and Atlas 2, Volume XI for the rest of the State (na2_ca_2yr6hr) (see http://www.nws.noaa.gov/oh/hdsc). These two datasets did not align properly and a ArcInfo GRID NIBBLE process was used to fill the gaps. Soil erodibility (K) is derived from the STATSGO soil dataset attribute KMEAN. We calculated the topographic length-slope factor (LS) using Van Remortel, Hamilton and Hickey's (2001) algorithm (rusle_ls_4_unix.aml,ver. 4, 12/18/2003) (see http://www.yogibob.com/slope/slope.html) on the elevation data set DEM9099_1. Caution is also urged because this model does not reflect many other sources of erosion in the landscape, such as roads, agricultural practices, or other disturbances that expose bare ground to the effects of rainfall. These data can only be used to indicate the portion of erosion potential that comes from the direct effects of wildland fire on the landscape, and not any other factors.