Comparison of modelling results to observed sediment delivery
Following the three October, 2016 rainfall events, a large amount of sediment was produced and accumulated on the surface of road 1, in constructed detention basins along road 2, and on the toe slopes downhill from the fire. Vollmer (2016) reported that 227 Mg of sediment were removed from Road 1 following the three big storm events after the wildfire. The WEPP Windows modelling based on the 22 hillslope profiles intersecting Road 1 resulted in an estimated average deposition of 40.4 kg m-2 (404 Mg ha-1 or an average depth of about 4-5 cm) on the road surface (Figures 5 and 7). The length of Road 1 within the fire is about 790 m, and the width is about 8 m. This means the total estimated deposition is 255 Mg, a number of similar magnitude to the amount of sediment removed.
We measured the total volume of the four sediment basins below road 2 to be 79.13 m3. The watershed topographies describing each of the sub-catchments above these sediment basins was entered into the WEPP Windows Watershed version, and the observed dimensions of each basin, assuming an “open” or weir type outlet was described in detail in the “Structures” file in the WEPP Windows Watershed Version interface. GeoWEPP delineated the watersheds above each of the sediment basins. For the observed climate, WEPP predicted that 139.6 Mg of sediment needed to be removed from the sediment basins following the October, 2016 storms. The amount of sediment removed from these four basins was estimated to be 134 m3 (about 120 Mg; Personal Communication, R. Wigart, Stormwater Coordinator for the Tahoe Basin, Eldorado County, California, 13 December 2016). Although not a formal validation, these comparisons suggest that the WEPP-predicted erosion and deposition rates following wildfire are reasonable.