CORRELATION OF AREA AND POINT MEASUREMENTS OF RAINFALL INTENSITIES IN A BURNED AREA SUBJECT TO DEBRIS FLOWS

Accurate estimates of rainfall intensities are essential in understanding the physics of post-fire runoff and developing debris flow warning systems that are based on intensity-duration thresholds. Rainfall intensity is a critical in debris flow production as the relation between infiltration rate and rainfall intensity controls overland flow production. To understand the variability of rainfall intensity estimates in debris-flow regions, a network of 7 tipping-bucket, recording rain gages was established in a burned area in the Verdugo Hills of Burbank, CA in 2005 as part of the USGS-NWS debris flow warning intensive research area. During the winter rains of 2005-2006, rainfall intensities (5-min) from these gages were inter-compared and compared to instantaneous estimates of rainfall intensities derived from a SMART-R radar unit positioned nearby. The SMART-R produces a 120-m grid of rainfall intensities, which represent conditions several hundred meters above the surface. This comparison addresses two questions: (1) Do closely spaced rain gages produce similar estimates of rainfall intensity, and (2) how well do the rainfall intensities from the SMART-R compare to the intensities measured by rain gages at 1-m above the surface?

Storms on 1-2 January and February 27-28^th 2006 produced overland flow, with the first storm transporting sediment. Total rainfall for the two storms varied more in the 7 rain gages (85.0 mm to 166.3 mm; standard deviation of 30.3 mm) than in the 3 radar cells that match the gages (196.8 to 198.8 mm; standard deviation of 1.0 mm). Two gages in the valley recorded more rainfall than four on the ridges although the difference was not significant (p=0.21). The 5-min rain intensity time series were highly correlated (r² = 0.84 to 0.95; analyzed for each storm separately). Comparisons between the radar and rain gage rainfall intensities produced lower correlations (r² = 0.42-0.64) than the inter-gage comparison. Notably, the peak rainfall intensities estimated with radar are higher than those estimated with rain gages. This may be due to evaporation of raindrops as they fall the height of radar measurement to where they are intercepted by the rain gages. These differences would be important in determining the threshold rain intensity for runoff production in burned basins.