How do geomorphic effects of rainfall vary with storm type and spatial scale in a post-fire landscape?

- Study compares post-fire erosion and channel change following different storm types.
- High intensity summer storms produced greatest localized hillslope erosion.
- Total rainfall above high intensity thresholds best predicts hillslope erosion.
- Long duration spatially extensive storm produced greatest channel change.
- Within the long storm, downstream channel aggradation followed high intensity rain.

In post-fire landscapes, increased runoff and soil erosion can cause rapid geomorphic change. We examined how different types of rainfall events in 2013 affected hillslope-scale erosion and watershed-scale channel change in two 14-16 km2 watersheds within the 2012 High Park Fire burn area in northern Colorado, USA. The first set of rainfall events was a sequence of 12 short, spatially variable summer convective rain storms, and the second was a > 200 mm week-long storm in September. We compared rainfall characteristics, hillslope sediment yields, stream stage, and channel geometry changes from the summer storms to those from the September storm. The summer storms had a wide range of rainfall intensities, and each storm produced erosion primarily in one study watershed. The September storm rainfall had less spatial variability, covered both watersheds, and its total rainfall depth was 1.5 to 2.5 times greater than the total summer rainfall. Because rainfall intensities were highest during some summer storms, average hillslope sediment yields were higher for summer storms (6 Mg ha− 1) than for the September storm (3 Mg ha− 1). Maximum storm rainfall intensities were good predictors of hillslope sediment yield, but sediment yield correlated most strongly with total depths of rainfall exceeding 10-30 mm h− 1 intensity thresholds. The combined summer storms produced relatively small changes in mean channel bed elevation and cross section area, with no clear pattern of incision or aggradation. In contrast, the sustained rain across the entire study area during the September storm led to extensive upstream incision and downstream aggradation. Because of different spatial coverage of storms, summer storms produced more total hillslope erosion, whereas the September storm produced the greatest total channel changes. At both scales, high intensity rainfall above a threshold was responsible for inducing most of the geomorphic change.