Functional and structural connectivity within a recently burned drainage basin

Studies examining post-wildfire sediment transport have often focused on changes to individual landscape compartments (planar slopes, rills, gullies, channels, or alluvial fans) or have captured coarse-scale hydrologic and sediment transport events at the drainage basin scale. We advance the understanding of functional and structural connectivity by quantifying changes of the morphodynamics of and sediment transport along seven rill-gully threads (RGTs) after two low intensity rainstorms in a burned basin from the 2008 Gap fire near Goleta, CA, USA. TLS surveys conducted within two months of the initial fire and three days after the rainfall events provide point clouds for high-resolution digital terrain models (DTMs). DTM differencing techniques and morphological sediment budgets from the RGTs showed discontinuous sediment transport along the extent of these two landscape compartments immediately after the rainfall. Surface runoff was unable to remove dry ravel deposits within the RGTs and implied a high degree of structural disconnectivity there. Dry ravel and runoff erosion from the contributing areas to the RGTs indicated functional and structural connectivity at this scale of analysis. The results provide clear evidence that small amounts of rainfall and gravity-induced erosion are interacting at different scales within the recently burned watershed to produce structural and functional disconnectivity along the RGTs. While the current system was transport-limited during the analyzed event, higher magnitude rainstorms may produce enhanced connectivity, resulting in the ability of surface runoff to remove the stored sediments and perhaps produce debris flows.