Fire Effects on Runoff Generation and Sediment Yield from a Coarse-Textured Sagebrush- Dominated Landscape

Post-fire increases in runoff and sediment yield from sagebrush rangelands are commonly attributed to fire- induced soil water repellency and/or reduction in canopy and ground cover. Recent research has demonstrated the strength of soil water repellency and its influence on runoff and sediment generation on burned and unburned sagebrush landscapes may exhibit significant annual fluctuation. The transient nature of soil water repellency complicates assessment of fire effects on hydrologic processes and determination of post-fire hydrologic recovery. Rainfall simulation and concentrated flow (rill) methodologies were applied on a coarse-textured sagebrush site in the Reynolds Creek Experimental Watershed, southwestern Idaho, USA. Simulations were conducted immediately pre- and post-burn and in each of three years following fire to determine fire effects on runoff and erosion. The influences of soil water repellency and changes in ground cover on hydrology and erosion processes were assessed using water drop penetration test and point frame methodologies. Runoff doubled following burning at the large plot scale (32.5 m2) and on coppice microsites at the small plot scale (0.5 m2). Runoff from rill processes was 3 times greater on burned than unburned plots immediately post- fire. Fire-induced increases in runoff generation were insignificant 1 year post-fire. Sediment yield from burned large and concentrated flow plots was 10 and 4 times greater respectively than on unburned plots immediately following and 1 year post-fire. Sediment yield on burned plots at the small plot scale was 70% greater immediately post-fire, but was greatly reduced 1 year following fire. Soils on burned and unburned hillslopes were strongly water repellent the year of the fire. The strength of water repellency was reduced by 50% 1 year post-fire and soils were slightly water repellent on burned large plots 2 years post-fire. The fire reduced litter cover by 50% and bare ground was increased from 20% to over 70%. Litter cover and percent bare ground on burned plots returned to near pre-burn levels 3 years post-fire. The greatest impact of burning was on sediment yield from rill processes. The combined effect of strong background water repellency and the increase in bare ground parentage allowed overland flow to concentrate into rills where rill flow width decreased and flow depth and velocity increased. Fire impacts on rill processes were persistent 2 years post-fire. The results indicate soil water repellency facilitates runoff on burned and unburned hillslopes, but the presence of ground cover mitigates sediment entrainment. These data suggest runoff and erosion from burned coarse-textured sagebrush hillslopes may require two to three years to return to background levels with or without the presence of strongly water repellent soil conditions.