Infiltration and interrill erosion rates after a wildfire in western Montana, USA

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© 2015. The 2000 Valley Complex wildfire burned in steep montane forests with ash cap soils in western Montana, USA. The effects of high soil burn severity on forest soil hydrologic function were examined using rainfall simulations (100 mm h-1 for 1 h) on 0.5-m2 plots. Infiltration rates, sediment yields and sediment concentrations were compared among three treatments: control (unburned and undisturbed); bare (unburned with all surface vegetation, litter, and duff removed prior to each simulation); and burned. Rainfall simulations were done immediately after the fire and repeated in 2001, 2002, and 2005. Soil moisture, water repellency, and understory canopy and ground cover were measured and related to infiltration rates and sediment yields. The unburned forest soil was water repellent at the mineral surface. This surface repellency was no longer detected after it was burned at high severity, but a post-fire water repellent soil layer was observed at 1-2 cm below the surface. The control plots had high ground cover (90% overall), infiltration of 44-48 mm, and very low sediment concentrations (median values of 0.1-0.6 g L-1) and sediment yields (6-54 g m-2) for all years despite changes in soil moisture and strong water repellency. The bare and control plots had similar water repellency values, but the interrill erosion in the bare plots was high throughout the study (624-1277 g m-2). In the year of the fire, the burned sites had high rates of soil water repellency (88%) and little ground cover (10%). This resulted in low infiltration rates (30 mm), high sediment concentrations (median value 21 g L-1), and high sediment yields (1157 g m-2). By 2005, the fire-altered water repellency decreased in occurrence (48%) and severity, and the ground cover increased (42%). This resulted in much greater infiltration (84 mm), lower sediment concentration (median value 0.5 g L-1), and lower sediment yields (15 g m-2) on the burned plots. The importance of ground cover for preventing interrill erosion was demonstrated by the very low sediment yields on the control plots as compared to the bare and burned plots. The strength and occurrence of water repellency in both the unburned and burned sites decreased as soil moisture increased; however, strong soil water repellency was detected at the soil surface whenever unburned soils were dry. Fire-altered soil water repellency influenced the infiltration capacity and increased runoff rates immediately after the fire; however, the loss of protective ground cover was a more significant factor for the increased sediment concentrations and sediment yields.

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