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ARS Home » Pacific West Area » Boise, Idaho » Northwest Watershed Research Center » Research » Publications at this Location » Publication #322345

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

item ROBICHAUD, PETER - Us Forest Service (FS)
item WAGENBRENNER, JOSEPH - Michigan Technological University
item Pierson Jr, Frederick
item SPAETH, KENNETH - Natural Resources Conservation Service (NRCS, USDA)
item ASHMUN, LOUSIE - Us Forest Service (FS)
item MOFFET, COREY - Samuel Roberts Noble Foundation, Inc

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2016
Publication Date: 1/29/2016
Citation: Robichaud, P.R., Wagenbrenner, J.W., Pierson Jr, F.B., Spaeth, K.E., Ashmun, L.E., Moffet, C.A. 2016. Infiltration and interrill erosion rates after a wildfire in western Montana, USA. Catena. 142:77-88.

Interpretive Summary: Runoff and erosion events on burned landscapes pose hazards to resources, property and infrastructure, and human lives. Although post-fire runoff and erosion have been widely investigated, studies of hydrologic and erosion responses from ash-capped soils are limited. In this study, we applied artificial rainfall to burned and unburned sloping forested sites to quantify fire impacts on infiltration, runoff generation, and erosion from ash-capped soils over a five year period and investigated relationships in soil water repellency and ground cover factors that influence runoff and erosion processes. Post-fire soil water repellency was a significant factor in the reduction of infiltration and increased runoff immediately after the fire. The loss of protective ground cover was the most significant factor for increased sediment concentrations and sediment yields following burning. Runoff and erosion were reduced five years following burning due to decreased occurrence of fire-altered soil water repellency and increased ground cover by vegetation and litter. The study provides a relative measure of the increased risk of runoff and erosion and the rate of hydrologic recovery following fire on sloping forested lands with ash-capped soils. The results provide a data set for future runoff and erosion modeling of burned forested landscapes and contribute insight to land manager and research assessments of runoff and erosion from steeply-sloping forested hillslopes.

Technical Abstract: The 2000 Valley Complex wildfire burned in steep montane forests with ash cap soils in western Montana, USA. The effects of high burn severity on forest soil hydrologic function was examined using rainfall simulations (100 mm h-1 for 1 h) on 0.5-m2 plots. Infiltration rates and sediment yields and concentrations were compared among three treatments—burned, bare (unburned with all surface vegetation, litter, and duff removed prior to each simulation) and control (undisturbed). 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 (3–27 g) 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 response was high throughout the study. In the year of the fire, the burned sites had high rates of soil water repellency (88%) and little ground cover protection (10%). This resulted in low infiltration rates (30 mm), high sediment concentrations (median value 21 g L-1), and high sediment yields (878 g). 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 of runoff), and lower sediment yields (8 g) 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. Post-fire soil water repellency was a significant factor in the reduced infiltration 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.