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Exposed mineral soil is an immediate result of forest fires with direct relevance on surface runoff and soil erosion. The goal of this study was to determine which topographic features influence the distribution of exposed mineral soil following wildfire in forested watersheds. In a field investigation 2 months after a simulated wildfire, ground cover was measured and a number of topographic variables, including slope, aspect, distance to streams, solar radiation, elevation, curvature, and profile curvature, were used to determine their relationships to postfire exposed mineral soil. The spatial distribution of fire effects along hill slopes was assessed using geographic information system (GIS), and a regressional relationship was derived to estimate postfire exposed mineral soil from key topographic variables, namely, aspect, solar radiation, and profile curvature. Solar radiation, aspect, and profile curvature were significantly correlated with the amount of soil exposed after the fire; however, aspect and profile curvature were the leading variables in the predictive regression model. Convex-shaped profile curvatures on south-facing slopes had higher values of exposed mineral soil than concave surfaces on north-facing slopes. These results can help land managers with decisions on prioritizing areas in need of pre- or postfire treatment.

The assessment of water yield from hillslopes into streams is critical in managing water supply and aquatic habitat. Streamflow is typically composed of surface runoff, subsurface lateral flow, and groundwater baseflow; baseflow sustains the stream during the dry season. The Water Erosion Prediction Project (WEPP) model simulates surface runoff, subsurface lateral flow, soil water, and deep percolation. However, to adequately simulate hydrologic conditions with significant quantities of groundwater flow into streams, a baseflow component for WEPP is needed. The objectives of this study were (1) to simulate streamflow in the Priest River Experimental Forest in the U.S. Pacific Northwest using the WEPP model and a baseflow routine, and (2) to compare the performance of the WEPP model with and without including the baseflow using observed streamflow data. The baseflow was determined using a linear reservoir model. The WEPP-simulated and observed streamflows were in reasonable agreement when baseflow was considered, with an overall Nash-Sutcliffe efficiency (NSE) of 0.67 and deviation of runoff volume (D v ) of 7%. In contrast, the WEPP simulations without including baseflow resulted in an overall NSE of 0.57 and D v of 47%. On average, the simulated baseflow accounted for 43% of the streamflow and 12% of precipitation annually. Integration of WEPP with a baseflow routine improved the model’s applicability to watersheds where groundwater contributes to streamflow.

Assessment of water yields from watersheds into streams and rivers is critical to managing water supply and supporting aquatic life. Surface runoff typically contributes the most to peak discharge of a hydrograph while subsurface flow dominates the falling limb of hydrograph and baseflow contributes to streamflow from shallow unconfined aquifers primarily during the non-rainy season. The Water Erosion Prediction Project (WEPP) model is a physically-based, distributed-parameter, continuous-simulation model. Recent improvements to WEPP include enhanced computation of evapotranspiration (ET) by incorporating the Penman-Monteith method into the model, and improved calculation of subsurface lateral flow by properly setting a restrictive layer and soil anisotropic ratios. These modifications have substantially improved the performance of the WEPP model for forested watersheds. In order to further enhance the model applicability, a baseflow component needs to be incorporated to adequately represent hydrologic conditions where significant quantities of ground water flow to streams.

Erosion following wildfire can be as much as 1000 times the erosion from an undisturbed forest. In August, 2005, the largest fire in the lower 48 states occurred in the Umatilla National Forest in Southeast Washington. Researchers from the Rocky Mountain Research Station assisted the forest in evaluating soil erosion risks using three different applications of the WEPP model. GeoWEPP was used to determine the onsite distribution of soil erosion. WEPP Windows Watershed Version was used to estimate peak runoff rates of each of the 33 small watersheds analyzed, and the ERMiT interface to WEPP was used to estimate the risk of erosion on selected hillslopes, and the benefits of mulching those slopes. About 38 percent of the burned areas was analyzed within the three days available for analysis. This paper summarizes the analytical methods, and the findings of the prediction runs.

The WEPP model was used to compare erosion rates from fuel management operations, including roads, to erosion following wildfire, the inevitable disturbance in the absence of fuel management, for climates across the western U.S. The results show that erosion from fuel management operations, including thinning and prescribed fire, are less than from wildfire, even when road erosion rates are included.