A simulation study to estimate the effects of wildfire and forest management on hydrology and sediment in a forested watershed, northwestern U.S.

Authors: SRIVASTAVA, ANURAG - Purdue University; WU, J - Washington State University; ELLIOT, W - Forest Service (FS); BROOKS, E - University Of Idaho; Flanagan, Dennis

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2018
Publication Date: 10/31/2018
Citation: Srivastava, A., Wu, J.Q., Elliot, W.J., Brooks, E.S., Flanagan, D.C. 2018. A simulation study to estimate the effects of wildfire and forest management on hydrology and sediment in a forested watershed, northwestern U.S. Transactions of the ASABE. 61(5):1579-1601. https://doi.org/10.13031/trans.12326.
DOI: https://doi.org/10.13031/trans.12326

Interpretive Summary: Wildfires in forests of the western United States have become more frequent and increasingly larger problems. This research involved use of a computer modeling study in the Colville National Forest in Washington State, where wildfires were simulated under undisturbed forest conditions, as well as the impacts of forest management practices (thinning, prescribed burns, etc.) to reduce fuel loads and burn severities. The WEPP (Water Erosion Prediction Project) model was applied to determine runoff and soil erosion rates in a watershed after simulated wildfire burns in the untreated area. The 777 hillslopes were ranked from highest to lowest sediment yield, to identify the most important ones for fuel treatment management. WEPP was then applied to hillslopes generating from 10 to 50% of the post-fire sediment yields, to examine the effect of treatments (thinning, prescribed burns) on the water balance and peak flows from the watershed. The model simulations showed that the treatments could increase water yield from an area and also alter hydrological processes. These results impact forest managers, scientists, and others involved in fire management and soil conservation efforts in forests at risk for wildfires. The results of this study may help guide forest managers in implementing fuel reduction treatments on forested watersheds to reduce the risk of wildfire.

Technical Abstract: Suitable fuel reduction treatments are needed in the Colville National Forest, WA, to reduce the risk of severe wildfire. This study aimed to identify high-risk erosion hillslopes following wildfire to aid in forest fuel reduction planning and to evaluate the effects of fuel treatments on the watershed hydrological response. The specific objectives were (i) to assess the soil burn severity associated with wildfires and use that information to identify critical hillslopes for forest fuel treatments, and (ii) to evaluate the potential changes in water yield and peak flows from pre-treatment (undisturbed forest) to post-treatment (thinning and prescribed burn) conditions, in the East Deer Creek watershed, a subwatershed of the Colville National Forest. Assessments were made using a modeling approach for hypothetical wildfire and fuel treatment scenarios. FlamMap, a fire behavior model, was used to predict the spatial distribution of wildfire intensity for a hypothetical event under current vegetation conditions. WEPP simulations were subsequently completed to obtain sediment and water yields based on fire intensity and topography. WEPP erosion estimations following a simulated wildfire showed hillslope sediment yield varying from zero to 49.4 Mg/ha/yr from the 777 hillslopes, which were ranked in descending order of sediment yield to identify critical hillslopes for fuel treatments. The WEPP model calibrated for a nearby gauged watershed was then applied to the East Deer Creek watershed for pre- and post-treatment conditions. At the watershed scale, the increase in water yield from pretreatment to post-treatment conditions ranged from 0.7 to 5.6 percent on hillslopes delivering 10-50 percent of the predicted post-fire sediment. Simulated water balance components at the treated hillslopes showed substantial changes. Surface runoff, subsurface lateral flow, and deep percolation increased 150% (5 mm), 50% (9 mm), and 40% (41 mm), respectively, whereas evapotranspiration (ET) decreased 23% (124 mm). The relative differences between pre- and post-harvest peak flows showed no clear trends as treatment area increased. The results suggest that thinning and prescribed burns to treated hillslopes in the East Deer Creek watershed may lead to an increase in water yield and significant alterations in hydrological processes.