Hydrologic and erosion responses to wildfire along the rangeland-xeric forest continuum in the western US: a review and model of hydrologic vulnerability

Authors: Williams, Christopher - Jason; Pierson Jr, Frederick; ROBICHAUD, PETE - Us Forest Service (FS); BOLL, JAN - University Of Idaho

Submitted to: International Journal of Wildland Fire
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2013
Publication Date: 2/6/2014
Citation: Williams, C.J., Pierson Jr, F.B., Robichaud, P.R., Boll, J. 2014. Hydrologic and erosion responses to wildfire along the rangeland-xeric forest continuum in the western US: a review and model of hydrologic vulnerability. International Journal of Wildland Fire. 23:155-172. DOI: http://dx.doi.org/10.1071/WF12161.

Interpretive Summary: Landscape plant community transitions and warming climate conditions across the western United States have altered fire regimes and present large-scale consequences relative to values-at-risk. Extensive conversion of shrublands to annual grasslands has increased the spatial extent and frequency of fires in wildland settings and along the urban-wildland interface. Wildfire activity is also increasing along transitions of rangeland vegetation to dry forests. The shift in fire regimes across the rangeland-dry forest transitions to larger-scale, more frequent and severe burning amplifies the spatial and temporal exposure and vulnerability of these landscapes to greater runoff and erosion and poses significant risks to habitat, water quality, property, and human life. This paper explores the potential hydrologic impacts of large-scale plant community transitions and altered fire regimes and proposes a conceptual framework for assessing post-fire hydrologic risk and recovery. Fire effects on hydrologic vulnerability are presented through a review of published field studies from semi-arid rangeland, woodlands, Mediterranean scrublands, and forested sites. We expand on these concepts relative to ongoing plant community transitions occurring throughout the western US. The manuscript provides an extensive review of fire effects on rangeland hydrology and highlights the need for additional research in lieu of the of ongoing altered fire regimes on western landscapes.

Technical Abstract: The recent increased wildfire activity across the rangeland-xeric forest continuum in the western United States presents landscape-scale consequences relative to runoff and erosion. Concomitant cheatgrass (Bromus tectorum L.) invasions, plant community transitions, and a warming climate in recent decades along grassland-shrubland-woodland-xeric forest transitions have promoted frequent and large wildfires, and continuance of the trend appears likely if warming climate conditions prevail. These changes potentially increase overall hydrologic vulnerability by spatially and temporally increasing soil exposure to runoff and erosion processes. Plot and hillslope scale studies demonstrate burning may increase event runoff and/or erosion by factors of 2-40 over small-plot scales and more than 100-fold over large-plot to hillslope scales. Reports of flooding and debris-flow events from rangelands and xeric forests following burning show the potential risk to natural resources, property, infrastructure, and human life. We present a conceptual model for evaluating post-fire hydrologic vulnerability and risk. We suggest that post-fire risk assessment of potential hydrologic hazards should adopt a probability-based approach that considers varying site susceptibility in conjunction with a range of potential storms and that determines the hydrologic response magnitudes likely to impact values-at-risk. Our review suggests that improved risk assessment requires better understanding in several key areas including quantification of interactions between varying storm intensities and measures of site susceptibility, the varying effects of soil water repellency, and the spatial scaling of post-fire response across rangeland to xeric forest plant communities.