Introduction
Soil erosion is one of the major concerns in forest management in forest watersheds everywhere. Disturbances such as forest roads, timber harvesting, and fire increase the likelihood of forest soil loss (Cornish, 2001; Croke, Mockler, Fogarty, & Takken, 2005; Elliot, 2013; Hairsine, & Grayson. 2003; Lane, Hairsine, Croke, & Takken, 2006; Luce & Black, 1999; Motha, Wallbrink, Sheridan & Noske, 2007;). Eroded sediment that is routed to forest streams or lakes can degrade the aquatic environment (McCormick, Riemen, & Kershner 2010) and impair beneficial uses of surface water resources (Elliot, 2013; Elliot, Miller, & Enstice, 2016; Smith, Sheridan, Lane, Nyman, & Haydon, 2011). Roads may accumulate, deliver, or increase sediment in runoff (Gucinski, Furniss, Ziemer, & Brookes. 2001), increasing the complexity of understanding and evaluating soil erosion and sediment transport processes in post-fire forest conditions.
Fire disturbance in forests increases runoff and erosion generally due to the decrease of soil organic matter (Ebel, 2012), loss of ground cover and on some soils, the tendency for the upper soil layers to become water repellant (DeBano, 2000; Doerr, Shakesby, & Walsh, 2000), all of which leave the soil more vulnerable to water erosion (Cerda & Lasanta, 2005; Moody, Shakesby, Pierson, Robichaud, Spaeth & Moffet, 2013; Robichaud, Cannon & Martin, 2013; Robichaud Wagenbrenner & Brown, 2010). Soil erosion following severe wildfire can be up to three magnitudes greater than before fire (DeBano, Neary & Ffolliot, 1998; Elliot, 2013). At a watershed scale, sediment and runoff response to wildfire are often influenced by burn severity and the amount and intensity of precipitation (Robichaud, 2005). An intense rainfall event following a high burn severity fire can greatly increase runoff, erosion and downstream sedimentation (Robichaud, Beyers & Neary, 2000; Moody et al., 2013). Topography is also a significant factor in post-fire soil erosion, with longer slopes and steeper slopes generating more sediment.
Road construction may change the underlying topography and alter surface hydrology or interact with geomorphic processes (Gucinski et al., 2001; Reid, 2010; Wemple, Swanson & Jones, 2001). Road networks linked with stream networks and may increase stream density within a watershed (Croke and Mockler, 2001; Gucinski et al., 2001; Wemple, Jones & Grant, 1996). Forest roads can also decrease the critical gradient for gully initiation and reduce the distance between onsite sediment generation and downstream channels (Katz, Daniels & Ryan, 2014). The road network may also increase watershed peak flow and sediment generation rates (Jones and Grant, 1996; Thomaz, Vestena & Ramos Scharrón, 2014). On the other hand, road segments can act as hydrological barriers that can change the natural flow path. Roads with upslope drains intercept hillslope and upstream runoff that can accumulate along road surfaces before drainage through culverts or overtopping of the accumulated runoff across the road surface (Elliot 2004; Gucinski et al., 2001; Wemple, Clark, Ross & Rizzo, 2017). Therefore flow accumulation and flow energy may be redistributed on hillslopes below road segments. This redistribution of runoff can result in altering surface soil erosion and sediment transport processes (Lane et al., 2006; Takken, Croke & Lane, 2008).
With the aid of GIS technology and advanced modelling methods, many researchers addressed the influence of road networks on watershed scale hydrology and sediment processes (Akay, Erdas, Reis & Yuksel, 2008; Araujo et al., 2014; Brooks, Boll, Deckert & Elliot, 2006; Grace, 2017; Parsakho Lotfalian, Kavian, & Hosseini, 2014; Soulis, Derkas & Papadaki, 2014;). Most of these researchers stated that road networks would accelerate runoff accumulation processes and enhance peak flow and sediment generation in natural watersheds. However, following forest fires, in which large amounts of sediment are generated, downstream roads may change prefire flow routes, erosion, and sediment transport. For example, a road segment may intercept sediment flow and become a site for deposition (MacDonald and Coe, 2008; Wemple et al., 2001). This implies that the road effect on sediment transport in disturbed forests may be different from that in the undisturbed conditions. The road hydrology effect is difficult to be evaluated spatially as road topography cannot be precisely measured using a coarse DEM. However, a high-resolution LiDAR (Light Detection and Ranging) DEM can capture the topographical details for precise hydrologic analyses (Persendt and Gomez, 2016; Yang, Ames, Fonseca, Anderson, Shrestha, Glenn & Cao, 2014;). The LiDAR DEM has also been used for road related flow path simulation (Sosa-Pérez and MacDonald, 2017). Thus it is possible with LIDAR data to characterize road-altered flow paths more precisely and estimate how road segments influence surface runoff and sediment transport in a montane watershed.
This study was carried out to evaluate the interactive effects of wildfire disturbance with roads on forest soil erosion. A GIS-based soil erosion model was applied to a fire-disturbed forest with internal roads. Sediment generation before and after the fire was calculated. The flow path network was delineated, and the spatial distribution of soil loss was estimated based on a high-resolution LiDAR DEM. The results of this study are intended to increase our understanding of how road segments influence surface runoff and post-fire sediment transport processes.