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.