Post-fire Sediment Research Articles

Empirical Models for Postfire Debris‐Flow Volume in the Southwest United States

AbstractImproving our ability to relate postfire debris‐flow volume to rainfall characteristics, terrain attributes, and fire severity is critical for quantifying postfire sediment yields from steep landscapes and predicting changes in debris‐flow hazards following fire. This is especially true in the Southwest United States (US) (Arizona and New Mexico), where fire activity has increased in recent decades. In this study, we present a database of 54 postfire debris‐flow volumes that we collected across the Southwest between 2010 and 2021. We use these data to develop a multiple linear regression model for postfire debris‐flow volume based on peak 30‐min rainfall intensity, watershed area greater than 23°, and a soil burn severity variable. We further propose a model that utilizes only rainfall and terrain variables, as well as a model that requires only terrain attribute and fire‐severity variables. These models are beneficial in scenarios where there are data limitations. We compare these new models with others developed in the western US to explore differences in the factors that control debris‐flow volume across geographic regions. We find that the models introduced here more accurately predict postfire debris‐flow volume in the Southwest relative to existing models. We also find that models that include sub‐hourly rainfall intensity perform better than those that do not, revealing the benefits of high‐resolution rainfall data for constraining postfire debris‐flow volume. Results improve our ability to forecast postfire debris‐flow volume in the Southwest and provide insights into relationships between rainfall intensity, terrain attributes, fire severity, and debris‐flow volume.

Exploring the inclusion of soil management practices in erosion models towards the improvement of post-fire predictions

Wildfires are recognized for having a strong impact on forest soils, a situation aggravated by inadequate pre-fire land management practices. Land management operations, such as plowing, are routinely carried out for cultural reasons and can impact soils for decades after their implementation. Therefore, it is crucial to take into account the pre-fire land management history when predicting post-fire sediment losses in burnt areas. This consideration is critical for a realistic assessment of soil erosion risk and, consequently, for effectively implementing emergency stabilization and/or rehabilitation measures.The aim of the study was to integrate pre-fire land management practices into erosion models, to enhance post-fire sediment losses predictions at slope scale. To accomplish this goal, both Multiple Linear Regression (MLR) and the revised-Morgan-Morgan-Finney model (revised-MMF) were applied in the Colmeal burnt area (Central Portugal). These models were adapted to account the impacts of different management options, specifically no plowing, downslope-plowing and contour-plowing, on the erosive response following a wildfire.The results revealed fluctuations in the performance of both models across different soil management, and over time since the wildfire. Despite the observed variability, it is important to highlight the positive outcomes achieved with the revised-MMF model over the three monitoring years where contour-plowing was applied. These results demonstrate that the best model performances are achieved when soil management is individualized and analyzed independently. Similarly, the MLR model exhibited improved performance when incorporating management practices into its predictions. This study confirms that disturbances on topsoil, whether caused by wildfires or soil management operations, play key roles in driving change in soil erosion. Hence, integrating these factors into models is essential for providing relevant information for the development of mitigation and/or restoration strategies in areas at high risk of erosion.

Postfire Sediment Mobilization and Its Downstream Implications Across California, 1984–2021

AbstractFire facilitates erosion through changes in vegetation and soil, with major postfire erosion commonly occurring even with moderate rainfall. As climate warms, the western United States (U.S.) is experiencing an intensifying fire regime and increasing frequency of extreme rain. We evaluated whether these hydroclimatic changes are evident in patterns of postfire erosion by modeling hillslope erosion following all wildfires larger than 100 km2 in California from 1984 to 2021. Our results show that annual statewide postfire hillslope erosion has increased significantly over time. To supplement the hillslope erosion modeling, we compiled modeled and measured postfire debris‐flow volumes. We find that, in northern California, more than 50% of fires triggering the top 20 values of sediment mass and sediment yield occurred in the most recent decade (between 2011 and 2021). In southern California, the postfire sediment budget was dominated by debris flows, which showed no temporal trend. Our analysis reveals that 57% of postfire sediment erosion statewide occurred upstream of reservoirs, indicating potential impacts to reservoir storage capacity and thus increased risk to water‐resource security with ongoing climate change.

Predicting Postfire Sediment Yields of Small Steep Catchments Using Airborne Lidar Differencing

AbstractPredicting sediment yield from recently burned areas remains a challenge but is important for hazard and resource management as wildfire impacts increase. Here we use lidar‐based monitoring of two fires in southern California, USA to study the movement of sediment during pre‐rainfall periods and postfire periods of flooding and debris flows over multiple storm events. Using a data‐driven approach, we examine the relative importance of terrain, vegetation, burn severity, and rainfall amounts through time on sediment yield. We show that incipient fire‐activated dry sediment loading and pre‐fire colluvium were rapidly flushed out by debris flows and floods but continued erosion occurred later in the season from soil erosion and, in ∼9% of catchments, from shallow landslides. Based on these observations, we develop random forest regression models to predict dry ravel and incipient runoff‐driven sediment yield applicable to small steep headwater catchments in southern California.

Impacts of barley (Hordeum vulgare L.) straw mulch on post-fire soil erosion and ground vegetation recovery in a strawberry tree (Arbutus unedo L.) stand

Rural fires are now a major societal concern across the world, especially where fire regimes have (apparently) intensified in terms of burnt area, intensity and recurrency. Among the indirect fire effects, fire-enhanced runoff and erosion have been an important focus of post-fire land and water management, in particular through emergency stabilization of hillslopes using a range of erosion mitigation measures. The most widely applied and – scientifically tested – measure is that of mulching with agricultural straw, in spite of concerns of introducing exogenous organic material and especially seeds of non-native higher plant species, including the straw species it- or themselves. So far, field studies in the present study region of north-central Portugal have preferred using endogenous forest residues but these studies concerned forest types for which such residues are easily available. The latter, however, is not the case for strawberry tree stands, so that straw mulch was selected in this study as a cheaper alternative to eucalypt or pine residues. This - apparently, first – post-fire erosion mitigation study in a strawberry tree (Arbutus unedo L.) stand aimed to compare post-fire sediment and organic matter losses as well as ground vegetation recovery without and with applying barley (Hordeum vulgare L.) straw mulch at a low rate of 2 Mg ha−1. The experimental set-up involved a randomized block design with a total of six geotextile-bounded erosion plots of 2 m by 5 m that were organized in three blocks, were installed and mulched roughly one month after the 17-October-2017 M-fire in inland Central Portugal, and monitored at 12 irregular intervals during the first two post-fire years. The principal findings were that: (i) especially the specific sediment losses without mulching over the first post-fire year were notably higher than those reported by the prior field studies in the region, in eucalypt and maritime pine plantations; and that the - low - mulching rate: (ii) was extremely effective in reducing these first-post-fire-year losses; but (iii) did not result in changes in the cover or floristic composition of the ground vegetation cover that were noteworthy and longer-lived than the first post-fire year.

Testing simple approaches to map sediment mobilisation hotspots after wildfires

Background The models currently used to predict post-fire soil erosion risks are limited by high data demands and long computation times. An alternative is to map the potential hydrological and sediment connectivity using indices to express the general properties of the burnt landscape.Aims In this study, we aimed to answer the question: Do these tools identify post-fire sediment mobilisation hotspots?Methods To achieve this, we assessed the spatial variability distribution of the location of soil erosion hotspots using the Index of Connectivity, Revised Universal Soil Loss Equation and the Sediment Export, and compared it with the simulation results of a more complex Landscape Evolution Model (LAPSUS model). Additionally, we evaluated statistical measures of association between the four tools.Key results The three tools tested in this study are suitable for identifying sediment mobilisation hotspots, where the erosion rates are above the 95th percentile, and differences between their performance are small.Conclusions The results indicate that these tools help locate extreme erosion locations in recently burnt areas.Implications These results can be considered for post-fire and water contamination risk management, especially for fast prioritisation of areas needing emergency post-fire intervention.

Monitoring of Post-Fire Bedload Transport Using Hydrophone in a Small Burnt Catchment, South Korea

Understanding the properties of wildfire-disrupted catchments is crucial for managing river floods and landslide risks. Using a hydrophone, we investigated the changes in sediment supply conditions in small mountainous catchments (30.8 ha) in southern Korea for 6 years (March 2014–December 2019). Bedload transport rates mostly increased in burned catchments 1–2 years post-wildfire (early post-fire sediment regime) but decreased 3 years post-wildfire owing to the coverage of burned slopes (late post-fire sediment regime). Landslides triggered 5 years post-wildfire increased the bedload transport rates by approximately one order of magnitude (post-landslide sediment regime). Although sediment activity decreased in wildfire-disturbed catchments after recovery of the ground-cover layer 1–3 years post-fire, thy increased during events such as landslides. Furthermore, even in undisturbed mountainous catchments (139.7 ha), bedload transport rates increased after intense rainstorm events (≥100.0 mm), with this change lasting for approximately 2 years. Our observations showed that the forest restoration after a wildfire should be planned and implemented from a min- to long-term perspective. In addition, during rainstorms, the relationship between flow and bedload transport showed large variability even in undisturbed catchments. Furthermore, surrogate monitoring using a hydrophone was useful in understanding the changes in bedload transport characteristics according to various supply conditions of the catchment.

Predicting the potential impact of forest fires on runoff and sediment loads using a distributed hydrological modeling approach

Forest fires profoundly affect the nature of watershed responses to precipitation, increases in runoff rate and volume, and post-fire sediment discharge. A post-fire watershed assessment to identify potential trouble spots for soil erosion and flooding can help land managers for decision-making on deploy mitigation, remediation measures, and restoration action plans. Predicting the potential impact of fires on runoff and soil erosion requires distributed hydrological modeling methods. This study used the Soil and Water Assessment Tool (SWAT), a rainfall-runoff model, to investigate the hydrological consequences of forest fires and their impact on sediment load under different burnt scenarios. For the pre-fire (baseline) scenario, the watershed is calibrated by using the measured discharge and suspended sediment concentration data from the weather station during the time step to estimate post-fire changes at the sub-catchment scale. The total water and sediment yield for the selected sub-catchment in the post-fire scenario with the highest spatial distribution of forest fire, due to the excessive surface runoff (an increase of 14.5%) increased significantly, resulted in 6.5 times increase in sediment yield. The results obtained from the study indicate a significant increase in sediment transport into the stream in the post-fire scenario. A large volume of soil losses occurred during the time step. In contrast, due to removing trees and vegetation from the ground, less evapotranspiration and infiltration occurred in selected sub-catchment areas after forest fires. In terms of watershed management, understanding the consequences of forest fires on hydrological services will be an effective method for sediment budget control. Therefore, applying the rainfall-runoff and erosion models for the watershed with developing plausible forest fire scenarios can be a productive method to deal with the threats and hydrological risks such as soil erosion and sediment transfer to the downstream environment.

Wildfires trigger multi-decadal increases in sedimentation rate and metal loading to subarctic montane lakes

We evaluated how two large wildfires affected the sedimentation rate and accumulation of lead (Pb), mercury (Hg), and cadmium (Cd) in sediment of four subarctic montane lakes in the Yukon, Canada. The wildfires occurred 60 and 20 years (1958, 1998) before sediment collection in 2018. Site-specific fire exposure was inferred from the charcoal accumulation histories in the lake sediments and the burned catchment area was determined from historical fire maps. The two major wildfires caused a two to five-fold increase in sedimentation rates and a two to eight-fold increase in sediment metal accumulation rates in Little Fox Lake. The mass accumulation rates of metals in Little Fox Lake sediment increased by a maximum of 2.7–4.7 mg Pb m−2 yr−1, 19–29 μg Hg m−2 yr−1 and 37–114 μg Cd m−2 yr−1 following wildfires. Modelling using elemental ratios of lithogenic sources suggested a large proportion of the Pb and Hg accumulating in post-fire sediment was from remobilized legacy anthropogenic pollution. In contrast, Cd fluxes were consistent with variation in catchment weathering. Impacts of wildfires were visible but more muted in the sediment of Little Braeburn Lake, whereas Fox Lake and Grayling Lake sediments showed little to no wildfire impact and served as a reference for external (long-range) metal deposition. Major changes to lake sediment geochemistry in Little Fox Lake were caused by the lack of vegetation and soil recovery in the catchment following the severe 1998 fire. Wildfire impacts were persistent in the lake more than 20 years after the last fire, with no sign of a return to pre-fire Pb, Hg, and Cd accumulation rates. This study shows that wildfires in northern montane catchments can significantly increase the rate of metal accumulation in affected lakes, thereby impeding recovery from reductions in anthropogenic air emissions of these metals.

Differential impacts of a wildfire and post-fire sedimentation event on platypus and fish populations in a Victorian upland river

Context Wildfire can affect freshwater ecosystems in many ways, notably when post-fire rainfall washes ash and sediment into waterways. Aims We investigated species-specific effects of bushfire and subsequent channel sedimentation on the abundance of platypus and fish populations in the upper Buffalo River, Australia. Methods Pre- and post-fire population surveys were conducted using fyke nets. Key results There was no evidence that fish numbers declined because of direct fire effects. However, native two-spined blackfish and Macquarie perch numbers dropped dramatically following post-fire sedimentation, whereas non-native redfin perch increased, most likely as a result of migration from a nearby lake. Platypus captures were consistently recorded at all survey sites both before and after the fire and sediment inflows occurred, with many juveniles being recorded in the first post-fire breeding season. Conclusions The platypus’s greater resilience to post-fire sediment inflows than that of native fish presumably reflects its reliance on lungs rather than gills and its ability to take refuge in burrows. It also has a broad diet, flexible foraging behaviour, is highly vagile and typically stores enough fat to support fasting for up to 2–3 weeks. Implications Management of fire-affected aquatic ecosystems must consider species-specific responses of freshwater vertebrates to fire.

A novel approach to estimating soil yield risk in fire prone ecosystems

Increasing fire activity in the American west heightens the need for natural resource managers to identify where risk associated with post-fire effects is greatest. This is particularly true for water resources, as many headwater forests are at risk for stand replacing fires. However, current methods to model post-fire erosion often have trade-offs between model area and spatial resolution. We introduce a novel approach to combine a process-based erosion model and state-and-transition simulation modeling to estimate post-fire sediment yield and identify areas of high risk. We demonstrate how this method can be applied at three scales: large watershed, sub-watershed, and single fire event. The combination of the erosion model and state-and-transition simulation model allowed us to identify areas of post-fire high sediment yield potential and increased likelihood of fire occurrence. This method can be used by land managers to prioritize pre-fire restoration practices or post-fire rehabilitation actions.

The Evolution of Sediment Sources Over a Sequence of Postfire Sediment‐Laden Flows Revealed Through Repeat High‐Resolution Change Detection

AbstractPostfire debris flows are particularly complex to study because they do not form discrete initiation locations and commonly involve multiple simultaneously operating erosional processes. Although recent work has begun to elucidate a more mechanistic understanding of postfire debris flows, there is still a paucity of detailed sediment budgets characterizing these events. In this study, we seek to understand how postfire sediment sources and erosional processes change over multiple storm cycles. To do this, we performed repeat high‐resolution change detection in a headwater catchment burned by the 2018 Holy Fire in the Santa Ana Mountains, California, USA. This included terrestrial laser scanning in a zero‐order catchment (0.95 ha) and unmanned aerial vehicle structure from motion of a headwater channel network (up to 6.5 ha). During the initial storm events that produced runoff‐generated debris flows, we found that the evacuation of dry ravel and prefire colluvium accounted for half of the eroded material. These initial flows also acted to clear out much of the material stored within downstream headwater channel networks. In subsequent storm events of equal or greater rainfall intensity, total erosion from the study site was subdued, and the relative importance of shallow hillslope erosion from interrill and rill erosion was increased, as has been noted in similar studies in the region. Overall, this suggests that channel sediment supplies may be more rapidly depleted than hillslope sources, which may drive a trend of decreasing sediment fluxes over time from burned headwater catchments subject to repeated runoff events.

Burn Severity Effects on Sediment and Nutrient Exports from Southeastern Forests Using Simulated Rainfall

Abstract Burn severity, commonly assessed as the amount of fuel consumed during fire, is an indicator of postfire sediment yield and erosion. This study examined the effect of burn severity on sediment and nutrient response in three different fire-adapted forest types of the Southeast. Soil and litter samples were experimentally burned to achieve increasing levels of fuel consumption. Simulated rainfall was applied to burned litter samples collected from pine, hardwood, and mixed hardwood-pine forests in the Clemson Experimental Forest. Runoff and leachate samples were collected and analyzed for sediment yield (kilograms per hectare) and total suspended solids (grams per liter); both runoff and leachate samples were analyzed for ammonium (NH4–), nitrate (NO3–), and orthophosphate (PO43–). Sediment yield and total suspended solids increased at only the highest burn severity treatment in all three forest types, with pine litter samples yielding significantly greater sediment in surface runoff than both mixed and hardwood samples. Burn treatment did not readily affect soluble nutrient concentrations in either runoff or leachate, but the data suggest that high-severity burning increases the availability of PO43– bound to sediment. This study suggests that high-severity burn patches function as sediment sources, but overall sediment and nutrient response to burning was minimal. Study Implications This study suggests that low-to-moderate burn severity, in terms of litter removal, is not an important indicator of sediment and nutrient exports from southeastern forests; precipitation in the immediate aftermath of fire (<48 hours) is not an effective transport mechanism for biologically available macronutrients; and that even at the highest burn severity treatment, sediment yield was relatively low.

PEMIP: Post-fire erosion model inter-comparison project

Land managers often need to predict watershed-scale erosion rates after disturbance or other land cover changes. This study compared commonly used hillslope erosion models to simulate post-fire sediment yields (SY) at both hillslope and watershed scales within the High Park Fire, Colorado, U.S.A. At hillslope scale, simulated SY from four models— RUSLE, AGWA/KINEROS2, WEPP, and a site-specific regression model—were compared to observed SY at 29 hillslopes. At the watershed scale, RUSLE, AGWA/KINEROS2, and WEPP were applied to simulate spatial patterns of SY for two 14–16 km2 watersheds using different scales (0.5–25 ha) of hillslope discretization. Simulated spatial patterns were compared between models and to densities of channel heads across the watersheds. Three additional erosion algorithms were implemented within a land surface model to evaluate effects of parameter uncertainty. At the hillslope scale, SY was only significantly correlated to observed SY for the empirical model, but at the watershed scale, sediment loads were significantly correlated to observed channel head densities for all models. Watershed sediment load increased with the size of the hillslope sub-units due to the nonlinear effects of hillslope length on simulated erosion. SY's were closest in magnitude to expected watershed-scale SY when models were divided into the smallest hillslopes. These findings demonstrate that current erosion models are fairly consistent at identifying areas with low and high erosion potential, but the wide range of predicted SY and poor fit to observed SY highlight the need for better field observations and model calibration to obtain more accurate simulations.

Exploring the effect of hydrological connectivity and soil burn severity on sediment yield after wildfire and mulching

AbstractSoil erosion can potentially threaten different resources inside and outside of burned areas, and the risk of water becoming contaminated with sediment may be particularly severe. Various postfire actions, such as applying straw mulch, have been carried out in northwestern Spain in recent years with the aim of mitigating the risk of soil erosion. Nonetheless, because of the short interval between summer wildfire and autumn rains, careful selection and prioritization of the areas to be treated is crucial. Changes in hydrological connectivity could be measured and used as a criterion for selecting such areas. However, studies addressing changes in hydrological connectivity as a consequence of forest fires are scarce. In the present study, we assessed the effects of fire and postfire helimulching on the hydrological connectivity and sediment loads in a forest catchment burned by a wildfire in August 2016. Sediment yields were recorded in 20 plots (180 m2). Hydrological connectivity was computed with a version of the Borselli index and two alternative weighting factors: the C factor from the Revised Universal Soil Loss Equation and a factor based on field surveys of soil burn severity. The effect of mulching was also considered. The results indicate that the version of the Borselli index based on field measurements of soil burn severity best reflects the susceptibility to postfire sediment delivery. Moreover, this method was also suitable for evaluating the effect of mulching on soil erosion. The study findings may help forest managers to plan postfire actions.

Effectiveness of straw bale check dams at reducing post-fire sediment yields from steep ephemeral channels

Post-fire flooding and elevated sediment loads in channels can pose hazards to people and structures within the wildland-urban interface. Mitigation of these hazards is essential to protect downstream resources. Straw bale check dams are one treatment designed to reduce sediment yields in small ephemeral catchments (<2ha). This study investigated their effectiveness in five paired catchments burned at high severity during the 2010 Twitchell Canyon Fire in Utah. Rainfall, ground cover and hillslope erosion rates were also measured during the two-year study. Adjacent paired catchments were physically similar and ranged in size from 0.2 to 1.6ha across pairs. Within pairs, one catchment was an untreated control and the other treated at a rate of four straw bale check dams ha−1. High intensity rainfall, erodible soils and slow regrowth contributed to the observed high hillslope sediment yields (> 60Mgha−1). 1- and 2-yr I30 return period rain events early in the study quickly filled the straw bale check dams indicating the treatment did not statistically reduce annual sediment yields. First year annual sediment yields across all catchments were 19.6 to 25.7Mgha−1. Once the check dams were full, they had limited storage capacity during the second post-fire year, allowing 3.8 to 13.1Mgha−1 of sediment to pass over the check dams. The mean mass of sediment trapped by individual straw bale check dams was 1.3Mg, which allowed them to trap a mean of 5.9Mgha−1 of sediment at the given treatment rate. Straw bale check dams trapped <50% of the total mass delivered from catchments with efficiency decreasing over time. Increasing straw bale check dam treatment rate in stable channels may improve trap efficiency. Application of this treatment in areas with lower expected rainfall intensities and less erodible soils may be justifiable.

Geomorphic complexity and sensitivity in channels to fire and floods in mountain catchments

Fires and floods are important drivers of geomorphic change. While the hydrologic and geomorphic effects of fires have been studied at the hillslope scale, we have much more limited data on post-fire runoff, channel changes, and inferred or measured sediment storage and delivery at larger scales. In this study we intensively documented channel changes over four years in two very similar ∼15 km2 catchments in the northern Colorado Front Range, Skin Gulch and Hill Gulch, that were burned primarily at high and moderate severity in June 2012. Ten and 11 cross sections and longitudinal profiles along the lower channel network were repeatedly surveyed in Skin Gulch and Hill Gulch, respectively. Summer thunderstorms generally caused deposition in the valley bottoms while incision resulted from the intervening baseflows and spring snowmelt, but a very high intensity summer thunderstorm in Skin Gulch just one week after burning caused much more deposition than in Hill Gulch. Fifteen months after burning both watersheds experienced an exceptional, long-duration flood resulting from a multi-day rainstorm with a several hundred year recurrence interval. This removed nearly all of the deposited post-fire sediment along with some of the older valley bottom deposits. The expansion and coarsening of the channel has greatly decreased the geomorphic sensitivity of both watersheds to future floods, and eliminated the more typical and persistent post-fire depositional signature. The channel changes in Skin Gulch after the fire and long-duration flood were much greater than in Hill Gulch, and this can be attributed to the much greater sediment deposition in Skin Gulch shortly after the fire, reduced geomorphic sensitivity in Hill Gulch resulting from a large erosional flood in 1976, and the spatial distribution of burn severity and storm rainfalls leading to lower peak flows in Hill Gulch. These results suggest that fires in the Rocky Mountains can trigger significant and dynamic hillslope and channel changes over sub-decadal timescales, but unusually long or intense rainstorms can cause larger and more persistent changes regardless of whether a catchment has recently burned.

Coupling wildfire spread and erosion models to quantify post-fire erosion before and after fuel treatments

Wildfires are known to change post-fire watershed conditions such that hillslopes can become prone to increased erosion and sediment delivery. In this work, we coupled wildfire spread and erosion prediction modelling to assess the benefits of fuel reduction treatments in preventing soil runoff. The study was conducted in a 68000-ha forest area located in Sardinia, Italy. We compared no-treatment conditions v. alternative strategic fuel treatments performed in 15% of the area. Fire behaviour before and after treatments was estimated by simulating 25000 wildfires for each condition using the minimum travel time fire-spread algorithm. The fire simulations replicated historic conditions associated with severe wildfires in the study area. Sediment delivery was then estimated using the Erosion Risk Management Tool (ERMiT). Our results showed how post-fire sediment delivery varied among and within fuel treatment scenarios. The most efficient treatment alternative was that implemented near the road network. We also evaluated other factors such as exceedance probability, time since fire, slope, fire severity and vegetation type on post-fire sediment delivery. This work provides a quantitative assessment approach to inform and optimise proactive risk management activities intended to reduce post-fire erosion.

A Simulation Study to Estimate Effects of Wildfire and Forest Management on Hydrology and Sediment in a Forested Watershed, Northwestern U.S.

Abstract. Suitable fuel reduction treatments are needed in the Colville National Forest, Washington, 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 (1) to assess the soil burn severity associated with wildfires and use that information to identify critical hillslopes for forest fuel treatments, and (2) 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 (EDCW), 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 0 to 49.4 Mg ha-1 year-1 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 EDCW for pre-treatment and post-treatment conditions. At the watershed scale, the increase in water yield from pre-treatment to post-treatment conditions ranged from 0.7% to 5.6% on hillslopes delivering 10% to 50% 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 EDCW may lead to an increase in water yield and significant alterations in hydrological processes. Keywords: Fuel treatments, Modeling, Peak flows, Sediment, Water yield, Wildfire.

Empirical models of annual post-fire erosion on mulched and unmulched hillslopes

Erosion is one of the primary land management concerns following wildfire. This study examines controls on post-fire hillslope-scale erosion for the 2012 High Park Fire in northern Colorado, develops simple empirical models for predicting post-fire sediment yields, and evaluates model performance on several nearby fires. From 2013 to 2015 we collected ground cover, rainfall, topographic, and sediment yield measurements from 29 convergent hillslopes; eight of these hillslopes had varying amounts of mulch applied to reduce erosion. From these data we examined correlations between annual sediment yield and three categories of predictor variables (ground cover, precipitation, and topography). Percent bare soil was the single largest control on sediment yield, followed by rainfall variables. Sediment yield generally decreased with flow path length, but the correlation was weak. The empirical models each predicted sediment yield with three variables: percent bare soil, one precipitation variable, and one topographic variable. The models had similar accuracy for the High Park Fire using varying combinations of precipitation and topographic variables (Nash-Sutcliffe coefficients 0.70–0.84). An empirical model predicting annual sediment yields as a function of percent bare soil, June–October precipitation, and the maximum flow path length had variable performance when applied to other fires in the same region, with predictions ranging from poor to good for individual fires and Nash-Sutcliffe coefficients of 0.26–0.32 for all fires combined. These tests show some promise for applying the empirical model to fires in the study region, but further model testing is needed to determine the range of conditions under which the model can be applied.