Negligible soil erosion in a burned mountain watershed, Canadian Rockies: field and modelling investigations considering the role of duff

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1 EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 36, (11) Copyright 11 John Wiley & Sons, Ltd. Published online October 11 in Wiley Online Library (wileyonlinelibrary.com) DOI: 1./esp.36 Negligible soil erosion in a burned mountain watershed, Canadian Rockies: field and modelling investigations considering the role of duff Y. E. Martin, 1,,3 * E. A. Johnson,,3 J. M. Gallaway 1 and O. Chaikina 1 1 Department of Geography, University of Calgary, Calgary, AB Canada Department of Geoscience, University of Calgary, Calgary, AB Canada 3 Biogeoscience Institute, University of Calgary, Calgary, AB Canada Department of Biological Sciences, University of Calgary, Calgary, AB Canada Received 3 December 1; Revised 13 August 11; Accepted 5 September 11 *Correspondence to: Y. E. Martin, Department of Geography, University of Calgary, 5 University Dr. NW, Calgary, Alberta, Canada TN 1 N. ymartin@ucalgary.ca ABSTRACT: Increased soil erosion in immediate post-wildfire years has been well documented in the literature, but many unanswered questions remain about the factors controlling erosional responses in different regional settings. The field site for the present study was located in a closed canopy, subalpine forest in Kootenay National Park, British Columbia that was subjected to a high-intensity crown fire in the summer of 3. Low soil erosion values were documented at the study site in the years immediately following the 3 wildfire, with estimates ranging from approximately 1-1 up to 1 tha -1. Following the wildfire, notable duff coverage (the duff layer is the combined fermentation and humus soil organic layers) remained above the mineral soil. This finding supports earlier studies documenting only partial duff consumption by high-intensity wildfires in the boreal forest of Canada. It is postulated that remnant duff coverage after many high-intensity wildfires impacts the hydrological and soil erosional response to rainstorm events in post-wildfire years. In particular, duff provides detention storage for infiltrating rainfall and, therefore, may inhibit the generation of overland flow. Furthermore, duff also provides a physical barrier to soil erosion. The Green Ampt model of rainfall infiltration is employed to better assess how interactions between rainfall duration/intensity and soil/duff properties affect hydrological response and the generation of overland flow. Model results show that duff provides an effective zone for detention storage and that duff accommodates all rainfall intensities to which it was subjected without the occurrence of surface ponding. In addition, the penetration of the wetting front is relatively slow in duff due to its high porosity and water storage potential. Copyright 11 John Wiley & Sons, Ltd. Introduction Post-wildfire soil erosion Wildfires are natural and recurrent processes in many ecosystems (Whelan, 1995; Johnson and Miyanishi, 1) and may affect both hillslope and fluvial geomorphic systems (DeBano, ; Martin and Moody, 1; Moody and Martin, 1a, 1b; Benda et al., 3; MacDonald and Huffman, ; Benavides-Solorio and MacDonald, 5; Cannon and Gartner, 5; Robichaud, 5; Martin, 7; Robichaud et al., 7; Moody and Martin, 9). The present study focuses on soil erosion following wildfire, specifically the erosion of mineral soil due to overland flow (which may occur in the form of sheetwash or rilling) and rainsplash. The general consensus in the published literature is that when post-wildfire soil erosion does occur, elevated erosion rates persist for several years following the wildfire event. While a great deal of research on post-wildfire soil erosion has been undertaken in the past several decades (Shakesby and Doerr, 6; Moody and Martin 9), the controls on the differing erosional responses for a full range of regional settings requires additional study for improved understanding (Shakesby and Doerr, 6). Infiltration, overland flow generation and soil erosion have been the subject of much investigation in non-wildfire settings (Emmett, 197; Boiffin and Monnier, 195; Moore and Burch 196; Bryan, ). A variety of factors associated with a post-wildfire environment may contribute to increased rates of soil erosion by overland flow. The lack of interception due to loss of vegetation allows rainfall to reach the mineral soil faster, increasing the likelihood of overland flow development. Evapotranspiration may be reduced by the lack of ground cover and live trees, thus increasing effective rainfall (Swanson et al., 19). Enhanced or fire-induced soil water-repellent layers in the soil are often thought to be a primary cause of notable soil erosion following wildfire (Savage et al., 197; De Bano et al., 1976; Dyrness, 1976; Giovannini, 199; Doerr and Moody, ; MacDonald and Huffman, ; Doerr et al., 5, 6). In addition to the physical and ecological factors described above, post-wildfire soil erosion requires rainfall intensities to exceed values required for initiation of overland flow and soil entrainment. Short-duration, high-intensity rainfall

2 9 Y. E. MARTIN ET AL. events may produce greater erosion than longer, low-intensity rainfall events (Spigel and Robichaud, 7), although further investigation about this topic is required. While individual field studies may document short, high-intensity rainfall bursts, it is unfortunate that most regional climate stations do not capture rainfall events at these scales, so there is a lack of understanding about the statistical return intervals for these shorter rainfall bursts. Finally, barriers to surface flow including loss of ground cover may be removed during the fire, increasing the connectivity and velocity of surface flow and sediment transport. Rainfall amounts and velocities reaching the ground surface may increase after wildfires due to canopy burning, and the bare surface may be exposed for some time period before revegetation occurs. Together this may lead to enhanced rainsplash erosion in post-wildfire years (McNabb and Swanson, 199; Terry and Shakesby, 1993), although overland flow is often considered to be the dominant post-wildfire erosive process in steep forested environments. Duff, hydrology and moisture To explain water movement and soil erosion in a forest both before and after wildfire occurrence, it is important to understand water movement in the duff layer (Keith et al., 1a, 1b). The duff layer (i.e. organic soil layers, see definition below) affects soil hydrology in several important ways: (i) it provides an upper layer of temporary storage during rainfall; (ii) it prolongs the time for percolation into the underlying mineral soil; (iii) it provides resistance to flow; and (iv) it provides protection of the mineral soil from water impact. In many forested landscapes, a duff layer the combined organic F (fermentation) and H (humus) layers (Van Wagner, 197) acts as an upper layer of temporary or detention storage during rainfall events that prolongs the time for percolation into mineral soil, thus inhibiting development of overland flow (Croft and Hoover, 1951). Unfortunately, the role of the duff layer in affecting infiltration has not been a focus of most physically-based hydrological modelling studies. Before wildfires, surface water flow on forested hillslopes often originates as saturation-excess overland flow (Dunne and Leopold, 197). Overland flow that develops moves either on top of the mineral soil and/or through the litter and duff layers. Lateral flow in the duff layer is limited to periods during and immediately after precipitation, with flow reduced by an order of magnitude within h after a precipitation event, and often with limited distances of lateral movement (<1m) (Keith et al., 1a, 1b). In addition to providing detention storage for rainfall, the duff layer protects the mineral soil from water impact and increases flow resistance. Thus, this duff layer may have a role in accounting for any lack of sediment transport (i.e. movement of mineral soil) in unburned forests during rainstorm events. In a post-wildfire environment, successful germination and establishment of tree seedlings usually occurs at locations where duff has been consumed, and this concern has been a primary motivating factor underlying studies of duff consumption (for a review of such studies see Miyanishi, 1). While both empirically-based regression models (Little et al., 196; Reinhardt et al., 199; Brown et al., 1991) and process-based models of heat transfer (van Wagner, 197) have been proposed, results have focused on the mean amount of duff removed across a landscape, with the latter point seeming to suggest that there is net duff removal everywhere across the landscape. However, recent ecological research has emphasized the distinct patches of deeply burned duff and unburned duff observed after high-intensity crown wildfires in the mixedwood boreal forest, and their causal mechanisms (Chrosciewicz, 1976; Dyrness and Norum, 193; Zasada et al., 193; Miyanishi, 1; Miyanishi and Johnson, ). Large, high-intensity crown wildfires do not necessarily result in uniform and complete removal of the duff layer. Duff layer consumption occurs by smoldering combustion during and after the flaming front passage, as opposed to flaming combustion (Dyrness and Norum, 193; Frandsen, 1991; Hungerford et al., 1995; Latham and Williams, 1; Miyanishi, 1). This occurs because duff is decomposing organic matter and will only burn by slow combustion of char, since the volatile content has been largely lost. Smoldering combustion of duff is controlled by its thickness, moisture content and bulk density (Miyanishi and Johnson, ). Smoldering combustion in thicker duff goes out more easily because it cannot maintain enough heat transfer to continue burning. Bulk density affects thermal diffusivity, which is defined as the ease with which a material absorbs heat from its surroundings. Moisture affects the latent heat of vaporization, decreasing the heat available for pyrolysis. As a result of these interacting factors, consumption of the duff layer during wildfires in many forests is often incomplete and heterogeneous at the scale of meters. Miyanishi and Johnson () found that for duff samples with similar bulk densities, critical combinations of soil moisture and depth were required for smoldering combustion. Generally, as soil moisture increases, the critical depth necessary for continuation of smoldering combustion increases. Study rationale and objectives Post-wildfire soil erosion is often studied and reported in regions where noteworthy erosion occurs (see Shakesby and Doerr, 6 for study locations including south-western USA, Mediterranean regions, Australia, and Moody and Martin, 9 for western USA studies). In some, or perhaps many of these investigations documenting high rates of post-wildfire soil erosion, the duff layer may have been removed and a water repellent layer may have developed in the soil. However, it is unfortunate that soil erosion often goes unreported in the refereed literature when negligible amounts occur and, therefore, explanations for these low rates have generally not been forthcoming. While the role of duff consumption has been considered in studies relating to wildfire and tree establishment, the full range of possible post-wildfire responses of duff and how this affects regional differences in post-wildfire soil erosion has not been systematically explored. The role of remnant duff coverage in affecting post-wildfire soil hydrological and erosional response has been explicitly addressed and/or discussed in a few studies, such as Robichaud (). However, in many post-wildfire soil erosion studies, detailed documentation and/or discussion of remnant duff coverage (or lack thereof) is seldom provided; it may be the case that the duff layer is completely consumed for high-intensity crown wildfires in these regional settings and so the issue has not been pursued in detail. However, as emphasized in our earlier discussion, recent ecological research suggests that duff consumption patterns following high-intensity crown wildfire may be complex in many regional settings and warrants additional physicallybased consideration in the context of post-wildfire soil erosion. Our study examines how environmental circumstances, in particular the presence of remnant duff coverage, may contribute to low rates of post-wildfire soil erosion despite the occurrence of a very high-intensity crown fire. Very low soil erosion rates observed during our field studies following the 3 Kootenay crown wildfire prompted us to seek and develop a process-driven explanation to understand why, in some instances, very high rates of post-wildfire soil

3 NEGLIGIBLE SOIL EROSION IN A BURNED MOUNTAIN WATERSHED 99 erosion occur and why in other cases negligible post-wildfire soil erosion occurs. Using an approach based on physical mechanisms, this paper examines how environmental circumstances, in particular post-wildfire duff coverage, rainfall intensity and infiltration, may together contribute to low rates of post-wildfire soil erosion in certain regional settings. A series of process-based model runs of rainfall infiltration into duff and soil are undertaken to better understand how such processes might affect the occurrence or not of post-wildfire overland flow generation (due to infiltration-excess) and soil erosion. To best demonstrate the progression of this study and our ideas, and in particular how the field results motivated our numerical modelling exercise, we have organized results and analysis under two main headings; (i) the field program; and (ii) the numerical modelling study. Kootenay National Park 93 1 Silt Fences Hawk Creek Weather Station N 1a 51 N Field Program: Motivation for Numerical Modelling Study Study area The location for the field component of this study is Hawk Creek Watershed, Kootenay National Park, south-eastern British Columbia (Figure 1). The region is underlain by folded and faulted sedimentary rocks (limestone and shale), with some other secondary rock types (e.g. quartzite), that were subjected to major erosion during the last glaciation. Hawk Creek is a th-order tributary of the Vermilion River, with a drainage area of km. Elevations range from 133 m at the outlet to 36 m in the upper slopes, with moderate hillslope gradients (generally <3º) in the lower third of the basin where the study plots are located. In the lower portions of the basin, soils are underlain by compact, massive debris flow deposits, morainal material and bedrock, with some evidence of colluvial reworking and deposition. Soils are mostly unconsolidated and unsorted, with significant amounts of cobbles and boulders interspersed within a matrix of mainly silty sand (sand often 7%) with low clay content (<%) (sandy loam). Climate is cordilleran with temperatures influenced by maritime air from the west or cold continental air masses from the north. The average annual rainfall is 3 mm, with summer rainfall dominated by convectional thunderstorms. Winter precipitation falls primarily as snow, with an average annual snowfall of about 17 cm that accounts for approximately 5 6% of the annual precipitation. Vegetation in the study area consists of subalpine forest, including lodgepole pine (Pinus contorta Loudon var. latifolia Engelm.) and Engelmann spruce (Picea engelmannii Parry ex. Engelm.). The duff layer of the soil in unburned forests is continuous and may be up to 15 cm thick. Crown fires are the major disturbance, with the fire season lasting from May to September and peak lightning activity occurring in July and August (Masters, 199; Reed, 1). The fire return interval was 6 years for the period , 13 years for the period and years for the period (Masters, 199; Reed, 1). The change in the 1th century is related to the Little Ice Age, and is found in other locations in the Canadian Rockies (Johnson and Larson, 1991; Reed, 199). The long interval between 19 and 19 has a very large confidence range (Reed, 1) because of the short time interval that the result represents. Lightning ignited two crown fires in the Vermilion valley in July 3, that merged and burned approximately 17 ha. These crown fires had flaming intensities greater at times than 3 kw m -1, indicating a very high-intensity fire. All trees were killed, the litter layer was completely consumed, and duff consumption was patchy (quantitative data on duff consumption are reported later in the paper) (Figure ). About 9% of the forest in Hawk Creek basin was burned. Vegetation recovery was rapid in the first year after the fire, with trees recruiting in locations where more of the duff layer was removed, which is typical in subalpine and boreal forests (Johnson, 199; Miyanishi and Johnson, ; Hesketh et al., 9). The previous wildfire in Hawk Creek watershed occurred in 135 (Masters, 199). Methods 116 W British Columbia Kootenay National Park, B.C. Alberta 95 1 km In spring, 1 silt fences were installed in the lower subalpine zone of Hawk Creek watershed, with elevations of the corresponding erosion plots within approximately m of one another (refer back to Figure 1). Aspect for all fences was SSW. Silt fences were located within a 1 km distance of each other to minimize rainfall variability. Slopes were approximately planar in the downslope direction. The design included stratification of the erosion plots across three gradient classes (6 15,16 5,6 35 ), with a mean area per plot of m. Plot areas were defined as the upslope contributing area above the silt fences, with the upper limits defined by notable breaks W 5 5 N Figure 1. Hawk Creek Watershed, Kootenay National Park, Canada. The grey shaded area on the large map represents Kootenay National Park. The 1 silt fences were all located within the black shaded area. The second weather station was situated approximately km NNE of the main weather station.

4 Y. E. MARTIN ET AL. Figure. Photographs showing the patchy nature of duff consumption after a crown wildfire. (a) Partially exposed tree roots; and (b) Mosaic pattern of duff consumption. Note the uneven nature of the ground surface due to partial duff consumption. For scale, trees in these photographs have DBH values ~ cm. in slope to a relatively level area across which minimal sediment transport would likely pass. Six hillslope plots were assigned to each gradient class. To act as a control, two silt fence traps were installed in an older 1 burn that was of similar fire intensity and that also occurred in Kootenay National Park. These locations had undergone significant revegetation since the wildfire. The silt fence methodology described by Robichaud and Brown () was followed for this study. Sediment traps consisted of a barrier of silt fence fabric supported by wooden or metal stakes installed across a section of hillslope (Figure 3). The fabric has small openings that allow water to seep through, but the barrier slows any surface flow, causing it to pond above the fabric. Ponding reduces flow velocity, resulting in settling of sediment. Widths of silt fences ranged from to 6 m, and the silt fence fabric had an opening size of 35 microns, equivalent to a sieve size of 1.5Φ (medium sand). Ponding of water above the fences may have resulted in settling of some portion of particles finer than 35 microns. Sediment collected in the traps was weighed in the field after each rain event, and adjusted to dry weight based on moisture content. Particle size distributions were determined for samples of sediment collected in the silt fence traps. In addition, surface soil samples were collected and analyzed from random locations in the vicinity of plots. Comparison of these surface samples with soils trapped by the silt fences allows for assessment of sediment sizes available for transport and the possible occurrence of selective transport. In addition, vertical soil sampling was undertaken for soil pits located in each gradient class; samples were taken at the surface, and then every cm down to maximum local rooting depths. Field investigations were undertaken to identify the properties of material underlying the soil layer. Duff coverage after the burn was estimated for quadrats within each of the 1 erosion plots. A wire mesh with 1 cm squares was placed on the ground, and Figure 3. Silt fence in Hawk Creek Watershed. Silt fences ranged in width from to 6 m. duff coverage at wire intersection points was recorded. For each erosion plot, data for six quadrats were collected in random locations. Standing tree density and tree bole dbh (diameter at breast height) were also recorded for each plot. Two meteorological stations were installed in clearings in the lower part of the basin; the primary station was located at an elevation of 135 m (see Figure 1), and the second station was located km NNE of the first station at an elevation 156 m. Rainfall was measured by tipping buckets and an electronic data logger that recorded the date and time of each tip. Rainfall was measured from June 15 to September 1, and June to October 1, 5. Initial soil erosion results are given in units of g m -1, and a non-parametric Friedman test was used to assess if soil erosion data collected in and 5 are significantly different from the control value of zero transport. There is no standard methodology for measurement of post-wildfire soil erosion, nor are there standard units for expressing post-wildfire soil erosion (see Table 3 in Shakesby and Doerr, 6). Shakesby and Doerr (6) addressed this issue when compiling a summary table of soil erosion values by converting all values into equivalent sediment yields (units of t ha -1 ). To allow our values to be compared with studies in the table of Shakesby and Doerr (6), our initial results (g m -1 )werealso converted into sediment yields (t ha -1 ). The conversion of soil erosion results, obtained using silt fences, into sediment yields requires either knowledge of or a sound basis for definition of the source area for sediment trapped in the fences. Since the exact source area is unknown, the following approach was adopted to obtain sediment yield estimates: in one case, the sediment source is assumed to be the entire hillslope plot, and in the second case the source area is assumed to be a characteristic length scale of disconnected overland flow. For the latter, a distance having order-of-magnitude 1 m was chosen, based on previous water budget studies for duff layers (Keith et al., 1a, 1b) and microtopographic studies in Hawk Creek (Martin et al., ). If rainsplash or disconnected overland flow (Martin et al., ) were major modes of transport, then the lower values for the source areas may be the most reasonable. Sediment yield results are expressed

5 NEGLIGIBLE SOIL EROSION IN A BURNED MOUNTAIN WATERSHED 11 as averages for the six individual plots falling under each gradient class. Rainfall intensity data during field seasons were derived from the tipping bucket measurements at the field site. A new event is defined herein as when a period >6 h lapses between tips. The two most commonly employed methods of deriving rainfall intensity data for tipping bucket rain gauges are the interval method and natural method (Fiser and Wilfert, 9). For the interval method, rainfall tips are counted within constant time intervals (for example, 1-min intervals) over the duration of the storm. Depending on the length of time interval chosen, the signal of short rainfall bursts may be dampened out using this approach. The natural method estimates the rainfall intensity between consecutive tips. For consistency with many other rainfall measurements, we adopt the interval method for the rainfall data collected at the field site. Field data are first reported using a 1-min time interval. For comparison with Environment Canada data, we also report rainfall data using a 1-h time interval. Results and analysis Soil erosion is expressed as dry mass per unit width (of silt fence) per rain event. Average soil erosion is calculated for the six plots within each gradient class to obtain mean values for a gradient class (Table I). Relatively small amounts of soil erosion occurred on three occasions in : July 19, August, and a series of rain events between August and August 9. Erosion occurred on May 15, June, and July 1 for 5. It is unknown with certainty if the sediment collected in the silt fences resulted from erosion due to overland flow or rainsplash, or some combination of the two processes (due to safety regulations of Parks Canada, field work in the study site was not possible during actual rainfall events). No evidence of rilling was evident in the hillslope plots during sediment collection. If the sediment did move via overland flow, then it is likely that the flow rates were low given the lack of evidence for rilling and the low transport rates (see values below). Moreover, flow paths would probably have been relatively disconnected, minimizing transport distances. Microtopographic studies, conducted in this same drainage basin, support the notion that surface irregularities limit the distance of connected water and sediment flow paths (Martin et al., ). The two silt fence traps in the 1 burn site produced no soil erosion. For data, there is a statistically significant difference from zero when all three gradient classes are combined at a 9% confidence level (P-value =.6). However, between individual gradient classes, there are no significant differences detected between control and burned plots. For 5, soil erosion results are not different from zero (P-value =.33), suggesting sufficient vegetation had re-established to further minimize surface erosion for the storm intensities observed in 5. When the sediment yield data for each transporting event and for the total amount over the year are examined, a notable gradient effect is observed, with erosion values increasing nearly an order of magnitude for each increase in gradient class (Table II). Such an increase in transport for steeper slopes would be expected for either overland-flow driven soil erosion or rainsplash erosion. When the lower value of the source area is adopted, the order-of-magnitude values of sediment yield for individual transporting events and for the annual total are respectively 1-1 and 1 tha -1, depending on the gradient class (note: there are some zero values for gradient classes A and B for the late August rainfall event). The grain sizes trapped by the silt fences relative to surface soil are based on soil samples taken from silt fences for the August, rain event and surface soil samples obtained in the nearby vicinity of silt fences. The August rain event had the highest rainfall intensity over the two seasons (about 3. mm per 1 min) and mobilized the largest amount of sediment. This event provides an opportunity for assessing silt fence efficiency and whether grain-size selective soil erosion occurred. Particle size analysis for trapped sediment revealed high percentages (65 5%) of particles <35 microns (the size of the openings in the fabric), indicating settlement did occur for particles smaller than silt fence openings. Most of the sediment collected in the silt fence traps fell in the size range < mm, indicating stresses associated with either overland flow or rainsplash were generally not sufficient to entrain Table II. Sediment yield (t ha -1 ) for soil erosion events in Gradient Contributing area (m ) July 19, Aug., Aug. 9, Total for Large contributing area a A (6-15 ) B (16-5 ) C (6-35 ) Small contributing area b A (6-15 ) B (16-5 ) C (6-35 ) Data are converted to t ha -1 for comparison with other studies. a Area contributing sediment to the silt fence is assumed to be the entire plot area to top of hillslope. b Area contributing sediment to the silt fence is assumed to be within 1 m upslope of the silt fence. Table I. Mean soil erosion for the first and second post-fire years a Date of causal rainfall event Group A (6 15 ): Soil erosion (g m -1 ) Group B (16 5 ): Soil erosion (g m -1 ) Group C (6 35 ): Soil erosion (g m -1 ) Control soil erosion (g m -1 ) 19 July August (two rain events) August 11.3 (multiple rain events) 5 15 May 3.1 June.6 1 June 1 a Rainfall statistics for the rainfall/soil erosion events are given in Table VI.

6 1 Y. E. MARTIN ET AL. larger particles. The < mm fractions for silt fence and hillslope samples were extracted for detailed comparison (Table III). The hillslope soil samples contained 5 5% of particles <35 microns, and silt fence samples contained 7 9%. The higher percentage of fines in the silt fence samples suggests that fines <35 microns were selectively entrained, transported and deposited in the silt fence (Hairsine and Rose, 199a, 199b for detailed descriptions of physical principles involved in overland flow erosion by sheetflow and rilling). For each sediment class, Table III shows the enrichment ratio of the silt fence samples relative to the hillslope samples. Results indicate selective transport of particles <.5 mm, with minimal mobilization of coarse sand. Some portion of fine sand and silt fractions may have been transported in suspension through the silt fence. The clay fraction was absent in the silt fence samples, suggesting that either it was not transported, or it all flowed through the silt fence. Particle size distributions for the vertical soil pits typically show a large percentage of sand (often >7%), silt (often ~%) and a small percentage of clay (<%). These soils are categorized as sandy loams, and they typically have high infiltration rates (order-of-magnitude 1 cm h -1 ; Mein and Larson, 1971, 1973; Rawls et al., 193), reducing the likelihood for Hortonian overland flow to develop. Furthermore, macropores created by dead and decaying plant roots are abundant in these post-wildfire soils, providing pathways for water to infiltrate the soil. While it is theoretically possible that a water repellent layer may have developed in the soil and been destroyed prior to the field season, there is no evidence to suggest that such a layer developed or played a role in promoting soil erosion by overland flow in the study area. Diamict was found to underlie most of the soils, with fabric analysis supporting a glacigenic origin for most sites, with the exception of the lower portion of the basin. The fabric, grain size distribution and geomorphic form of underlying material in the lower portion of the basin suggest that it was originally deposited as a result of fluvial and debris flow processes operating on a large alluvial fan. Overall, the material underlying the soil shows a high degree of compaction and strength, with low permeability. Based on a knowledge of duff water budgets and smoldering combustion, it is not unexpected that post-wildfire duff coverage in the study plots after the 3 burn was found to be significant (Table IV). Average duff coverage for the six plots falling under each gradient class was 5%, 6% and 35% for low-, medium- and high-gradient grouped plots, respectively. The percentage of duff layer coverage correlates inversely with hillslope gradient (Figure (a)). Given the similar stand composition and the relatively uniform depth and bulk density of the unburned duff layer for the study plots Table III. Sediment class Enrichment of transported sediment by particle size % silt fence sample % hillslope sample %silt fence / % hillslope a Coarse sand (.5 mm) Medium sand. (.5.5 mm) Fine sand (.5.5 mm) Silt (..5 mm) Clay (<. mm) 1. a Values greater than 1 indicate transport enrichment and hillslope depletion. Table IV. Percentage of duff coverage for quadrats in the 1 soil erosion plots. Four quadrats were measured in each erosion plot a Plot b Slope ( ) Sample 1 Sample Sample 3 Sample Average A A A A A A B B B B B B C C C C C C a There are no measurements of duff depth before the wildfire. However, values from nearby unburned forests averaged 15 cm. b A plots are in the low gradient class (6 15º), B plots are in the medium gradient class (16 5º), C plots are in the high gradient class (6 35º). (see also Miyanishi and Johnson, ), the most notable factor affecting differing rates of duff consumption may be the moisture content. Steeper slopes are generally expected to be drier than lower-gradient slopes when duff depth and bulk density are the same, which would lead to greater duff consumption (Bridge and Johnson, ; Miyanishi and Johnson, ; Keith et al., 1a, 1b). Hence, smoldering combustion was probably more effective in removing duff from these steeper slopes (Miyanishi and Johnson, ). In addition to the slope effect on remnant duff coverage, duff layer consumption was also observed to be dependent on whether a location was under a tree canopy before the fire, which leads to more interception and drier duff. It is difficult to disentangle to what extent the higher erosion values on steeper slopes result from the greater driving force for sediment transport on steep slopes and to what extent the lower duff coverage (and, therefore, lower soil protection) on steep slopes was a significant factor (Figure (b), (c)). Finally, rainfall associated with soil erosion events documented in the field study was evaluated to assess if unusually low rainfall was a possible cause for the low values of soil erosion observed. Unfortunately, there is often not enough standardization of how rainfall intensity data are collected, analyzed and reported by various monitoring agencies and in the research literature, making comparisons among results problematic. It is also worth nothing that field studies for particular research projects may have the ability to collect more detailed information about rainfall intensity than meteorological stations monitored by Environment Canada and other government agencies, which are often relied on for understanding longerterm rainfall trends. Depending on the time length of bins chosen for the interval approach, different results can be obtained. As one example, rainfall intensities for 1-min, 3-min and 6-min time intervals for the July 19, event are shown in Table V. The peak values for the 1-min intervals have an equivalent hourly rainfall intensity of 15.6 mm h -1, whereas the larger-sized bins show much lower hourly rainfall intensities. It is possible that short, high-intensity rainfall bursts (time scales <<1h) may

7 NEGLIGIBLE SOIL EROSION IN A BURNED MOUNTAIN WATERSHED 13 a) Duff remaining (%) b) Soil erosion (t ha -1 ) Soil erosion (t ha -1 ) c) Tangent slope Erosion event #1 Erosion event # Erosion event #3 Total erosion Tangent slope 6 Duff remaining (%) Figure. Duff coverage, slope gradient and soil erosion for the three gradient classes. (a) Average duff coverage vs. average slope gradient; (b) average slope gradient vs. soil erosion; (c) average duff coverage vs. soil erosion. Note that is the first post-fire year. Table V. Rainfall intensity data for the July 19, event for quantifying rainfall amount in various time intervals 1-min bins Time interval (min) mm per 1 min mm per 1 min min bins Time Interval (min) mm per 3 min mm per 1 min min bins Time Interval (min) mm per 1 min a Data are initially binned into different intervals: 1-min bins, 3-min bins, 6-min bins. These binned rates are then translated into equivalent values in units of mm h -1 for comparison. play a notable role in the timing and amounts of overland flow, and that rainfall intensity data available for many meteorological stations and studies may be binned at too coarse a resolution to provide meaningful values for the purpose of analyzing post-wildfire overland flow and associated soil erosion. Rainfall statistics using the interval method for the rainfall events are given in Table VI. The largest sedimenttransporting event in coincides with the storm of highest instantaneous rainfall intensity on August (maximum of 3. mm per 1 min). However, the average rainfall intensity over the duration of this storm was lower than for the July 19 event. Neither saturation overland flow nor infiltration-excess overland flow probably occurred as this rainfall event took place after days of no precipitation; sediment most likely moved by rainsplash and/or restricted areas of limited surface flow. The second largest sediment-transporting event occurred during the July 19 rain event, which showed the next highest value of maximum rainfall intensity (.6 mm per 1 min) and the highest average rainfall intensity. However, an earlier storm on June 3 with reasonably high rainfall intensities (total rainfall of mm over 3 h 15 min; mean rainfall rate of.1 mm min -1 ; maximum rainfall intensity of.6 mm per 1 min) generated no sediment in the fences. Both the June 3 and July 19 events occurred on dry soils, with little to no precipitation Table VI. Rainfall statistics for events triggering soil erosion Date a Duration b Total rainfall (mm) Mean rainfall rate over duration of storm (mm per min) Range of rainfall intensity (mm per 1 min) July 19 h min to.6 Aug. Part 1 7 h 35 min 6..1 to 1. Part 3 h 51 min to 3. Aug. -9 Part 1 3 h 1 min 3..5 to 1. Part 36 h min to 1. Part 3 6 h min..11 to. Part h min.6.5 to. Part 5 3 h 9 min to 1. Part 6 13 h min to. Part 7 h 6 min..63 to. a June 3, rainfall event is not shown as it resulted in no soil erosion. b If a time period >6 h occurred between tips of the rain gauge, then a new event is defined.

8 1 Y. E. MARTIN ET AL. occurring in the previous week. Total amount of rainfall, duration of rainfall, and intensity of rainfall were all lower for the July 19 event than for the June 3 event. A large total amount of rainfall (~76 mm) occurred over the period August to August 9 and was delivered in several defined storm events. These rainfall events resulted in rainsplash and/or some surface flow that delivered small amounts of sediment to some traps. The peak rainfall intensity associated with this event had a value of 1 mm per 1 min, which is not a particularly high rainfall intensity. Rainfall intensity measurements collected by Environment Canada, binned into 1-h time periods, are available for several stations in the regional vicinity of our field site. The first station is located in Kootenay National Park (station elevation of 117 m) where years of rainfall intensity data are available (with a total of 53 rainfall events; the same criterion of 6 h between events is used to define a new rainfall event); a second station in nearby Yoho National Park (station elevation of 119 m) has 17 years of data available (with a total of 95 rainfall events). Our field data were re-analyzed using 1-h bins and are compared with the longer-term rainfall record. Figure 5 shows results for the two data sets, including histograms of average hourly rainfall intensity for each rainfall event, maximum rainfall intensities (mm h -1 ; this peak value may have occurred during any one discrete hour within a storm of any duration) and total rainfall (mm) for all storms. Table VII shows a comparison of our data with the regional data. The maximum rainfall intensities for the July 19 rainfall event and the second storm on August fall within the top ~ 1% of values found for the regional data, indicating that these maximum rainfall intensities were high. Some of the individual storms within the August 9 period had high maximum rainfall intensities, while others did not. Average rainfall intensities for the July 19 event and the second event on August were also high, falling within the top.5% to 6.% of the regional values. The events during the period August 9 did not result in high average rainfall intensities. The total rainfall amounts for some of the events during the August 9 period were high, and the total rainfall amounts were also reasonably high for the August events. The July 19 storm did not have a particularly high amount of total rainfall. Modelling the Role of Duff in Infiltration Background To further explore our finding of low soil erosion rates and the possible role of duff in influencing these low values, we utilized a) b) Frequency 15 5 KNP Average hourly rainfall intensity (mm/hr) Frequency YNP Average hourly rainfall intensity (mm/hr) c) d) Frequency Maximum rainfall intensity (mm/hr) KNP KNP Frequency e) f) Frequency Maximum rainfall intensity (mm/hr) YNP YNP Total rainfall (mm) Total rainfall (mm) Figure 5. Frequencies of rainfall event parameters for Kootenay National (KNP) and Yoho National Park (YNP). (a) Average hourly rainfall intensity (KNP); (b) average hourly rainfall intensity (YNP); (c) maximum rainfall intensity (KNP); (d) maximum rainfall intensity (YNP); (e) total rainfall (KNP); (f) total rainfall (YNP). Note the change in y-axis scale for different plots. Frequency

9 NEGLIGIBLE SOIL EROSION IN A BURNED MOUNTAIN WATERSHED 15 Table VII. Comparison of rainfall intensity values for soil erosion events with regional rainfall data Rainfall for soil erosion events Comparison with rainfall data for Kootenay National Park a Comparison with rainfall data for Yoho National Park b Maximum rainfall intensity Average rainfall intensity July 19 July 19: July 19: 3.6 mm/h % % Aug. Aug. Aug. Part 1:. mm/h Part 1: 19.-.% Part 1:.-3.5% Part : 6 mm/h Part :.7-.9% Part :.3-.% Aug. -9 Aug. -9 Aug. -9 Part 1: 3. mm/h Part 1: % Part 1: % Part : 3.6 mm/h Part : % Part : % Part 3:. mm/h Part 3: % Part 3: % Part :. mm/h Part : % Part : % Part 5:. mm/h Part 5: 19.-.% Part 5:.-3.5% Part 6 : 1.6 mm/h Part 6 : % Part 6: % Part 7:. mm/h Part 7: % Part 7: % July 19 July 19 July mm/h.5-.%.9-1.% Aug. Aug. Aug. Part 1:.775 mm/h Part 1: 7.% Part 1: % Part : 1.75 mm/h Part :.6-.9% Part : % Aug. -9 Aug. -9 Aug. -9 Part 1: 1.6 mm/h Part 1:.6-.% Part 1:.7-.% Part :.519 mm/h Part :.6-.% Part : % Part 3:. mm/h Part 3: % Part 3: % Part :.1 mm/h Part : % Part : % Part 5:.33 mm/h Part 5: % Part 5: % Part 6:.36 mm/h Part 6: % Part 6 : % Part 7:.67 mm/h Part 7: % Part 7:.7-1. Total rainfall July 19 July 19 July mm % % Aug. Aug. Aug. Part 1: 6. mm Part 1: % Part 1: % Part : 7. mm Part : % Part : % Aug. -9 Aug. -9 Aug. -9 Part 1: 3. mm Part 1:.-.66% Part 1:.73-.% Part : 19. mm Part : % Part : % Part 3:. mm Part 3:.9% Part 3: % Part :.6 mm Part : % Part : % Part 5: 1. mm Part 5: 9.93% Part 5: % Part 6: 5. mm Part 6.3% Part 6: 5.-6.% Part 7:. mm Part 7: % Part 7: % a Hawk Creek is located ~ km from Kootenay National Park rain gauge. b Hawk Creek is located ~5 km from Yoho National Park rain gauge. The two right-hand columns indicate the approximate percentile range in which data for the three rainfall events fall in comparison with the two regional rainfall data sets (e.g. a value of % for a particular parameter would indicate that our data are in the top % of values for that regional data set). a soil infiltration model to explore hydrological response of duff and various soil types to rainstorms of varying duration and intensity, representing a wide range of storm return intervals. A modelling approach allowed us to gain critical insights as to why our field site experienced minimal soil erosion, and the conditions and combinations of rainfall characteristics, duff and/or soil types that can generate overland flow, thus allowing for the possibility of soil erosion. Most studies undertaking physically-based hydrological modelling have focused on infiltration and overland flow generation for soil layers (Ahuja, 193; Selker et al., 1999; Wang et al., 1999), and not on the detailed hydraulic characteristics and physical mechanisms associated with the duff layer. As indicated earlier in this paper, there has been some consideration of how the duff layer impacts post-wildfire erosion, but detailed examination of the hydraulic characteristics and physical mechanisms associated with infiltration of rainfall through the duff layer requires much further investigation. The two components in our model of duff/soil and hydrological response are: (i) atmospheric input of rainfall into the system; (ii) duff or mineral soil characteristics. The limited number of hydrological studies explicitly considering the duff layer may be because it is not considered a limiting factor for infiltration, due to its very high infiltration capacities. Limited information exists about the properties of duff that are necessary for its incorporation into physical models of rainfall infiltration. Duff properties in our study were obtained from Raaflaub and Valeo,, 9; Keith et al., 1a, 1b; Valeo (pers. comm.); Hayashi (pers. comm.). There are several possible fates for rainfall if it enters into the coupled duff/soil system (Laurén and Mannerkoski, 1; Raaflaub and Valeo,, 9; Keith et al., 1a, 1b). The infiltration capacity of the duff layer most often exceeds the rainfall rate (and that of the underlying mineral soil), and water enters the duff layer. The duff and underlying mineral soil generally have contrasting infiltration capacities, with duff values generally >>soil values. When duff remains on the ground surface after wildfires, the duff layer would be expected to provide detention storage, thus slowing down the infiltration of water into the mineral soil (Croft and Hoover, 1951). This

10 16 Y. E. MARTIN ET AL. would delay the time to ponding (or inhibit ponding altogether) versus what would happen if no duff layer were present. Hence, even soils of limited infiltration capacities may be able to cope with relatively high rainfall rates (keeping in mind that effective rainfall may be increased due to post-wildfire changes to interception and evapotranspiration) because of the buffering role played by the duff layer. If the wetting front through the duff layer reaches the mineral soil, water may begin to infiltrate into the mineral soil layer. If soil infiltration capacity is limited relative to the input of water across the duff/soil interface, possible surface ponding and/or lateral flow through the duff layer or along the ground surface may begin to occur. The duff layer also provides a physical barrier to soil erosion, limiting erosion by any lateral water flow that develops on slopes. For the situation when most, or perhaps all, of the duff layer is removed, the following modifications occur. The possibility for detention storage by the duff layer is eliminated. Therefore, the threshold rainfall intensity for surface ponding to occur would be expected to be lower, thus increasing the possibility for soil erosion by overland flow. Moreover, removal of duff means that it can no longer provide resistance to soil erosion. It is also possible that soil water repellency may develop, although in our model runs we did not consider this scenario. Methods To explore how duff/soil properties and rainfall intensity interact to influence rainfall infiltration and the development of surface ponding and/or overland flow due to infiltration-excess, the Green Ampt infiltration model was utilized (Green and Ampt, 1911). This model of rainfall infiltration accounts for a number of variables, including soil suction head, porosity, hydraulic conductivity and time, and has been studied and expanded over a period of many decades (Fok, 197; Mein and Larson, 1971, 1973; Chu, 197; Ahuja, 193; Rawls et al., 193; Selker et al., 1999; Wang et al., 1999; Parsons and Muñoz-Carpena, ). Parsons and Muñoz-Carpena () developed Fortran source code for the Green Ampt model that is available for use (GAmpt); this code is utilized in our study. Bedient and Huber () outline the equations within the Green Ampt model that form the basis for the modelling component of our study; the reader is referred to this source for additional details not covered below. The Richard s equation for unsaturated flow in the subsurface, on which the Green- Ampt model is based, has @z @z where θ is the volumetric moisture content, z is distance below the ground surface (cm), c (θ ) is capillary suction (pressure) (in cm of water), and K(θ ) is unsaturated hydraulic conductivity (cm s -1 ). Solution of this equation is only possible in certain cases. Green and Ampt (1911) developed one of the most widely used approaches to do so, based on a series of assumptions: (i) as rain continues to fall and water infiltrates, the wetting front advances at the same rate with depth and there is a well-defined wetting front; (ii) the volumetric water contents remain constant above and below the wetting front as it advances; and (iii) the soil-water suction immediately below the wetting front remains constant with both time and location as the wetting front advances. The original Green Ampt equation was developed for the period of time after surface saturation. Since the introduction of (1) the original equation, extensions of the original Green Ampt equation have been made to describe the time period before surface saturation (Mein and Larson, 1971, 1973; Bedient and Huber, ). Both of these stages are now considered. First, the amount of water infiltrated at the time of surface saturation must be calculated, which then allows the time to ponding to be determined. The following equation is used to calculate the volume of water that has infiltrated at the time of surface saturation (F S has units of cm, and assumes unit area): F S ¼ M d c= ð1 i=k S Þ () where K S is saturated hydraulic conductivity (cm hr -1 ), i is rainfall intensity (cm hr -1 ), and M d is initial moisture deficit: M d ¼ θ S θ i (3) where θ S is the saturated volumetric moisture content (cm 3 cm -3 ) and θ i (cm 3 cm -3 ) is the initial volumetric water content. The time to ponding (t p in hours) is then given by: t p ¼ F S i Until the time to ponding is reached (the time period before surface saturation), the rate of infiltration, f, is equal to the rainfall rate, i (both with units of cm h -1 ). The rate of infiltration after surface saturation has been reached (f p ) is: f p ¼ K S 1 þ M dc F where F is infiltration volume (cm). Note that this is the situation specifically covered by the original Green Ampt model (1911). Our model runs consisted of either just one mineral soil layer or the duff layer, and allowed comparison of results for different materials. A focus was placed on the time to ponding; the time when surface ponding is reached represents a critical and necessary stage for the initiation of overland flow. The soil and duff input variables for our model runs are provided in Table VIII, and are based on the references cited in the table. Environment Canada has compiled rainfall intensity duration data for >5 stations across Canada (Environment Canada, 1). The input rainfall intensities for our model runs cover storm durations ranging from 1 minutes to 1 h (Table IX), and rainfall intensities are defined for storm return intervals ranging from years to years. The input rainfall intensity duration values represent averages of data for four of the closest meteorological stations to Kootenay National Park. Results and analysis The Green Ampt infiltration model was run for duff and five soil types (sand, sandy loam, silt loam, clay loam, silty clay; note that the predominant soil type at the field site was sandy loam). In the first set of model runs, the initial water content was set equal to the residual water content (the lowest possible value). For model results, time to ponding is particularly important as it represents a necessary condition for overland flow initiation. Results for all of the input storm durations are shown in Figure 6(a) (f). Short-duration, high-intensity rainstorms produced ponding for a larger number of soil types and in a () (5)