Effectiveness of needle cast at reducing erosion after forest fires

Transcription

1 WATER RESOURCES RESEARCH, VOL. 39, NO. 12, 1333, doi: /2003wr002318, 2003 Effectiveness of needle cast at reducing erosion after forest fires C. D. Pannkuk and P. R. Robichaud Rocky Mountain Research Station, Forest Service, U.S. Department of Agriculture, Moscow, Idaho, USA Received 10 May 2003; accepted 11 August 2003; published 3 December [1] Needle cast from partially burnt conifer trees coonly occurs after forest fires. The effectiveness of needles in reducing soil erosion was investigated in this study. Two needle types, ponderosa pine and fir needles, were used at four different cover amounts (0, 15, 40, and 70 percent) on granitic and volcanic derived soils. Simulated rainfall was used to examine interrill erosion; added inflow was used to determine rill erosion in a laboratory setting. After a series of runs, data showed that sediment delivery was greater for the granitic soil compared with the volcanic soil. fir needles were more effective at reducing interrill erosion compared with the ponderosa pine needles. pine needles, because of their shape and being bundled together, often caused minidebris dams to form. The minidebris dams formed by ponderosa pine needles reduce flow within the rill, resulting in less rill erosion than the fir needles. A 50 percent cover of fir needles reduced interrill erosion by 80 percent and rill erosion 20 by percent. A 50 percent cover of ponderosa pine needles reduced interrill erosion by 60 percent and rill erosion by 40 percent. We also compared the effectiveness of using stream power, rather than shear stress, to model rill erosion. Stream power was a better predictor of sediment load than shear stress. detachment rates based on stream power decreased with increasing cover for both needle types. These results challenge the use of shear stress detachment rates in current erosion models and provide insight into the use of stream power detachment rates. INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1860 Hydrology: Runoff and streamflow; 1851 Hydrology: Plant ecology; KEYWORDS: erosion, runoff, wildfires Citation: Pannkuk, C. D., and P. R. Robichaud, Effectiveness of needle cast at reducing erosion after forest fires, Water Resour. Res., 39(12), 1333, doi: /2003wr002318, Introduction [2] Although many studies on erosion processes on agricultural lands have examined the effects of crop cover on low slopes and mechanically disturbed soils, there have been no studies investigating the effects of conifer needle cover on steep slopes after forest fires. e is a natural and important part of the disturbance regime for forested ecosystems especially in the western United States [Agee, 1993]. Wildfires generally burn in mosaics with portions of the area burned in low, moderate, and high severity conditions as defined by Ryan and Noste [1983] and DeBano et al. [1998]. e effects on erosion are related to the amount of forest floor organic matter left protecting mineral soil and sometimes the creation of water repellent conditions. [3] Land management agencies develop postwildfire watershed rehabilitation plans to evaluate the risks of soil erosion and flooding. These plans consider burn severity, climate, and values at risk during their assessment [Robichaud et al., 2001] and make recoendations on actions to be taken to control flooding and reduce erosion. [4] Needle cast from partially burnt conifer trees is often present in areas of low or moderate burn severity. These needles generally fall to the soil surface within several Copyright 2003 by the American Geophysical Union /03/2003WR ESG 1-1 months after the fire. Relationships between tree diameter, scorch height, tree density, and the mass of needles that fall to the soil surface after a wildfire were described by Brown [1978], Moeur [1981], Keane et al. [1996], and Reinhardt et al. [1997]. A dense forest stand with high tree scorch height will produce more needle cast than a low scorch height in a sparse forest stand. [5] Long and short needles are the main two types of conifer needles. Long needles, usually bundled, are found on species such as ponderosa pine (Pinus ponderosa), coulter pine (Pinus coulteri), and white pine (Pinus monticola). These needles are approximately 140 long and curved and generally grow in bundles of two to five, held together by the fascicle. Once on the ground, these curved needles may settle about 5 above the soil surface for the majority of their length while the fascicle and tips touch the ground. [6] Short needles grow on fir (Pseudotsuga menziesii), grand fir (Abies grandis), white spruce (Picea glauca), and western larch (Larix occidentalis). These needles are approximately 25 long and straight and generally grow individually from the branch. On the ground, they appear to be in more direct contact with the soil surface than the ponderosa pine needles. [7] The benefits of surface cover at reducing erosion are well established. Surface cover reduces water erosion by decreasing the area susceptible to raindrop impact [Foster, 1982], which reduces overland flow velocities and causes

2 ESG 1-2 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES Table 1. Needle Length and Midpoint Cord-to-Arc Dimensions for the and Needles Used in the Experiment, n = 100 Length, Midpoint Cord-to-Arc, Mean SD a Mean SD fir pine a Standard deviation. sediment deposition in detention storage [Laflen, 1983]. Although postfire needle cover has been observed to reduce erosion on forest soils, the effects have not been systematically investigated. [8] In current erosion models, such as the Water Erosion Prediction Project (WEPP) [Nearing et al., 1989; Flanagan and Livingston, 1995; Laflen et al., 1997] or the revised universal soil loss equation (RUSLE) [Renard et al., 1997], surface cover is considered a very important factor in controlling erosion. In RUSLE, surface cover subfactor (SC) relationships indicate a reduction in soil loss for a given fraction of surface cover by bfs SC ¼ e ð Þ ð1þ where b is a fitted coefficient and Fs is the fraction of surface cover. Robichaud [1996] showed similar relationships for residual forest floor conditions in forest environments and suggested a b value of [9] Sediment load is estimated in WEPP using a steady state sediment continuity equation: slopes (30 percent or above), critical shear is virtually nonexistent, as detachment has been observed to occur with even the smallest flow. [10] Recently, stream power w (g s 3 ) has been investigated as a better model for rill erosion and is calculated by w ¼ r w gsq where r w (g cm 3 ) is the density of water, g (cm s 2 )isthe gravitational constant, S (m m 1 ) is the hydraulic gradient, and q (cm 2 s 1 ) is the unit discharge of water [Nearing et al., 1997]. [11] Surface cover can influence rill development and erosion rate within the rill by disrupting the flow in the rill. s actively erode and evolve morphologically over a short time period. Hydraulic roughness of rills decreases as flow increases [Gilley et al., 1990], suggesting that surface cover in the rill would be less effective as flow increases. As rill sidewalls slough and headwalls develop, the surface cover often falls into the rill. Surface cover can form minidebris dams of needles, bark, small sticks, and decomposed plant material. These dams, up to 500 cm 2 in area, can reduce velocities in the rills. Small pools upstream of these minidebris dams can trap sediment. If flow increases, the minidebris dams can break and release accumulated sediment. [12] Surface cover traditionally was measured in mass per unit area. However, this time-consuming method has been abandoned in favor of measuring percent cover. Assuming uniform distribution, percent surface cover can be related to mass of the cover by ð5þ y ¼ 1 e bx 100 ð6þ dg=dx ¼ D f þ D i ð2þ where G is the sediment load (kg s 1 m 1 ), x is the distance downslope (m), D f is the rill erosion rate (kg s 1 m 2 ), and D i is the interrill sediment delivery (kg s 1 m 2 )[Nearing et al., 1989]. erosion is generally predicted by the excess shear relationship D f ¼ K r ðt t c Þ ð3þ where D f (kg s 1 m 1 ) is the shear stress detachment rate, K r (s m 1 ) is the soil erodibility for rills, t (kg s 2 m 1 )is the hydraulic shear of flowing water, and t c (kg s 2 m 1 )is the critical shear [Foster and Meyer, 1972; Elliot and Laflen, 1993]. Hydraulic shear stress is calculated by t ¼ g w r h S ð4þ where g w (N m 3 ) is the specific weight of water, r h (m) is the rill hydraulic radius, and S (m m 1 ) is the hydraulic gradient. Current process-based erosion models such as WEPP use a baseline value based on soil type for hydraulic conductivity, rill erodibility, interrill erodibility, and critical shear stress. These values are adjusted according to the amount of incorporated residue, roughness, roots, sealing and crusting, and freeze-thaw effects. Critical shear stress is the lowest flow at which soil detachment begins. On steep Figure 1. Photo of (a) fir needles at 53 percent cover and (b) ponderosa pine needles at 52 percent ground cover.

3 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES ESG 1-3 Figure 2. Schematic view of the experimental setup with a ponderosa pine needle treatment on the left and a bare soil treatment on the right. The inset provides a side view. where y is the percent surface cover, b is a coefficient that varies with cover type, and x is mass per unit area (kg ha 1 ) [Wischmeier and Smith, 1978]. The b coefficient has been established for many residue types, including pine needles, which is [Gilley et al., 1991]. [13] The objective of this study is to quantify the effects of needle type and percent cover on postfire runoff, sediment concentration, and sediment delivery for interrill and rill erosion. We compared hydraulic shear stress to stream power in predicting sediment load and detachment rates. Experiments were conducted on two forest soil types under controlled conditions. The results have important implications for understanding needle cast surface cover effects on erosion and the use of stream power rather than shear stress for predicting sediment load. 2. Methods 2.1. Needle Collection and Determination of Percent Cover [14] pine and fir needles were collected from the Wenatchee National Forest in Washington after a 1998 wildfire. In some areas, the trees were killed, but the needles had not burned. The dead needles were

4 ESG 1-4 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES Table 3. Main Runoff Effects for the Granitic and Volcanic With Inflows by Needle Type, Cover Amounts, and Inflow Rates Degrees of Freedom Granitic Volcanic Granitic, Lmin 1 Volcanic, Lmin 1 Figure 3. Ground cover versus needle mass for fir and ponderosa pine needles. collected, and a subsample of 100 needles of each type was characterized (Table 1). [15] To determine percent needle cover, needles were placed uniformly by hand on a 0.25 m 2 white board in 2.5 g increments. Four lights were placed on each side of the board to eliminate shadow effects and a digital picture was taken. Ninety successive pictures were taken as each increment of 2.5 g of needles was added until the board was nearly 100 percent covered (Figure 1). The pictures were cropped and calibrated to the intensity of the four corner marks drawn on the board with image processing software [SPSS, Inc., 1999]. The adjusted intensity was then applied to the picture, and percent cover was determined by dividing the sum of the colored pixels by the total number of pixels Experimental Procedures [16] Two surface soils (0 10 cm) were collected after wildfires from intensely burned areas where the entire forest floor was consumed by the fire. The first soil was granitic (Dystric cryochrepts), from the Salmon National Forest in Idaho, and the second soil was volcanic (Alic vitrixerands), collected from the Wenatchee National Forest in Washington. Both soils were sandy loams with percent gravel and only 1 percent clay. The volcanic soil contained a small fraction (less than 5 percent by weight) of gravel-sized pumice that has a bulk density less than water, allowing it to float. Both soils were passed through a 10 sieve to remove roots and large rocks and then air-dried to approximately 10 percent gravimetric water content. Needle type (N) fir 1.83a a 2.32a pine 1.75b 2.17b Ground cover (C), a 2.32a a 2.21ab a 2.28ab b 2.16b Inflow rates (I ), L min c 0.28d b 1.93c a 2.67b 3.9 ND b 4.09a N 1 1 S c S c C 3 3 S c S d I 2 3 S c S c N C 3 3 S c S d N I 2 3 NS e NS C I 6 9 S c S c N C I 6 9 S c S d a Different letters within column groups indicate a significant difference at b No data. c Significant at a = d Significant at a = 0.1. e Not significant. [17] For each experimental run, one soil type was placed into a 4 m long 1 m wide 0.2 m deep box over a filter fabric (Figure 2). The loose soil was then screed level, 10 below the top of the box frame. The soil was compacted to the measured bulk densities of 1.2 g cm 3 for the granitic soil and 1.0 g cm 3 for volcanic soil with 10 percent water content. The compacted layer was then screed to 60 below the top of the box. To simulate loose soil found iediately after wildfire due to combustion of organic matter, a layer of sieved soil was placed on top and leveled with a double U-shaped screed 30 below the top of the box. The U shape created two depressions or channels with a 10 percent side slope to prevent water from flowing along the edge of the box. The two channels in the soil box Table 2. Main Runoff Effects for the Granitic and Volcanic From Rain Only by Cover Amounts Granitic, L min 1 Volcanic, L min 1 Table 4. Main Sediment Concentration Effects for the Granitic and Volcanic From Rain Only by Cover Amounts Granitic, g L 1 Volcanic, g L 1 Needle Cover, Needle Cover, a a 0.58a 0.36a 0.36a a 0.57a 0.32a 0.16a a 0.53a 0.22a 0.26a a 0.54a 0.25a 0.28a a Different letters within columns indicate significant difference at 0 223a a 223a 77a 77a b 121b 47a 30c 40 34c 93b 21ab 52b 70 14c 59c 13b 32c a Different letters within columns indicate significant difference at

5 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES ESG 1-5 Table 5. Main Sediment Concentration Effects for the Granitic and Volcanic s With Inflows by Needle Type, Cover Amounts, and Inflow Rates Degrees of Freedom Granitic Volcanic Granitic, gl 1 Volcanic, gl 1 Needle type (N ) fir 194b a 168a pine 213a 145b Ground cover (C), 0 307a 186a b 186a c 153b 70 73d 101c Inflow rates (I ), L min b 44c a 187b a 187b 3.9 ND b 208a N 1 1 S c S c C 3 3 S c S c I 2 3 S c S c N C 3 3 S c NS N I 2 3 NS d S e C I 6 9 S c S c N C I 6 9 S c S c a Different letters within column groups indicate a significant difference at b No data. c Significant at a = d Not significant. e Significant at Figure 4. Average normalized sediment delivery for interrill erosion as a function of the ground cover with an exponential regression fit for each needle type. [19] Needles were weighed to approximate 15, 40, and 70 percent surface cover (equation (6)) and spread evenly on the box. The soil box was positioned under a modified Purdue-type rainfall simulator [Meyer, 1995] (Figure 2). To simulate interrill erosion, rainfall was applied throughout the entire run at a rate of 34 h 1 and was monitored using four rain gages, two on each side of the soil box. For the first 10 min only rain was applied. Between 10 and 15 min, 1.5 L min 1 inflow was added to simulate rill erosion and upslope contribution. At 15 min the inflow was increased to 2.4 L min 1, and at 20 min the inflow was allowed two rills to form side by side; thus two treatments could be run simultaneously. The soil box was placed on an adjustable frame and a trough was placed at the lower edge to collect runoff from each side. [18] Before the experiments were started, sieved surface soils were consolidated by applying simulated rainfall with the soil box in a level position. Simulated rainfall was applied at 18 h 1 until ponding occurred, approximately 12 min for granitic soil and 25 min for volcanic soil. The soil was then left to equilibrate for at least 12 but no more than 24 hours. Once soils were equilibrated the soil box was adjusted to a 40 percent slope. Two inflow manifolds were positioned at the top of the soil box to introduce water at rates of 1.5, 2.4, and 3.9 L min 1. The inflow simulated the effect of upslope runoff contributions. Table 6. Main Sediment Delivery Effects for the Granitic and Volcanic From Rain Only by Cover Amounts Needle Cover, Granitic, g min 1 Volcanic, g min a a 128a 34a 34a 15 64a 70a 19a 12a 40 20a 49a 7a 18a 70 7a 31a 6a 12a a Different letters within columns indicate significant difference at a = Table 7. Main Sediment Delivery Effects for the Granitic and Volcanic s With Inflows by Needle Type, Cover Amounts, and Inflow Rates Degrees of Freedom Granitic Volcanic Granitic, gmin 1 Volcanic, gmin 1 Needle type (N ) fir 440a a 506a pine 441a 392b Ground cover (C )() 0 623a 509a b 514a c 464a d 309b Inflow rates (I )(L min 1 ) 0 62c 18d b 398c a 512b 3.9 ND b 868a N 1 1 NS c S d C 3 3 S d S d I 2 3 S d S d N C 3 3 S d S e N I 2 3 NS S e C I 6 9 S d S d N C I 6 9 S d S e a Different letters within column groups indicate a significant difference at b No data. c Not significant. d Significant at a = e Significant at

6 ESG 1-6 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES Table 8. Fraction of Length That Had Minidebris Dams, Mean Widths, and Depths Measured After Each Experimental Run Granitic Volcanic Needle Cover, Minidebris Dams, Width, Depth, Minidebris Dams, Width, Depth, Minidebris Dams, Width, Depth, Minidebris Dams, Width, Depth, 0 0b a 25c 94a 0b 25c 94a 0b 27c 95b 0b 27b 95a 15 1b 29c 93a 6b 36b 79ab 0b 28c 107a 6b 32ab 87ab 40 4ab 43b 69b 27a 53a 59bc 0b 34b 104ab 45a 55a 67b 70 6a 55a 46c 37a 35bc 35c 6a 40a 74c 52a 56a 27c a Different letters within columns indicate significant difference at increased to 3.9 L min 1. The inflow and rain were stopped after 25 min or when the plot began leaking from the bottom. Twelve of the 21 soil boxes leaked during the 3.9 L min 1 inflow on the granitic soil before the completion of the run. For this reason, only inflow rates of 1.4 and 2.5 L min 1 were reported for granitic soil. [20] Six runoff samples were taken during each rain and inflow combination from the base of the soil box. Three samples were taken before a velocity measurement was made, and three were taken after. The six samples were averaged to calculate runoff, sediment concentration, and sediment delivery. [21] Water velocity was measured at each of the inflow rates with a salt tracer solution that could be detected with a conductivity probe. Conductivity probes were placed 1 and 3 m from the top of the plot. After three runoff samples were taken, the probes were positioned in the rill and 20 ml of the salt tracer solution was poured onto the energy dissipater (Figure 2). The probes were wired to a data logger that stored the conductivity reading and the time at which the peak salt solution passed by the probe. Flow width and depths were measured at the 1 and 3 m marks in the soil box at the time the velocity measurements were taken. [22] After each experimental run, generally only one rill was formed on each side of the soil box. width and depth were measured at 1, 2, and 3 m down the length of the plot. Usually the rill was filled with both needles and soil that caused minidebris dams. The fraction of the rill that was made up of minidebris dams was calculated by dividing the width of minidebris dam by the total length of the rill. After all measurements were made, the needles were removed from the surface and the soil was removed from the box. The soil was air-dried and the process began again Statistical Design [23] The statistical design of the experiment is a balanced incomplete block [SAS Institute, 1989]. This requires each of the four cover amounts (0, 15, 40, and 70 percent) and the two needle types ( fir and ponderosa pine) to be run simultaneously with every other treatment combination. A total of 21 soil boxes or 42 side-by-side runs were completed (six replications of seven treatments). Zero percent cover was compared to both fir and ponderosa pine needle cover. type was not statistically compared in this study because the soil box could not be mixed with the two soil types. A coin toss was used to decide which needle and cover combination was applied to the right or left side of the soil box. When treatments were significantly different, means were tested with a protected least significant difference (LSD) at the a = 0.05 level unless otherwise noted [SAS Institute, 1989]. When interactions were significant, contrasts were used to examine simple effects. 3. Results and Discussion 3.1. Needle Mass to Cover Relationship [24] A regression analysis was carried out to determine the relationship between needle mass and surface cover Figure 5. Unit sediment load as a function of stream power for both soil and needle types. Figure 6. Unit sediment load as a function of shear stress for both soil and needle type.

7 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES ESG 1-7 Figure 7. Unit sediment load as a function of stream power for granitic and volcanic soils. using the form of equation (6). The R 2 for ponderosa pine needles was 0.97 compared to an R 2 of 0.80 for fir needles (Figure 3). The b coefficient was for the ponderosa pine needles and for fir needles Runoff and Infiltration [25] Although runoff was greater for granitic soils than for volcanic soil, the runoff amount did not change significantly with needle cover when rain only was applied (Table 2). With added inflow, however, runoff was significantly lower under the ponderosa pine needles compared to the fir needles (Table 3). Runoff was significantly lower at the 70 percent cover compared to 0 percent cover for both soil types, indicating increased infiltration, whereas at all other needle cover amounts runoff was similar. Thus infiltration rates did not change as inflow was added. The granitic soil had a mean infiltration rate of 26.6 h 1, and the volcanic soil was 29.6 h 1. [26] When examining the interaction between needle type, cover, and inflow on the granitic soil, the runoff was greater with 40 percent fir needles at the 2.4 L min 1 inflow compared to every other inflow, needle, and cover combination. While sampling runoff, oftentimes a flow concentration sample would be collected from a breaking minidebris dam that had formed and could no longer pool the ponding water behind the dam. This threeway interaction can only be explained as an experimental error Sediment Concentration [27] When rain only was applied, sediment concentration decreased with increasing needle cover for all treatments except on the volcanic soil where ponderosa pine needles were applied (Table 4). The apparent lack of direct soil contact with the ponderosa needles due to their length, arc, and being bundled caused less effect on sediment concentration than the fir needles, which are in better contact with the soil surface. Where the differences were not significant at every cover amount, the trend was evident that the needles protected the soil surface from raindrop impact and caused sediment deposition in detention storage, therefore reducing the sediment concentration in runoff. [28] On granitic soil, when inflow was added to the top of the plot, sediment concentrations in the first two runoff samples were generally higher compared to the next four. The particles most susceptible to transport are entrained early in the process. On the volcanic soil, however, sediment concentration was low on the first sample and then the same for all the other samples. Silica in the volcanic soil has a low particle density, which may contribute to a more uniform entrainment rate. [29] With added inflow, sediment concentration was greater with ponderosa pine needle cover than with fir needles for the granitic soil; in volcanic soil the opposite was true (Table 5). The pumice in the volcanic soil interacts with the ponderosa pine needles, forming more minidebris dams in the rill. As needle cover increased, sediment concentration decreased for both soil types except on the volcanic soil with 0 and 15 percent cover (Table 5). As inflow increased, sediment concentration increased only on the volcanic soil between the 2.4 and 3.9 L min 1 flow. [30] The interactions between needle type, cover, and inflow indicate that on the granitic soil the fir needles at 70 percent cover over all inflow rates reduced sediment concentration significantly more than the ponderosa pine needles at 70 percent cover. On the volcanic soil, however, the ponderosa pine needles at 70 percent Figure 8. Unit sediment load as a function of stream power for (a) fir and (b) ponderosa pine needles for each cover amount. Detachment is the slope of the line for each cover amount.

8 ESG 1-8 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES Figure 9. Normalized stream power detachment rate as a function of ground cover for fir and ponderosa pine needles. cover showed lower sediment concentrations at the 2.4 and 3.9 L min 1 compared to the fir needles Sediment Delivery [31] Sediment delivery is the product of sediment concentration and runoff. When rain only was applied, sediment delivery decreased slightly with increasing cover (Table 6). Sediment delivery was averaged for each soil type and then normalized to bare soil sediment delivery rates. Normalized sediment delivery was then compared to ground cover and fit to an exponential curve with R 2 values of 0.58 for ponderosa pine and 0.82 for fir (Figure 4). The shapes of the exponential curves for fir needles and crop residue are similar [Wischmeier and Smith, 1978; McCool et al., 1997]. This indicates fir needles provided protection (i.e., reduced sediment delivery) similar to that seen in agricultural studies of crop residue cover. The exponent in equation (1) is 3.2 for fir and 1.8 for ponderosa pine needles. With 50 percent fir needle cover, sediment delivery was reduced by 80 percent, whereas the ponderosa pine needle sediment delivery was reduced by only 60 percent. [32] During the rill portion of the experiment, when inflow was added, there was no difference in needle type for sediment delivered with the granitic soil. In contrast, significant differences were seen with the volcanic soil, where fir was greater compared to ponderosa pine needles (Table 7). As needle cover increased, sediment delivery significantly decreased at each cover amount for the granitic soil, but it only decreased significantly at 70 percent cover on the volcanic soil. When inflow rates increased, sediment delivery also significantly increased for both soils. [33] The interactions between needle type and inflow show that the fir needles protected both soils better than the ponderosa pine needles at lower inflow rates. On the volcanic soil at high inflow rates, however, the ponderosa pine needles were more effective than the fir needles Minidebris Dams [34] The portion of the rill in which minidebris dams formed increased, though not always significantly, as needle cover increased for both soils (Table 8). The fir needles created fewer minidebris dams than the ponderosa pine needles. Since the bundled pine needles are longer and curved, they often interlock. This makes them less likely to move down the rill and more likely to reduce velocities and cause sediment to deposit within the rill. In addition, the floating pumice in the volcanic soil combined with the ponderosa pine needles to cause tighter minidebris dams and increase the occurrence of minidebris dam formation. This did not occur with the granitic soil (Table 8). width generally increased, and rill depth generally decreased as cover increased. This is due to the decrease in flow velocities and an increase in hydraulic friction caused by the presence of minidebris dams. The presence of needles disperses water flow to a wider area, resulting in less downward scouring Detachment Prediction [35] Stream power was a better predictor of sediment load than shear stress (Figures 5 and 6). The scatter of points between sediment load and shear stress is typically seen in erosion studies [Elliot et al., 1989; Laflen et al., 1991; Norton and Brown, 1992; Brown and Norton, 1994]. The difference between stream power and shear stress is the additional velocity term (q) in stream power (equation (5)). Since stream power appears to be a better predictor of sediment load, the two soils were plotted separately, which showed that the granitic soil was more erodible that the volcanic soil (Figure 7). Further investigating stream power, we separated the data by needle type and cover amounts to predict sediment load. A linear regression, forced through the origin, was determined for each needle type and cover amount (Figures 8a and 8b). The slope of each calculated regression line is the rate of unit sediment load (g s 1 cm 1 ) per unit of stream power (g s 3 ) for each cover amount. This stream power detachment rate (s 2 cm 1 ) decreases with the addition of cover and is dependent on needle type. As needle cover increased, the slopes progressively decreased. In other words, with increasing stream power, cover decreases the rate at which sediment loading increases. In models, however, ground cover is a continuous variable, and thus stream power detachment rates were averaged and normalized to the bare soil surface conditions and then fit to an exponential curve for each needle type (Figure 9). Fifty percent ponderosa pine needle cover reduced the detachment rate by 40 percent, whereas 50 percent fir needle cover reduced the detachment rate by 20 percent. Many soil erosion models coonly address differences in soil erodibility through baseline or bare soil erodibility parameters and adjust due to management-induced cover. In forest environments, cover is generally present unless affected by natural or management disturbances, and ground cover is often more important than soil type in determining erodibility [Robichaud, 1996]. 4. Conclusions [36] Rainfall simulation experiments indicate that needle cast from burnt conifer trees reduces postfire erosion rates. fir needles were more effective at reducing interrill erosion because they had better ground contact than the ponderosa pine needles. pine needles, on the other hand, were more effective at reducing rill erosion

9 PANNKUK AND ROBICHAUD: REDUCING EROSION AFTER WILDFIRES ESG 1-9 because they tend to form minidebris dams. Infiltration rates were not affected by needle cover amount. Sediment delivery was greater for the granitic soil compared to the volcanic soil. [37] When comparing rill erosion models, stream power was a better predictor of sediment load than shear stress. The rill detachment rates based on stream power decreased with increasing cover for both needle types. The normalized, sediment delivery and stream power, detachment rates provide an estimate of soil erosion reduction due to the needle cover for both interrill and rill erosion. Needle cast cover was more effective at reducing interrill erosion than rill erosion rates; however, the magnitude of rill erosion was higher than interrill erosion. References Agee, J. K., e Ecology of Pacific Northwest Forests, 493 pp., Island Press, Washington, D. C., Brown, J. K., Weight and density of Northern Rocky Mountain conifers, USDA For. 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