mpats of Fire on Hydrology and Erosion in Steep Mountain Big Sagebrush Communities Frederik B. Pierson, Peter R. Robihaud, Kenneth E. Spaeth, Corey A. Moffet Abstrat ntrodution Wildfire is an important eologial proess and management issue on western rangelands. Major unknowns assoiated with wildfire are its affets on vegetation and soil onditions that influene hydrologi proesses inluding infiltration, surfae runoff, erosion, sediment transport, and flooding. Post wildfire hydrologi response was studied in big sagebrush plant ommunities on steep slopes with oarse-textured soils. Signifiant rill erosion was observed following both thunderstorm and rapid snowmelt events. Rainfall simulation and ontrolled overland flow tehniques were used to study postfire effets on infiltration, and interrill and rill erosion proesses on burned and adjaent unburned areas. Results indiate that burn severity and the development of water repellent soil onditions play signifiant roles in determining infiltration and interrill erosion rates, partiularly on shrub oppie dunes haraterized by high surfae litter aumulations. The most dramati and long-lasting affet of fire was on rill erosion proesses by reduing ground over needed to slow and spread water as it moves aross the soil surfae. Ongoing researh efforts are aimed at haraterizing the hydrologi impats of presribed fire used as a tool to manage vegetation and mitigate the impats of atastrophi wildfire events. Keywords: rangeland, fire, hydrology, erosion, infiltration, water repellent soils. Fire is a natural omponent of the ntermountain sagebrush-steppe eosystem (Wright and Bailey 1982) with return periods of 25 to 100 years, depending on ommunity type and natural fuel load and distribution. Fuel and land management ativities in the past entury have disturbed fire frequenies putting wildland values suh as soil and water quality at greater risk from devastating wildfires. Higher runoff rates from severely burned landsapes an lead to flooding and inreased risk to human life and property. nreased soil erosion over natural levels following wildfire an lead to loss of soil produtivity and a deline in rangeland health. Hydrologi onsequenes of fire have been losely examined in forested eosystems, but few studies have examined impats of wildfire on rangeland hydrology. Many studies have shown an inrease in runoff and erosion rates the first year following fire, with reovery to pre-fire rates generally within five years (Wright and Bailey 1982). Timing and extent of reovery is highly dependent on preipitation, slope and vegetation type (Branson et al. 1981, Knight et al. 1983, Wright et al.1982, Wilox et al. 1988). The USDA-ARS Northwest Watershed Researh Center (NWRC) has been investigating the interation of wildfire and rangeland hydrologi proesses on steep, oarse textured, sagebrush dominated landsapes in daho and Nevada. Researh objetives are aimed at quantifying impats of wildfire on infiltration apaity, runoff, and erosion, gaining insight into the proesses Pierson is a researh hydrologist, U.S. Department of Agriulture, Agriultural Researh Servie, Northwest Watershed Researh Center, Boise, D 83712. E-mail: fpierson@nwr.ars.usda.gov. Robihaud is a researh engineer, U.S. Department of Agriulture, Forest Servie, Roky Mountain Researh Station, Mosow, D 83843. Spaeth is a rangeland hydrologist, U.S. Department of Agriulture, Natural Resoure Conservation Servie, Boise, D 83712. Moffet is a researh assoiate, University of daho, Boise, D 83712. The First nterageny Conferene on Researh in the Watersheds, 27-30 Otober 2003, Benson, AZ 1
involved and determining how long fire effets persist. The NWRC is also implementing a presribed fire researh program on Reynolds Creek Experimental Watershed (RCEW) to examine the hydrologi impats of using presribed fire as a vegetation and fuels management tool. Study Areas Study areas are loated on the Boise Front immediately above the ity of Boise, daho (Eighth Street Fire), and in the Pine Forest Range, 50 km south of Denio, Nevada (Denio Fire). Both sites have vegetation onsisting of big sagebrush (Artemisia tridentata)/ daho fesue (Festua idahoensis) ommunities. Some areas are haraterized by inreases in three-awn (Aristida spp), Sandberg s bluegrass (Poa sandbergii), and rabbitbrush (Chrysothamnus spp). Soils on both sites were derived from granite and onsist of well drained, fine gravelly oarse sandy loams, on slopes of 35 to 45%. Both sites were severely burned by wildfire. More omplete desriptions of study sites are presented by Pierson et al. (2001, 2002). Methods At both loations burned sites were ompared to unburned sites with the same soil type, slope and vegetation as that found before the fire. Sampling on both burned and unburned sites was stratified based on oppie (areas strongly influened by the existene of a shrub) and interspae areas (areas between shrubs primarily dominated by grasses and forbs) (Pierson et al. 1994). A portable osillatingarm rainfall simulator with speifiations as desribed by Meyer and Harmon (1979) was used to ahieve intermittent rainfall similar to naturally ourring rainfall. Simulations were run on natural plots, 0.5 m 2 in size, without pre-wetting. Sampling was onduted in late summer when soil moisture for all plots was extremely low (<10% generally). The Boise Front sites were sampled one year following the fire and the Denio sites were sampled immediately following the fire. Rainfall was applied at a rate 67 mm hr -1 on the Boise Front and 85 mm hr -1 on the Denio sites. Runoff samples were olleted at one- or two-minute time intervals throughout the 60-minute simulation and analyzed for runoff volume and sediment onentration. nfiltration apaity for eah time interval was alulated as the differene between measured rainfall and measured runoff. A flow regulator was used to apply rill flow rates of 7, 12 and 15 l/min to six randomly seleted points at eah site to simulate the rill development proess. Flow samples were olleted 4-m down slope of the release point. Suspended sediment samples for both rainfall and rill proedures were weighed, dried at 105 o C then re-weighed to determine runoff rate and soil loss. Flow veloity in eah rill was measured by releasing a onentrated salt solution into the rill and using eletrial ondutivity probes to estimate the mean travel time of the salt over a known rill length. Rill erosion sampling was done in early spring while soils were still moist. Vegetative over and ground over were also estimated for eah rainfall simulation plot and rill using a ombination of pinpoint and oular estimation proedures. Further desription of the methodology an be found in Pierson et al. (2001, 2002). Results and Disussion Both the Boise Front and Denio study areas were severely burned with virtually all vegetation and litter being onsumed during the fire. Bare ground for all burned sites was greater than 95% resulting in inreased soil exposure to the erosive fores of raindrop impat and overland flow. Loss of surfae litter, vegetative basal over and assoiated mirotopographi relief also redued surfae storage of water ruial for reduing runoff and inreasing infiltration. Past studies have shown that fire an vaporize organi ompounds on the soil surfae and distill them downwards, reating onentrated water repellent layers within the upper the soil (De Bano 1981, Wright and Bailey 1982). Suh water repellent soil onditions are generally temporary (Robihaud 2000b, De Bano et al. 1967), so that during the first minutes (or longer) of rainfall, water beads on or near the soil surfae and quikly runs off the plot. Water repelleny then deteriorates as rainfall ontinues, resulting in a gradual reovery in the infiltration apaity of the soil. The following results demonstrate the effets of these fire impats on ritial hydrologi and erosion proesses. nfiltration The redution in infiltration apaity due to the fire was onentrated on the oppie mirosites for both study sites. nfiltration rates for oppies were redued by over 30% in the initial stages of runoff (Figures 1 and 2). After one hour of rainfall, the initial fire-indued redutions in infiltration rates had 2
reovered to a point where no signifiant differenes in final infiltration rates ould be deteted (Table 1). Coppie mirosites are generally dominated by heavy litter aumulations providing ample organi material for the reation of hydrophobi ompounds that in turn reate water repellent soils. Pierson et al. (2001) defined a soil water repelleny index (WR) to quantify the perent redution in infiltration apaity during the initial stages of the infiltration proess. Burned oppies exhibited an average of 16 and 32% redution in infiltration due to water repelleny on the Boise Front and in Denio, respetively (Table 1). Runoff began within 5 min for all burned and unburned mirosites (Table 1). Also, all mirosites reahed their peak runoff rates from 5 to 14 min following the start of rainfall (Table 1). Figure 2. nfiltration rate for burned and unburned mirosites, Denio Fire, Nevada. Coppie rates within a level of time labeled with the same letter () are signifiantly different (P 0.05). Figure 1. nfiltration rate for burned and unburned mirosites, Boise Front, daho. Coppie rates within a level of time labeled with the same letter (C) are signifiantly different (P 0.05). Under very dry onditions, unburned soils with suffiient waxy substanes on the soil surfae an also reate water repellent soil onditions (DeBano et al. 1967, DeBano and Rie 1973). This phenomenon was observed for unburned interspae mirosites at both loations. Unburned interspaes had soil water repelleny values of 17% on the Boise Front and 9% in Denio (Table 1). Final infiltration rates for unburned interspaes were lower than all other burned or unburned mirosites (Table 1). Runoff Fire had a signifiant, but small affet on the initiation of overland flow on all burned sites. Both sites showed a onsistent and rapid runoff response to intense rainfall for all burned and unburned mirosites. Runoff on burned oppies onsistently began approximately 2 min after the start of rainfall. The greatest impat of fire in these steep oarsetextured sagebrush systems is not on generating runoff, but its effet on the dynamis of runoff as it begins to move down slope over the soil surfae. A year after the fire on the Boise Front, signifiant rilling was observed following a short intense thunderstorm that flooded the town of Boise, daho. n Denio, signifiant rilling was observed on the burn following numerous rapid snow melt events during the winter following the fire. As a relative measure of how a burned hillslope responds to onentrated overland flow, varying rates of water were released at points on the landsape and the rate of disharge (l/min) was measured at 4 m down slope (Figure 3). At a release rate of 7 l/min, no water reahed 4 m on the unburned sites. An average of 2 l/min reahed 4 m on the burned sites after the fire and was nearly zero after one growing season. At a flow rate of 12 l/min, water was just beginning to reah 4 m on the unburned sites, while the burned sites flowed at over 4.5 l/min after the fire. Flow delined to less than 1 l/min on the burn after two growing seasons. At a release rate of 15 l/min, the unburned sites flowed at less than 0.3 l/min on the unburned sites. Flow on the burned sites dramatially inreased to over 8 l/min after the burn and still flowed at 2 l/min after two growing seasons of vegetation reovery. The First nterageny Conferene on Researh in the Watersheds, 27-30 Otober 2003, Benson, AZ 3
Erosion Figure 3. Rill disharge rate for burned and unburned sites on the Denio Fire, Nevada. Vegetation, litter, roks and other forms of ground over reate barriers that slow and spread water movement aross the soil surfae allowing more time for water to infiltrate over a larger surfae area. Fire removes most of these barriers and allows the water to onentrate into rills where flow depth and veloity inrease. This signifiantly dereases runoff response time and inreases the runoff volume delivered to the bottom of a hillslope and into a stream hannel. After the Denio fire, nearly all the soil ground over was removed exposing over 95% bare ground ompared to less than 5% bare ground on the unburned sites (Figure 4). t took two growing seasons and three winters for litter aumulation to redue the amount of bare ground on the burned sites to near 50% (Figure 4). A ground over of 60% is often used to gauge adequate hydrologi protetion of a site (Gifford 1985, Robihaud 2000a). These results indiate that while 50% vegetation and litter over does indue a signifiant redution in overland flow, the site is not yet fully proteted ompared to unburned onditions. Fire had far greater impat on rill erosion ompared to interrill erosion. nterrill erosion was relatively low for all burned and unburned mirosites after both fires. The only signifiant fire effet observed was for burned oppie mirosites on the Denio fire (Table 1). To examine the impat of the fire on rill erosion, the sediment onentration of the rill disharge olleted 4 m down slope was used as a relative index to ompare burned and unburned sites (Figure 5). No measurable sediment was olleted on the unburned sites at any of the flow rates applied. At a flow rate of 7 l/min, over 175 g/l of sediment were measured after the Denio fire and dropped to zero after two vegetation growing seasons. At a flow rate of 12 l/min, over 110 g/l of sediment were measured for the burned sites after the fire and near 10 g/l of sediment were measured after two growing seasons. At a flow rate of 15 l/min, burned sites exhibited sediment onentrations of over 75 g/l after the burn and less than 10 g/l after two growing seasons. The observed deline in post-fire sediment onentrations as the rill flow rates inreased was due to the rills beoming armored. Rills beame sediment limited as the detahable sediment was removed by lower flow rates. Figure 5. Rill sediment onentration for burned and unburned sites on the Denio Fire, Nevada. 4 Figure 4. Ground over for burned and unburned sites on the Denio Fire, Nevada. The veloity of the water flowing in a rill is diretly related to its ability to detah and arry sediment. As an example, the rill veloity at the 15 l/min release rate for the unburned sites was onsistently under 0.05 m/s throughout the study (Figure 6). Suh a veloity is apparently insuffiient to detah and arry sediment down slope in a fully proteted system with high ground over of vegetation and
Table 1. Hydrologi and erosion variables for the Boise Front and Denio study sites. Values aross a row followed by different letters are signifiantly different (p<0.05). Burned Burned Unburned Unburned Coppie nterspae Coppie nterspae Final nfiltration Rate (mm hr -1 ) Boise Front, D 54.2a 60.4a 64.7a 53.7a Denio, NV 55.5ab 60.0a 55.9ab 40.4b Water Repelleny ndex (%) Boise Front, D 16.3a 16.1a 3.3b 17.3a Denio, NV 32.2a 11.6b 5.3b 9.4b Time to Runoff (min) Boise Front, D 2.5b 4.3a 5.4a 2.1b Denio, NV 1.8b 2.0ab 3.0a 2.0ab Time to Peak Runoff (min) Boise Front, D 12.4a 12.8a 14.0a 13.6a Denio, NV 5.6a 5.5a 7.8a 6.3a Sediment/ Runoff Ratio (kg ha -1 mm -1 ) Boise Front, D 3.5a 3.8a 1.2a 0.9a Denio, NV 13.6a 8.7ab 5.1b 6.4b litter. The veloity for burned sites was near 0.21 m/s after the burn and delined to 0.075 m/s after two growing seasons. This indiates that a veloity of 0.075 m/s an still ause rill erosion in a system proteted by only 50% ground over. to keep in mind that these systems have a ertain degree of natural water repelleny when they are extremely dry. This an onfuse and onfound the assessment of fire-indued water repelleny often inventoried and used after wildfires to determine the need for post-fire mitigation. n general, the greatest impat of the fire is not on infiltration, but on overland flow dynamis and rill erosion. Whether burned or unburned, these types of sites are going to rapidly generate runoff under intense rainfall. The impat of the fire is that it leaves sites devoid of barriers to slow and store runoff one it begins to move. The water then onentrates and inreases in veloity resulting in greater erosive energy. Water moves more rapidly down slope and ultimately into stream hannels potentially ausing downstream flood damage. Figure 6. Rill flow veloity for a rill flow rate of 15 l/min for burned and unburned sites on the Denio Fire, Nevada. mpliations The results of these studies provide a relative index of the inreased risk of runoff and erosion and the rate of hydrologi reovery following fire in oarsetextured sagebrush dominated systems. Fire in these systems an indue signifiant water repelleny, partiularly in areas dominated by shrubs with large aumulations of litter to fuel the intensity of the fire and provide the hydrophobi substanes neessary to reate water repellent soils. However, it is important Future Researh The natural fire yle of many rangeland eosystems has been disrupted by past management and fire suppression ativities. The result has been a dramati inrease in atastrophi wildfires aross the western United States over the past deade. To redue the threat of wildfire and return these eosystems to a healthy state means to return them to a natural fire yle. The NWRC in addition to the work on the hydrologi impats of wildfire has established a long-term presribed fire researh program on RCEW in southwest daho. The goals of the researh program are to: Determine the shortand long-term effets of presribed fire on runoff, erosion, water quality and vegetation and forage in The First nterageny Conferene on Researh in the Watersheds, 27-30 Otober 2003, Benson, AZ 5
rangeland eosystems; develop pre-fire assessments for prediting post-fire soil, vegetation and hydrologi responses; and investigate interations of presribe fire and grazing for reduing the threat of wildfire. The NWRC intends to transfer researh findings through deision support tools to help managers determine the risk of wildfire, to explore options for reduing suh risk through the use of presribed fire, and to predit post-fire impats and optimize rehabilitation strategies. Referenes Branson, F.A., G.F. Gifford, K.G. Renard, and R.F. Hadley. 1981. Rangeland hydrology, 2 nd edition. Kendall/Hunt Publishing Co., Dubuque, A. DeBano, L.F. 1981. Water repellent soils: a state of the art. USDA Paifi Southwest Forest and Range Exp. Sta. Gen. Teh. Report PSW-46, Berkeley,CA. DeBano, L.F., J.F. Osborn, J.S. Krammes, and J. Letey, Jr. 1967. Soil wetability and wetting agents: our urrent knowledge of the problem. USDA Forest Servie Res. Paper. Paifi Southwest Forest and Range Exp. Sta.PSW-43, Berkeley, CA. DeBano, L.F., and R.M. Rie. 1973. Water-repellent soils: their impliations in forestry. Journal of Forestry. 71:220-223. Gifford, G.F. 1985. Cover alloation in rangeland watershed management (a review). n B. Jones and T. Ward eds., Watershed Management in the Eighties, Denver, CO, April 30 May 1, 1985., pp 23-31. Amerian Soiety of Civil Engineers, New York, NY. Knight, R.W., W.H. Blakburn, and C.J. Sifres. 1983. nfiltration rates and sediment prodution following herbiide/fire brush treatments. Journal of Range Management. 36:154-157. Pierson, F.B., S.S. Van Vator, W.H. Blakburn, and J.C. Wood. 1994. norporating small sale spatial variability into preditions of hydrologi response on sagebrush rangelands. Soil Siene of Ameria Speial Publiation #38. p 23-34. Soil Siene Soiety of Ameria, n. Madison, W. Pierson, F.B., P.R. Robihaud, and K.E. Spaeth. 2001. Spatial and temporal effets of wildfire on the hydrology of a steep rangeland watershed. Hydrologi Proesses. 15:2905-2916. Pierson, F.B., D.H. Carlson, and K.E. Spaeth. 2002. mpats of wildfire on soil hydrologial properties of a steep sagebrush-steppe rangeland. nternational Journal of Wildland Fire. 11: 145-151. Robihaud, P.R. 2000a. Fire effets on infiltration rates after presribed fire in northern roky mountain forest, USA. Journal of Hydrology. 231-232:220-229. Robihaud, P.R. 2000b. Forest fire effets on hillslope erosion: what we know. Watershed Management Counil Networker, Winter 2000, Davis, CA. Wilox, B.P., M.K. Wood, and J.M. Tromble. 1988. Fators influening infiltrability of semiarid mountain slopes. Journal of Range Management. 41:197-206. Wright, H.A., and A.W. Bailey. 1982. Fire eology. John Wiley and Sons, New York, NY. Wright, H.A., F.M. Churhill, and W.C. Clark. 1982. Soil loss, runoff, and water quality of seeded and unseeded steep watersheds following presribed burning. Journal of Range Management. 35:382-385. Meyer, L.D., and W.C. Harmon. 1979. Multipleintensity rainfall simulator for erosion researh on row sideslopes. Trans. Amerian Soiety of Agriultural Engineers. 22:100-103. 6