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VEGETATION The vegetation cover for the study area \Va>, taken from a GIS database developed for the Wasatch.Cache National Forest with the use of satellite remote sensing. This database was not evaluated in detail for this study but is uleful for illustrating the various types of vegetation which would become fuel in the event of a wildfire. Vegetation was considered in the groun d ,:over portion of the PSIAC model. This factor evaluat,:d the ,effective cover of vegetation and rock within the study area. Vegetation plays an important role in detennining where water repellent (hydrophobic) conditions may develop with a wildfire. Development of a water repellent coondition can increase surface runoff and increase the potentil~ for a debris flow event at the mouth of a watershed. In a study prepared for the Davis County Planning Commission (1980) it is noted that vege'tation such as oakbrush and bluebunch wheatgrass playa key role in acting as "check dams" for overland flow even on 70 p'ercent slopes, This type of vegetation is important in controlling the development of rills and gullies following a fire in these zones, The largest portion of the study area ',egetation is composed of the Oak-Maple complex. This complex is usually situated on very productive soils, however the plant production beneath the maple canopy is oftell sparse, If disturbed, many plant species could be expec;ted to establish quite readily. In some sites, maple is the only woody species; in others, oak and other shrub!: .Ire scattered throughout (Forest Service, 1975), The GIS vegetation database can be used for further evaluation of the hydrologic processes occuring in the study area and can be useful in the developmenl of emergency mitigation alternatives. PRECIPITATION Farmer and Fletcher (1971) report that 87 percent of all rainstorms occuring in the foothill zone cf the Wasatch Range in Davis County are less than six hours in duration. Four or five storms may be expected to occ:ur each month from June to September in the Davis County area. A storm is defined as continuous rainfall for at lealit one hour delivering at least 0,1 inch of rain, A storm even at this small magnitude could produce damaging s.:diment and debris. laden flows downstream, TOPOGRAPHY The topography in the study area is ste'ep and rugged. Topography is a key element in determining runoff and the potential for sediment damage following a fire event The Slope Map can be used in mitigation plan selection and analysis to highlight areas with extreme slopes within a burn area. Most of the watersheds in the study received a maximum topography rating of 20 due to th.: very steep upland slopes and alluvial fan development at thl' outlets. HYDROPHOBICITY (Water Repellency) A fire can consurne and vaporize orgarlic:; to a point where the vapor will concentrate to fonn a water repellent Mayer or hydrophobic soil (Debano, 1981). A simplistic definition is that a waxy covering on the leaf lilter liquefies and penetrates the soil until enough depth is achi eved for the waxes to cool and return to a solid state around the soil particles (USDA.Forest Service,1978). This wate". repellency 174 ". is strongest in coarse. textured soils (Debano, 1981) and can increase overland flow and sediment yield through the formation of a rill network in the bum area (Wells, 1987). '," Many of the soil units in the study area are coarse-textured and have potential for the development of a hydrophobic condition. The potential for development of a hydrophobic layer was given little weight in the PSIAC ratings for the post-fire condition. This a reasonable assumption because examination of 9 post-fire rehabilitation reports prepared by the Forest Service for fires located within the Wasatch National Forest showed only small areas of hydrophabic sail development High-Intensity burns as well a~ some Moderate-Intensity burns create the ideal environment far hydraphobic soil development (USDA-Forest Service, 1978). Hydraphobic soil conditians fallawing fires have been reported for forested watersheds, particularly in Southern California chaparral where this layer can migrate downward a few inches from the surface by extreme heat (Debano, 1981). A study on water-repellency in fir and spruce farests in northwestern Montana shawed an increase in water- repellency in the top 4 inches of sail after a controlled burn (Debyle, 1973). Sails in a drier pine and fir farest in the same area would likely react similarly (Parret, 1987), Soils that have experienced the most intense heat during a fire can be tested by pouring water on the soil just beneath the surface. The hydrophabic layer can be 1/2 inch to 3 inches beneath the soil surface and up to 1 inch thick (Forest Service, 1978), CHANNEL EROSION A burn increases the potential far overland flow, rill and gully develapment and creates a condition where runoff can rapidly concentrate tOo a main stem channel and cause bank and channel degradation, Reports of debris flows alang the Wasatch Range in Davis Caunty show that a major partion of the debris flow volume is fram the main.stem channel banks and bed. A channel erosian factor is cansidered in the PSIAC sediment yield madel but the model does nat estimate the potential for a debris flaw event The channel erasion factor is weighted fram 0 to 25 (worst) where a rating af25 would involve continuously or frequently eroding banks with deep flows of long duration and active headcuts and degradation in tributary channels. A rating of 0 wauld involve wide shallow channels with flat gradients and short flow duration or channels with massive rack, large boulders or well. vegetated banks, The character af channel materials in the study area is typically coarse, consisting af a mixture af gravel, sand, and boulders, Excessive peak flows generated fram a burned area after a significant storm event can suspend and transport huge volumes of channel sediment and debris, Studies of mountain streams after watershed disturbance show that the sediment transport rate is a function of the supply af sail or sediment in the stream rather than increased runoff (Booker and others, 1993), This suggests the importance af retarding sediment an hill slopes until vegetatian is reestablished in the burn area. However, Booker and others (1993) concluded that an.site erasion and sediment cantral measures that increase infiltration and soil moisture shauld not be used on slopes that have a high probability of landslide failure. The GIS database can be used tOo quickly locate slope failures within a burned area and an assessment can be made to detenoine if on-site mitigatian measures are appropriate.