Forests and Water: A State of the Art Review for Colorado

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2.4. Effect of Fires on Runoff The effect of wild and prescribed fires on runoff is highly variable because they can affect both the vegetation canopy as well as the physical properties of the soil. At one level, the effect of fires on runoff is similar to forest harvest, as any reduction in the forest canopy will alter the amount of interception and transpiration. Fires may also consume some or all of the litter and duff layers, and this also can increase runoff by reducing the total water storage capacity at a given site (Kittredge, 1948). The greater concern is the potential for high- severity fires to alter the surface organic layer and mineral hori- zons in ways that reduce infiltration and increase runoff and erosion rates. The reduction in infiltration can oc- cur by several processes. High - severity fires consume all of the surface litter and duff. Sustained soil heating can burn off much of the organic matter in the top few centimeters of the mineral soil. In this situation there is little protection of the soil surface from rainsplash, and the disaggregated soil particles and ash can clog up the larger soil pores that are crucial for maintaining the high infiltration rates typical of forested areas (Terry and Shakesby, 1993). The second important change is the generation of a wa- ter repellent (hydrophobic) layer at or slightly below the soil surface. Fires can generate a water repellent layer by volatilizing hydrophobic compounds in the organic material, and some of these are driven downwards where they condense on cooler soil particles at or below the soil surface (DeBano, 1981; Letey, 2001). The amount and type of these compounds varies with the type of vegetation, while the depth at which these compounds condense is a function of soil heating. In hotter, slower - moving fires there is more soil heating and these volatile compounds may be deposited at depths of 2 -6 inches be- low the soil surface. In low- temperature and faster -mov- ing fires there is less soil heating and these compounds condense closer to the soil surface. Stronger water repel- lent layers are associated with increasing burn severity, as high- severity fires vaporize more organic compounds and thereby generate a stronger and more continuous water repellent layer (Tiedemann et al., 1979; DeBano, 1981). The development of a water repellent layer is of concern because this can greatly reduce infiltration rates. Once rainfall saturates the thin layer of ash or soil above the water repellent layer, any additional rainfall will run off as overland flow. The development of a post -fire water repellent layer has been most extensively studied in chaparral ecosystems, 17 but post -fire water repellency has also been documented under a number of different conifer species, including ponderosa pine (Helvey, 1980), lodgepole pine (Meeu- wig, 1971), and Douglas -fir (Helvey, 1980). Post -fire water repellent layers are believed to be less likely in vegetation types, such as aspen, that have less surface fuels and fewer secondary compounds (MacDonald et al., 2000). Post -fire water repellency is generally believed to be more severe in areas with coarse - textured soils. Coarse - textured soils have a much lower particle surface area than fine - textured soils, and this effectively results in a greater concentration of the water repellent compounds per unit surface area (Meeuwig, 1971; DeBano, 1981). Changes in soil moisture will also affect the strength of a water repellent layer. If moisture is present, a water repellent soil will slowly wet up due to the strong hy- draulic gradient and movement of water vapor (DeBano, 1981). As a water repellent soil wets up, there usually is usually a threshold at which a soil ceases to be water repellent (Crockford et al., 1991; Dekker and Ritsema, 1994; Doerr and Thomas, 2000). Upon drying the water repellent conditions can be re- established ( Shakesby et al., 1993). The persistence of a post -fire hydrophobic layer will depend on the initial strength and thickness of the hy- drophobic layer and the animal activity, plant regrowth, and physical and chemical processes that collectively act to break down the hydrophobic layer (DeBano, 1981). Soil water repellency usually returns to pre -burn condi- tions in no more than six years (Dymess, 1976; DeBano, 1981), and several studies have documented a much more rapid recovery (e.g., DeByle, 1973; Reeder and Jurgensen, 1979). Until recently there has been relatively little work on the development and persistence of post -fire soil water repellency in Colorado. In most cases the presence of a water repellent layer has been inferred from the observed post -fire increases in runoff and erosion (e.g., Moms and Moses, 1987). However, in the summer of 2000 detailed measurements of soil water repellency were made on five fires in the Colorado Front Range that burned from 1 to 22 months earlier (Huffman et al., 2001). Strong water repellency was found in ponderosa and lodgepole pine forests that burned at high or moderate severity, regard- less of whether the fire was a wildfire or a prescribed fire. Areas that burned at low severity generally had little or no more water repellency than unburned areas. Soil water repellency was strongest at the soil surface,