2020 Issue

31 vegetation on slowing surface runoff velocities. Loss of surface litter, vegetative basal cover, and the associated microtopographic relief also reduce surface storage of water crucial for reducing runoff and increasing infiltration (Pierson, Jr. et al., 2003). The removal of vegetation due to fires increases runoff as surface runoff velocities increase, decreasing the time available for infiltration. Fires also change soil characteristics. Fires induce temperatures at ground level, reaching 600 to 700 degrees centigrade. Burning vegetation, especially chaparral, releases oils, resins, and waxy fats stored in plants and plant litter as intense heat vaporizes the vegeta- tion (McPhee, 1989). The soil acts as an insulator, keeping temperatures a few centimeters below the surface much cooler. This temperature difference allows condensation of vaporized substances, forming a hydrophobic layer. This layer is impermeable and prevents water from reaching all but the first few inches of soil. It also slows evaporation through the soil (Ainsworth and Doss, 1995). The extent and depth of a hydrophobic layer both depend on the type of soil, the fire intensity, and antecedent soil moisture. Clay soils tend to resist the formation of a hydrophobic layer. Sandy and sandy loam soils are far more susceptible to hydrophobic conditions (DeBano 1987). If a drop of water is placed on a pre-burn sample of sandy loam soil, the water will all but disappear. If the same water drop is placed on a post-burn sample, the drop will ball up and may remain there for hours. Water quickly saturates the thin layer of permeable soil above the hydrophobic zone because a vegetative canopy is not slowing it down. Slower infiltration rates result in an increased intensity of surface runoff and erosion. These changes to the soil and vegetation lead to higher soil erosion rates. Figure 2 shows the expected probabilities of debris flow in water - shed areas due to impacts from the Dollar Fire area above Starvation Reservoir. Figure 2 - Expecter Change to Soil Erosion and Debris Flows - Dollar Fire Figure 3: Sediment in Strawberry River Below Timber Canyon after Dollar Fire — July 2018 leads to bulking of flows, where entrained sediment increases the volume of runoff. Vegetation, litter, rocks, and other forms of ground cover create barriers that slow and spread water movement across the soil surface, allowing more time for water to infiltrate over a larger surface area. Fire removes most of these barriers and allows the water to concentrate into rills. Rills allow increased flow depth and velocity. Higher flow depths and velocities significantly decrease runoff response time and increase runoff volume in streams (Pierson, Jr. et al., 2003). Several studies have been conducted to determine the influence of fire on the volume and peak runoff from watersheds. Work by Davis (1977) suggests that many post-fire flows are debris flows. In the watersheds that Davis studied, he found that bulking ratios in runoff ranged from 0.5% to 2.5% by volume for normal flows to 40% to 60% by volume for post-fire flows. Bulking can increase runoff volumes and peaks significantly. However, it will not be evaluated further in this study. Veenhuis (2002) studied two burned watersheds in New Mexico. He noted that storm flows increased dramatically after the wildfire. Peak flows in each of these two water - sheds increased to about 160 times the maximum-re- corded flood before the fire. As vegetation reestablished itself in the second year, the annual maximum peak flow was reduced to approximately 10 to 15 times the pre-fire annual maximum peak flow. During the third year, maxi - mum annual peak flows were reduced to about three to five times the pre-fire maximum peak flow. In the 22 years since the La Mesa wildfire, flood magnitudes have not completely returned to pre-fire magnitudes. The number of larger than normal peak flows seems to be most pro - nounced for three years after the fire. (Veenhuis, 2002). Other studies also indicate significant increases in the runoff after a fire (Pierson, Jr. et al., 2003; Nasseri, 1988; Wondzell et al., 2003). Figure 3 shows sediment deposited in the Strawberry River floodplain after rainfall in July 2018. Changes to Runoff After Fires Fire changes the soil and vegetation characteristics of a watershed. The changes result in higher runoff rates and more erosion within the watershed. Erosion of sediment