Effects of Fire on Soil Properties, Erosion and Hydrologic Regime of Zrebar Lake Watershed

Forest herbs due to decrease of runoff coefficient and the kinetic energy of raindrops, is known as a key factor in controlling runoff and soil conservation. Many physical (hydrophobicity, electrical conductivity, pH, particle size distribution, color and temperature regimes), chemical (quality and quantity of organic matter, nutrient availability) and biological (Microbial biomass, soil invertebrates living community) soil properties can be affected by forest fires. Fire not only reduces forest herbs, vulnerability against splashing rain but also has strong effects on the hydrological cycle and soil loss. despite of repeated fires, there are very few studies about fire impact on natural resources of the west of the country, especially the city of Marivan, in Kurdistan province so this study aimed to investigate the short-term fire impacts on soil properties, Hydrologic regime, soil erosion and sedimentation of Zrebar Lake watershed in west of Iran.
Considering the importance of the slope on the hydrological response of the watershed, slope classes of the Zrebar Lake watershed were mapped. Therefore, effects of fire on hydrological characteristics, erosion and sedimentation were studied by the establishment of twelve 0.25 square meter plots in three replications at two dominant slope classes (0 to 30 and 30 to 60%) in burned and natural areas . The first plots in the burned and natural sections, was established randomly and two other plots with the similar conditions at a distance of 1.5 meters from each other were established. Garden Spray Simulator with constant pressure was used to fall rain from half a meter height for thirty minutes with an intensity of about 2 mm min-1 and 1 mm droplet diameter according to the general weather conditions of the studied area. For every five minutes, runoff and sediment were collected. Runoff volume by weighting and suspended sediment concentration by drying at 105°c were measured. Infiltration and interception by water balance assessment and subtracting the simulated rainfall volume and runoff were calculated. Soil samples were also collected in triplicate by auger drilling method to assess the effects of fire on different characteristics of topsoil (depth of 5 cm) asthe samples 1 and 2 in the slope class of 30 to 60%, and samples 3 and 4 in slope class of 0 to 30%, at the burned and natural sections respectively. In the laboratory, after determination of soil texture, the moisture content, pH and EC by portable laboratory instruments, the percentage of total organic carbon and organic matter bye wet oxidation were measured. Soil water repellency by Water Drop Penetration Times (WDPT) method in the field was calculated. Statistical analysis for comparing the average hydrophobicity of the soil, runoff and sediment of both natural and burned treatments by independent t-test and for equality of variances by the Levene test was performed using SPSS 21 software.
The results showed no effect of fire on soil texture because the fire severity was low to moderate as the color of ash was black. The fire caused to 46 percent reduction of soil moisture and also 24 percent reduction of soil organic matter and 29 percent reduction of total organic carbon at 0% to 30% and 30% to 60% slope classes respectively. Slight increase in pH and electrical conductivity were another effects of fire. At the slope of less than 30%, the average water drop penetration time were 1.24 and 0.70 S in burned and natural treatments and at the slope of 30 to 60 % slope class were 4.52 and 1.04 S respectively. Independent t-test results showed that, the water repellency differences of natural (t=3.466, p=0.003) and burned (t=7.364, p=0.000) treatments were significant. Levene test and independent t-test results showed significant difference in the average of runoff at confidence level of 95% in both slope classes. At the slope of less than 30%, differences of sediment average of burned and natural plots with a significant level of 0.012 were significant. But in slope class of 30% to 60%, despite of large different sediment concentration output from the burned and natural plots at the beginning of the experiment, there was no significant difference between them. The results showed that fire leads to low soil moisture content, total organic carbon and soil organic matter. Slight increase of pH and EC and also lack of effect on soil texture, in both slope classes were also observed. Ash on the forest floor caused to soil water repellency and therefor decreased soil infiltration and increased runoff and soil erosion.