Effect of uphill gradient on runoff and soil loss in forest road (A case study: Kohmiyan forest, Azadshahr)

Different parts of forest roads and uphill are usually the main sources of surface runoff, sediment, water resources pollution in forest streams and soil loss. Today, soil erosion and its consequences is a global matter of concern. This study was conducted in Kohmiyan forest near-Azadshahr in order to assess the effect of forest road uphill gradient in controlling the runoff production and soil loss by a rainfall simulator. Treatments include four levels of gradients (>20%, 20-40%, 40-60% and 60-80%). Other parameters such as soil moisture content, resistance to penetration, vegetation cover and precipitation were considered constant. Then, the amount of runoff and soil loss in each level were collected at an intensity of 80 mm/h with 15 min duration and sampled at three minute intervals. The results indicated that the average amount of total runoff in level 1 (gradient >20) to level 4 (60-80%) were 5.97, 10.14, 14.27 and 18.60, respectively. Sediment concentration were 4.15, 5.87, 7.84 and 10.68 g/l-1; runoff coefficient were 12.01, 20.25, 28.51 and 37.20 percent and total soil loss were 21.65, 54.67, 108.13 and 191.51 gr, respectively for each one square meter. The results of ANOVA showed there is significant difference between the amount of runoff and soil loss due to road gradient. The results of Duncan test also indicated that with increasing uphill gradient, the amount of runoff and sediment increased significantly.