Simulation runoff and sediment yield in a harvested forest (Case study: Zailakirood Basin, northern Iran)

In this research, the impacts of forest operation and road construction on runoff and sediment yield were investigated in Zailakirood Basin. As there was only one hydrometric station and sediment measurement at the outlet of the basin, estimation of runoff and sediment from different parts of the Basin was not possible which limits our understanding of the effective factors. We used Soil and Water Assessment Tools (SWAT2012) and SWAT-CUP and SUFI-2 algorithm to model the monthly runoff and sediment yield at the outlet of the Basin. The values of the objective function, NS, and the coefficient of determination, R2, for runoff were 0.72 and 0.88, respectively, for calibration and 0.83 and 0.89 for validation. For sediment yield, we obtained R2 0.51 and 0.56 for calibration and 0.66 and 0.73 for validation, respectively. Overall, we concluded that the SWAT model performance and capabilities were acceptable for simulating the monthly runoff and sediment yield in the Zailakirood Basin. Based on the results of sensitivity analysis for runoff,SOL_BD and CH_N2 were determined as the most sensitive parameters, whereas CH_N2 and CN2 were identified as the most sensitive parameters for sediment yield. It can be concluded that forest harvesting reduced the forest canopy and compact the soil of the skidding path, decreased the permeability and capacity of soil water storage, and increased the speed of runoff into rivers. Road construction changed the direction of runoffs to the rivers, increasing the speed of surface runoff and soil erosion. Overall, deforestation resulted in increases in the peak flows and sediment yield.