Effective Factors on Runoff Generation and Hydrologic Sensitivity in a Mountainous Watersheds (A Case Study: Farsan Watershed, Upstream of Karoun River)

Aims Affecting factors on runoff generation in mountainous areas, where the hydrological processes are complex, play an important role in the recognition of hydrological phenomena. The aim of the present study was to simulate the water balance of Farsan Basin using the SWAT model. Materials Methods In this semi-distributed research, SWAT model was used to simulate the monthly runoff the basin of interest. The study area was Farsan watershed, it is the part of Beheshtabad Basin. Basin’s curve number was estimated using a remotely sensed NDVI. The calibration and validation of the model were carried out by using the SUFI2 Algorithm (sequential uncertainty fitting) for two periods, one from 2001 to 2011 and another from 2012 to 2015. Findings The threshold depth of water in the shallow aquifer to start evaporation (REVAPMN) had the least sensitivity, while the soil evapotranspiration (ESCO), the time delay of the transferring water from the last soil profile to the groundwater level (GW_DELAY), and curve numbers in normal condition (CN2) were the most sensitive factors, respectively. To evaluate the simulation, R2 (coefficient of determination), bR2 (weight correlation coefficient), and NS (Nash Sutcliffe model efficiency) at the calibration stage were 0.63, 0.33, and 0.57, respectively. Whereas at the validation phase, these coefficients were found to be 0.69, 0.68, and 0.52, respectively. Conclusion A proper specification of these sensitive parameters may be the key factor for runoff simulations. The impact of change in surface parameters may have a great influence in both generating runoff and mountain hydrology.