Evaluation of the Applicability of SWAT Model for Watershed Modeling: Upper Part of Wabi Shebelle River Basin, Ethiopia

Abstract

Catchment modeling has been an outstanding science for investigation of the hydrological response of watershed. Different researchers found out that SWAT can efficiently simulate the hydrology of a catchment. In this study also, focus has given to evaluate the applicability of the model in simulating the spatial water availability in the upper part of Wabishebelle River Basin. The water balance equation is the driving force in SWAT for hydrological modeling. Climate and spatial data are processed and used to study the hydrology of a catchment. Model sensitivity analysis, auto calibration and validation have been done to find out optimum model parameters that results in acceptable R 2 and ENS· The sensitivity analysis was done using the LH-OA T that is embedded in SW AT and is based on Monte Carlo simulation technique. Monthly observed discharge for calibration period is used to detect the sensitive parameters. To achieve acceptable value of model efficiency manual and auto calibration method is adopted. The auto calibration method uses the parasol optimization method that uses the shuffled complex evolution algorithm developed at University of Arizona. PET is calculated using the penman monteith method and runoff computation using the SCS curve. Routing computation is done with variable storage method. Since water availability within the basin is considered only the shallow aquifer is treated in this study. The most sensitive parameters that result in peak flows in the simulated result are, Alpha_BF, cn2, Gwqmn, Gw_delay, Esco and sol_Awc. The coefficient of determination and ENs for the calibration period (1998-2001 ) are 0.86 and 0.64 respectively, during the validation period (2003-2005) these values are 0.78 and 0.57 respectively. The average annual water balance for the calibration and validation period shows groundwater contributes much and surface runoff next for the water yield in the respective periods. The water yield in each sub basin for the period 1998-2005 is: 559 . 86 , 509.89 , 410 . 77, 435. 42 and 530.85 in mm for Wabi up to Melka Wakena, Wabi-Melka Wakena to Ulul,Ulul, Wabi-Ulul to Robe and Robe respectively.