Evaluation of Subsurface Drainage Performance by Using SWAP Model (Case Study Behshahr Ran Drainage System)

A transient drainage simulation model، SWAP، was used to evaluate the performance of subsurface drainage system. SWAP model was calibrated by measured daily data including water table depth، drain discharge rate and soil and water drain salinity collected from Behshahr Ran drainage system for 120 days during 1385. Calibration of SWAP model was done by inverse modeling via linking with WinPEST model. In order to calibrate drainage quantity parameters، two objective functions were defined to minimize difference between measured and simulated values of the water table depth and drain discharge rate، simultaneously. To calibrate drainage quality parameters، another objective function was also defined to minimize difference between measured and simulated values of soil salinity. There were good agreements between measured and simulated values of drain discharge rate and water table depth. The absolute error of estimation was 7 and 4 % for water table depth and drain discharge rate، respectively. Measured cumulative drainage was 7. 5 % (5. 3 mm) greater than its simulated value. The SWAP model could also simulate soil and drainage water salinity with a reasonable accuracy. The results of this study indicated that the performance of the SWAP model could be considerably improved using inverse modeling.