Evaluation of DRAINMOD by a Physical Model to Simulate the Performance of Subsurface Drainage at the Mid and End Season in Paddy Fields

The complexity of drainage process in paddy fields and time consumption of field tests make simulation models to be used inevitably for assessing subsurface drainage systems performance. The objective of this study was to evaluate DRAINMOD model and the effects of drainage management of paddy fields on the salinity of drainage water. In this regard, a physical model with 3m in length, o.6m width and 1m height was constructed and equipped with controlled drainage. The corrugated drainage pipe, covered with geotextile was laid at a depth of 70 cm in the box. The tank was filled with silty loam soil with the same density and layering as in the paddy fields. After rice cultivation and during the experiment, the soil solutions were collected from the depths of 40, 50 and 70 cm and their TDS were measured in the laboratory. In order to evaluate the accuracy of DRIANMOD model for simulation of qualitative and quatitive subsurface drainage water, the solute transport parameters including dispersivity, tortousity factor, molecular diffusion coefficient, precipitation limit and the hydraulic conductivity were obtained through minimizing differences between the observed and the estimated salt solusion and drainage volume using genetic algorithm optimization method. The results showed that the model could simulate the performace of drainage system very well at before and after the midseason (NRMSE < 20%) and excellent at the mid and end season (NRMSE < 10 %). The TDS and drainage volume were estimated by average RMSE of 63.27 mg/l and 0.0145 cm3 respectively. The comparison of estimated and observed values showed a better performance for the model at the non-submerged conditions. The results of calibration and validation of model showed that the DRAINMOD-S model is capable to simulate the performance of subsurface drainage in paddy fields.