Simulation of Trenched Subsurface Drainage Performance in a Physical Model scale Paddy Fields with HYDRUS-2D Model

Poor drainage system in paddy fields with heavy texture soils not only makes it difficult to harvest rice mechanically at the time of rainfall, but also the use of non-conventional Waters (drainage water reuse) or increased salinity of irrigation water in long-term solute accumulation Soil and its sodification. Also, such lands are not able to eliminate the conditions of waterlogging in wet season and limited the second crop cultivation. Therefore, to avoid salinizing and remove the waterlogging problem in paddy fields the construction of drainage system is necessitated. Installation of subsurface drainage in sand and gravel trench results in better porous conditions around the drainage pipe and thus improves drainage efficiency. Selecting the optimum trench geometry from a technical and economic point of requires simulation abilities. In this study, HYDRUS-2D simulator model was used to evaluate the performance of trenched subsurface drainage with sand and gravel envelope in paddy fields. In order to calibrate the HYDRUS-2D model, a physical model of controllable trenched subsurface drainage was constructed in paddy fields and the rice plant was cultivated in its tank. In the growth period, soil solution in various depths and distances from drain and the drain water of mid and end season drainage was sampled. The electro conductivity (EC) of samples were measured in laboratory. Simulation results showed that the model has a good accuracy in simulating water movement, total dissolved solid of drain water and solute of soil solution. R2, RMSE and nRMSE in water movement validation were obtained 0.982, 2.8 cm and 4.55 percent, respectively and in the solute transfer validation stage part were obtained 0.419, 0.035 mg/cm3 and 10.56 percent, respectively. The results of sensitivity analysis also showed that soil saturated hydraulic conductivity parameters and longitudinal diffusivity coefficient with sensitivity coefficient of 2.6 cm/day and 0.47 cm had more effect on water and solute movement respectively.