Investigation of the Dimensions of Open Drains in Paddy Fields of Land Consolidation Project

Construction of drainage systems is inevitable in paddy fields to control the level of waterlogging during rice harvest and cultivation of the second crop. Finding the best design option of drainage is very important from the point of view of environmental and economic issues. This study was conducted with the aim of investigating the performance of open drains including pressure head and depletion flux in controlling the water level in the cultivation of rice plants and the second plant in paddy fields. In this regard, the performance of open drains during rice and the second crop growth period was simulated using HYDRUS-2D model. To calibrate and validate HYDRUS-2D model, the required information was collected from 130 hectares of paddy fields in Nodeh, Jirsar and Nupashan villages in Soumesara region, in 2019. Soil texture, bulk and specific density, porosity and infiltration rate were measured. Also, pressure head of water was measured monthly in the region by piezometers. After validation, different open drain design options (bottom width and depth of drain) were run in HYDRUS-2D and its results were compared. The simulation results showed that the HYDRUS-2D had a reasonable accuracy in simulating the changes in water pressure head. The statistical indexes including R2, RMSE, nRMSE and MAE in the calibration stage were 0.98, 4.39 cm, 1.6%, and 3.55 cm, respectively, and 0.98, 4.33 cm, 1.67%, and 3.37 cm in the validation stage, respectively. The results showed that the open drain with a depth and bottom width of, respectively, 200 and 170 cm had the best performance for controlling water table during rice and the second crop cultivation, with a discharge rate of 726440 and 169960 cm3 per unit of drain length, and the reaction coefficient of 0.293 and 0.583 per day, respectively.