Excessive soil nitrogen losses as a result of the inappropriate design of subsurface drainage systems have given rise to different environmental problems. The drain spacing and depth play a substantial role in the quality and quantity of the drain outflow into the environment. In this research, a simple but comprehensive simulation model using a system dynamic approach for the water and nitrogen cycle was used to simulate the impact of drain depth and spacing on nitrate and ammonium losses in sugarcane farmland at Imam Khomeini agro-industrial Company. Sixteen scenarios were modeled including the combination of four drain spacing (60, 70, 80 and 90 m) and four different drain depths (1.1, 1.4, 1.7 and 2.0 m) to compare the effect of drain spacing and depth on the amount of ammonium losses through runoff, nitrate and ammonium losses through drainage water, nitrogen losses via the denitrification process and nitrogen uptake by the plant. The results indicated that through the increasing of drain spacing and the reducing of drain depth nitrogen losses in the form of denitrification and runoff would increase; and the nitrate and ammonium losses through the drainage water would decrease. Furthermore, the amount of applied urea fertilizer has a significant impact on the amount of nitrogen losses. So, based on the results the optimal tile-drainage system density in this region would be a drain spacing of 80 m and depth of 1m, in as such that the total drainage and runoff losses would be reduced up to an acceptable level. Therefore, the optimum design of subsurface drainage systems based on environmental criteria could aid in the control of nitrogen pollution on the farm-level.
