A Determination of the Optimum Remaining Time on Water Table Management to Decrease Nitrogen Loss in Drained Agricultural Lands

The management of water table is introduced as one of the major parts of the solutions in water resources management. This method could play a significant role in protecting water resources، increasing irrigation efficiency، preserving nutrients in soil، and protecting the quality of surface downstream، and as well، of groundwater. Towards this end، a laboratory study has been done on the effects of water table management on control of leaching and discharging of nitrogen in sandy loam soils. Water table was controlled at a depth of 30 cm from soil surface through three drainpipes installed at 80 cm depth and while the volume and quality of effluent drained water (in terms of different nitrogen compounds، namely: nitrite، nitrate، ammonium، and ammoniac were evaluated at retention times of 2، 4 and 5 days within the framework of a physical model. Results show 29. 4 percent decrease in drainage discharge at a retention time of 5 days and while 27. 7 percent decrease in effluent nitrate density at 4 days past، as compared with the free drainage water. Through a statistical comparison of the results، the most appropriate time to control water table was attained after 4 days past. Given the present findings، duration of retention time in modern drainage systems is one of the management variables، and while a provision of correct and opportune usage of this management method، (Through an increase in consumption efficiency of water and fertilizer) would be hopeful in developing stable agriculture and as well improving the quality of water resources and consequently the protection of the environment.