USING THE DRAINMOD-N MODEL TO STUDY EFFECTS OF DRAINAGE SYSTEM-DESIGNAND MANAGEMENT ON CROP PRODUCTIVITY, PROFITABILITY AND NO3-N LOSSES IN DRAINAGE WATER

The environmental impacts of agricultural drainage have become a criti cal issue. There is a need to design and manage drainage and related w ater table control systems to satisfy both crop production and water q uality objectives. The model DRAINMOD-N was used to study long-term ef fects of drainage system design and management on crop production, pro fitability, and nitrogen losses in two poorly drained soils typical of eastern North Carolina (NC), USA. Simulations were conducted for a 20 -yr period (1971-1990) of continuous corn production at Plymouth, NC. The design scenarios evaluated consisted of three drain depths (0.75, 1.0, and 1.25 m), ten drain spacings (10, 15, 20, 25, 30, 40, 50, 60, 80, and 100 m), and two surface conditions (0.5 and 2.5 cm depressiona l storage). The management treatments included conventional drainage, controlled drainage during the summer season and controlled drainage d uring both the summer and winter seasons. Maximum profits for both soi ls were predicted for a 1.25 m drain depth and poor surface drainage ( 2.5 cm depressional storage). The optimum spacings were 40 and 20 m fo r the Portsmouth and Tomotley soils, respectively. These systems howev er would not be optimum from the water quality perspective. If the wat er quality objective is of equal importance to the productivity object ive, the drainage systems need to be designed and managed to reduce NO 3-N losses while still providing an acceptable profit from the crop. S imulated results showed NO3-N losses can be substantially reduced by d ecreasing drain depth, improving surface drainage, and using controlle d drainage. Within this context, NO3-N losses can be reduced by provid ing only the minimum subsurface drainage intensity required for produc tion, by designing drainage systems to fit soil properties, and by usi ng controlled drainage during periods when maximum drainage is not nee ded for production. The simulation results have demonstrated the appli cability of DRAIN-MOD-N for quantifying effects of drainage design and management combinations on profits from agricultural crops and on los ses of NO3-N to the environment for specific crop, soil and climatic c onditions. Thus, the model can be used to guide design and management decisions for satisfying both productivity and environmental objective s and assessing the costs and benefits of alternative choices to each set of objectives. (C) 1998 Elsevier Science B.V.