Agricultural drainage is a primary source of excessive nitrogen (N) in surface waters, leading to significant waterquality problems in streams and estuaries in many locations around the world. Although there have been only a few fieldstudies of the effect of drain depth and spacing on N loss to surface waters, the data that exist indicate that N loss increasesas subsurface drains are placed closer together. Some simulation model studies agree with these trends, others do not.Published field data from Indiana and North Carolina were plotted as a function of drainage intensity (DI), which was definedas the steady-state drainage rate when the water table at a point midway between the drains is coincident with the surface.Trends for NO3-N loss as a function of DI were similar for soils in the two states in spite of large differences in weather andsoil conditions. These data indicated that the magnitude of NO3-N loss in drainage waters is strongly dependent on DI.Simulations were conducted to examine effects of drain depth, spacing, and soil properties on processes that affect NO3-Nloss from drained soils. The use of DI explained or normalized the effect of these variables on some of the processes but notothers. Results showed that, in addition to its effect on DI, drain depth appears to have a significant impact on NO3-N losses.
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Skaggs et al. (2005) studied this question.