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Determination of nitrate loading per unit area from a field to a subsurface drain requires an estimate of the area contributing flow to the drains. In random drainage systems or systems with irregularly spaced drains, the contributing area is often unknown. Presented are the development and application of a method for using drain outflow rates to estimate drain spacing for incomplete drainage systems that in effect drain most of the intended area. An optimization routine is used to determine the drain spacing that minimizes the difference between observed and DRAINMOD-simulated tile outflows.

The method was used to determine the effective spacings of tile drains installed in four fields in the Little Vermilion River watershed in east-central Illinois. The drain spacing information estimated the nitrate loading rates for regions within the watershed. Under the prevailing conditions of the watershed, DRAINMOD performance was relatively insensitive to surface storage, the depth of the impermeable layer from the ground surface, diameter of the tile drains, and the lateral hydraulic conductivities in all the soil layers of the soil profile, with the exception of the layer in which the drains were located. Random tile drains in the agricultural fields on Drummer/Flanagan soils in east-central Illinois have an effective region of influence of 100 m. Effective drain spacing was not site specific, whereas hydraulic conductivity was highly variable and site specific.