ESTIMATING DRAIN SPACING OF INCOMPLETE DRAINAGE SYSTEMS

Determination of nitrate loading per unit area from a field to a subsu rface 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 deve lopment and application of a method for using drain outflow rates to e stimate drain spacing for incomplete drainage systems that in effect d rain most of the intended area. An optimization routine is used to det ermine the drain spacing that minimizes the difference between observe d and DRAINMOD-simulated rile outflows. The method was used to determi ne the effective spacings of tile drains installed in four fields in t he Little Vermilion River watershed in east-central Illinois. The drai n spacing information estimated the nitrate loading tares for regions within the watershed Under the prevailing conditions of the watershed, DRAINMOD performance was relatively insensitive to surface storage, t he depth of the impermeable layer from the ground surface, diameter of the rile drains, and the lateral hydraulic conductivities in all the soil layers of the soil profile, with the exception of the layer in wh ich the drains were located. Random tile drains in the agricultural fi elds on Drummer/Flanagan soils in east-central Illinois have an effect ive region of influence of 100 m. Effective drain spacing was not site specific, whereas hydraulic conductivity was highly variable and site specific.