Subsurface drainage systems are a significant source of nitrate pollution to surface water supplies in the flat watersheds of East Central Illinois. Many of these watersheds are drained exclusively by subsurface tile and ditch flow. To solve localized drainage problems, many tile drainage systems were installed with an irregular or random design. The purpose of this study is to apply DRAINMOD-N to irregular tile systems and develop a methodology to determine the effective drain spacing of irregular drain systems based on system geometry and effective drainage area of isolated tile lines. This study also examines the application of a cell-based DRAINMOD-N with surface runoff routing to model flow infields with irregular ststems. DRAINMOD-N was used to model tile flow and nitrate load from two monitored, irregular tile systems. The results of this study show that over a 2-year calibration period, for Drummer soil type, the effective lateral distance of influence for individual tile lines is 13 m. The average daily flow deviation between observed and simulated flow was 1.2 mm/day. Cell-based analysis with surface routing of irregular tile systems provided a lower deviation (0.96 mm/day). The results of the nitrate experiment showed an average monthly deviation of 3.02 kg/ha-N.
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