Systematic Analysis Of Drainage Events In Free Draining And Managed Subsurface Drainage Systems

Understanding the hydrologic controls that regulate outflow from free and managed subsurface drainage systems during drainage events can offer improved insight on the overall functioning and effectiveness of the systems so that they can be better managed or retrofitted to increase their environmental benefits. This study used drainage, precipitation, water table, and soil moisture data from a monitoring site located in east central Indiana to investigate the event hydrology of 22 drainage events in free and managed subsurface drainage systems. Relationships between event drainage volume, drain flow hydrograph metrics, column soil moisture, water table depth, and precipitation characteristics were explored to determine the effect of precipitation characteristics and antecedent conditions on drainage volumes, reduction in drainage volumes, peak outflows, the time to peak, and the mechanisms by which runoff is generated in managed and free draining subsurface drainage systems. Drainage water management reduced event drainage volume and peak flows by 22% ± 12% and 29% ± 16% respectively, and increased the time to peak of drainage by 98% ± 52%. Higher total precipitation and precipitation time spread promote more infiltration throughout the course of the event and thus greater drainage volumes in managed and free draining systems, while the average precipitation intensity did not correlate with drainage volumes in both systems. Peak flows in free draining quadrants were positively affected by higher precipitation totals and the average precipitation intensity that can increase the infiltration rate. In managed quadrants, the antecedent soil moisture appeared to play a more important role in affecting peak flows than precipitation characteristics. The time to peak in the free draining quadrants decreased with higher average precipitation intensity and increased with higher precipitation time spread. As the average precipitation intensity increased the runoff potential increased on both managed and free draining quadrants. Saturation excess ponding or possibly overland flow occurred in events that have a low average precipitation intensity, and a high precipitation time spread. Field observations indicate that saturation excess overland flow is more pronounced in managed quadrants because the water tables level rise higher than the water table of their free draining counterpart