Subsurface Drainage and Its Management in the Upper Midwest Tile Landscapes

Hydrologic modeling in combination with field measurements is used to understand subsurface drainage patterns and how drainage water management may impact subsurface drainage in the upper Midwest, particularly in Iowa. Long-term measured subsurface drainage data from an on-going drainage study in north-central Iowa was used to calibrate and validate DRAINMOD, a deterministic hydrologic model to simulate a soil-water regime of surface and subsurface water management systems. The model was then used with long-term (1945-2004) weather records with the objective of understanding the timing, duration, and volume of subsurface drainage flow patterns. Controlled drainage, a drainage management practice, was considered in the modeling simulations to assess its potential to reduce subsurface drainage in the upper Midwest. In the north-central Iowa, approximately 45% of the annual subsurface drainage occurs in the months of April and May, and approximately 80% of the annual subsurface drainage has occurred by the end of June. When simulating controlled drainage practices, there was approximately a 16% reduction in the volume of annual subsurface drainage but most of this reduction was reflected in increased surface runoff. The timing of subsurface drainage in these landscapes specifically during the spring coincides with the time of planting, crop germination, and early crop development. This coincident may limit the effectiveness of drainage management practices such as controlled drainage to reduce subsurface drainage and thereby nitrate-nitrogen export in the north-central Iowa.