The researchers studied how managing drainage water in the upper Midwest, especially in Iowa, affects subsurface drainage. They used a model to simulate water flow in soil with different drainage systems. They found that in north-central Iowa, most subsurface drainage happens in April and May, with 80% by the end of June. Using controlled drainage reduced annual subsurface drainage by 16%, but this mostly increased surface runoff. This drainage timing during spring when crops are planted limits the effectiveness of drainage management in reducing nitrate-nitrogen export in the area.