Simulations were made for corn productuion over a period of 25 yrs using normal planting and harvesting dates for the various locations. The number of working days during the one-month period prior to seedbed preparation and SEW30 values during the growing season were predicted for each year of simulation. These results showed that trafficability during the seedbed preparation period is heavily dependent on the factors controlling the rate of subsurface drainage: drain spacing, depth, and soil properties. Surface drainage had a relatively small effect on trafficability for the cases considered. SEW30 values are dependent on both surface and subsurface drainage. In most cases several alternative combinations of surface and subsurface drainage can be used to satisfy a given SEW30 limit. Drainage requirements for trafficability during the seedbed perparation periods were about the same for three locations.