Tile Drainage Increases Surface Runoff?

[Heather White]

Hello all,

I recently successfully calibrated and validated a SWAT model for a midwestern agricultural watershed. When performing tile drainage scenario analysis, increasing the total area under tile drainage resulted in increased surface runoff. 

I tried reducing CN2 and increasing R2ADJ,  yet the model still outputs unreasonable values for surface runoff. 

Has anyone ever ran into this problem, or has suggestions to solve it? 

Thank you,

Heather