Figure 12. Average annual total phosphorus transport in drainage outflow as measured at the field edge of 12 sites in eastern North Carolina. Note that phosphorus transport is greater in surface flow than in subsurface runoff, and is four times higher in agricultural drainage than in forestland drainage.
Bar Graph of conventional drainage and controlled drainage vs. total phosphorus (lbs/ac/yr)

Controlled drainage results in a net 35 percent reduction of phosphorus.