This study presents a method to determine efficient environmental targets at a watershed level. Efficient targets are devised by estimating abatement cost and cost of environmental damages and minimizing their sum. The method was applied to a case study of phosphorus pollution in a watershed in Oklahoma. Several cumulative scenarios with alternative abatement options were simulated and efficient targets were determined. As the number of abatement options at disposal to agricultural sources increased, their optimal abatement expanded relative to the abatement at the point source. Efficient targets were found to be dependent on the choice of policy that stimulates abatement.