A field scale model for predicting the surface losses of pesticides (Pesticide Losses In Erosion and Runoff Simulator, PLIERS) was developed. PLIERS accounts for pesticide losses by degradation and volatilization, the washoff of pesticides from plant canopy and surface residue, the adsorption and desorption of pesticides to and from soil particles, and the movement of pesticides in the dissolved and adsorbed phases. Hydrologic data are generated by the comprehensive watershed model, FESHM; which contains an extended sediment detachment and transport algorithm. PLIERS uses first order rate equations to describe degradation and volatilization, and pesticide washoff. The adsorption of pesticides to individual particle size classes is estimated using the Freundlich equation. Movement of atrazine and 2,4-D in runoff and sediment was measured on twelve field plots under simulated rainfall. The plots were treated with conventional or no-tillage in combination with one of three residue levels (0, 750, and 1500 kg/ha). Runoff and sediment losses were found to increase with decreasing residue cover for both tillage systems. No-till reduced sediment loss and total runoff volume by 98 and 92 percent, respectively, compared to conventional tillage. Concentrations of atrazine and 2,4-D ir1 runoff and sediment were greater from the no-till plots than from the conventional plots but the total losses were less. Both pesticides were carried predominately in the dissolved phase. Averaged over all plots, the atrazine losses were 2.9 percent of applied amount for conventional tillage and 0.3 percent for no-tillage. The corresponding values for 2,4-D were 0.3 percent and 0.02 percent. PLIERS was validated using data from the rainfall simulator field plot studies. Agreement between predicted and observed data was very good for dissolved pesticide losses and satisfactory for adsorbed pesticide losses. In addition, the effects of tillage type and residue level were reflected in PLIERS predictions. PLIERS shows great potential as a flexible planning tool since it could be used with any comprehensive hydrologic model and is able to predict the losses of pesticides under various field conditions. / Master of Science