Foliar washoff causes a loss of copper-based pesticides sprayed on crops, leading to an increase in the number of applications and contamination of the soil with Cu. In field studies, the variables that determine the amount of Cu loss are difficult to control. An experimental setup based on a rotating shear device (RSD) was used to estimate the influence of physical factors in the loss of Cu due to washoff of three copper-based fungicides: copper oxychlorhide (CO), Bordeaux mixture (BM), and a mixture of copper oxychlorhide and propylene glycol (CO-PG). Full factorial designs were used to model the loss of Cu from fungicides sprayed on the polypropylene surface of the RSD. Variables in the experiments were rotation speed, wash water volume, and fungicide dose. Good reproducibility was obtained for Cu loss, with a coefficient of variation less than 8%. Mean Cu losses were 27.0, 33.0, and 13.5% of the copper applied in fungicide for the BM, CO, and CO-PG, respectively. Empirical equations were obtained to calculate Cu losses from the rotation speed, wash water volume, and dose, as well as their interactions. CO losses were consistent with a model of particle detachment in which such losses depended on a threshold boundary shear stress required to initiate particle motion. Also, percent CO losses were found to be significantly correlated with the linear momentum at the surface boundary. The momentum values obtained in the RSD tests were similar to those estimated for a rainfall event of 20 mm h(-1) lasting 10 min. The most important mechanism in the loss of CO was the erosion of Cu-bearing particles.