The pesticide paddy field model (PADDY-model) was improved by considering the water balance in a paddy plot, and applied to the evaluation of runoff characteristics of rice herbicides from a paddy field. This model focused particularly on granule formulation, and has been developed for predicting pesticide concentration of water and soil in paddy fields. Recently, not only granule formulations but also floable formulations have been widely utilized as herbicides on paddy fields in Japan. From this viewpoint, field experiments and numerical simulations were performed to examine the applicability of PADDY-model to floable formulations. A comparison between the calculated results and the observed data for the pesticide concentrations in paddy water showed that the PADDY-model could approximately reproduce the observed of herbicide concentration, and that this model could be applied to floable formulations. A scenario analyses was conducted to evaluate the impact of physicochemical properties of pesticide, as well as irrigation water management, on the behavior of rice herbicides in paddy fields. As a result, the behavior of pesticide (of which adsorptivity and volatility were low) was attributable to vertical percolation. Furthermore, surface runoff amounts of pesticide under management of spill-over irrigation, deep ponding irrigation, and non-tilled cropping were larger compared with the runoff amount under the normal irrigation regardless of physicochemical properties. In the cases of mamegement for shallow ponding and intermittent irrigations, vertical percolation amounts increased and surface runoff amounts decreased in comparison with those obtained in the case of normal irrigation management.