A method is proposed for the equitable allocation of impacts of groundwater pumping on streamflow. The method is intended for cases in which the pumping activity of multiple entities has impacts on streamflow and these impacts are computed by perturbation. It is shown that when the response of streamflow to pumping is nonlinear, simple methods for impact calculation can fail. The proposed method is developed for the case when there are four entities that impact streamflow. The method relies on the calculation of impacts by perturbation of the simulation model from different base pumping levels. When four entities are evaluated, 16 runs of the simulation model are required. It is shown the proposed method produces estimated impacts for each individual entity that are equitable because they meet the requirement that the impacts of each entity sum to the total impacts of all entities acting together and the impacts attributed to each entity do not depend on the order of calculation. A brief example demonstrates the approach.