Abstract: The spatial heterogeneity of the underlying surface in a basin has a significant impact on runoff generation.The water storage capacity curve can effectively address the issue of spatial heterogeneity in runoff generation.When the instantaneous unit hydrograph is used for calculating surface runoff confluence, it is considered as the net rain with uniform spatial distribution.In this scenario, local runoff does not account for local confluence.In this paper, the spatial correlation between confluence time and water storage capacity was considered, and their joint distribution was constructed by a Copula function.The spatial distribution characteristics of confluence time at various storage capacities were manifested in the form of conditional probability.Furthermore, a concept of an isovolumetric instantaneous unit hydrograph was proposed by using the isovolumetric water storage capacity as the calculation unit.This approach was equivalent to the variable unit hydrograph method for calculating the confluence of surface runoff, thereby unifying the calculation process for local runoff and confluence.Therefore, an isovolumetric instantaneous unit hydrograph based hydrological model(IIUHH) was constructed, and various forms of IIUHH were developed based on the type of Copula function.Finally, the Qingjiang River upstream was selected as a case study.The results showed that Clayton Type IIUHH performed better in the study area.Compared with the Xin′anjiang model, the Nash efficiency coefficient can be increased by 4% to 12%,the Kling-Gupta efficiency coefficient can be increased by 13% to 27%,and the flood peak relative error can be reduced by 26% to 36%.The simulation accuracy of flood peak and flood processes has been significantly enhanced.The research results can be used for high-precision runoff prediction in a basin.