Results showed that a 10% increase in rainfall rate could result in 1.3 times increase instream discharge, surface runoff, and water outflow. Three simulation scenar-ios were then performed to gain a better understanding of the impacts of different rainfall rates and stormintensities on stream discharge, surface water runoff from forest land, and water outflow from the JFLwatershed outlet. The HSPF model wascalibrated and validated with available measured data prior to its applications. This study ascertained impacts of potential future rainfall change on hydrological pro-cesses at the Jianfengling (JFL) tropical mountain rainforest watershed in Hainan Island, China usingthe BASINS (Better Assessment Science Integrating Point and Nonpoint Sources)-HSPF (HydrologicalSimulation Program-FORTRAN)-CAT (Climate Assessment Tool) modeling system. Climate change over the past several decades has resulted in shifting rainfall pattern and modifying rain-fall intensity, which has exacerbated hydrological processes and added the uncertainty and instability tothese processes.