Understanding how rainfall, terrain, and use of the landscape affect stream flow and water resources availability is important for planning how to use water resources. In the Caribbean, climate models predict less annual rainfall, and potentially more intense rainfall in storm events. This will mean difficult choices as managers plan and allocate water resources to satisfy domestic, industrial, agricultural, energy, scenic, and wildlife needs. Being able to simulate future scenarios of stream flow will be important, and a first step is developing and calibrating models that simulate stream flow and accurately reflect historical conditions. Forest Service scientists explored and quantified how land cover change has an impact on simulated stream flow in the tropical island setting of Puerto Rico where streams and rivers are short, steep, and storm events can have high precipitation rates. With modeling, they can use real or projected precipitation and other information to predict how water flows across the land. In this study, the scientists used the Precipitation Runoff Modeling System (PRMS) to compare stream flow simulations based on five static parameterizations of land cover with those based on dynamically varying parameters derived from four land cover scenes for the period 1953-2012. The PRMS simulations based on static land cover illustrated consistent differences in simulated stream flow across the island. They determined that the scale of the analysis makes a difference: large regions with localized areas that have undergone dramatic land cover change may show negligible difference in total stream flow, but stream flow simulations using dynamic land cover parameters for a highly altered subwatershed clearly demonstrate the effects of changing land cover on simulated stream flow. Incorporating dynamic parameterization in these highly altered watersheds can reduce the predictive uncertainty in simulations of stream flow using PRMS. Hydrologic models that do not consider the projected changes in land cover may be inadequate for water resource management planning for future conditions.