Ecosystem processes are strongly affected by the magnitude, timing, and variability of water flows. As such, our understanding of biogeochemical and ecological processes is enhanced when our ability to track water flow and storage within ecosystems is improved. In this study, we investigated how climatic variability and land-cover change affected the terrestrial water balance of a temperate forest region of the north-central United States (46? N, 89? W). We used a well-validated process-based ecosystem model (IBIS) to investigate the magnitude, timing, and variability of evapotranspiration, surface runoff, and drainage rates across a continuum of time scales, from daily to interannually. We found that from 1951 to 2000, climatic variability imposed a large, detectable signal on the surface water balance, but years with anomalously high or low precipitation only perturbed the surface water balance from the longer-term average for a year or two. Conversely, land-cover change (i.e., forest to grassland) resulted in subtler, persistent changes, which caused large cumulative changes in hydrology on a decadal time scale. As a result, in the context of this study, climatic variations typically had a strong effect on the surface water balance in the short-term, but land-cover change was more influential on water balance over the long-term. Through this study, we improved our understanding of the historical causes of water cycle variability, which allows us to better identify the hydrology of this system and other forested regions. Ultimately, this may allow us to develop a better understanding of how forested ecosystems could respond to multiple drivers of global change.