Controls of climatic variability and land cover on land surface hydrology of northern Wisconsin, USA

Ecosystem processes are strongly affected by the magnitude, timing, and variability of water flows. As such, our understanding of biogeochemical and ecological processes can be enhanced when our ability to track water flow and storage within ecosystems is improved. We assess how climatic variability and land cover change affect water flow and storage within a temperate forest region of the north central United States (46 degrees N, 89 degrees W). We use a well-validated process-based ecosystem model (IBIS) to investigate evapotranspiration, surface runoff, and drainage rates across a continuum of time scales. We found from 1951 to 2000, climatic variability imposed a large, detectable signal on both annual and seasonal surface water balance that resulted in changes in total runoff that ranged from 30% to 200% of the 50-year average. Conversely, land cover change resulted in subtler, persistent changes (i.e., forest to grassland changed total runoff by 10% annually), which were not detectable from year to year. If, however, changes in land cover persist, within 6 years the cumulative difference from land cover change became slightly more than two standard deviations of annual runoff variability, and within 15 years the accumulated differences were greater than changes between the largest and smallest runoff events within the 50-year period. 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 (season or year-to-year variations), but land cover change had influence on water balance over the long-term (6 years and beyond). These changes in hydrology from land cover were detectable as subtle, yet persistent differences that accumulate as changes in magnitude and shifts in seasonal cycles. Through this, we provide a process-based context for understanding the historical causes of water cycle variability, which allows us to better identify the hydrology of this system. Ultimately, this allows for improved understanding of how forested ecosystems could respond to multiple drivers of global change.