The hydrologic cycle evolves over time, with landscape changes driving differences in evapotranspiration, runoff, and groundwater recharge while climate change affects the timing and magnitude of precipitation as well as temperature. These changes also affect how sediment moves across the landscape and through watersheds. In this dissertation, I examine how land use changes and climate change both affect the movement of water and sediment through watersheds in the Great Lakes Basin.Extreme cases of land use change, such as the logging and forest fires that affected large swaths of the Great Lakes in the late 19th and early 20th centuries, can greatly increase both streamflow and sediment transport. Chapter 1 utilizes the process-based Landscape Hydrologic Model (LHM) to examine the hydrologic effects of land use change from the forested pre-settlement condition to clearcut, burned, and modern land uses in the northwestern corner of Michigan's Lower Peninsula. I show that extensive fires could have increased streamflow by 160% relative to the virgin forest landscape and 96% relative to the logged scenario. Chapter 2 focuses on modeling of the Jordan River watershed, showing that logging may have increased sediment transport in the river by up to 34% compared to pre-settlement conditions and a watershed-wide fire could have increased the sediment transport capacity by as much as 166% above the pre-settlement levels. A reach-based sediment budgeting tool, the Sediment Impact Assessment Methods (SIAM), highlights the possibility of complex system responses to land use change over time.Chapter 3 explores the potential impacts of climate change on sediment yield and dredging costs in the adjacent Maumee and St. Joseph River watersheds where I project that dredging costs may change in opposite directions (-8 to -16% in the St. Joseph but +1 to +6% in the Maumee). This difference between the two watersheds is driven by differences in the proportion of farmland and assumptions about how farmers will respond to a changing climate. I also show that there is a large variation in sediment yield and sediment discharge predictions because of the differences among the various Global Climate Model (GCM) projections.Rather than downscale and run all of the GCM projections, many researchers average a subset of the projections together and use the ensembled climate data as the input to hydrologic models. In Chapter 4, I compare different climate change scenario ensembling methodologies to determine if they produce the same results. I show that a climate ensemble produces significantly (p < 0.05) different hydrologic model results than the ensemble of the hydrologic model results based on the individual climate scenarios. I also demonstrate a method for selecting a subset of climate ensemble members that captures most of the range of the hydrology outputs while also matching the hydrologic and sediment results from the entire set.Global change causes complex hydrologic and sediment responses that need to be carefully considered in modeling and management. Land use mediates the effects of climate change and there are potential feedback mechanisms that will depend on how farmers and other land managers respond to the changing climate. This dissertation provides some of the information needed to identify and understand these interactions.
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