Large-scale hydrologic modeling of the Michigan and Wisconsin agricultural regions to study impacts of land use changes.

摘要

Hydrologic variables in the Great Lakes region have been altered relative to pre-settlement conditions in response to major land use changes during the past 150 years. One of the goals of the present work is to develop a baseline scenario relative to which the impacts of land use changes on hydrological and environmental processes can be evaluated. In addition, this study can help in quantifying the potential impacts of future projected changes in land use in order to mitigate the negative impacts of these changes, especially in regard to a shift toward urbanization and second-generation bioenergy crop production derived from lignocellulosic crops. The present study explores the relationship between land use changes and hydrologic indicators within the agricultural regions of Michigan and Wisconsin. Two sets of land use data, the circa 1800 county base and the 2001 National Land Cover Dataset, were used to set up the Soil and Water Assessment Tool (SWAT) model. First, sensitivity analyses were performed based on both pre-settlement and current land use scenarios. Results showed that parameter sensitivity analysis may not always explain how the variation in model output can be attributed to different sources of variation in the model input. Therefore, effort should be made to determine the true importance of sensitive parameters by considering their placement in model algorithms. The model was then calibrated against measured daily streamflow data obtained from eight U.S. Geological Survey gauging stations. The impacts of land use changes were studied at three scales: subwatershed, watershed, and basin. At the subwatershed scale, most of the hydrologic behavior can be described by percent change in land cover. However, the trend was more apparent for land use conversion from mixed forest to urban and agriculture lands than for other land use conversions. At the watershed scale, significant differences were observed based on the long-term average hydrologic variables under the current and pre-settlement scenarios. In addition, an increase in evapotranspiration (up to 16.5%) and surface runoff (up to 93.9%) contribution to streamflow, a decrease in recharge to aquifers (up to -51.5%) and baseflow (up to -50.1%), and mixed impacts on water yield (-21.5% to 24.6%) were detected. However, at the basin scale, more than 70% of the study area experienced decreased lateral subsurface flow and recharge to aquifers, while 65% of the area experienced increased overland flow and minor changes in evapotranspiration and water yield.