TEMPORAL CHANGES IN STREAMFLOW AND ATTRIBUTION OF CHANGES TO CLIMATE AND LANDUSE IN WISCONSIN WATERSHEDS 1 Rabi Gyawali, Steve Greb, and Paul Block 2 ABSTRACT: Previous historic trends analyses on 21st Century hydrologic data in the United States generally focus on annual flow statistics and have continued to use USGS hydro-climatic data network (HCDN) stations, although post-1988 diversions and runoff regulations are not reflected in the HCDN. Using a more recent data- set, Geospatial Attributes of Gages for Evaluating Streamflow, version II (GAGES II), compiled by Falcone (2012), which includes more watersheds with reference conditions, a comprehensive analysis of changes in sea- sonal, and annual streamflow in Wisconsin watersheds is demonstrated. Given the pronounced influence of seasonal hydrology in Wisconsin watersheds, the objective of this study is to elucidate the nature of temporal (annual, seasonal, and monthly) changes in runoff. Considerable temporal and regional variability was found in annual and seasonal streamflow changes between the two historic periods 1951-1980 and 1981-2010 considered in the study. For example, the northern watersheds show relatively small changes in streamflow discharge rang- ing from À6.0 to 4.2%, while the southern watersheds show relatively large increases in streamflow discharge ranging from 13.1 to 18.2%. To apportion streamflow changes to climate and nonclimatic factors, a method based on potential evapotranspiration changes is demonstrated. Results show that nonclimatic factors account for more than 60% of changes in annual runoff in Wisconsin watersheds considered in the study. (KEY TERMS: streamflow; Wisconsin watersheds; potential evapotranspiration; climate; landuse; attribution.) Gyawali, Rabi, Steve Greb, and Paul Block, 2015. Temporal Changes in Streamflow and Attribution of Changes to Climate and Landuse in Wisconsin Watersheds. Journal of the American Water Resources Association (JAWRA) 51(4): 1138-1152. DOI:10.1111/jawr.12290 INTRODUCTION Understanding historical and current changes in hydrologic conditions is critical for proper assessment of plausible future conditions and proactive mitigation measures (WMO, 2009). Water resources applications in various sectors including agriculture, urban drain- age, reservoir management, and disaster preparedness are dependent on changing hydrologic conditions. Additionally climate change in the northern latitudes is expected to produce significant hydrologic changes toward earlier runoff timing, loss of snow accumula- tion, warming winters, and increased precipitation (IPCC, 2013). As a baseline for hydro-climate change studies, it is of importance to understand these antici- pated hydrologic changes relative to historic condi- tions, thus providing motivation for proper hydrologic characterization. Trend analysis is commonly used to assess histori- cal changes in hydrologic data (e.g., precipitation, streamflow, and temperature). Twentieth Century 1 Paper No. JAWRA-14-0028-P of the Journal of the American Water Resources Association (JAWRA). Received January 22, 2014; accepted December 24, 2014. © 2015 American Water Resources Association. Discussions are open until six months from print publication. 2 Research Hydrologist (Gyawali) and Senior Research Hydrologist (Greb), Bureau of Science Services, Wisconsin Department of Natural Resources, 2801 Progress Rd., Madison, Wisconsin 53716; and Assistant Professor (Block), Civil & Environmental Engineering, University of Wisconsin — Madison, Madison, Wisconsin 53706 (E-Mail/Gyawali: Rabi.Gyawali@wi.gov). JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1138 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 51, No. 4 AMERICAN WATER RESOURCES ASSOCIATION August 2015