Seasonal and Regional Variability in Hydrological and Drought Responses in the Great Lakes Region: Impacts of SPEI Data Window Selection on Trend Detection

This study investigates seasonal and spatial variability in hydrological and drought-related responses across watersheds in the Great Lakes region, focusing on streamflow, baseflow, and the Standardized Precipitation Evapotranspiration Index (SPEI). Using daily streamflow data from 70 USGS sites (1960-2022), baseflow was separated via the HYSEP method, and watersheds were delineated using the D8 algorithm with high-resolution elevation data. Trend analyses employed Mann-Kendall and Pettitt break tests to detect temporal shifts in hydrological and climatic variables. Two SPEI datasets were compared: a full-period dataset spanning 1901-2022 and a truncated dataset aligned with the streamflow record (1960-2022). Results reveal pronounced spatial heterogeneity, with the northern region exhibiting the greatest variability and temporal dispersion in break years for baseflow and total flow. SPEI break years generally precede hydrological shifts, indicating that meteorological drought signals often emerge before hydrological responses. However, the truncated SPEI dataset shows closer temporal alignment with streamflow changes, emphasizing the critical influence of data window selection on drought interpretation. Seasonal analyses highlight stronger agreement between SPEI and hydrological trends during fall and winter, while summer shows lower correspondence, particularly in the northern region, suggesting complex interactions between climatic drivers and watershed processes. These findings underscore the importance of considering temporal alignment and regional heterogeneity when using drought indices for hydrological assessment and water resource management under changing climate conditions.