Among numerous drought indices, the Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) are widely used for meteorological droughts, while the Standardized Streamflow Index (SSI) is commonly employed for hydrological droughts. These indexes are typically calculated for various timescales (from a week to several years). As drought characteristics are sensitive to the indexes and the timescales, their knowledge-informed selections are desired. Furthermore, their selections should consider seasonality, as the cause of hydrological drought can vary in different seasons, especially for streams fed by rainfall and snowmelt/ice-melt. Therefore, in this study, we demonstrate the differences in the characteristics of SPI- and SPEI-based droughts between summer and winter. We then propose a selection approach to determine the suitable time scale for SSI using the total water storage anomaly based on the notion that watershed storage modulates hydrological drought, and then the suitable meteorological drought index (SPI or SPEI) and its time scale, according to the correlation between meteorological and hydrological droughts for each season. The case study is conducted in the head watershed (above Banff) of the Bow River in Alberta. The results indicate significant differences between SPI- and SPEI-based droughts in terms of severity and duration, particularly during summer, as well as the drought onset time. The timescales of 3- and 6-month are most appropriate for SSI, while 12- and 18-month SPI is most suitable for meteorological droughts in summer and winter, respectively. This study offers valuable insights for improved drought monitoring.
