Expanding the applications of the standardized streamflow index through regionalization

The Standardized Streamflow Index (SSI) has frequently been used to quantify drought by comparing periods of streamflow against a river's historical values. This study expands upon previous SSI methodologies by creating a more flexible, regionalized version of the metric for Iowa, a Midwestern state located in the central United States. Five drought regions were developed for Iowa that largely correspond to the state's Major Land Resource Areas. Several United States Geological Survey gauges were identified within each drought region and streamflow data were used to calculate daily water yields from 1960 to the present. SSI values calculated for both individual river sites and the entire drought region provide insights into the relative dryness of Iowa watersheds over the past several decades. The SSI methodology can be used to evaluate river dryness with limited data records and water yields of specific streams can be compared to each other and across the overall region. This study demonstrates the potential for developing versions of the SSI that enable real-time calculations at a daily scale at locations with limited historical streamflow data. The new SSI metric may be a valuable tool for decision-makers as state and federal agencies continue to identify and manage drought.