Use of a standardized runoff index for characterizing hydrologic drought

Many current metrics of drought are derived solely from analyses of climate variables such as precipitation and temperature. Drought is clearly a consequence of climate anomalies, as well as of human water use practices, but many impacts to society are more directly related to hydrologic conditions resulting from these two factors. Modern hydrology models can provide a valuable counterpart to existing climate-based drought indices by simulating hydrologic variables such as land surface runoff. We contrast the behavior of a standardized runoff index (SRI) with that of the well-known standardized precipitation index (SPI) during drought events in a snowmelt region. Although the SRI and SPI are similar when based on long accumulation periods, the SRI incorporates hydrologic processes that determine seasonal lags in the influence of climate on streamflow. As a result, on monthly to seasonal time scales, the SRI is a useful complement to the SPI for depicting hydrologic aspects of drought.