Methods for Calculating a Standardized Streamflow Index Using the Drought Risk Atlas

The National Drought Mitigation Center’s Drought Risk Atlas (DRA, found at droughtatlas.unl.edu), which is built on data from weather stations in the National Weather Service’s Cooperative Observer Network, has the best, most complete long-term histories of both precipitation and temperature data. At the next DRA update, there will be approximately 4000 weather stations in the DRA. During the next update to the DRA, point measured streamflow data from the United States Geological Survey’s Hydro-Climatic Data Network (HCDN-2009) will also be added to the DRA. This update will include a standardized streamflow index (SSI) calculated from data collected by the 743 HCDN-2009 stations. The USGS selected these 743 stations for inclusion in HCDN-2009 because they had the longest data records and because they primarily reflected responses to meteorological conditions without significant human interference. The station-based SSI is calculated using streamflow data from the streamflow measurements in the DRA. Two SSI time series are calculated for each respective station, one which fits the streamflow data to a gamma distribution, and the other to the Tweedie distribution. Goodness of fit results for both distributions are compared to look for regions where one distribution works better or worse than the other.