Product Description
The USGS Streamflow Percentile products compare recent streamflow conditions with historical observations at individual USGS streamgages. Percentiles are calculated for 7-, 14-, and 28-day periods to support hydrological drought monitoring across the United States and Puerto Rico.
Data Source and Period of Record
Daily mean streamflow observations are obtained from the U.S. Geological Survey (USGS), using discharge parameter 00060, reported in cubic feet per second (cfs).
The historical reference period extends from January 1, 1945, through October 8, 2026, based on the historical database used to develop the product. Individual stream gages have varying periods of record, and not all stations have observations spanning the full period.
Methodology
Daily discharge observations are averaged over rolling 7-, 14-, and 28-day periods, requiring at least 4, 7, and 14 valid observations, respectively. Historical percentile distributions are calculated separately for each stream gage and time of year using a 15-day window (±7 days) to account for seasonal differences in streamflow.
Positive historical rolling-average values are log-transformed to calculate the 2nd, 5th, 10th, 20th, 30th, 50th, 70th, 80th, 90th, 95th, and 98th percentile thresholds. At least 100 observations are required for each historical distribution.
Recent rolling-average streamflow values are compared with the corresponding historical thresholds, with percentile ranks determined through linear interpolation. Values at or below the 2nd-percentile threshold are assigned to 0, while values at or above the 98th-percentile threshold are assigned 100.
Interpretation and Availability
Lower percentiles indicate below-normal streamflow conditions relative to the historical record for that location and time of year. Higher percentiles indicate above-normal conditions. Percentiles are station-specific and do not represent absolute differences in discharge between rivers.
Products are updated through an automated processing workflow and are available as shapefiles, GeoJSON, and PDF/PNG maps. Coverage varies with station availability and historical data completeness. Zero-flow conditions may not be fully represented in the historical percentile distributions.
Data Source: USGS National Water Information System (NWIS), Daily Values, Parameter 00060.