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Drought and High-flow Categories

Define default categories of drought and high-flow to use on maps.

From email to hydrologists: Background information on WaterWatch

Current streamflow conditions and real-time data are categorized based on the following scale that is common to both WaterWatch and the National Water Dashboard:

  • All time high = highest value
  • Much above normal = >90th percentile
  • Above normal = 76th-90th percentile
  • Normal = 25th-75th percentile
  • Below normal = 10th-24th percentile
  • Much below normal = <10th percentile
  • All time low = lowest value

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Drought / low-flow conditions

  • “Below normal” streamflow is shown if flows are calculated to be within one of these categories:
  • “Extreme Hydrologic Drought” -- "Low" indicates that the estimated streamflow is a new record low for this day of the year,
  • “Severe Hydrologic Drought” -- "<=5" indicates that the estimated streamflow is less than or to the 5th percentile for this day of the year,
  • “Moderate Hydrologic Drought” -- "6-9" indicates that the estimated streamflow is between the 6th and 9th percentile for this day of the year,
  • “Below Normal” -- "10-24" indicates that the estimated streamflow is between the 10th and 24th percentile for this day of the year.

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These categories are available for 7-day, 14-day, 28-day, monthly average streamflow and 7-day, 14-day, 28-day, monthly area-weighted (by HUC8) runoff using variable (day of year) percentile thresholds

Flood / high-flow conditions High flow sites are shown if flows are calculated to be within one of these categories:

  • "<95" indicates that the estimated streamflow is less than the 95th percentile for all days of the year,
  • "95-98" indicates that the estimated streamflow is between the 95th and 98th percentiles for all days of the year,
  • ">=99" indicates that estimated streamflow is greater than the 99th percentile, and
  • "Not ranked" indicates that a flow category has not been computed. Common reasons for a "Not ranked" category are insufficient historical data or no current streamflow estimates.

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These categories are available for real-time instantaneous values by comparing to daily value fixed (all days of year) percentile thresholds