Pennsylvania Drought Condition Monitoring
Stream Flows (Surface Water)
Both the Commonwealth of Pennsylvania and the USGS use "percentiles" in regard to both stream-flow and groundwater statistics. Every day, USGS stream-gage records are used to compute an average flow of the last 30 days preceding that day (called the “30-day moving average daily flow”), that serves as a stream-flow indicator. The stream-flow indicators are then compared with statistical flow values known as “percentiles” derived from historic stream-gage records. A flow percentile is a value on a scale from 0 to 100 that indicates the percent of the time on that given date throughout the gage period of record that flow has been equal to or below that value.
The Pennsylvania Department of Environmental Protection has defined that an average flow over the last 30 days having a percentile range of 10 to 25 as entry into drought watch; 5 to 10 as entry into drought warning; and 0 to 5 as entry into drought emergency. Suitable stream gages with adequate periods of record do not exist in each of the 67 counties; therefore, surrogate stream-flow gages are used for some counties, as indicated by the triangles on the surface-water and composite drought-monitoring maps. The term "Exceedances" is sometime used to describe drought statistics and may be considered the complement of percentiles; i.e., a 10% exceedance is equivalent to a 90th percentile value, a 75% exceedance is equivalent to a 25th percentile value, etc.