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Pennsylvania Drought Condition Monitoring

Stream Flows (Surface Water)

Stream flows are monitored in terms of chance of occurrence, or exceedence, of 30-day average flows, where the average flows are based upon the period of record at the USGS stream gage. The Pennsylvania Department of Environmental Protection has defined the 30-day average flow value that is exceeded 75% of the time (the 75% exceedence flow) represents entry into drought watch; the 90% exceedence flow indicates entry into drought warning; and the 95% exceedence flow indicates 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.

Note that the Commonwealth of Pennsylvania uses the term "exceedence" in its drought management activities. The USGS uses "percentiles" in regard to both stream flow and ground water statistics. Exceedances can be considered the complements of percentiles; i.e., a 10% exceedence is equivalent to a 90 percentile value, a 75% exceedence is equivalent to a 25 percentile value, etc.