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Pennsylvania Drought Condition Monitoring

Ground Water

Ground water levels are monitored in terms of chance of occurrence, or exceedence, of 30-day average levels, where the average levels are based upon the period of record at the particular USGS monitoring well. The 75%, 90% and 95% exceedence levels were chosen by The Pennsylvania Department of Environmental Protection to represent entry into watch, warning and emergency, respectively.

Suitable monitoring wells with adequate periods of record do not exist in each of the 67 counties; therefore, surrogate monitoring wells are used for some counties, as indicated by the triangles on the ground water and composite drought monitoring maps. On the ground water map, arrows point to the county whose well is being used as the surrogate.

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.