Pennsylvania Drought Condition Monitoring
Both the Commonwealth of Pennsylvania and the USGS use "percentiles" in regard to both stream-flow and groundwater statistics. Groundwater levels are used to indicate drought status in a manner similar to stream flows. Every day, groundwater levels in USGS observation wells are used to compute an average level of the last 30 days preceding that day (called the “30-day moving average groundwater level”), that serves as a ground water indicator. The groundwater indicators are then compared with statistical groundwater-level values known as “percentiles” derived from historic observation-well records. A percentile is a value on a scale from 0 to 100 that indicates the percent of the time on that given date throughout the observation well period of record that water levels have been equal to or below that value.
The Pennsylvania Department of Environmental Protection has defined that groundwater percentile ranges of 10 to 25, 5 to 10, and 0 to 5 to represent entry into watch, warning and emergency, respectively. Suitable observation wells with adequate periods of record do not exist in each of the 67 counties; therefore, surrogate wells are used for some counties, as indicated by the triangles on the ground water and composite drought monitoring maps. On the groundwater map, arrows point to the county whose well is being used as the surrogate.