This report presents the results of a study by the U.S. Geological Survey (USGS), in cooperation with the Chester County Water Resources Authority (CCWRA), to update the drought-trigger water levels for the Chester County observation-well network. The Chester County observation-well network was established in 1973 through a cooperative agreement between the CCWRA and the USGS to monitor local ground-water levels and trends and to determine drought conditions.

In 1990 and again in 1997, drought-warning and drought-emergency water-level triggers were determined for the majority of wells in the existing Chester County observation-well network of 23 wells. Since 1997, the Chester County observation-well network expanded to 29 wells, some of the original wells were destroyed, and additional monthly water-level observations were made to allow for better statistical relations. Because of these changes, new statistics for water-level triggers were required.

For this study, 19 of the 29 wells in the observation-well network were used to compute drought-trigger water levels. An additional 'drought-watch water-level trigger' category was developed to make the Chester County drought-trigger water-level categories consistent with those implemented by the Pennsylvania Department of Environmental Protection (PaDEP). The three drought-trigger water-level categories, as defined by PaDEP are 1) 'drought watch' when at the 75th-percentile level; 2) 'drought warning' when at the 90th-percentile level; and 3) 'drought emergency' when at the 95th-percentile level.

A revised methodology, resulting from longer periods of record representing ground-water and climatic conditions and changes in local water use, has resulted in some observed differences in drought-trigger water levels. A comparison of current drought-trigger water levels to those calculated in 1997 shows the largest mean annual change in percentile values was in northeastern Chester County. In this northeastern region, the average 90th-percentile water levels changed by as much as 1.84 feet (17.5 percent) from those determined in 1997. Other observation wells in the county generally exhibited a smaller mean annual change in percentile values; a typical change was less than 0.5 foot. While most revisions in the drought-trigger water levels generally can be attributed to different methodology and (or) the additional years of data collected under existing climatic conditions, anthropogenic influences, such as the cessation of local pumping stresses, also were shown to cause changes in the drought-trigger water levels determined in this study.