EXPLORING WHETHER LOCALIZED FLASH FLOODING EVENTS HAVE RECENTLY INCREASED OVER SOUTHEASTERN PENNSYLVANIA USING SEMIVARIOGRAMS

Localized flash flooding events have been observed across southeastern Pennsylvania. This study explores whether these geohazards have been increasing over the last 15 years. To address this temporal and spatial question, a quantitative analysis is conducted employing semivariograms (SVG; Matheron 1963). SVGs show the spatial variability of a phenomenon (e.g., precipitation) by calculating the semivariance (i.e., the sum of the squared differences; SSD) between observed data values for permutations of paired locations at varying distances. In this study, 15-minute precipitation data from five United States Geological Survey (USGS) stations located in Chester County, Pennsylvania, spanning the period 2009-2023, are used. Two SVGs were constructed representing the "initial" (2009-2016) and "more recent" (2016-2023) halves of the data period. A comparison of the SVG plots' appearances reveals an increase in precipitation variability in the "more recent" SVG as compared to the "initial" SVG. In addition, the "more recent" SVG shows a greater variability in SSD values with increasing station distance, an expected result since precipitation amounts often vary considerably over long distances.