Quantifying the place of karst aquifers in the groundwater to surface water continuum: A time series analysis study of storm behavior in Pennsylvania water resources

SummaryThough karst aquifers have commonly been identified, with respect to their behavior, as intermediate between ground and surface water, their putative location between these end members is generally descriptive rather than quantitative. Autocorrelation and spectral analysis of data from four karst springs, three wells, and eight stream gauges in Pennsylvania illustrate that specific karst water resources exhibit widely varying inertia with lag times that overlap those of groundwater and surface water. When analyzed in the frequency domain, the same data reveal distinctive patterns for each type of water resource.The four springs display characteristic lag times ranging from 5 to 25 days, compared to 1–10 days for streams and 11–46 days for wells. Physically, karst waters may behave as a mix of porous media, fracture, and open-channel flow, but in temporal terms the balance of this mix results in a range of system behaviors with characteristic periodicities evident in the karst aquifers. In the frequency domain, karst aquifers manifested slow flow paths as a gradual fall-off at lower frequency and quick flow paths as a flattening at high-frequency.Our comparison of water resources across different time periods revealed that the period considered can have strong effects on results. One spring displayed characteristic lag times of 12 and 25 days for two different time spans. To directly compare water resources over relatively short time scales, precipitation inputs must be similar and data sets must cover the same period; otherwise, substantial differences in lag times can be due to data collection and input differences rather than system characteristics. This limitation is less when the same data are analyzed in the frequency domain.