MULTI-DECADE PRECIPITATION AND STREAM FLOW CYCLES IN EASTERN PA: IMPLICATIONS FOR MANAGING WATER RESOURCES AND STORM WATER

Decisions about water resource management and storm water are often made based on estimates of conditions likely to be exceeded with a given frequency. For example, the 100-year flood and the Q7,10 (lowest mean flow over 7 consecutive days which recurs every 10 years, on average) are two stream extremes that are characterized this way. The value of such statistics is based on the assumption of stationarity; i.e., the hydrologic processes underlying the statistic create no trends in the mean rate of occurrence of events. However, it is also generally recognized that there are at least two possible causes of non-stationarity: monotonic effects related to urbanization and other human modifications of watersheds, and cyclic effects resulting from climate cycles such as those related to ENSO.

I investigate precipitation and stream flow records from eastern Pennsylvania that provide evidence for significant cyclic changes in hydrologic fluxes over the last century. For example, precipitation at Philadelphia, PA since 1872 (average 41.2 in/year) has gone through three cycles averaging 35 years in length. During the 15-20 year long "wet" phases of the cycle precipitation exceeds the 130-year average by 3 in/year. During the "dry" phases precipitation falls below the mean by 3 in/year. Precipitation cycles are evident in stream flow records. For example, Brandywine Creek at Chadds Ford, PA maintained an average flow of 419 cfs from 1962 to present. However, during part of a dry phase (1962-1969) flow averaged more than 75 cfs lower than average; and during the succeeding wet phase (1972-1983) averaged more than 75 cfs higher than average. Low flows during dry periods progressively tap groundwater, leading to drought conditions. The drought watches and warnings in Pennsylvania and New Jersey this year and in recent years are part of the most recent dry phase which started in 1986. If cyclic behavior continues, a wet phase will begin within a few years.