Trend Analysis of Precipitation and Runoff As a Basis of Design and Operation of Pumped Storage Water Supply Infrastructure in New Jersey

Droughts in the 1880's, the 1930's, the 1960's and the 1980's have been the basis of design for the water supply systems in New Jersey. But as demand exceeded the available supply, reservoirs constructed to store runoff from 300 square miles had to be augmented by pumping stations that skimmed water from an additional 500 square miles. However, even that was not enough to preserve storage during droughts that occurred from 1990 to 2002. Consequently, the purveyors had to operate those systems under emergency conditions based on State regulations. This paper provides an historic and more recent analysis of rainfall runoff trends to determine if those emergency operating rules would be sufficient if present trends continue or if new facilities are needed. The results are ambiguous, because the median trends tend to increase at the same time as frequency and magnitude high and low flow in each subwatershed changes differently. From a drought management perspective, one critical question is, "Would pumped storage systems be capable of skimming enough in the future?". The operation of pumping stations to refill reservoirs is the single most important strategy in drought management in New Jersey. As such, the frequency and magnitude of streamflows, in the range of 200 million gallons per day (mgd) to 500 mgd, especially in the Spring, becomes critical. Since Hurricanes and northeasters have occurred in the last 10 years, one might assume there is plenty to skim. However from a multi year perspective -high flows will be followed by low flows as nature reverts to a mean over decades. From that perspective, if the rainfall during the past 10 years averages 55 inches, the probability that rainfall would average 37 inches during the next ten years increases as long as the long term mean for New Jersey is an average annual rainfall of 46 inches. Therefore, even if long term trends increase, a shorter term evaluation remains appropriate. In this instance, a trend analysis before 1970 and after 1980 indicates design, operation and management of the pumped storage water supply system may have to change. An initial finding was that more pumping may have to occur in the Fall and Winter since streamflows in the Spring are decreasing.