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Summary of Annual Hydrologic Conditions - 2000
Volume 1: Surface Water
Precipitation and Reservoir Contents
Hydrologically, water year 2000 was a relatively uneventful year for most of New Jersey, especially when compared to the droughts and flooding that occurred during the previous year. Monthly precipitation (spatially weighted average throughout New Jersey) was above normal for October, below normal for November through February, much above normal for March, below normal for April and May, then above normal June through September when compared to normal monthly precipitation from 1961-90. Statewide, total average precipitation was approximately 1 inch above normal for water year 2000 and approximately 2 inches more than that recorded the previous year. The winter months were the 24th driest and the 21st warmest of the 106 years of monthly precipitation and monthly mean temperatures recorded for New Jersey. Snow covered parts of northern and central New Jersey from January 13 to February 22. (David Robinson, New Jersey State Climatologist, Rutgers University, oral commun., 2001). Rainfall was well distributed throughout the summer months (about 1 inch above normal for June through September) with one exception, an unusual thunderstorm that stalled over northwestern New Jersey from August 11-14.
The August 11-14 storm produced various intensities of rainfall in areas of northwestern New Jersey. Rainfall totals were highest in southeastern Sussex County and northwestern sections of Morris County. The heaviest rain fell during a 6-hour period on August 12. During August 11-14, privately owned rain gages in Jefferson Township and on Sparta Mountain (Sussex and Morris Counties, respectively) recorded 18.65 inches and 14.11 inches, respectively. The rain gages are operated by Sussex County Weather Network, LLC. Rain gages within a 10-mile radius of Sparta Township recorded the most rainfall in the State. In some locations, record flooding resulted. Four dams failed completely, and 26 others were damaged. Some bridges were washed away and more than 100 rescues were made.
Three National Weather Service (NWS) precipitation stations in Newark, Trenton, and Atlantic City have been selected as index sites for precipitation. During water year 2000, precipitation totals were above normal at the Newark and Atlantic City NWS index stations and below normal at the Trenton NWS index station. The Newark station recorded 44.63 inches, which is 102 percent of the 30-year reference-period (1961-90) mean. The Atlantic City station recorded 47.63 inches, which is 118 percent of the 30-year mean. The Trenton station recorded 36.41 inches, which is 83.2 percent of the 30-year mean. Monthly precipitation at the three NWS stations, along with the 30-year mean is shown in figure 1.
Monthly mean temperatures were below normal for October, above normal for November through June, and below normal for July through September when compared to New Jersey mean monthly temperatures for 1961-90. The July 2000 monthly mean temperature equaled the coolest mean temperature in 106 years of record, which occurred in July 1914.
Combined usable contents of the 13 major water-supply reservoirs in New Jersey were 64.7 billion gallons at the end of September 1999, which is 123 percent of the 30-year mean (normal) contents for the end of September and 80.5 percent of capacity. Reservoirs were replenished as a result of the heavy rainfall from Tropical Storm Floyd on September 15-17, 1999, and thus recovered from a deficit due to drought conditions. Combined usable contents increased to a maximum of 80.4 billion gallons by the end of March 2000, which is 115 percent of normal contents for the end of March and 100 percent of capacity. Reservoir levels declined during the summer because of an increased demand for water supplies. By September 30, 2000, combined usable contents totaled 69.9 billion gallons, which is 132 percent of normal contents for the end of September and 86.9 percent of capacity (fig. 2). The term 'usable contents' is used here as a measure of the total volume of water that can be removed from a reservoir without pumping, and does not account for the volume of water below the bottom of the lowest outlet or pipe (sometimes referred to as dead storage).
Three gaging stations, located in north, south, and central New Jersey, are considered index stations for statewide streamflow conditions. Streamflow at the index station in northern New Jersey (South Branch Raritan River near High Bridge) averaged 112 ft3/s for the water year, which is 91.1 percent of the 1919-2000 average. Streamflow at the index station in southern New Jersey (Great Egg Harbor River at Folsom) averaged 73.8 ft3/s, which is 86.2 percent of the 1926-2000 average. The observed annual mean discharge for the Delaware River at Trenton was 12,340 ft3/s, which is 106 percent of the 1913-2000 average. The Delaware River is highly regulated by reservoirs and diversions. Monthly mean discharge at each of these index gaging stations during the current water year and the long-term normal monthly discharge are shown in figure 3. Annual mean discharge at each of these index gaging stations and the mean annual discharge for the period of record are shown in figure 4.
Annual mean discharges at 46 gaging stations that had 40 years or more of continuous records and mean annual discharge for the period of record at each gaging station are shown in table 1. The difference is listed as percent difference. Discharge at 36 of the 46 gaging stations was below normal for water year 2000. Eight of the ten gaging stations with above-normal flow recorded flow that was less than 10 percent above normal. Several gaging stations that monitor heavily regulated rivers were not included in this comparison because of large artificial deficits related to regulation. The criterion of assessing gaging stations with 40 years or more of record was used in order to encompass at least one of the 30-year drought cycles that New Jersey has experienced.
The first notable flooding of water year 2000 was in Gloucester and Salem Counties and was the result of nearly 5 inches of rain that fell on March 21. Monmouth County experienced flooding after approximately 5 inches of rain fell on July 26. Areas of Atlantic County flooded on August 4 after an intensive rainfall. The most notable flooding for water year 2000 occurred in Sussex and Morris Counties on August 11-14.
Floods occurred after thunderstorms deposited as much as 18 inches of rain on areas of northwestern New Jersey during August 11-14. The heaviest rainfall was at the headwaters of the Wallkill, Musconetcong, and Rockaway River basins. Flood peaks at gaging stations on Lake Hopatcong, Musconetcong River, Green Pond Brook, Rockaway River and Russia Brook tributary were the highest ever recorded (table 2). Table 2 includes peak flow and stage at 14 gaging stations in northern New Jersey that experienced greater than a 2-year flood event during the period August 12-15. The gaging stations with peaks of record are in close proximity because the rainfall was localized. The peak flow attenuated downstream from these gaging stations, thus the recurrence intervals were low.
Following the storms in Sussex and Morris Counties, the New Jersey Department of Environmental Protection, Dam Safety Section, inspected more than 50 dams. They found complete failure at 4 dams (Seneca Lake Dam, Tomahawk Dam, Furnace Dam, and Edison Dam, all in Sussex County) and damage to another 26 dams throughout Sussex and Morris Counties (NJDEP, 2000, http://www.state.nj.us/dep/nhr/engineering/damsafety/).