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Summary of Annual Hydrologic Conditions - 1999

Volume 1: Surface Water

Precipitation and Reservoir Contents

Water year 1999 precipitation began with a record dry period and closed with heavy rainfall from Tropical Storm Floyd that ended the drought. Precipitation was below normal statewide during October, November, and December, continuing a trend that began in July 1998. July 1998 through December 1998 was the driest six-month July through December period in 105 years of record for New Jersey (David Robinson, New Jersey State Climatologist, Rutgers University, oral commun., 2000); statewide average precipitation was more than 11 inches below normal. A drought warning was declared on December 14 by the Delaware River Basin Commission as a result of the long period of dry weather. Precipitation was above normal during January, February, and March. January was the third wettest on record, and a near record snowfall of 10 inches occurred mid-March. The drought warning was cancelled on February 2 because the January rainfall replenished depleted reservoir supplies in the upper Delaware River Basin in New York.

The period April through July was the second driest early-to-middle growing season of the century (David Robinson, New Jersey State Climatologist, Rutgers University, oral commun., 2000). This, along with declining reservoir levels, led Governor Christine Whitman to declare a water emergency in New Jersey on August 5. A water emergency places mandatory restrictions on outdoor, non-essential water use statewide, such as watering lawns, washing cars, and operating display fountains.

Precipitation was above normal for August, but streamflow and reservoir levels remained low, and the drought continued. September began with some precipitation from Tropical Storm Dennis, but not enough to end the drought. Relief finally came when Tropical Storm Floyd together with a western storm system produced as much as 14 inches of rain. This rainfall raised New Jersey reservoir levels enough that Governor Christine Whitman cancelled the drought emergency in northern and central New Jersey on September 27, but a statewide drought warning remained due to below normal ground-water levels.

During water year 1999, precipitation was below normal at the Newark and Atlantic City National Weather Service (NWS) stations and above normal at the Trenton NWS station. The Newark station recorded 39.72 inches, which is 90.3 percent of the 30-year mean. The Atlantic City station recorded 39.01 inches, which is 96.8 percent of the 30-year mean. The Trenton station recorded 46.8 inches of precipitation, which is 107 percent of the 30-year reference-period (1961-90) mean. Monthly precipitation at the three NWS stations, along with the 30-year mean is shown in figure 1. The difference between precipitation for water year 1999 prior to Tropical Storm Floyd and normal precipitation for the same period is shown in table 1. A drastic increase in total precipitation and percent of normal precipitation in northern and central New Jersey (Newark and Trenton NWS stations) was the result of rainfall from Tropical Storm Floyd two weeks prior to the end of water year 1999 (table 2).

Figure 1. Monthly precipitation at three National Weather Service locations.
Figure 1



Table 1
Table 2


Monthly mean temperatures were above normal for October, normal for November, above normal for December through February, slightly below normal for March, and above normal for April through September when compared to New Jersey mean monthly temperatures for the period 1961 to 1990. The long stretch of higher than normal temperatures during the summer increased evapotranspiration, which stressed the already depleted water supplies; thus, drought conditions were compounded.

Combined usable contents of the 13 major water-supply reservoirs in New Jersey were 45.7 billion gallons at the end of September 1998, which is 86.6 percent of the 30-year mean (normal) contents for the end of September and 56.8 percent of capacity. Combined usable contents increased to a maximum of 77.4 billion gallons by the end of May 1999, which is 106 percent of normal contents for the end of May and 96.3 percent of capacity. Reservoir levels declined alarmingly during the summer because of low precipitation, above normal temperatures, and increased demand for water supplies. By September 30, 1999, the combined usable contents had recovered from the deficit, as a result of the heavy rainfall from Tropical Storm Floyd. At the end of water year 1999, combined usable contents totalled 64.7 billion gallons, which is 123 percent of normal contents for the end of September and 80.5 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).

Figure 2. Combined usable
contents of 13 major
water-supply reservoirs.
Figure 2



Streamflow at the index site in northern New Jersey (South Branch Raritan River near High Bridge) averaged 89.5 ft3/s for the water year, which is 72.8 percent of the 1919-99 average. Streamflow at the index station in southern New Jersey (Great Egg Harbor at Folsom) averaged 61.5 ft3/s, which is 71.8 percent of the 1926-99 average. The observed annual mean discharge of the Delaware River at Trenton was 7,750 ft3/s, which is 66.4 percent of the 1913-99 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.

Figure 3. Monthly discharge at index gaging stations. Figure 4. Annual mean discharge at index gageing stations.
Figure 3 Figure 4

At the beginning of water year 1999, during October, November, and December, streamflows were below normal as a result of the record dry period. The lowest monthly mean discharges on record for December were recorded at the northern and southern index stations (South Branch River near High Bridge and Great Egg Harbor River at Folsom, respectively) and at a number of other gaging stations throughout the state. Mandated releases from three major reservoirs in New York to the Delaware River were reduced because of the drought warning issued on December 14. Additional reductions in mandated water releases from the three New York reservoirs to the Delaware River were made on December 23.

The third wettest January on record (including snow melt) resulted in minor flooding throughout portions of New Jersey. Monthly mean streamflows at the three index gaging stations were above normal for January and near or above normal for February and March. Some minor flooding occurred in northern and central New Jersey on March 22.

Statewide, precipitation was below normal from April through July, and monthly streamflow averages declined steadily to below normal. Minimum monthly mean discharge reached record lows at 37 continuous gaging stations with 20 years or more of record (table 3). All streams throughout New Jersey would have experienced near-record low flows except for the fact that some streams were maintained artificially with releases from reservoirs and sewage-treatment plant effluent. August precipitation totals were above normal, but monthly mean streamflow remained below normal.


Table 3


Tropical Storm Dennis (September 6) was greatly overshadowed by Tropical Storm Floyd, which combined with a storm system from the west (rain began September 15 and ended September 17) to produce flooding of historic proportions in many areas of the state. New record highs were established for instantaneous peak discharges at 32 gaging stations with more than 20 years of record (table 4). The 100-year flood-frequency discharges were exceeded at 17 long-term gaging stations. Flood levels on the Raritan River at Bound Brook exceeded all known flood levels since the Bound Brook area was first settled around 1690. Several stream-gaging stations were destroyed or severely damaged by the high waters. One station was washed off of its foundation and could not be located. The New Jersey Department of Environmental Protection, Dam Safety Section inspected more than 50 dams and found complete failure at 3 dams (Kirbys Mill Dam, Burlington County; Bostwick Lake Dam, Cumberland County; and Spencer Detention Basin Dam, Morris County). This inspection also found notable damage to another 21 dams in 9 counties throughout New Jersey. Dam failures also were reported in nearby New York State. Because of the extremely dry conditions prior to Tropical Storm Floyd, most of the reservoirs in New Jersey had plenty of storage space available. This helped to reduce flooding downstream from the reservoirs.


Table 4


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