New Jersey Water Science Center
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Summary of Annual Hydrologic Conditions - 2005
Volume 1: Surface Water
Three gaging stations, located in north, south, and central New Jersey, on the South Branch Raritan River, Great Egg Harbor River, and the Delaware River, respectively, are considered index stations for statewide streamflow conditions. Monthly mean discharges at the three index gages were above or close to average from October through April and then generally below average from May through September, during water year 2005 (fig. 1). Annual mean discharge at each of these index gaging stations was above the annual mean for the period of record for the third consecutive year (fig. 2).
Streamflow at the index station in northern New Jersey (South Branch Raritan River near High Bridge) averaged 131 ft3/s for the water year, which is 107 percent of the 1919-2005 average. Peak flow for the water year was 3,920 ft3/s on April 3; the recurrence interval is greater than 10 years. The lowest daily mean flow was 18 ft3/s, recorded September 11-13, and 23-25, which is lower than the 99-percent flow duration.
Streamflow at the index station in southern New Jersey (Great Egg Harbor River at Folsom) averaged 85.9 ft3/s, which is 101 percent of the 1926-2005 average. Peak flow for the water year was 346 ft3/s on April 5; the recurrence interval is from 2 to 5 years. The lowest daily mean flow was 24 ft3/s, recorded on September 24-30, which is less than the 98-percent flow duration.
The observed annual mean discharge for the Delaware River at Trenton was 15,470 ft3/s, which is 131 percent of the 1913-2005 average. Peak flow for the water year was 242,000 ft3/s on April 4; the recurrence interval is greater than 100 years. The lowest daily mean flow was 2,520 ft3/s, recorded on September 20, which is about the 95-percent flow exceedance. The Delaware River is substantially regulated by reservoirs and diversions.
Eleven floods and flash floods occurred during the 2005 water year due to heavy rainfall, as documented by the National Oceanic and Atmospheric Administration's National Weather Service (http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~storms). Table 1 lists the dates of the events and the affected counties. The most widespread event this year occurred at the end of March through the beginning of April. Many surface water gages throughout the state had peak-of-year discharges during this period.
The greatest flood since 1955 occurred on the main stem of the Delaware River on April 2-3, 2005 due to the spring rain event. A rainstorm that passed through the region on March 28-29, 2004, saturated soils and caused rivers to rise. A second rainstorm occurred the following week on April 2-3, 2004, for a total of 3 to 6 inches of rain in a seven-day period. In addition to the rainfall, a substantial snowpack with a water equivalency of up to 6 inches was present throughout the Catskill and Pocono Mountains from eleven winter storms that occurred January through March. The snow melted from the warmer temperatures and heavy rains, and exacerbated the record flows. Peak discharges were the third highest for the period of record, and had recurrence intervals greater than 100 years. Flooding on the main stem of the Delaware River had occurred just seven months earlier, on September 19, 2004. Peak discharges from the September event had recurrence intervals ranging from 40 to 70 years.
For the remainder of the water year, a general decreasing trend in streamflow was evident throughout the state. The months of June through September were all in the top 10 for warmth, and August and September were ranked as the 3rd and 5th driest, respectively. The State of New Jersey issued a drought watch on September 13, 2005. A drought watch indicates that the NJDEP is closely monitoring drought indicators including precipitation, streamflow, reservoir contents, groundwater levels, and water demands. Under a drought watch, the public was asked to voluntarily cut back on water use.
Annual mean discharges for water year 2005 at 45 selected gaging stations that had 40 years or more of continuous record and mean annual discharge for the period of record at each gaging station are shown in table 2. The differences are listed as percent difference, and ranged from -24.6 to 79.1 percent. The majority of the sites had discharges near or above the period of record mean. For 16 of the 46 sites, the difference between the current and the period of record annual means was within ±5 percent. Of the remaining 29 sites, 21 had discharges greater than the period of record mean. During both the 2004 and 2003 water years, discharges at 45 of 46 gages were above the historical mean. In contrast, during the 2002 and 2001 water years, 46 and 40 gages out of 46, respectively, had discharges below the historical mean. 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 approximately 30-year drought cycles that New Jersey has experienced.
The combined usable contents of 13 major water-supply reservoirs in New Jersey ranged from 48.1 to 99.3 percent of the 80.4 billion gallon capacity for the entire water year. The combined contents at the end of each month exceeded the 1961-1990 monthly averages from October to May then dropped steadily from June until September. Precipitation was considerably above normal in the months prior to the 2005 water year, and approximately normal for the first half of the water year in northern New Jersey, which maintained the higher reservoir levels. The second half of the water year had below normal precipitation, with corresponding lower reservoir contents. Combined usable contents of the 13 major water-supply reservoirs was 76.7 billion gallons at the end of September 2004, which is 145 percent of the 30-year mean (normal) contents for September, and 95.4 percent of capacity. Combined usable contents climbed steadily from September 2004 to a maximum for the water year of 79.5 billion gallons by the end of March 2005; this is 114 percent of normal contents for the end of March and 99.3 percent of capacity. Reservoir levels remained high for April, then declined steadily during the summer because of a combination of below normal precipitation, evaporation, and increased demand for water supplies. By September 30, 2005, combined usable contents totaled 38.7 billion gallons, which is 73.3 percent of the 30-year mean (normal) contents for the end of September and 48.1 percent of capacity (fig. 3). 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 pipe (sometimes referred to as dead storage).
Precipitation and Temperature
Monthly spatially weighted average-precipitation values using data from several dozen stations throughout New Jersey, along with the statewide long-term monthly means (1895-2005), can be accessed at http://climate.rutgers.edu/stateclim_v1/data/njhistprecip.html. For water year 2005, the spatially weighted values for 7 of 12 months were above the long-term mean (November through January, March, April, June and July were above their respective means, as shown in Figure 4). Water year 2005 is the 25th driest for the period of record. The statewide spatially weighted average-precipitation total was 39.38 inches, which is 5.36 inches less than the long-term mean-annual precipitation from 1895 to 2005. The average annual precipitation for New Jersey is approximately 45 inches. Rankings of monthly precipitation in New Jersey for water year 2005 as compared to water years 1896-2005 are listed in table 3.
Three National Weather Service (NWS) precipitation stations in Newark, Trenton, and Atlantic City have been selected as index sites for precipitation. Water year 2005 precipitation totals were below normal at the Newark and Atlantic City index sites and slightly above normal at the Trenton index site. The Newark station recorded 32.21 inches, which is 14.04 inches below normal or 69.6 percent of the 30-year reference-period (1971-2000) mean. The Atlantic City station recorded 38.21 inches, which is 2.99 inches below normal or 94.1 percent of the 30-year mean. The Trenton station recorded 42.91 inches, which is 1.52 inches above normal or 104 percent of the 30-year mean. Monthly precipitation at the three NWS stations, along with the 30-year mean, is shown in figure 5.
Eight of the 12 monthly mean temperatures in the 2005 water year (determined from spatially weighted average temperatures recorded throughout New Jersey) were above the long-term mean monthly average (1895- 2005) (fig. 6). The October monthly mean was 0.4 degrees Celsius below the long-term average temperature. Monthly mean temperatures were 1.4 and 1.0 degrees Celsius above average for November and December. January averaged 0.1 degrees Celsius below average. The low temperatures of January caused streams to ice over at times. This ice cover led to ice jams and flooding on a number of streams. The monthly mean for February was 1.6 degrees Celsius above average. The monthly mean for March was 1.5 degrees Celsius below average. April was 1.5 degrees Celsius above average; May was 1.9 degrees Celsius below average; and June through September was up to 3.3 degrees Celsius above average. Monthly mean temperatures for the 2005 water year are listed in table 4 and are compared by rank to the historical monthly means (water years 1896-2005). August was the warmest August on record, with September being the second warmest on record. Temperature data can be accessed at http://climate.rutgers.edu/stateclim-v1/data/njhisttemp.html.