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U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004-5152

Statistical Analysis of Long-Term Hydrologic 

Records for Selection of Drought-Monitoring 

Sites on Long Island, New York

In cooperation with

NEW YORK STATE DEPARTMENT OF ENVIRONMENTAL CONSERVATION

Precipitation

Monitoring

Drought-Monitoring Indices

Ground-Water

 Monitoring

Surface-Water Monitoring
This page has been left blank intentionally.
Statistical Analysis of Long-Term Hydrologic 

Records for Selection of Drought-Monitoring 

Sites on Long Island, New York

By Ronald Busciolano

In cooperation with New York State Department of Environmental Conservation

Scientific Investigations Report 2004-5152

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

For additional information write to:

U.S. Geological Survey

2045 Route 112, Bldg. 4

Coram, NY 11727

This report is available on-line at http://ny.usgs.gov

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Busciolano, R., 2005, Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, 

New York: U.S. Geological Survey Scientific Investigations Report 2005-5152, 14 p.
iii

Figures

1. Location and major physiographic features of Long Island, N.Y. ..................................................2

2. Major hydrogeologic units along a generalized north-south section through the groundwater system of Long Island, N.Y.........................................................................................................3

 3. Major hydrogeologic units, hydraulic heads, and ground-water flow paths along a 

generalized north-south section of Long Island in an area near the Nassau-Suffolk 

County boundary, under natural (predevelopment) conditions......................................................6

 4. Ground-water levels on Long Island, N.Y., March-April 1997: A. Water-table altitude in 

the upper glacial aquifer. B. Potentiometric-surface altitude in the Magothy aquifer ..............8

5. Locations of the selected precipitation-monitoring stations, streamflow-gaging 

(discharge) stations, and monitoring wells on Long Island, N.Y. ...................................................9

6. Relation between heads (water levels) in the upper glacial and Magothy aquifers at 

five well pairs on Long Island, N.Y., during selected 3-year periods...........................................10

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope .......................................................................................................................................2

Methods..................................................................................................................................................2

Site Selection ...............................................................................................................................2

Data Analysis................................................................................................................................4

Previous Drought Studies....................................................................................................................4

Hydrogeology and effects of human activity (pumping and sewers) ...................................................5

Surficial Geology...................................................................................................................................5

Upper Glacial Aquifer..................................................................................................................5

Magothy Aquifer..........................................................................................................................5

Relation between the Upper Glacial and Magothy Aquifers................................................7

Effects of Human Activity on Ground-Water Levels .......................................................................7

Ground-Water Pumping..............................................................................................................9

Sewer Systems...........................................................................................................................11

Delineation of Human Effect on Ground-Water Levels .......................................................13

Statistical Analysis of Long-Term Hydrologic Records .........................................................................15

Precipitation.........................................................................................................................................15

Stream Discharge...............................................................................................................................19

Ground-Water Levels .........................................................................................................................22

Selection of Drought-Monitoring Sites ....................................................................................................28

Precipitation.........................................................................................................................................29

Stream Discharge...............................................................................................................................40

Ground-Water Levels .........................................................................................................................40

Future Implementation of Drought-Monitoring Indices ........................................................................41

Summary and Conclusions.........................................................................................................................45

Acknowledgments.......................................................................................................................................45

References Cited..........................................................................................................................................45
iv

7. Distribution of A. population density, B. upper glacial and Magothy aquifer public-supply 

wells and large-scale sewer districts, C. inferred human-impact on aquifer zones 

and selected drought-monitoring-index stations on Long Island, N.Y. .......................................12

8. Annual ground-water pumpage from Long Island’s major aquifers, by county, 1985-99: 

A. upper glacial (water-table) aquifer. B. Jameco aquifer. C. Magothy aquifer. 

D. Lloyd aquifer. ....................................................................................................................................14

9. Mean precipitation on Long Island, N.Y., 1951-65: A. Mean annual. B. Mean for warm 

season (April through September). C. Mean for cool season (October through March) ........16

10. Monthly precipitation and exceedance statistics for eight selected stations on or near 

Long Island, N.Y., 1951-2000. ..............................................................................................................17

11. Monthly mean discharge and exceedance statistics for eight selected streamflowgaging stations on Long Island, N.Y., 1951-2000 or period of record...........................................26

12. Monthly water levels and exceedance statistics for 20 selected monitoring wells on 

Long Island, N.Y., 1951-2000 or period of record.............................................................................35

13. Composite-average monthly precipitation and exceedance statistics for eight selected 

stations on or near Long Island, N.Y., 1951-2000.............................................................................41

14. Composite-average monthly mean discharge and exceedance statistics for two streamdischarge index groups in the (A) moderate-human-impact zone, and (B) low-human-impact 

zone, on Long Island, N.Y., 1951-2000................................................................................................42

15. Composite-average water levels and exceedance statistics for three ground-waterlevel index groups in the (A) significant-human-impact zone, (B) moderate-humanimpact zone, and (C) low-human-impact zone, on Long Island, N.Y., 1951-2000. ......................44

Tables

1. Well pairs used in analysis of the hydraulic connection between the upper glacial 

and underlying Magothy aquifer, Long Island, N.Y. .........................................................................11

2. Public-supply ground-water withdrawals on Long Island, N.Y., by aquifer and county, for 

1985-89, 1990-94, and 1995-99. .............................................................................................................13

3. A. Annual minimum, mean, and maximum precipitation volume, and volumes for selected 

exceedance percentiles, at the eight selected precipitation-monitoring 

stations on or near Long Island, N.Y., 1951-2000.........................................................................19

B. Monthly minimum and maximum precipitation volume, and means for driest 

and wettest months, at the eight selected precipitation-monitoring stations on or 

near Long Island, N.Y., 1951-2000. ................................................................................................19

C. Notable short- and long-term trends in precipitation at the eight selected precipitationmonitoring stations on or near Long Island, N.Y., 1951-2000.....................................................20

4. A. Annual minimum, mean, and maximum discharge, and selected exceedance 

percentiles at the eight selected streamflow- gaging (discharge) stations on Long 

Island, N.Y., 1951-2000 or period of record...................................................................................21

B. Monthly minimum and maximum mean discharge, and means for lowest and highest 

months, at the eight selected streamflow-gaging (discharge) stations on 

Long Island, N.Y., 1951-2000 or period of record.........................................................................22

C. Notable trends in discharge at the eight selected streamflow-gaging (discharge) 

stations on Long Island, N.Y., 1951-2000 or period of record ....................................................23

5. A. Annual mean ground-water levels, selected exceedance percentiles, and screen 

depths, at the 20 selected monitoring wells on Long Island, N.Y., 1951-2000 or 

period of record................................................................................................................................28
v

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

million gallons per day (Mgal/d) 0.0438 cubic meter per second (m3

/s)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Conversion Factors, Abbreviations and Datums

Other abbreviations used in this report

milligrams per liter (mg/L)

million gallons (Mgal)

billion gallons (Bgal)

cubic feet per second (CFS)

autoregressive integrated moving average (ARIMA)

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

B. Monthly minimum and maximum ground-water levels, and means for lowest 

and highest months, at the 20 selected monitoring wells on Long Island, N.Y., 

1951-2000 or period of record.........................................................................................................29

C. Notablel trends in ground-water levels at the 20 selected monitoring wells on 

Long Island, N.Y., 1951-2000 or period of record.........................................................................30

6. Summary of degree of human impact and primary monitoring use of eight selected 

streamflow-gaging (discharge) stations and 20 selected monitoring wells on Long 

Island, N.Y., 1951-2000 or period of record...................................................................................43
Statistical Analysis of Long-Term Hydrologic Records 

for Selection of Drought-Monitoring Sites on Long 

Island, New York

By Ronald Busciolano

Abstract

Ground water is the sole source of water supply for more 

than 3 million people on Long Island, New York. Large-scale 

ground-water pumpage, sewering systems, and prolonged 

periods of below-normal precipitation have lowered groundwater levels and decreased stream-discharge in western and 

central Long Island. No method is currently (2004) available 

on Long Island that can assess data from the ground-watermonitoring network to enable water managers and suppli­ers 

with the ability to give timely warning of severe water-level 

declines.

This report (1) quantifies past drought- and humaninduced changes in the ground-water sys­tem underlying 

Long Island by applying statistical and graphical methods 

to precipitation, stream-discharge, and ground-water-level 

data from selected monitoring sites; (2) evaluates the relation 

between water levels in the upper glacial aquifer and those 

in the underlying Magothy aquifer; (3) defines trends in 

stream discharge and ground-water levels that might indicate 

the onset of drought conditions or the effects of excessive 

pumping; and (4) discusses the long-term records that were 

used to select sites for a Long Island drought-monitoring 

network.

Long Island’s long-term hydrologic records indicated 

that the available data provide a basis for development of a 

drought-monitoring network. The data from 36 stations that 

were selected as possible drought-monitoring sites— 8 precipitation-monitoring stations, 8 streamflow-gaging 

(discharge) stations, 15 monitoring wells screened in the upper 

glacial aquifer under water-table (unconfined) conditions, 

and 5 monitoring wells screened in the underlying Magothy 

aquifer under semi-confined conditions—indicate that 

water levels in western parts of Long Island have fallen and 

risen markedly (more than 15 ft) in response to fluctuations 

in pumpage, and have declined from the increased use of 

sanitary- and storm-sewer systems. Water levels in the central 

and eastern parts, in contrast, remain relatively unaffected 

compared to the western parts, although the effects of human 

activity are discernible in the records.

The value of each site as a drought-monitoring indicator 

was assessed through an analysis of trends in the records. 

Fifty-year annual and monthly data sets were created and 

combined into three composite-average hydrographs— precipitation, stream discharge, and ground-water levels. Three 

zones representing the range of human effect on ground-water 

levels were delineated to help evaluate islandwide hydrologic 

conditions and to quantify the indices. Data from the three 

indices can be used to assess current conditions in the groundwater system underlying Long Island and evaluate water-level 

declines during periods of drought.

Introduction

Long Island extends about 120 mi northeastward from 

the southeastern part of the mainland of New York State and 

is surrounded by the Atlantic Ocean, Long Island Sound, 

New York Bay, and the East River estuary (fig. 1). The 

island contains four counties—Kings, Queens, Nassau, and 

Suffolk—that together encompass about 1,450 mi2.

Kings County and most of Queens County obtain most 

of their water from upstate reservoirs, whereas Nassau County 

and Suffolk County obtain water solely from wells that tap 

the underlying ground-water system, which consists of a 

sequence of unconsolidated sediments of Late Cretaceous and 

Pleistocene age that overlie a southeastward sloping surface of 

crystalline bedrock (fig. 2). 

The U.S. Geological Survey (USGS), in cooperation with 

Federal, State, and local agencies, has continuously monitored 

ground-water levels and stream discharge on Long Island for 

more than 50 years. The monitoring data provide a basis for 

analysis of changes resulting from natural and human-induced 

stresses in the ground-water system. In the past, prolonged 

periods of below-normal precipitation, together with increases 

in large-scale pumping from public-supply production wells 

and sewering by completion of extensive sanitary-sewer and 

storm-sewer systems (which export water from the system), 

have produced major water-level declines. These declines, 
in turn, have resulted in decreased stream discharge and 

local saltwater encroachment into the ground-water system; 

therefore, the ground-water system requires close monitoring. 

No method is currently (2004) available on Long Island that 

can assess data from the ground-water-monitoring network to 

enable water managers and suppliers with the ability to give 

timely warning of severe water-level declines.

In 2000, the USGS, in cooperation with the New York 

State Department of Environmental Conservation, began 

a study to analyze the long-term records of 36 selected 

precipitation, stream-discharge, and ground-water sites on 

Long Island for possible use as drought-monitoring stations. 

The objective was to provide water managers with a means 

of defining when the ground-water system underlying Long 

Island is being adversely affected by drought or excessive 

pumping—the first step in the development of a droughtwarning system that can provide timely data on the current 

status of the ground-water system. The resulting information 

can be used in times of drought to determine the effects waterconservation programs are having on the ground-water system.

Purpose and Scope

This report (1) quantifies past drought- and humaninduced changes in the ground-water system underlying 

Long Island, New York by applying statistical and graphical 

methods to long-term precipitation, stream-discharge, and 

ground-water-level records from selected monitoring sites, (2) 

evaluates the relation between water levels in the upper glacial 

aquifer and those in the Magothy aquifer to discern whether 

water-table fluctuations can be an indicator of changes in 

the underlying Magothy aquifer, (3) defines trends in stream 

discharge and ground-water levels that might indicate the 

onset of drought conditions or the effects of excessive 

pumping, and (4) describes and rates the long-term data to 

identify which sites are appropriate for inclusion in a Long 

Island drought-monitoring network.

Methods

Long-term records from 13 National Weather Service 

(NWS) long-term precipitation-monitoring stations, 18 USGS 

streamflow-gaging (discharge) stations, and 40 USGS groundwater-monitoring wells (30 screened in the upper glacial 

aquifer and 10 screened in the underlying Magothy aquifer) 

were compiled and evaluated to determine which of these 

stations could be used as representative sites for a droughtmonitoring network.

Site Selection

Sites were selected if (1) their records were complete 

(measurement frequency at least monthly for most of the 

period of record), and (2) the period of record began before 

or during the 1962-66 drought, which is the longest period of 

consistently below-average precipitation on record for Long 

Island and is regarded as a benchmark indicator for natural 

(not human induced) declines in the ground-water system. 

Some monitoring wells did not meet the criteria listed above 

EXPLANATION

40° Glacial moraine 

45' 

41°

74° 73°45' 30' 15' 73° 72°45' 30' 15' 72° 

New York 

Suffolk

County

Long Island Sound 

Nassau

County

Great South Bay 

Atlantic Ocea

Outwash 

n 

Plain 

 Kings

County

Queens

County

Base from U.S. Geological Survey State base map, 1:100,000, 1979,

Universal Transverse Mercator projection, NAD 27, Zone 18

0 5 10 MILES 

0 5 10 KILOMETERS 

Connetquot

River

East

River

New York 

 Bay 

LOCATION MAP 

New York 

Map

Area

Connecticut 

New

Jersey

 Hill 

Harbor 

Moraine 

Peconic River 

Ronkonkoma 

Nissequogue River

Moraine 

North Fork 

Peconic

 Bay

South Fork 

Figure 1. Location and major physiographic features of Long Island, N.Y. (Modified from McClymonds and Franke, 1972, fig. 2.)

 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
Figure 2. Major hydrogeologic units along a generalized north-south section through the ground-water system of 

Long Island, N.Y. (Section represents an area near the boundary between Nassau and Suffolk Counties.) (Modified 

from McClymonds and Franke, 1972, fig. 3.)

EXPLANATION

GLACIAL DEPOSITS

DELTAIC AND MARINE DEPOSITS

Atlantic Ocean

Long Island

Sound

Bedrock

Vertical scale is greatly exaggerated

Datum is NGVD 29

400

FEET

-400

-800

-1,200

-1,600

-2,000

-2,400

0

NORTH SOUTH

0

0

5 MILES

5 KILOMETERS

Gardiners Clay

Magothy aquifer

Raritan confining unit

Lloyd aquifer

Jameco Gravel

Upper glacial aquifer

Brown, fine to coarse sand and

gravel with some interbedded silt

and clay

Gray clay with silt and fine sand.

Gardiners clay contains glauconite

and some interbedded gravel

Gray, fine to coarse sand with

interbedded silt, clay and lignite.

Gray or white, fine to coarse sand

and gravel with some interbedded

silt and clay

Purpose and Scope 
but were nevertheless selected as potential drought-monitoring 

sites because they are in areas, or screened in parts of the 

ground-water system, where data are lacking. Most of the 

wells selected for study are on or close to the glacial moraines 

that extend east-west along the spine of Long Island (fig. 1), 

where ground-water levels show the greatest response to 

precipitation. 

Data Analysis

Data from each site were analyzed by graphical and 

statistical methods to identify which could be used as 

representative indicators of drought conditions. Data plots 

and hydrographs for each site were created and compared 

to identify the sites at which the effects of human-induced 

stresses, such as pumping or sewering, were most severe.

The precipitation and stream-discharge data generally 

were collected by daily measurement or by continuousrecording equipment. Water-level data were generally 

collected at monitoring wells monthly, although the 

measurement interval for some wells may have been 

variable. Water-level records from wells with more than 

one measurement per month were averaged to obtain a 

monthly mean, whereas means for wells with less than one 

measurement per month were estimated, if possible, by 

interpolation of values from preceding and subsequent months; 

if this estimation was impossible, the data were omitted.

The data analysis was based on water years unless 

otherwise stated. A water year is defined by the 12-month 

period beginning on October 1 and ending on September 30 

of the following year and is designated by the calendar year in 

which the 12-month period ends.

The initial data set was analyzed, and all redundant 

and unrepresentative sites were removed. The final data set 

consists of 8 precipitation-monitoring stations, 8 streamflowgaging (discharge) stations, 15 monitoring wells screened in 

the upper glacial aquifer, and 5 monitoring wells screened in 

the underlying Magothy aquifer. Data sets for sites with more 

than 50 years of record were trimmed to include only the last 

50 years (1951-2000); for sites with less than 50 years of 

record, the entire period of record was used. Various statistics 

were calculated for each site—50-year mean and monthly 

mean; departure and cumulative departure from mean; 

annual and monthly 10th, 25th, 50th (median), 75th, and 90th 

exceedance values; and autoregressive integrated moving 

average (ARIMA) and polynomial trend lines. An ARIMA 

trend of 12-month frequency was used for the precipitation 

and stream-discharge data and 18-month frequency for the 

ground-water-level data. Trends were computed by averaging 

the 12 or 18 months prior to and after the value of interest.

The term “normal” generally is used as a standard of 

comparison for a particular value over a specific 30-year 

period (Langbein and Iseri, 1960). In this report, a period of 

50 years was selected for comparison so that the long-term 

drought of the 1960’s, and the 10 years preceding it, were 

included in the data set for comparative purposes.

Exceedance values are used in this report to quantify how 

a mean for a selected period relates to the long-term (1951-

2000) record. An exceedance value of 100 percent indicates 

that the value of interest was exceeded 100 percent of the time 

and, thus, is less than all other measured values for the period 

of record (record low); an exceedance value of 50 percent 

indicates that half of the values are above the value of interest, 

and half are below it; and an exceedance value of 0 percent 

indicates that the value of interest is greater than all other 

measured values for the period of record (record high).

Data are considered to be in the well-above-normal range 

if they are above the 10-percent exceedance percentile, in the 

above-normal range if they are between the 10-percent and 

25-percent exceedance percentiles, in the normal range if 

they are between the 25-percent and 75-percent exceedance 

percentiles, in the below-normal range if they are between 

the 75-percent and 90-percent exceedance percentiles, and in 

the well-below-normal range if they are below the 90-percent 

exceedance percentile.

Long-term records were reviewed, and the utility of each 

site as a drought indicator was assessed through an analysis 

of temporal trends in the records. These assessments, in 

combination with a review of islandwide population density, 

distribution of large sewer systems, and the distribution of 

public-supply wells were used to develop a map delineating 

three aquifer “human-impact zones” that reflect the extent 

to which pumping and sewers have affected ground-water 

levels—significant (high), moderate, or insignificant (low). 

Monthly data from sites in each of these three zones were then 

combined to create multiple composite-average hydrographs. 

The composite-average hydrographs for each data type 

(precipitation, stream discharge, and ground-water level) in the 

low-human-impact zone were selected as drought-monitoring 

indices. The composite-average hydrographs for the other two 

areas are to be monitored for human-induced changes that 

could be detrimental to the ground-water system underlying 

Long Island.

Previous Drought Studies

Several USGS studies have evaluated the effects of 

drought on Long Island. The most comprehensive of these 

studies (Cohen and others, 1969), analyzed the hydrologic 

effects of the 1962-66 drought on Long Island’s groundwater system. Miller and Frederick (1969) and Franke and 

McClymonds (1972) also studied the hydrologic effects of that 

drought.

Other selected USGS reports that study historical trends 

and the effects of drought in the ground-water system are 

Fowler (1992), which reviewed the effects of drought on 

ground-water levels, streamflow, and reservoir levels in 

Indiana; and Schreffler (1997), which studied drought-trigger 

ground-water levels and historical trends in Chester County, 

Pennsylvania.

 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
Hydrogeology and Effects of Human 

Activity (Pumping and Sewers)

Precipitation is the only natural source of freshwater 

on Long Island. Under natural (undeveloped) conditions, 

about 50 percent of the precipitation that falls on the land 

surface recharges the ground-water reservoir (Franke and 

McClymonds, 1972; Aronson and Seaburn, 1974; Peterson, 

1987), but this percentage can vary locally, depending on 

the climate, geography, and land use. The other 50 percent 

is lost either through evapotranspiration or to the sea as 

overland runoff to streams. In developed areas, much of the 

overland flow is diverted through a system of storm drains 

that discharge either to recharge basins or stream channels. 

Much of the precipitation that reaches the water table moves 

laterally and discharges to streams and the saltwater bodies 

that surround the island; the remainder infiltrates downward 

to recharge the deep (Magothy and Lloyd) aquifers (fig. 3). 

Water in the deep aquifers also moves seaward along lateral 

and upward gradients that result in flow into the overlying 

aquifers. The boundaries of the ground-water system are the 

water table, the freshwater-saltwater interface (fig. 3), and the 

bedrock surface.

Under natural conditions, the rate of horizontal groundwater flow through the ground-water system ranges from a 

few feet to several hundred feet per year. The rate of horizontal 

flow in most aquifers is 10 to 100 times faster than the 

rate of vertical flow (Nemickas and others, 1989) because 

stratification and clay layers within the aquifers and between 

aquifers impede vertical flow.

Human activities on Long Island have produced large 

changes in the quantity, movement, and quality of ground 

water in many parts of the island. The major causes for these 

changes are (1) increased ground-water pumping, which has 

created local and widespread drawdowns; (2) storm sewers 

and sanitary sewers, which discharge wastewater to the 

surrounding bays and ocean, thereby removing water from the 

ground-water system and contributing to water-level declines; 

and (3) roads, parking lots, and other impervious surfaces that 

prevent the infiltration of precipitation and decrease recharge 

to the ground-water system. The installation of numerous 

ground-water-recharge basins (that divert storm-water runoff 

back into the ground-water system) has helped to minimize 

water-level declines in some areas of Long Island.

The ground-water system underlying Long Island consists 

of three major aquifers—the upper glacial, the Magothy, and 

the Lloyd—and various other smaller hydrogeologic units of 

limited extent (fig. 2). These units consist of a sequence of 

unconsolidated glacial, lacustrine, deltaic, and marine deposits 

of clay, silt, sand, and gravel that range in age from Upper 

Cretaceous to Pleistocene and overlie a southward sloping 

surface of Precambrian and (or) Paleozoic-aged crystalline 

bedrock. The geology and hydrology of Long Island are 

discussed in detail in many reports, for example, Veatch 

and others (1906), Fuller (1914), Suter and others (1949), 

Cohen and others (1968), Jensen and Soren (1974), Soren 

and Simmons (1987), and Smolensky and others (1989). This 

report pertains to the two uppermost units—the upper glacial 

and Magothy aquifers, whose hydrogeologic properties and 

water-level configurations are described below.

Surficial Geology

Most of the major features of the present-day topography 

of Long Island are a result of Pleistocene glaciation and are 

oriented in belts or ridges parallel to the island’s length. The 

most prominent features are two east-west-trending morainal 

ridges (Ronkonkoma and Harbor Hill moraines) that traverse 

the island (fig. 1); these moraines are interpreted as the 

terminal moraines of two major ice advances. Many small, 

less continuous ridges probably were deposited between or 

at the terminus of smaller lobes of the ice sheet, or as other 

types of glacial features. An outwash plain slopes southward 

from the base of the Harbor Hill Moraine in Kings and Queens 

Counties, and from the Ronkonkoma Moraine in Nassau and 

Suffolk Counties, to the southern shore.

Upper Glacial Aquifer

The upper glacial aquifer of Pleistocene age (fig. 2) is 

the uppermost unit in Long Island’s ground-water system and 

forms the present-day land surface of Long Island, except 

where it is overlain by Holocene-age or man-made deposits. 

The upper glacial aquifer contains the water table throughout 

the island, except in parts of central and eastern Nassau 

County and western Suffolk County, where its entire thickness 

is unsaturated; in these areas the water table is in the upper 

part of the Magothy aquifer, which is unconfined (under watertable conditions) and functions hydraulically as part of the 

upper glacial aquifer. The upper glacial aquifer unconformably 

overlies the Magothy aquifer throughout most of Long Island.

Magothy Aquifer

The Magothy aquifer (Matawan Group-Magothy 

Formation, undifferentiated, of upper Cretaceous age) is 

the most extensive hydrogeologic unit in the ground-water 

system underlying Long Island (fig. 2). The Magothy aquifer 

underlies most of Long Island and the offshore waters, except 

in parts of western and northern Kings and Queens Counties, 

northern Nassau County, and northwestern and northeastern 

Suffolk County, where it has been removed by erosion 

and glacial scour. The aquifer unconformably overlies the 

Raritan confining unit (clay member of the Raritan Formation 

of Upper Cretaceous age) throughout Long Island and is 

hydraulically confined by the Gardiners Clay of Pleistocene 

age in most of Kings County, south-central Queens County, 

and southern Nassau and Suffolk Counties. The Magothy 

aquifer is overlain by Pleistocene-age deposits of the upper 

glacial aquifer in the area north of the Gardiners Clay. In this 

Hydrogeology and Effects of Human Activity (Pumping and Sewers) 
Figure 3. Major hydrogeologic units, hydraulic heads, and ground-water flow paths along a generalized north-south 

section of Long Island in an area near the Nassau-Suffolk County boundary, under natural (predevelopment) conditions. 

(Modified from Franke and Cohen, 1972, fig. 2.)

water 

table 

Bedrock 

Vertical scale is greatly exaggerated 

Datum is NGVD 29 

400 

FEET 

-400 

-800 

-1,200 

-1,600 

-2,000 

-2,400 

0 

NORTH SOUTH 

Magothy aquifer 

upper glacial 

aquifer 

Lloyd aquifer 

Raritan confining unit 

Saltwater 

EQUIPOTENTIAL LINE--Number is 

hydraulic head, in feet above NGVD 29

GROUND-WATER FLOW PATH--Arrow 

indicates direction of flow 

FRESHWATER-SALTWATER INTERFACE 

Atlantic Ocean Long Island 

Sound 

Saltwater 

10

70

80

80

70

20

20

60

60

50

50

30

30

40

40

40

0 

0 

5 MILES 

5 KILOMETERS 

Gardiners Clay 

1 0 

SALTWATER 

EXPLANATION 

 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
area, the uppermost part of the Magothy aquifer is unconfined 

and functions hydraulically as part of the water table.

Relation between the Upper Glacial 

and Magothy Aquifers

The upper glacial aquifer on Long Island is recharged by 

precipitation that infiltrates downward through the unsaturated 

zone. The water table (located in the upper glacial aquifer and 

parts of the uppermost Magothy aquifer) generally parallels 

land surface; it gradually rises from the western part of Long 

Island to form an east-west-trending mound in Nassau and 

western Suffolk Counties that is dissected by a low region 

in west-central Suffolk County beneath the Nissequogue 

and Connetquot River drainage basins (fig. 4A). The water 

table rises again in central Suffolk County, then gradually 

declines toward the eastern end of Long Island, where it splits 

into northern and southern lobes beneath the Peconic River 

drainage basin (fig. 4A).

The Magothy aquifer is recharged by downward 

movement of water from the overlying upper glacial aquifer. 

Small silty or clayey layers within the Magothy aquifer 

cause the water to become increasingly confined with depth. 

The potentiometric-surface altitude of the Magothy aquifer 

generally parallels the overlying water table (fig. 4A,B); it 

gradually rises from the western part of Long Island to form 

an east-west-trending mound in Nassau and western Suffolk 

Counties, then declines to form a low region in west-central 

Suffolk County beneath the Nissequogue and Connetquot 

River drainage basins. The potentiometric surface rises again 

in central Suffolk County, then gradually declines toward 

the eastern end of Long Island. A second, smaller mound is 

evident in the central part of the South Fork of eastern Long 

Island (fig. 4B). Parts of the Magothy aquifer are naturally 

saline (chloride concentration greater than 250 mg/L; 

Lusczynski and Swarzenski, 1966) on the North and South 

Forks (Nemickas and Koszalka, 1982).

Water levels in the upper glacial aquifer generally 

parallel those in the Magothy aquifer as a result of the good 

hydraulic connection between the two hydrogeologic units 

in areas where confining units are absent. Five upper glacial 

and Magothy aquifer monitoring-well pairs were selected for 

a water-level comparison to measure the lag time between 

an increase in pressure head from precipitation entering 

the water table and the head (water-level) responses in the 

underlying Magothy aquifer. An increase in pressure head 

does not indicate the rate of downward water movement from 

land surface to the Magothy aquifer, which could take years 

or decades; it does, however, indicate the degree of hydraulic 

connection between the two aquifers, which is useful for 

drought or pumpage analysis.

Well pairs in areas that are hydrologically similar to 

those of the 15 monitoring wells screened in the upper glacial 

aquifer, and 5 monitoring wells screened in the underlying 

Magothy aquifer that were selected as drought-monitoring 

wells (along and near the ground-water divide) were 

evaluated; locations are shown in figure 5. Hydrographs of 

the five well pairs (fig. 6) were created for selected 3-year 

time periods, which were identified based on two criteria: (1) 

water levels in both wells of a pair were measured at least 

monthly, and (2) the record encompassed a period of waterlevel fluctuations related to changes in precipitation. The 

water levels in each paired well were compared to determine 

whether wells screened in the upper glacial aquifer could be 

used as indicators of general conditions in the underlying 

Magothy aquifer. This information would be useful for 

drought monitoring in areas where deep wells are sparse, 

such as in central and eastern Suffolk County. Results of the 

comparison are summarized in table 1.

The data from the four selected well pairs that had 

measurement intervals of 1 month (A, B, C, and D) indicate no 

discernible pressure-head lag time (fig. 6). Well pair E, which 

has a weekly measurement interval, indicates a pressure-head 

lag time of as much a 1 week at a few points in the record. 

However, the short lag times (less than 1 month) at all five 

well pairs indicate that most water-table wells on Long Island 

that are screened above the unconfined part of the Magothy 

aquifer can be used to evaluate heads (water levels) in the 

underlying Magothy aquifer where deep monitoring wells are 

lacking.

Effects of Human Activity on 

Ground-Water Levels

The abundance of fresh ground water on Long Island 

has allowed extensive growth and development during the 

past 100 years. The island’s population increased from about 

2 million in 1900 to about 7 million in 2000 (U.S. Bureau 

of Census, 2000). The greatest increase since the 1940’s 

has occurred in the central and eastern parts of the island as 

the population expanded eastward from Kings and Queens 

Counties into Nassau and Suffolk Counties. Today, population 

densities range from less than 10 persons per acre in most 

parts of eastern, central, and northwestern Suffolk County and 

in parts of northern Nassau County, to more than 180 persons 

per acre in parts of Queens and Kings Counties (fig. 7A). 

This wide range in population density is reflected by groundwater levels, which have been drawn down in some parts of 

the island (areas with high density), but not in others (areas 

with low density). The ground-water system in low density 

areas remains relatively unchanged from predevelopment 

conditions.

About 3.1 million people on Long Island rely on water 

pumped from about 1,000 public-supply wells (fig. 7B) that 

together withdraw about 400 Mgal/d (Nemickas and others, 

1989) from the ground-water system. Large-scale pumping in 

the past caused severe ground-water-level declines that have 

led to saltwater intrusion (Lusczynski, 1952; Perlmutter and 

Soren, 1962); this, in turn, required the curtailment of longterm pumping in most of Kings and Queens Counties. The 

Hydrogeology and Effects of Human Activity (Pumping and Sewers) 
Figure 4. Ground-water levels on Long Island, N.Y., March-April 1997: A. Water-table altitude in the upper glacial aquifer. 

B. Potentiometric-surface altitude in the Magothy aquifer. (Modified from Busciolano, and others, 1998, pls. 1 and 2.)

LOCATION 

MAP 

New York 

Map 

Area 

New

Jersey

New York 

Connecticut 

Long Island Sound 

Base from U.S. Geological Survey State base map, 1979 

0 5 10 MILES 

0 5 10 KILOMETERS 

Atlantic Ocean 

LOCATION 

MAP 

New York 

Map 

Area 

New

Jersey

New York 

Connecticut 

Long Island Sound 

0 5 10 MILES 

0 5 10 KILOMETERS 

Atlantic Ocean 

A. Upper Glacial Aquifer 

B. Magothy Aquifer 

NORTHERN LIMIT OF MAGOTHY AQUIFER 

70 

50 

30 

10 

50 

30

70 

50 

30 

10 

50 

30 

10 

10 

10 

WATER-TABLE CONTOUR -- Shows

 altitude of water-table surface. Contour

 interval 10 feet. Datum is NGVD 29.

50 

50 

POTENTIOMETRIC CONTOUR -- Shows

 altitude of potentiometric surface. Contour

 interval 10 feet. Datum is NGVD 29.

Peconic 

River 

Peconic 

River 

Nissequogue River 

Nissequogue River 

Connetquot 

 River 

Connetquot 

 River 

NORTH FORK 

SOUTH FORK 

SOUTH FORK 

NORTH FORK 

EXPLANATION

EXPLANATION

40°

45' 

41°

74° 73°45' 30' 15' 73° 72°45' 30' 15' 72° 

40°

45' 

41°

 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
water-table recovery that followed has caused local flooding of 

basements and subways that were installed during periods of 

water-table decline.

The installation of large sanitary and storm-sewer 

systems (fig. 7B), and the increase in impervious surfaces, 

such as roads and parking lots, have caused regional waterlevel declines; these declines, in turn, have caused a decrease 

in ground-water storage and a loss of freshwater habitat 

through the lowering of pond and lake levels and the reduction 

of stream length and discharge. A brief history of the largescale pumping and sewering activities on Long Island during 

the past century is given below.

Ground-Water Pumping

Nassau County, Suffolk County, and parts of eastern 

Queens County rely solely on water pumped from the 

underlying ground-water system. The upper glacial aquifer 

was the principal source of water supply throughout these 

areas for several decades, but its gradual contamination by 

surface sources and septic waste resulted in the widespread 

curtailment of its use for public supply. The upper glacial 

aquifer remains a major source of water supply in eastern 

parts of Suffolk County, but the Magothy aquifer is now the 

principal source of water supply in parts of eastern Queens 

County, most of Nassau County, and most of western and 

central Suffolk Counties. The deepest aquifer, the Lloyd 

aquifer, remains mostly undeveloped, except in parts of 

eastern Queens County and along the north and south shores 

of Nassau and western Suffolk Counties, where it is a major 

source of water.

In 1900, Long Island had a population of about 2 

million, which resided primarily in Kings and western and 

central Queens Counties. The first large-scale pumping from 

the ground-water system began in Kings County during the 

late 19th century; the water supply for most of the island 

before that was from local surface-water sources. Pumpage 

increased rapidly throughout the early 1900’s, and by 1916 the 

combined pumpage for Kings, Queens, and Nassau Counties 

was estimated to be about 225 Mgal/d (Thompson and 

Leggette, 1936). By 1917, the first water tunnel carrying water 

from upstate reservoirs to New York City was completed, 

and ground-water pumping in western Long Island decreased 

sharply.

By the mid-1930’s and early-1940’s, pumpage in western 

Long Island had again increased sharply, and by 1947, 

saltwater intrusion in Kings County necessitated the shutdown 

of all public-supply wells in the New York Water Service 

Corporation’s Flatbush system. The shutdown of the Flatbush 

system caused a water-level rise of 19 ft in some parts of 

central Kings County (Lusczynski, 1952).

From the late 1940’s through the 1960’s, most of Queens 

County and parts of Nassau County became increasingly 

urbanized, and pumpage in these areas continued to increase. 

By the early 1970’s, wells in western Queens County were 

pumping more than 60 Mgal/d (Chu and others, 1997), which 

led to new saltwater intrusion in southern Queens County; by 

1974, most public-supply wells in western Queens County 

(New York Water Service Corporation’s Woodhaven system) 

were shut down. This curtailment of pumping caused a waterlevel rise of more than 10 ft, which resulted in basement 

flooding and foundation damage in some parts of eastern 

Kings and western Queens Counties (Soren, 1976).

Figure 5. Locations of the selected precipitation-monitoring stations, streamflow-gaging (discharge) stations, and 

monitoring wells on Long Island, N.Y.

40°

45'

41°

74° 73°45' 30' 15' 73° 72°45' 30' 15' 72°

New York

Map

Area

EXPLANATION

LOCATION

MAP

NEW

JERSEY

NEW YORK

CONNECTICUT

Long Island Sound

Base from U.S. Geological Survey State base map, 1979

0 5 10 MILES

0 5 10 KILOMETERS

Atlantic Ocean

£

£

£ £

£

£

£

£

p

p

p

p

£

p

p

p

p p

.

K1194

Q1249

N1614 N1616

N1259 S1806

S1810

S1812 S3513 S5517

S4271

S3543

S8833

. .

N7493

N2528

S15622

S33380

S6455

 Central

 Park

 Setauket

 Valley

 Stream

 Massapequa

 Creek

 Mill Neck

 Creek

Nissequogue

River

Connetquot

River

Carmans River

Peconic

River

Precipitation station and identifier

Streamflow-gaging (discharge) station and identifier

Monitoring well and identifier

S16783

S48579

N6294

. . .

S29776 S29778

S24770

S24771

S22577

S22579

. Monitoring well pair and identifier

 Bensonhurst

 East

Meadow

 Brook

 LaGuardia

 Airport Mineola

 Upton

 Riverhead Bridgehampton

Upton

Peconic River

S3513

N6294

N2528

Hydrogeology and Effects of Human Activity (Pumping and Sewers) 
Figure 6. Relation between heads (water levels) in the upper glacial and Magothy aquifers at five well pairs on Long Island, 

N.Y., during selected 3-year periods. (Well pairs are listed in west-to-east order. Screened interval shown in parentheses.

Locations are shown in figure 5.)

E. Wells S5517 and S6455 35 

38 

41 

44

1950 1951 1952

TIME, IN YEARS 

S-5517 (85-91)

S-6455 (952-962)

A. Wells N6294 and N2528 58 

59 

60 

61 

62 

63 

1966 1967 1968

N-6294 (34-37)

N-2528 (278-282)

D. Wells S22579 and S22577 37 

39 

41 

43 

1994 1995 1996 

S-22579 (200-210)

S-22577 (724-734) 

B. Wells S29778 and S29776 77

79 

81 

83 

85 

87 

1978 1979 1980

S-29778 (158-168)

S-29776 (710-720)

C. Wells S24771 and S24770 47 

49 

51 

53 

55

57 

1994 1995 1996 

S-24771 (117-127)

S-24770 (424-434)

WATER LEVEL, IN FEET

IN REFERENCE TO NGVD 29

WATER LEVEL, IN FEET

IN REFERENCE TO NGVD 29

WATER LEVEL, IN FEET

IN REFERENCE TO NGVD 29

WATER LEVEL, IN FEET

IN REFERENCE TO NGVD 29

WATER LEVEL, IN FEET

IN REFERENCE TO NGVD 29

10 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
During the 1970’s and early 1980’s, pumpage continued 

to increase in eastern Queens County, Nassau County, and 

western Suffolk County as the population increased in these 

areas. By the mid-1980’s, more than 60 Mgal/d were being 

pumped from wells in eastern Queens County, and by the 

late-1980’s, new concerns about saltwater intrusion and 

ground-water contamination caused the Jamaica Water Supply 

Company to sharply decrease pumpage in central and eastern 

Queens County (table 2). In response, ground-water levels 

rose more than 20 ft in these areas and caused severe basement 

and subway flooding. Annual 1985-99 pumpage, by county 

and aquifer, is plotted in figure 8.

Ground-water pumpage in Nassau and Suffolk Counties 

continued to increase during the late-1980’s and 1990’s (table 

2, fig. 8), and saltwater intrusion developed locally. The 

current areas of major concern are in southwestern Nassau 

County, where water is pumped from the Lloyd aquifer 

(Terracciano, 1997), and in northern Nassau County, where a 

combination of increased pumpage and complex stratigraphy 

have led to saltwater intrusion (Stumm, 2001; Stumm and 

others, 2002; Stumm and others, 2004).

Sewer Systems

In the early 1900’s, most homes on Long Island disposed 

of wastewater through private septic-waste systems that 

returned the water directly to the water-table aquifer. The 

rapid urbanization and population growth that followed 

resulted in the gradual contamination of this aquifer by these 

systems until the water in some places was impotable. This 

contamination led to the installation of extensive sanitary 

sewer systems in the heavily populated parts of western Long 

Island. The first systems were installed in Kings and Queens 

Counties in the early and mid-1900’s; Nassau County Sewage 

Disposal District 2, in southwestern Nassau County, was 

completed in the early 1960’s (fig. 7B), and Nassau County 

Sewage Disposal District 3 (in southeastern Nassau County) 

and the Southwest Sewer District (in southwestern Suffolk 

County) were completed in the mid-1980’s (fig. 7B).

All of the sewer systems described above slowed the 

contamination of the upper glacial aquifer but resulted in 

major declines in regional ground-water levels by diverting 

the water to treatment plants and to the surrounding bays 

and ocean, instead of returning it to the aquifer. Water 

levels during the first 15 years after the completion of the 

Nassau County Sewage Disposal District 2 in the 1960’s 

declined markedly, and by the mid-1970’s, had reached a new 

equilibrium at about 9 ft below pre-sewer conditions (Franke, 

1968; Garber and Sulam, 1976; Sulam, 1979).

The lowering of ground-water levels in the upper glacial 

aquifer has decreased the base flow of nearby streams. Base 

flow of East Meadow Brook, in south-central Nassau County, 

decreased by about 45 percent during 1965-74 (Pluhowski 

and Spinello, 1978); 75 percent of this decrease was attributed 

to the effects of sanitary sewers and the remainder to storm 

sewers, which route storm runoff to streams, where it flows to 

tidewater.

1 

Table 1. Well pairs used in analysis of the hydraulic connection between the upper glacial and underlying Magothy aquifer,

Long Island, N.Y. 

[Locations are shown in figure 5.] 

Well pair and county Wells and aquifer

Screened

depth

(feet below

land surface)

Variation in 

head (water-level) 

difference 

(feet) Remarks

A. Northeastern Nassau Shallow well N6292 Upper glacial 34-37 0.25 Deep-well head 

Deep well N2528 Magothy 278-282 generally higher

B. Western Suffolk Shallow well S29778 Magothy (water table) 158-168 0.25 - 0.5 Shallow-well 

Deep well S29776 Magothy 710-720 head generally

C. West-central Suffolk Shallow well S24771 Upper glacial 117-127 Shallow-well 

head generally 

higher 

Deep well S24770 Magothy 424-434

D. Central Suffolk Shallow well S22579 Upper glacial 200-210 Shallow-well 

head generally 

higher 

Deep well S22577 Magothy 724-734

E. East-central Suffolk Shallow well S5517 Upper glacial 85-91 Shallow-well 

head generally 

higher 

Deep well S6455 Magothy 952-962

higher

3.5 - 4.0

0.25 - 0.5

3 - 4

Hydrogeology and Effects of Human Activity (Pumping and Sewers) 11
Figure 7. Distribution of A. population density, B. upper glacial and Magothy aquifer public-supply wells and large-scale sewer 

districts, C. inferred human-impact on aquifer zones and selected drought-monitoring-index stations on Long Island, N.Y.

Kings 

County 

Queens 

County 

Nassau 

County 

Suffolk 

County 

A. Population Density 

C. Human-Impact Zones and Locations of Drought-Monitoring-Index Stations

0 5 1 0 2 0 M i l e s 

EXPLANATION 

Population density, in persons per acre 

(U.S. Census Data for 2000)

0.00 - 9.99

10.00 - 19.99 

20.00 - 59.99 

60.00 - 179.99

180.00 - 1100.00 

B. Public-Supply Wells and Large-Scale Sewer Districts 

New York 

New Jersey 

Connecticut 

Long Island Sound 

Atlantic Ocean 

74° 73° 72° 

New York 

New Jersey 

Connecticut 

Long Island Sound 

Atlantic Ocean 

EXPLANATION 

 Nassau County Sewer District 2 

 Nassau County Sewer District 3 

 Suffolk County Southwest Sewer District

 Approximate position of the water-table divide 

PUBLIC-SUPPLY WELLS 

 Upper glacial aquifer 

 Magothy aquifer 

EXPLANATION 

HUMAN-IMPACT ON AQUIFER 

Significant 

Moderate 

Low 

Unspecified 

Drought-monitoring-index well 

Drought-monitoring-index stream

New York 

New Jersey 

Connecticut 

Long Island Sound 

Atlantic Ocean 

40°

45' 

41° 

40°

45' 

41° 

40°

45' 

41° 

12  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
A study of the expected effects of the Nassau County 

Sewage Disposal District 3 and the Suffolk County Southwest 

Sewer District (Kimmel and others, 1977) indicated that, when 

equilibrium is reached, water-table declines could be as much 

as 18 ft in Nassau County and 8 ft in western Suffolk County 

(Reilly and others, 1983; Buxton and Reilly, 1985; Reilly 

and Buxton, 1985). Potentiometric-surface declines in the 

Magothy aquifer could be comparable, and stream base flow 

in those areas could decrease by 70 to 90 percent. Much of 

this predicted decline has already occurred in parts of Nassau 

County and has begun to occur in western Suffolk County.

Delineation of Human Effect on GroundWater Levels

Three zones representing the range of human effect on 

ground-water levels, “human-impact zones”, were delineated 

to help evaluate islandwide hydrologic conditions and to 

quantify the drought-monitoring indices described further 

on. Four data sets were the basis for zone delineation: 

(1) population density, (2) locations of large sanitary- and 

storm-sewer systems, (3) locations of public-supply wells 

screened in the upper glacial and Magothy aquifers, and (4) 

long-term records of selected monitoring wells and streamdischarge stations.

The first zone (significant-human impact) encompasses 

Kings and Queens Counties and parts of western Nassau 

County (fig. 7C). It represents population densities greater 

than 20 persons per acre (fig. 7A). The construction of 

impervious surfaces, such as roads, parking lots, and 

buildings, and the installation of large sewer systems before 

the 1950’s, generally decreased recharge to the ground-water 

system. Historically, this area has relied heavily on ground 

water, but overpumping in the past has caused severe saltwater 

intrusion in Kings and Queens Counties that has necessitated 

the cessation of ground-water pumpage for most public use. 

In western Nassau County, where ground water is still the sole 

source of potable water, pumping caps, conservation, recharge 

basins, and aquifer monitoring have helped to avert severe 

saltwater intrusion problems.

Ground-water and surface-water records from wells and 

streams in this western zone indicate that overpumping and 

lack of aquifer protection has severely affected the groundwater system. Large increases in pumpage, and the reductions 

in aquifer recharge caused by sewer systems and impervious 

surfaces, have obscured all natural water-level fluctuations 

in the hydrologic record. Therefore, this western area is 

designated as a zone of significant-human impact (fig. 7C).

The second zone (moderate-human impact) encompasses 

most of Nassau County and parts of western Suffolk County. 

The zone generally represents population densities of 10 

to 20 persons per acre (fig. 7A), but includes some parts of 

T able 2. Public-supply ground-water withdrawals on Long Island, N.Y., by aquifer and county, for 1985-89, 1990-94, and 1995-99.

[Data from the New York State Department of Environmental Conservation.] 

Aquifer Years 

Ground-water withdrawals 

Kings County

Billions 

of 

gallons 

Per cent 

of total 

Billions 

of 

gallons 

Per cent 

of total 

Billions 

of 

gallons 

Per cent 

of total 

Billions 

of 

gallons 

Per cent 

of total 

Billions 

of 

gallons 

Per cent

of total 

Upper glacial 1985-89 0 0 12.6 10.0 12.7 10.1 100.7 79.9 126.0 19.0

1990-94 0 0 12.2 9.4 11.8 9.1 105.4 81.5 129.4 19.4

1995-99 0 0 9.4 6.8 10.6 7.7 117.5 85.5 137.5 19.1

Magothy 1985-89 0 0 38.7 7.5 296.4 57.6 179.3 34.9 514.4 77.3

1990-94 0 0 28.6 5.5 291.2 56.2 198.6 38.3 518.4 77.6

1995-99 0 0 26.7 4.8 303.2 54.2 229.7 41.0 559.6 77.9

Lloyd 1985-89 0 0 2.3 9.3 22.5 90.7 0 0 24.8 3.7

1990-94 0 0 0.2 1.0 19.6 99.0 0 0 19.8 3.0

1995-99 0 0 0 0 21.0 98.6 0.3 1.4 21.3 3.0

Totals 1985-89 0 0 53.6 8.1 331.6 49.8 280.0 42.1 665.2 100

1990-94 0 0 41.0 6.1 322.6 48.3 304.0 45.6 667.6 100

1995-99 0 0 36.1 5.0 334.8 46.6 347.5 48.4 718.4 100

Queens County Nassau County Suffolk County Total

Hydrogeology and Effects of Human Activity (Pumping and Sewers) 13
northern Nassau and northwestern Suffolk Counties that are 

less densely populated. Impervious surfaces are less extensive 

in this zone than in the western zone, but the installation of 

large sewer systems (fig. 7B) since the 1950’s has severely 

decreased aquifer recharge. The construction of numerous 

recharge basins in parts of this zone has helped to offset the 

loss of recharge through the sewer systems and has minimized 

water-level declines north of the ground-water divide. This 

central area relies solely on ground water for public-water 

supply.

Ground-water and surface-water records from wells 

and streams in the central area indicate that large increases 

in pumpage and a decrease in recharge have greatly affected 

ground-water levels. Water levels have declined sharply 

(greater than 15 ft) in some areas during the past 50 years; 

these declines began in the western areas during the 1950’s, 

and more recently have been detected in central and eastern 

areas as the population and urbanization expanded eastward. 

These declines have altered the ground-water system such 

that human-induced changes are now a primary cause of 

A. Upper glacial (water-table) aquifer

0

5,000

10,000

15,000

20,000

25,000

30,000

Queens County Nassau County Suffolk County

B. Jameco aquifer

0

250

500

750

1,000

1,250

1,500

1,750

2,000

C. Magothy aquifer

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

D. Lloyd aquifer

0

1,000

2,000

3,000

4,000

5,000

6,000

WATER YEAR

PUMPAGE, IN MILLIONS

OF GALLONS

PUMPAGE, IN MILLIONS

OF GALLONS

PUMPAGE, IN MILLIONS

OF GALLONS

PUMPAGE, IN MILLIONS

OF GALLONS

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Figure 8. Annual ground-water pumpage from Long Island’s major aquifers, by county, 1985-99: A. Upper glacial (water-table) 

aquifer. B. Jameco aquifer. C. Magothy aquifer. D. Lloyd aquifer.

14  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
fluctuations in the stream-discharge and water-level records. 

Natural fluctuations remain visible, but are partly masked by 

the human-induced changes. Therefore, this central area is 

designated as a zone of moderate-human impact (fig. 7C).

The third zone (low-human impact) encompasses eastern 

and central parts of Suffolk County. It generally represents 

population densities of less than 10 persons per acre (fig. 7A). 

Impervious surfaces are less extensive than in the western and 

central zones, and no large sewer systems have been installed. 

This area also relies solely on ground water for public-water 

supply.

Ground-water and surface-water records from wells and 

streams in this eastern area indicate that most parts of the 

ground-water system remain relatively unaffected by human 

activity, and natural fluctuations are still the primary cause of 

changes in the hydrologic record. Therefore, this eastern area 

is designated as a zone of low-human impact (fig. 7C).

Statistical Analysis of Long-Term 

Hydrologic Records

Various types of hydrologic data have been collected on 

Long Island for more than 100 years. Long-term precipitation 

data collection by the NWS began in the mid- to late 1800’s, 

and ground-water levels were first measured by the USGS in 

the early 1900’s. Long-term surface-water-data collection by 

the USGS in most parts of the island began in the 1930’s and 

1940’s.

Precipitation

Precipitation is the sole source of all fresh ground 

water on Long Island. Seasonal or long-term fluctuations in 

precipitation, and the associated recharge rate, are reflected 

by changes in water levels in the ground-water system. Mean 

annual precipitation on Long Island ranges from about 40 in/yr 

across southernmost Nassau County and on the eastern tip 

of the North Fork to about 50 in/yr across central and westcentral Suffolk County (fig. 9A). Mean summer precipitation 

generally is equal to mean winter precipitation throughout 

the island; 5 to 10 percent of the water equivalent of winter 

precipitation is in the form of snow (Miller and Frederick, 

1969).

Precipitation on Long Island can be divided into a 

growing-season (warm-season) component (fig. 9B) and a 

dormant-season (cool-season) component (fig. 9C). The type 

and intensity of precipitation, and the amounts that reaches the 

water table as recharge, differ seasonally. The warm season 

(April through September) is characterized by convective 

storms that generally form in advance of an eastward moving 

cold front or during periods of local atmospheric instability. 

Occasionally, tropical cyclones will move up from southern 

coastal areas and produce large quantities of rain. Both types 

of storms are typically characterized by relatively short periods 

(hours) of intense precipitation that produce large amounts 

of surface runoff and little recharge. Most precipitation that 

enters the unsaturated zone is quickly absorbed by vegetation 

and lost through evapotranspiration; therefore, little or no 

recharge reaches the water table during summer.

The cool season (October through March) is 

characterized by large, low-pressure systems that originate in 

the southern and central United States and move northeastward 

along the Atlantic coast or the western side of the Appalachian 

Mountains. Storms of this type are characterized by long 

periods (days) of steady precipitation in the form of rain, snow, 

and ice, and tend to produce more recharge than the summer 

storms because they have a longer duration and occasionally 

result in snowmelt. In addition, precipitation that enters the 

unsaturated zone is not taken up by the dormant vegetation, 

and, therefore, infiltrates to the water table as recharge; thus, 

most aquifer recharge occurs during the cool season.

Of the 13 active precipitation stations on or near Long 

Island, only 8 have continuous periods of record that go 

back to 1951 and were selected as possible indicators of 

islandwide precipitation. These stations are: in Central Park 

in central New York County, in Bensonhurst in southwestern 

Kings County, at LaGuardia Airport in northern Queens 

County, in Mineola in central Nassau County, in Setauket 

in north-central Suffolk County, in Upton in central Suffolk 

County, in Riverhead in northeastern Suffolk County, and 

in Bridgehampton in southeastern Suffolk County. Station 

locations are shown in figure 5.

Annual and monthly precipitation data were compiled 

for each of the eight stations. The period of record at each site 

was trimmed to include only the last 50 years of record (1951-

2000); this period was selected to include the 1960’s drought 

as well as the 10 years preceding it, for comparative purposes; 

the station at Bensonhurst is missing 2 years (1952-53) of 

data. The 50-year hydrographs and boxplots for each of the 

eight stations (fig. 10) include the annual and monthly 10th, 

25th, 50th (median), 75th, and 90th exceedance percentiles, 

and the ARIMA trend line.

The long-term precipitation records from each of the 

eight selected stations are summarized in table 3 and figure 10. 

The minimum, mean, and maximum annual precipitation for 

1951-2000, and the values for selected exceedance percentiles 

are given in table 3A, and the minimum and maximum 

monthly precipitation and mean precipitation for the driest and 

wettest months on record are given in table 3B. Notable shortand long-term trends in precipitation are given in table 3C. 

The stations are presented in west-to-east geographical order.

In summary, the records for all eight stations generally 

indicate similar long-term trends, although most of the Long 

Island stations lack the large extremes seen at Central Park, 

probably because they are subject to the moderating effects 

of Long Island’s coastal climate. The deficit of the 1960’s 

was the major feature in the record for most stations, but 

stations on eastern parts of Long Island show a much smaller 

cumulative deficit for this period than in western parts.

Statistical Analysis of Long-Term Hydrologic Records 15
Base from U.S. Geological Survey State base map, 1979

40°

45'

41°

74° 73°45' 30' 15' 73° 72°45' 30' 15' 72°

New

Jersey

Connecticut

Long Island Sound

Kings

Atlantic Ocean

County

Queens

County

Nassau

County

New York 

Suffolk

County

A. MEAN ANNUAL PRECIPITATION

50

Line of equal average precipitation, 

 1951-65. Interval 1 and 2 inches 

 with supplementary 0.5-inch 

 lines (dashed)

44

44

42

42

42

42

42

42

40

40 40

44 44

44

46

46

46

48 50

44

44

44

New

Jersey

Connecticut

Long Island Sound

Kings

Atlantic Ocean

County

Queens

County

Nassau

County

New York 

Suffolk

County

C. MEAN COOL-SEASON PRECIPITATION

21

21

22

23

23

23

23

22

21

24.5

24.5

24.5 24

24 24

25 26

27

24

26

25

25

0 5 10 MILES

0 5 10 KILOMETERS

New

Jersey

Connecticut

Long Island Sound

Kings

Atlantic Ocean

County

Queens

County

Nassau

County

New York 

Suffolk

County

B. MEAN WARM-SEASON PRECIPITATION

19

20

20

20

21

21 21.5

21

21 21

21

22

23

22

23

Line of equal average precipitation, 

 1951-65. Interval 1 and 2 inches 

 with supplementary 0.5-inch 

 lines (dashed)

27

Line of equal average precipitation, 

 1951-65. Interval 1 and 2 inches 

 with supplementary 0.5-inch 

 lines (dashed)

40°

45'

41°

40°

45'

41°

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

EXPLANATION

EXPLANATION

Figure 9. Mean precipitation on Long Island, N.Y., 1951-65: A. Mean annual. B. Mean for warm season (April 

through September). C. Mean for cool season (October through March). (Modified from Miller and Frederick, 

1969, pl. 1.)

16  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
50 50 50 50 50 50 50 50 50 50 50 50

A. Central Park

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

B. Bensonhurst

0

2

4

6

8

10

12

0

2

4

6

8

10

12

C. LaGuardia Airport

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

D. Mineola

0

2

4

6

8

10

12

14

16

1950

WATER YEAR

Precipitation

90 Percent Exceedance Percentile

75 Percent Exceedance Percentile

50 Percent Exceedance Percentile (median)

25 Percent Exceedance Percentile

10 Percent Exceedance Percentile

12-Month ARIMA Trend

EXPLANATION

0

2

4

6

8

10

12

14

16

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S

50 50 50 50 50 50 50 50 50 50 50 50

48 48 48 48 48 48 48 48 48 48 48 48

50 50 50 50 50 50 50 50 50 50 50 50

Figure 10. Monthly precipitation and exceedance statistics for eight selected stations on or near Long Island, N.Y., 

1951-2000. (Station locations are shown in figure 5.) 

Statistical Analysis of Long-Term Hydrologic Records 17
50 50 50 50 50 50 50 50 50 50 50 50

50 50 50 50 50 50 50 50 50 50 50 50

E. Setauket 50 50 50 50 50 50 50 50 50 50 50 50

F. Upton

G. Riverhead

H. Bridgehampton

50 50 50 50 50 50 50 50 50 50 50 50

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

1950

WATER YEAR

Precipitation

90 Percent Exceedance Percentile 75 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

25 Percent Exceedance Percentile 10 Percent Exceedance Percentile 12-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

18  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York

Figure 10. (continued) Monthly precipitation and exceedance statistics for eight selected stations on or near Long Island, N.Y., 

1951-2000. (Station locations are shown in figure 5.)
Mean annual precipitation ranges from 43.14 in. 

(LaGuardia Airport) to 48.85 in. (Upton); the average for 

all eight stations was 45.52 in. This value is close to the 

combined Long Island long-term precipitation average of 44 

in/yr calculated by Cohen and others (1968). Mean monthly 

precipitation values for all stations are fairly uniform, although 

the wettest and driest months are variable throughout the 

island. Data from all eight stations indicate conditions that 

are representative of Long Island and are to be included in the 

composite-average precipitation index (described later in the 

report).

Stream Discharge

Most streams on Long Island are broad and shallow and 

have gentle gradients that average about 10 ft/mi (Cohen and 

Table 3A. Annual minimum, mean, and maximum precipitation volume, and volumes for selected exceedance percentiles, at the eight 

 selected precipitation-monitoring stations on or near Long Island, N.Y., 1951-2000. 

[Amounts are in inches. %, percent. Locations are shown in figure 5.] 

† Percentage of time during which the given amount is exceeded.

* Proximity to Long Island makes this station representative of conditions on western Long Island. 

Table 3B. Monthly minimum and maximum precipitation volume, and means for driest and wettest months, at the eight selected 

precipitation-monitoring stations on or near Long Island, N.Y., 1951-2000. 

[Amounts are in inches. Locations are shown in figure 5.] 

Station name 

Annual 

mean 

Y ears 

of 

record 

Annual minimum Annual maximum Exceedance percentile†

50%

(median)

Central Park * 45.98 50 29.19 1965 71.15 1984 59.69 52.21 44.98 38.20 33.92 

Bensonhurst 44.55 48 30.48 1965 66.01 1984 55.18 50.43 43.57 37.87 34.74 

La Guardia Airport 43.14 50 25.55 1965 60.78 1984 56.79 47.60 44.33 36.85 31.92 

Mineola 44.67 50 25.82 1965 70.33 1984 56.31 50.04 42.95 38.22 35.53 

Setauket 44.93 50 30.19 1995 63.00 1984 53.72 48.71 46.14 38.66 36.27 

Upton 48.85 50 35.59 1995 68.79 1989 59.99 55.11 47.71 43.11 37.59 

Riverhead 46.01 50 30.99 1966 65.16 1984 58.11 50.24 46.50 40.11 35.87

Bridgehampton 47.05 50 30.76 1966 64.04 1958 60.36 52.33 46.84 39.86 36.42 

Station name 

Monthly minimum Monthly maximum 

Driest monthly 

mean 

W ettest monthly 

mean 

Volume

Central Park 0.14 Oct. 1963 14.01 Apr. 1983 3.12 Feb. 4.22 Mar. 

Bensonhurst .20 Dec. 1955 11.12 Aug. 1971 3.09 Feb. 4.52 July 

La Guardia Airport .06 Oct. 1963 16.05 Aug. 1955 2.94 Feb. 4.21 Aug. 

Mineola .11 Oct. 1963 15.60 Aug. 1955 3.18 Feb. 4.31 Mar. 

Setauket .26 Oct. 1963 12.56 June 1972 3.15 Feb. 4.13 Mar.

Upton .18 Oct. 1963 13.01 Jan. 1979 3.30 July 4.83 Mar. 

Riverhead .19 June 1994 12.42 Aug. 1954 3.25 July 4.30 Nov. 

Bridgehampton .11 Oct. 1963 14.58 June 1982 3.00 July 4.63 Mar.

Volume Year Volume Year 10% 25% 75% 90%

Month Year Volume Month Year Volume Month Volume Month

Statistical Analysis of Long-Term Hydrologic Records 19
1 

Table 3C. Notable short- and long-term trends in precipitation at the eight selected precipitation-monitoring stations on or near 

Long Island, N.Y., 1951-2000. 

[Values are in inches. no., number; --, not applicable. Locations are shown in figure 5.] 

Station 

Period of record

Long or

short term Pe riod No. of years

1951-2000 

mean 

Above or 

below 

Surplus 

(inches) 

Deficit

(inches)

Central Park Short term 1953-59 7 45.98 below -- 54.60 

1961-66 6 below -- 68.53 

1971-75 5 above 58.81 -- 

1980-82 3 below -- 11.16 

1989-91 3 above 38.51 -- 

1996-98 3 above 21.23 -- 

Long term 1951-70 20 below -- 128.09 

Bensonhurst Short term 1962-66 5 44.55 below -- 41.10 

1971-75 5 above 38.77 -- 

1980-82 3 below -- 19.52 

1989-91 3 above 20.80 -- 

1996-98 3 above 20.20 -- 

Long term 1962-70 9 below -- 55.30 

La Guardia Airport Short term 1951-53 3 43.14 above 25.63 -- 

1962-68 7 below -- 70.09 

1971-75 5 above 34.46 -- 

1980-82 3 below -- 19.86 

1989-91 3 above 20.78 -- 

1996-98 3 above 18.17 -- 

Long term 1962-70 9 below -- 79.76 

Mineola Short term 1982-84 3 44.67 above 31.73 -- 

1989-91 3 above 33.27 -- 

1992-95 4 below -- 23.71 

1996-98 3 above 14.80 -- 

Long term 1962-72 10 below -- 85.53 

Setauket Short term 1958-60 3 44.93 above 17.22 -- 

1961-66 6 below -- 44.48 

1972-76 5 above 22.85 -- 

1982-84 3 above 25.45 -- 

1989-91 3 above 19.50 -- 

Long term 1961-68 8 below -- 50.88 

1972-79 8 above 41.57 -- 

Upton Short term 1962-66 5 48.85 below -- 32.19 

1982-84 3 above 27.49 -- 

1985-88 4 below -- 38.16 

1989-91 3 above 26.98 -- 

1992-95 4 below -- 30.61 

Long term 1972-79 8 above 25.10 -- 

Riverhead Short term 1958-61 4 46.01 above 20.90 -- 

1962-66 5 below -- 37.96 

1982-84 3 above 28.37 -- 

1989-91 3 above 25.30 -- 

1996-98 3 above 19.42 -- 

Long term 1962-71 10 below -- 60.19 

1972-79 8 above 29.98 -- 

Bridgehampton Short term 1952-55 3 47.05 above 26.65 -- 

1958-61 4 above 25.74 -- 

1982-84 3 above 20.42 -- 

1989-91 3 above 24.65 -- 

1996-98 3 above 38.63 -- 

Long term 1952-61 10 above 35.37 -- 

1962-71 10 below -- 67.43 

Period of record

Relation to mean Cumulative value

20 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
others, 1968). The streambeds consist mainly of sand and 

gravel and are highly permeable; thus, Long Island’s streams 

are in direct hydraulic contact with the upper glacial aquifer. 

Under natural (undeveloped) conditions, 95 percent of the total 

stream discharge originates as ground-water seepage (Franke 

and McClymonds, 1972); the remaining 5 percent consists of 

direct runoff. Accordingly, the flows of Long Island’s streams 

closely reflect changes in the water-table altitude.

Discharge data have been collected continuously since 

the early 1930’s at most of Nassau County’s major streams, 

and since the early 1940’s at most of Suffolk County’s major 

streams. Annual and monthly mean discharge data were 

compiled for the eight streams (fig. 5) that were selected 

as indicators of ground-water conditions in the underlying 

ground-water system. Totals for water year 2000 were not 

computed for most streams in Nassau County because data 

collection ceased in March or April of that year. The period 

of record at each station was trimmed to include only the last 

50 years of record (1951-2000); this period was selected to 

include the 1960’s drought as well as the 10 years preceding 

it, for comparative purposes. The 50-year hydrographs and 

boxplots for each of the eight streams (fig. 11) include the 

annual and monthly 10th, 25th, 50th (median), 75th, and 90th 

exceedance percentiles, and the ARIMA trend line.

The long-term discharge records from each of the 

eight selected streamflow stations are summarized in table 

4 and figure 11. The minimum, mean, and maximum 

annual discharge for 1951-2000, and the values for selected 

exceedance percentiles are given in table 4A, and the 

minimum and maximum monthly discharge, and mean 

discharge for the lowest and highest months on record are 

given in table 4B. Notable trends in discharge, and the stations 

relation to human impact are given in table 4C. The stations 

are presented in west-to-east geographical order.

In summary, discharge records for all eight streams 

indicate the effects of the 1960’s drought; the monthly mean 

discharges were well below average from the early 1960’s 

through the early 1970’s (fig. 11). Precipitation returned to 

near normal by the late 1960’s, but stream discharge did not 

return to normal until the early 1970’s because ground-water 

levels were well below normal and did not recover until the 

wet period in the early 1970’s.

Stream discharge in western parts of Long Island, 

particularly Valley Stream, East Meadow Brook, and 

Massapequa Creek, has never returned to pre-drought levels 

as a result of regional water-level declines from sanitary- and 

storm-sewer systems in Nassau and western Suffolk Counties. 

Stream discharge in central and eastern parts of Long Island 

returned to near or slightly above normal conditions after the 

drought.

Table 4A. Annual minimum, mean, and maximum discharge, and selected exceedance percentiles at the eight selected streamflow-

 gaging (discharge) stations on Long Island, N.Y., 1951-2000 or period of record. 

[Discharge values are in cubic feet per second (ft 3 Locations are shown in figure 5.] 2 mi , square miles; %, percent. 

* Percentage of time during which the given amount is exceeded. 

Station name 

Station 

n umber 

Drainage 

area 

(mi 2 ) 

Annual 

mean 

Y ears 

of 

record 

Lowest 

annual mean 

Highest 

annual mean Exceedance percentile* 

V alue Year Value Year 10% 25% 

50% 

(median) 75% 90% 

V alley Stream at 

V alley Stream

01311500 4.5 1.96 46 0.11 1986 8.86 1956 5.35 1.82 1.22 0.58 0.39 

East Meadow 

Brook at Freeport

01310500 31 11.6 49 2.08 1995 23.3 1961 20.7 15.5 10.4 5.97 3.67 

Mill Neck Creek 

at Mill Neck

01303000 11.5 8.81 49 5.59 1966 12.1 1984 10.7 10.1 8.49 7.48 6.79 

Massapequa Creek 

at Massapequa

01309500 38 9.62 49 2.27 1995 19.4 1973 16.8 13.2 8.71 5.91 3.78 

Connetquot River 

near Oakdale 

01306500 24 38.6 50 24.9 1966 52.5 1984 48.5 42.2 38.2 34.2 30.0 

Nissequogue River 

near Smithtown

01304000 27 43.3 50 27.0 1966 58.9 1991 52.3 47.0 43.9 38.0 34.8 

Carmans River 

Y aphank

01305000 71 24.4 50 12.9 1967 37.7 1979 32.5 28.6 24.3 20.2 17.0 

Peconic River 

at Riverhead 

01304500 75 38.3 50 16.1 1966 67.9 1984 55.5 43.5 36.1 29.2 23.1 

/s). 

Statistical Analysis of Long-Term Hydrologic Records  21
Ground-Water Levels

Ground-water levels on most of Long Island are generally 

highest in March, April, or May and lowest in September, 

October, or November. This pattern can be attributed largely to 

seasonal variations in natural recharge and partly to seasonal 

pumping patterns. This pattern is no longer evident in western 

parts of Long Island, however, where large-scale groundwater pumping and the loss of recharge (through sanitary- and 

storm-sewer systems) have caused large declines in groundwater levels.

Ground-water-level data have been collected continuously 

since the early 1900’s at many wells on Long Island. Waterlevel data from the 15 upper glacial aquifer wells and the 

5 Magothy aquifer wells that were selected as droughtmonitoring sites (fig. 5) were compiled. All of the selected 

water-table wells are screened in the upper glacial aquifer 

except well N1616, which is screened in the upper part of the 

Magothy aquifer. Magothy wells are screened in the middle or 

bottom part of the aquifer. Like the stream-discharge records, 

the period of record for each site was trimmed to include only 

the last 50 years of record (1951-2000), to include the 1960’s 

drought as well as the 10-year period preceding the drought, 

for comparative purposes. For wells with less than 50 years 

of record, the entire period of record was used in the analysis. 

The 50-year hydrographs and boxplots for each of the 20 

wells (fig. 12) include the annual and monthly 10th, 25th, 

50th (median), 75th, and 90th exceedance percentiles, and the 

ARIMA trend line.

The long-term water-level records from each of the 

20 selected wells are summarized in table 5 and figure 12. 

The mean water levels for 1951-2000, values for selected 

exceedance percentiles, and the screen zone depths are given 

in table 5A, and the minimum and maximum water levels, 

and mean water levels for the lowest and highest months on 

record are given in table 5B. Notable trends in water level, and 

the wells relation to human impact are given in table 5C. The 

wells are presented in west-to-east geographical order.

In summary, water levels at most wells, excluding those 

in the westernmost part of Long Island, indicate the effects 

of the 1960’s drought and were well below average from the 

early 1960’s through the early 1970’s (fig. 12). Precipitation 

had returned to near normal by the late 1960’s, but water levels 

remained below average until the early 1970’s, when a period 

of above-normal precipitation occurred.

Water levels on western Long Island show the effect of 

extensive urbanization; those farther east reflect the eastward 

decrease in human activity. Water levels at wells K1194, 

Q1249, and N7493, in western Long Island (fig. 5), generally 

do not show the declines associated with the 1960’s drought 

because they were already well below normal as a result of 

heavy pumping and sewering. Water levels in other western 

Table 4B. Monthly minimum and maximum mean discharge, and means for lowest and highest months, at the eight selected 

streamflow-gaging (discharge) stations on Long Island, N.Y., 1951-2000 or period of record. 

[Discharge values are in cubic feet per second (ft 3 /s). Locations are shown in figure 5.] 

* Result of regulation. 

 Result of temporary construction upstream. 

Station 

n umber 

Monthly minimum Monthly maximum 

Lowest 

monthly mean 

Highest 

monthly mean 

Station name Value Month Year Value Month Year Value Month Value Month 

V alley Stream at 

V alley Stream

01311500 0.00 multiple times 16.8 Aug. 1955 1.50 Sep. 2.84 Apr. 

East Meadow 

Brook at Freeport

01310500 0.03 Aug. 1995 39.7 Aug. 1955 8.71 Sep. 16.1 Apr. 

Mill Neck Creek at 

Mill Neck 

01303000 4.10 July 1966 17.9 July 1984 8.14 Sep. 9.76 Mar.

Massapequa Creek 

at Massapequa

01309500 0.59 Aug. 1995 33.4 Apr. 1953 6.68 Sep. 14.0 Apr. 

Connetquot River 

near Oakdale 

01306500 17.3* Nov. 1982 70.4 June 1998 33.0 Sep. 45.1 Apr. 

Nissequogue River 

near Smithtown 

01304000 22.1 Aug. 1966 76.1 Oct. 1990 38.8 Sep. 49.4 Apr. 

Carmans River at 

Y aphank

01305000 9.35 Jan.

Peconic River at 

Riverhead 

01304500 10.8 Aug. 1966 109 Mar. 1979 26.9 Sep. 53.2 Apr. 

†

†

† 1967 49.2 June 1984 22.1 Oct. 27.6 Apr.

22  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
Table 4C. Notable trends in discharge at the eight selected streamflow-gaging (discharge) stations on Long Island, N.Y., 1 9 5 1 -2 0 0 0 or period of recor

[Discharge values are in cubic feet per second (ft3/s). mi2, square miles; %, percent; ARIMA, autoregressive integrated moving average. 

Significant period of

below normal annual

discharge in relation

to annual mean

AR IMA trend at or below

monthly exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

e e xceedanc

90%

e e xceedanc

Valley Stream at Valley Stream (0 1 3 11 5 0 0 ). Drainage area 4 .5 mi2 . Significant-human-impact zone. Not useful for drought monitoring.Long-term (46-year) annual mean discharge (1955-2000) 1.96 ft3/s; minimum annual mean discharge 0.11 ft3/s, 1986; maximum annual mean discharge 8.87 ft3/s, 1956.

1963-721980-831985-88

-1.82-1.85-1.85

196519811986

Decreased from well above normal to below normal,

1955-65; increased to normal, 1966-68; remained nearnormal, 1969-80; decreased to below normal, 1980-81;increased to normal, 1982-84; decreased to belownormal, 1985-86; increased to normal, 1987-90;remained near normal, 1991-95; increased to abovenormal, 1996-97; decreased to normal, 1998-2000.

1964-67, 1971,

1980-83, and1986-88.

1965, and 1987-

88.

• Data missing from 1951 to 1955.• Decline from 1955 to 1960 attributed to increased pu

Queens County and western Nassau County; sharp deto effects of 1960’s drought; slight rise since mid-198pumping in eastern Queens County.

• Discharge never returned to pre-drought levels; attrib

pumping and sewering.

• Long-term averages and exceedance values strongly 

urbanization mask fluctuations associated with precipmonitoring site; useful for monitoring human-induce

E as t Meadow Brook at F reeport (0 1 3 1 0 5 0 0 ). Drainage area 3 1 mi2 . Moderate-human-impact zone. Not useful for drought monitoring.Long-term (49-year) annual mean discharge (1951-1999) 11.6 ft3/s; minimum annual mean discharge 2.08 ft3/s, 1995; maximum annual mean discharge 23.3 ft3/s, 1961.

1964-681970-721981-831985-891991-99

-9.08-5.70-5.88-8.30-9.53

19661971198119881995

Increased from normal to above normal, 1951-52;

remained above normal, 1953-61; decreased to wellbelow normal, 1962-64; increased to above normal,1965-78; decreased to normal, 1979-82; remained innormal range, 1983-84; decreased to below normal,1985-87; increased to normal, 1988-90; decreased towell below normal, 1991-95; increased to normal,1996-97; decreased to below normal, 1998-1999.

1965-67, 1971,

1982, 1986-88,1992-99.

1966, 1987-88,

and 1994-96.

• Data missing for 2000.• Sharp decline from 1961 to 1964 attributed to effects 

since late-1970’s attributed to increased pumping and 

• Long-term averages and exceedance values strongly 

primary source of variability in record; not useful as for monitoring human-induced changes.

Mill Neck C reek at Mill Neck (0 1 3 0 3 0 0 0 ). Drainage area 11 .5 mi2 . Moderate-human-impact zone. Not useful for drought monitoring.Long-term (49-year) annual mean discharge (1951-1999) 8.81 ft3/s; minimum annual mean discharge 5.59 ft3/s, 1966; maximum annual mean discharge 12.1 ft3/s, 1984.

1963-721981-831986-891992-97

-3.21-1.72-1.72-2.01

1966198219881995

Increased from normal to above normal, 1951-52;

remained above normal, 1953-61; decreased to wellbelow normal, 1962-66; increased to well abovenormal, 1967-78; decreased to below normal, 1979-81;increased to above normal, 1982-84; decreased tobelow normal, 1985-88; increased to normal, 1989-90;decreased to below normal, 1991-95; increased tonormal, 1996-99.

1964-72, 1982,

1987-88, 1993-97.

1965-71, and

1982.

• Data missing for 2000.• Sharp decline from 1961 to 1966 attributed to effects 

since late-1970’s attributed to increased pumping and 

• Long-term averages and exceedance values strongly 

primary source of variability in record; not useful as for monitoring human-induced changes.

Locations are shown in fig
Massapequa Creek at Massapequa (01309500). Drainage area 38 mi2. Moderate-human-impact zone. Not useful for drought monitoring.

Long-term (49-year) annual mean discharge (1951-1999) 9.62 ft3/s; minimum annual mean discharge 2.27 ft3/s, 1995; maximum annual mean discharge 19.4 ft3/s, 1973.

1964-721981-831985-891991-99

-6.41-3.22-6.13-7.35

1966198119861995

Increased from normal to above normal, 1951-52;

remained above normal, 1953-61; decreased to belownormal, 1962-66; increased to above normal, 1967-72;decreased to normal, 1973-76; increased to well abovenormal, 1977-78; decreased to normal, 1979-81;increased to above normal, 1982-83; decreased tobelow normal, 1984-86; increased to normal, 1987-90;decreased to well below normal, 1991-95; increased tonormal, 1996-99.

1965-69, 1971,

1982, 1986-89,1992-99.

1966, 1986-88,

and 1993-97.

• Data missing for 2000.• Sharp decline from 1961 to 1966 attributed to effect

since late-1970 s attributed to increased pumping anCounty and western Suffolk County.

• Long-term averages and exceedance values strongly 

primary source of variability in record; not useful as for monitoring human-induced changes.

Connetquot River near Oakdale (01306500). Drainage area 24 mi2. Low-human-impact zone. Useful for drought monitoring.Long-term (50-year) annual mean discharge (1951-2000) 38.6 ft3/s; minimum annual mean discharge 24.9 ft3/s, 1966; maximum annual mean discharge 52.5 ft3/s, 1984.

1963-721981-831985-881992-96

-13.68

-8.33

-11.45

-9.19

1966198119881995

Increased from normal to above normal, 1951-55;

decreased to normal, 1956-62; decreased to well belownormal, 1963-66; increased to normal, 1967-72;remained in normal range, 1973-76; increased to abovenormal, 1977-78; decreased to below normal, 1979-81;increased to above normal, 1982-83; decreased tobelow normal, 1984-87; increased to well abovenormal, 1988-90; decreased to below normal, 1991-95;increased to normal, 1996-2000.

1951, 1963-72,

1981-82, 1986-88, 1994-96,and 2000.

1965-71, 1982,

1986-88, and1995.

• Sharp decline from 1962 to 1967 attributed to effect• Long-term averages and exceedance values slightly 

source of variability in record; useful as a drought-m

Nissequogue River near Smithtown (01304000). Drainage area 27 mi2. Low-human-impact zone. Useful for drought monitoring.Long-term (50-year) annual mean discharge (1951-2000) 43.3 ft3/s; minimum annual mean discharge 27.0 ft3/s, 1966; maximum annual mean discharge 58.9 ft3/s, 1991.

1951-521963-721981-821986-881994-96

-7.42

-16.25

-7.57-7.02-8.43

19511966198119881995

Increased from below normal to above normal, 1951-55;

decreased to normal, 1956-62; decreased to well belownormal, 1963-66; increased to normal, 1967-72;remained in normal range, 1973-76; increased to abovenormal, 1977-78; decreased to normal, 1979-81;increased to above normal, 1982-83; decreased tonormal, 1984-87; increased to well above normal,1988-90; decreased to below normal, 1991-95;increased to above normal, 1996-97; decreased tonormal, 1998-2000.

1951-52, 1963-72,

1981-82, 1986-88, 1994-96,and 2000.

1951, 1964-72,

1982, 1987-88,and 1995.

• Sharp decline from 1962 to 1967 attributed to effect• Long-term averages and exceedance values slightly 

source of variability in record; useful as a drought-m

Significant periods of

below normal annual

discharge in relation

to annual mean

AR IMA trend at or below

monthly exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

e e xceedanc

90%

exceedance

Table 4C. (Continued) Notable trends in discharge at the eight selected streamflow-gaging (discharge) stations on Long Island, N.Y., 1 9 5 1 -2 0 0 
Carmans River at Yaphank (01305000). Drainage area 71 mi2. Low-human-impact zone. Useful for drought monitoring.

Long-term (50-year) annual mean discharge (1951-2000) 24.4 ft3/s; minimum annual mean discharge 12.9 ft3/s, 1967; maximum annual mean discharge 37.7 ft3/s, 1979.

1951-521964-721981-821986-891992-96

-6.23

-11.48

-4.23-7.30-8.22

19511967198119881995

Increased from below normal to above normal, 1951-55;

decreased to normal, 1956-62; decreased to well belownormal, 1963-66; increased to above normal, 1967-73;decreased to normal, 1974-76; increased to well abovenormal, 1977-78; decreased to normal, 1979-81;increased to well above normal, 1982-83; decreased tobelow normal, 1984-87; increased to above normal,1988-90; decreased to below normal, 1991-95;increased to normal, 1996-2000.

1951, 1965-72,

1981-82, 1987-88, and 1994-96.

1965-71, 1988,

and 1995-96.

• Sharp decline from 1962 to 1967 attributed to effects o• Long-term averages and exceedance values slightly ske

source of variability in record; useful as a drought-mon

Peconic River at Riverhead (01304500). Drainage area 75 mi2. Low-human-impact zone. Useful for drought monitoring.

Long-term (50-year) annual mean discharge (1951-2000) 38.3 ft3/s; minimum annual mean discharge 16.1 ft3/s, 1966; maximum annual mean discharge 67.9 ft3/s, 1984.

1951

1963-721981-821985-881992-96

-15.15-22.16-16.71-15.58-16.80

19511966198119861995

Increased from below normal to above normal, 1951-58;

decreased to normal, 1959-62; decreased to well belownormal, 1963-66; increased to above normal, 1967-72;decreased to normal, 1973-76; increased to well abovenormal, 1977-78; decreased to normal, 1979-81;increased to above normal, 1982-83; decreased tobelow normal, 1984-87; increased to normal, 1988-90;decreased to below normal, 1991-95; increased tonormal, 1996-2000.

1951-52, 1963-71,

1981-82, 1986-88, 1992-96,and 2000.

1951, 1964-69,

1971, 1981,1986-88, and1995.

• Sharp decline from 1962 to 1967 attributed to effects o• Long-term averages and exceedance values slightly ske

source of variability in record; useful as a drought-mon

Significant periods of

below normal annual

discharge in relation

to annual mean

AR IMA trend at or below

monthly exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

exceedance

90%

exceedance

Table 4C. (Continued) Notable trends in discharge at the eight selected streamflow-gaging (discharge) stations on Long Island, N.Y., 1 9 5 1 -2 0 0 0 o
Figure 11. Monthly mean discharge and exceedance statistics for eight selected streamflow-gaging stations on Long Island, 

N.Y., 1951-2000 or period of record. (Station locations are shown in figure 5.)

D. Massapequa Creek

0

5

10

15

20

25

30

35

0

5

10

15

20

25

30

35

A. Valley Stream

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

C. Mill Neck Creek

4

7

10

13

16

4

7

10

13

16

B. East Meadow Brook

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

DISCHARGE, IN CUBIC FEET

PER SECOND

DISCHARGE, IN CUBIC FEET

PER SECOND

DISCHARGE, IN CUBIC FEET

PER SECOND

DISCHARGE, IN CUBIC FEET

PER SECOND

1950

WATER YEAR

Precipitation

90 Percent Exceedance Percentile 75 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

25 Percent Exceedance Percentile 10 Percent Exceedance Percentile 12-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S

49 49 49 49 49 49 49 49 49 49 49 49

49 49 49 49 49 49 49 49 49 49 49 49

49 49 49 49 49 49 49 49 49 49 49 49

46 46 46 46 46 46 46 46 46 46 46 46

26  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
H. Peconic River

10

20

30

40

50

60

70

80

90

100

110

10

20

30

40

50

60

70

80

90

100

110

F. Nissequogue River

20

30

40

50

60

70

80

20

30

40

50

60

70

80

G. Carmans River

5

10

15

20

25

30

35

40

45

50

5

10

15

20

25

30

35

40

45

50

E. Connetquot River

15

25

35

45

55

65

75

15

25

35

45

55

65

75

DISCHARGE, IN CUBIC FEET

PER SECOND

DISCHARGE, IN CUBIC FEET

PER SECOND

DISCHARGE, IN CUBIC FEET

PER SECOND

DISCHARGE, IN CUBIC FEET

PER SECOND

1950

WATER YEAR

Precipitation

90 Percent Exceedance Percentile 75 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

25 Percent Exceedance Percentile 10 Percent Exceedance Percentile 12-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S

50 50 50 50 50 50 50 50 50 50 50 50

50 50 50 50 50 50 50 50 50 50 50 50

50 50 50 50 50 50 50 50 50 50 50 50

50 50 50 50 50 50 50 50 50 50 50 50

Figure 11. (Continued) Monthly mean discharge and exceedance statistics for eight selected streamflow-gaging stations on 

Long Island, N.Y., 1951-2000 or period of record. (Station locations are shown in figure 5.)

Statistical Analysis of Long-Term Hydrologic Records  27
parts of Long Island, particularly at wells N1614 and N1616 

(fig. 5), have never returned to pre-drought levels; a result of 

regional water-level declines caused by increased pumpage 

and the completion of sanitary- and storm-sewer systems in 

Nassau and western Suffolk Counties. Water levels in central 

parts of Long Island returned to near normal after the drought, 

but have been generally declining since the mid- to late 1980’s 

as a result of increased pumpage and the effects of sanitaryand storm-sewer systems. Only in eastern parts of Long 

Island do water levels still generally reflect natural conditions, 

although the effects of human activity are becoming 

discernible in the record.

Selection of Drought-Monitoring Sites

Analysis of long-term hydrologic records on Long Island 

indicates that the data available provide a sufficient basis for 

development of a drought-monitoring network. Data from 

the 8 precipitation-monitoring stations, 8 streamflow-gaging 

(discharge) stations, 15 monitoring wells screened in the 

upper glacial aquifer, and 5 monitoring wells screened in the 

underlying Magothy aquifer selected as possible droughtmonitoring sites reflect the wide variation in hydrologic 

conditions across Long Island.

Table 5A. Annual mean ground-water levels, selected exceedance percentiles, and screen depths, at the 20 selected monitoring 

wells on Long Island, N.Y., 1951-2000 or period of record. 

[Values are in feet above (+) or below (-) NGVD 29; %, percent. Locations are shown in figure 5.] 

* Percentage of time during which the given amount is exceeded. 

Well

number

Station

number Aquifer

Screen zone

(feet below

land surface)

Annual

mean

Start of 

period 

of

record

Number

of

values

used for 

statistics

Exceedance percentile*

10% 25%

50%

(median) 75% 90%

K1194 404059073520702 Upper glacial 52-55 5.75 1951 264 10.39 10.04 8.93 0.46 -0.27 

Q1249 404240073443401 Upper glacial 85-88 18.96 1951 192 27.92 25.44 20.16 17.55 3.65 

N7493 404237073433701 Magothy (deep) 349-353 13.76 1968 348 24.29 22.74 11.09 7.55 6.09 

N1614 404446073392904 Upper glacial 50-53 57.06 1951 580 67.33 61.48 55.22 52.19 50.38 

N1616 404554073351502 Magothy (shallow) 65-68 75.79 1951 592 82.14 80.22 75.71 71.25 69.26 

N2528 405101073343401 Magothy (middle) 278-282 67.54 1953 400 71.81 70.53 68.05 65.21 61.78 

N1259 404317073291105 Upper glacial 38-41 50.40 1951 587 54.60 53.18 50.77 48.06 45.58 

S1806 404442073240503 Upper glacial 41-45 54.45 1951 582 57.93 56.59 54.76 52.30 50.37 

S15622 405250073180801 Magothy (middle) 437-457 40.83 1969 196 43.92 42.64 40.75 39.20 37.59 

S1810 404614073164404 Upper glacial 52-55 50.46 1951 578 53.93 52.27 50.63 48.71 47.03 

S1812 404958073085001 Upper glacial 46-50 46.27 1951 463 48.95 47.84 46.51 44.70 43.38 

S33380 404932073055902 Magothy (deep) 840-850 49.06 1968 347 51.94 50.66 49.01 47.34 46.09 

S3513 405146073031801 Upper glacial 63-65 63.88 1951 536 66.92 65.72 64.52 62.42 59.62 

S5517 405149072532201 Upper glacial 85-91 40.90 1951 543 44.42 43.08 41.20 39.04 36.87 

S6455 405223072523402 Magothy (deep) 952-962 38.18 1951 411 40.44 39.35 38.13 37.02 36.14 

S4271 405743072425701 Upper glacial 100-105 10.89 1951 561 12.31 11.61 10.90 9.98 9.55 

S3543 405037072390301 Upper glacial 56-58 18.24 1951 520 20.37 19.18 18.22 17.11 16.10 

S16783 410634072223601 Upper glacial 20-24 2.06 1958 349 2.85 2.35 1.99 1.67 1.41 

S8833 405756072173501 Upper glacial 10-13 16.08 1951 280 17.98 17.26 16.17 14.90 14.09 

S48579 410316071535501 Upper glacial 53-56 3.31 1974 213 3.80 3.51 3.31 3.06 2.89 

28  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
As discussed previously, a map showing three degrees 

of human impact on water levels (significant, moderate, and 

low) was created to qualify the changes in the ground-water 

system underlying Long Island (fig. 7C). These zones reflect 

the extent to which ground-water levels have been affected by 

increased urbanization.

The hydrologic data were used to calculate 50-year 

composite annual and monthly averages for six indices—an 

islandwide precipitation index representing the eight stations; 

two stream-discharge indices, one representing a moderatehuman-impact zone (three streams) and one representing a 

low-human-impact zone (four streams); and three water-level 

indices, one representing a significant-human-impact zone 

(three wells), one representing a moderate-human-impact 

zone (six wells), and one representing a low-human-impact 

zone (nine wells). These composites were created to simplify 

comparison among data sets, to minimize the effects of local 

fluctuations in the record, and to present the relative condition 

of Long Island’s water-level conditions in an easy-to-view 

format. The following section discusses each of these indices.

Precipitation

The long-term precipitation records from the eight 

selected precipitation stations on or near Long Island (fig. 5) 

indicate a general similarity among stations, although some 

in the western part of the island show more variability in the 

record than the eastern part. A 50-year composite-average 

hydrograph and boxplot of monthly precipitation for the eight 

selected stations are shown in figure 13. Data for each month’s 

composite average was calculated only for months in which 

Table 5B. Monthly minimum and maximum ground-water levels, and means for lowest and highest months, at the 20 selected 

monitoring wells on Long Island, N.Y., 1951-2000 or period of record. 

[Values are in feet above (+) or below (-) NGVD 29. Locations are shown in figure 5.]

Station 

n umber 

Minimum Maximum 

Lowest monthly 

mean 

Highest monthly 

mean 

W ell 

n umber

K1194 404059073520702 -1.88 Oct. 1966 11.30 Mar. 1995 3.55 Oct. 6.70 Aug. 

Q1249 404240073443401 -5.66 Mar. 1982 29.56 May 1953 11.14 Mar. 22.41 Dec. 

N7493 404237073433701 3.73 Sep. 1982 27.82 June 1997 13.00 Oct. 14.56 June 

N1614 404446073392904 48.42 Dec. 1970 72.26 May 1953 56.36 Dec. 57.59 May 

N1616 404554073351502 66.82 Jan. 1996 84.94 June 1953 75.41 Dec. 76.27 June 

N2528 405101073343401 59.12 Feb. 1967 73.59 Sep. 1984 66.55 Jan. 68.47 June 

N1259 404317073291105 41.64 Oct. 1995 57.60 Feb. 1978 49.67 Nov. 51.24 Apr. 

S1806 404442073240503 46.97 Jan. 1967 62.37 June 1984 53.54 Dec. 55.54 May 

S15622 405250073180801 34.33 Apr. 1969 47.09 Jan. 1980 40.18 Feb. 41.37 Jan. 

S1810 404614073164404 43.30 Feb. 1967 56.28 July 1984 49.75 Dec. 51.28 May 

S1812 404958073085001 40.09 Feb. 1967 51.97 Mar. 1979 45.60 Jan. 47.09 Apr. 

S33380 404932073055902 43.90 Aug. 1995 54.30 Apr. 1979 48.76 Dec. 49.51 May 

S3513 405146073031801 56.06 Mar. 1967 69.91 May 1979 63.27 Dec. 64.41 July 

S5517 405149072532201 33.34 Mar. 1967 47.43 July 1998 39.97 Dec. 41.72 June 

S6455 405223072523402 33.82 Dec. 1966 42.50 Apr. 1979 37.54 Nov. 38.83 June 

S4271 405743072425701 8.20 Aug. 1966 14.24 Aug. 1984 10.74 Aug. 11.24 May 

S3543 405037072390301 14.94 Nov. 1986 22.53 July 1984 17.66 Jan. 18.86 June 

S16783 410634072223601 0.89 July 1964 4.11 Mar. 1998 1.71 Aug. 2.55 Mar. 

S8833 405756072173501 12.84 Mar. 1982 19.31 June 1998 14.68 Nov. 16.84 June 

S48579 410316071535501 2.46 Dec. 1976 4.30 May 1998 3.06 Nov. 3.58 May 

Value Month Year Value Month Year Value Month Value Month

Selection of Drought-Monitoring Sites  29
[Values are in feet above (+) or below (-) NGVD 29. ft, feet; %, percent; ARIMA, autoregressive integrated moving average. Locations are shown in figure 5.]

S fi igni cant periods of

below normal water level

in relation to annual mean

AR IMA trend at or below monthly

exceedance percentiles

(part or all of each year)

P r e iod

Maximum

departure Year AR IMA trend in relation to annual mean

75%

exceedance

90%

exceedance

Well K 11 9 4 (4 0 4 0 5 9 0 7 3 5 2 0 7 0 2 ). Screened 5 2 -5 5 ft below land surface in the upper glacial aquifer. Significant-human-impact zone. Not useful for drought monitorin

Long-term (50-year) mean water level (1951-2000) +5.75 ft; minimum water level -1.88 ft, October 1966; maximum water level +11.30 ft, March 1995.

1951-73 -7.63 1966 Increased about 2 ft from well below normal to below

normal, 1951-55; remained below normal, 1956-64;

increased about 9 ft to normal, 1965-79; remained nearnormal, 1980-88; increased about 1 ft to above normal,

1989-90; remained above normal, 1990-2000.

1951-57 and

1959-62.

1951-1952. • ARIMA trend and exceedance values skewed by sp

• Sharp rise from the mid-1960’s to late-1970’s attrib

western Queens County.

• Long-term averages and exceedance values strong

urbanization mask fluctuations associated with premonitoring site; useful for monitoring human-indu

Well Q 1 2 4 9 (4 0 4 2 4 0 0 7 3 4 4 3 4 0 1 ). Screened 8 5 -8 8 ft below land surface in the upper glacial aquifer. Significant-human-impact zone. Not useful for drought monitorin

Long-term (50-year) mean water level (1951-2000) +18.96 ft; minimum water level -5.66 ft, March 1982; maximum water level +29.56 ft, May 1953.

1960-90 -24.62 1982 Well above normal, 1951-52; decreased about 8 ft to

normal, 1953-54; decreased about 15 ft to well below

normal, 1955-75; remained well below normal, 1976-79; increased about 2 ft to below normal, 1980-87;increased about 13 ft to normal, 1988-89; remained in

normal range; 1990-2000.

1959-61, 1966,

1972, 1985-91,and 1993.

1959, 1961,

1964-66, 1972,and 1985-90.

• ARIMA trend and exceedance values skewed by sp• Sharp decline in mid-1950’s attributed to increased 

gradual decline in late-1950’s to mid-1970’s attribeffects of the 1960’s drought; sharp rise since earlypumping in eastern Queens County.

• Long-term averages and exceedance values strong

urbanization mask fluctuations associated with pre

monitoring site; useful for monitoring human-indu

Well N7 4 9 3 (4 0 4 2 3 7 0 7 3 4 3 3 7 0 1 ). Screened 3 4 9 -3 5 3 ft below land surface in the deep part of the Magothy aquifer. Significant-human-impact zone. Not useful for dr

Long-term (33-year) mean water level (1968-2000) +13.76 ft; minimum water level +3.73 ft, September 1982; maximum water level +27.82 ft, June 1997.

1968-83 -10.03 1982 Decreased about 6 ft from normal to well below normal,

1968-82; increased about 19 ft to above normal, 1983-91; remained above normal, 1992-93; increased about 3ft to well above normal, 1994-98; decreased about 2 ft

to above normal, 1999-2000.

1972-83. 1981-83 • Data missing from 1951 to 1967.

• Sharp rise since early-1980’s attributed to decrease• Long-term averages and exceedance values strong

urbanization mask fluctuations associated with premonitoring site; useful for monitoring large human

Well N1 6 1 4 (4 0 4 4 4 6 0 7 3 3 9 2 9 0 4 ). Screened 5 0 -5 3 ft below land surface in the upper glacial aquifer. Moderate-human-impact zone. Not useful for drought monitoringLong-term (50-year) mean water level (1951-2000) +57.06 ft; minimum water level +48.42 ft, December 1970; maximum water level +72.26 ft, May 1953.

1965-781980-841985-891994-96

-8.64-7.91-5.74-6.39

1971198319891996

Well above normal, 1951-53; decreased about 5 ft to

above normal, 1954-62; decreased about 11 ft to wellbelow normal, 1963-67; remained well below normal,1968-71; increased about 7 ft to normal, 1972-79;decreased about 5 ft below normal, 1980-82; remained

in normal range, 1983-2000.

1966-74, 1982,

and 1987.

1967-72. • Gradual decline from 1954 to 1962 attributed to in

eastern Queens County and western Nassau Counattributed to effects of 1960’s drought; slight rise sdecreased pumping in eastern Queens County.

• Water levels never returned to pre-drought levels; 

pumping and sewering.

• Long-term averages and exceedance values strong

urbanization partially mask fluctuations associated drought-monitoring site; useful for monitoring hum

Table 5C. Notable trends in ground-water levels at the 20 selected monitoring wells on Long Island, N.Y., 1951-2000 or period of record.

.
Well N1 6 1 6 (4 0 4 5 5 4 0 7 3 3 5 1 5 0 2 ). Screened 6 5 -6 8 ft below land surface in the shallow part of the Magothy (water-table) aquifer. Moderate-human-impact zone. Not use

Long-term (50-year) mean water level (1951-2000) +75.79 ft; minimum water level +66.82 ft, January 1996; maximum water level +84.94 ft, June 1953.

1965-731981-841985-90

1991-2000

-7.51-3.21-6.81-8.97

1967198319891996

Increased about 3 ft from above normal to well above

normal, 1951-53; decreased about 2 ft to above normal,1954-62; decreased about 12 ft to well below normal,1963-67; increased about 11 ft to above normal, 1968-79; decreased about 5 ft to normal, 1980-82; remainednear normal, 1983-84; decreased about 5 ft to belownormal, 1985-87; increased about 4 ft to normal, 1988-90; decreased about 6 ft to well below normal, 1991-96;remained below normal, 1997-2000.

1966-71, 1987-88,

and 1993-2000.

1967-68, 1995-

97, and 2000.

• Gradual decline from 1954 to 1962 attributed to increa

western Nassau County; sharp decline from 1963 to 19drought; gradual decline since late-1970’s attributed to in central and eastern Nassau County.

• Long-term averages and exceedance values strongly sk

primary source of variability in record; not useful as a for monitoring human-induced changes.

Well N2 5 2 8 (4 0 5 1 0 1 0 7 3 3 4 3 4 0 1 ). Screened 2 7 8 -2 8 2 ft below land surface in the middle part of the Magothy aquifer. Moderate-human-impact zone. Not useful for drougLong-term (48-year) mean water level (1953-2000) +67.54 ft; minimum water level +59.12 ft, February 1967; maximum water level +73.59 ft, September 1984.

1963-731981-831986-89

1992-2000

-8.42-2.34-3.60-5.67

1967198319891996

Above normal, 1953-62, decreased about 11 ft to well

below normal, 1963-67; increased about 11 ft to abovenormal, 1968-77; decreased about 5 ft to normal, 1978-88; remained near normal, 1989-91; decreased about 4ft to below normal, 1992-96; remained near normal,1997-2000.

1964-72 and

1994-2000.

1966-69. • Data missing from 1951 to 1952.

• Sharp decline from 1963 to 1967 attributed to effects o

since late-1970’s attributed to increased pumping and Nassau County.

• Long-term averages and exceedance values strongly sk

primary source of variability in record; not useful as a for monitoring human-induced changes.

Well N1 2 5 9 (4 0 4 3 1 7 0 7 3 2 9 11 0 5 ). Screened 3 8 -4 1 ft below land surface in the upper glacial aquifer. Moderate-human-impact zone. Not useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +50.40 ft; minimum water level +41.64 ft, October 1995; maximum water level +57.60 ft, February 1978.

1964-721981-831985-89

1991-2000

-4.79-2.30-5.99-8.76

1966198319881996

Above normal, 1951-62; decreased about 6 ft to below

normal, 1963-67; increased about 7 ft to above normal,1968-78; decreased about 4 ft to normal, 1979-81;remained near normal, 1982-83; decreased about 5 ft tobelow normal, 1984-86; increased about 3 ft to normal,1987-90; decreased about 5 ft to well below normal,1991-95; increased about 2 ft to below normal, 1996-97; remained below normal, 1998-2000.

1966-68, 1986-89,

and 1992-2000.

1987-88 and

1993-2000.

• Sharp decline from 1963 to 1967 attributed to effects o

since late-1970’s attributed to increased pumping and County and western Suffolk County.

• Long-term averages and exceedance values strongly sk

primary source of variability in record; not useful as a for monitoring human-induced changes.

Well S1 8 0 6 (4 0 4 4 4 2 0 7 3 2 4 0 5 0 3 ). Screened 4 1 -4 5 ft below land surface in the upper glacial aquifer. Moderate-human-impact zone. Not useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +54.45 ft; minimum water level +46.97 ft, January 1967; maximum water level +62.37 ft, June 1984.

1963-731981-831985-891992-98

1999-2000

-7.48-2.14-3.95-6.38-3.33

19671983198919952000

Increased about 2 ft from normal to above normal, 1951-

52; remained above normal, 1953-62; decreased about 8ft to well below normal, 1963-67; increased about 9 ftto above normal, 1968-78; decreased about 3 ft tonormal, 1979-81; increased about 2 ft to above normal,1982-83; decreased about 5 ft to below normal, 1984-

86; increased about 5 ft to above normal, 1987-90;

decreased about 6 ft to below normal, 1991-95;increased about 2 ft to normal, 1996-97; remained nearnormal, 1998-2000.

1964-73, 1987-88,

and 1993-2000.

1965-71 and

1995-96.

• Sharp decline from 1963 to 1967 attributed to effects o

since late-1970’s attributed to increased pumping and County and western Suffolk County.

• Long-term averages and exceedance values moderatel

longer primary source of variability in record; not useuseful for monitoring human-induced changes.

S fi igni cant periods of

below normal water level

in relation to annual mean

AR IMA trend at or below monthly

exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

exceedance

90%

exceedance

Table C. (Continued)Notable trends in ground-water levels at the 2 0 selected monitoring wells on Long Island, N.Y., 1 9 5 1 -2 0 0 0 or period of record.
Well S1 5 6 2 2 (4 0 5 2 5 0 0 7 3 1 8 0 8 0 1 ). Screened 4 3 7 -4 5 7 ft below land surface in the middle part of the Magothy aquifer. Low-human-impact zone. Useful for drought mon

Long-term (32-year) mean water level (1969-2000) +40.83 ft; minimum water level +34.33 ft, April 1969; maximum water level +47.09 ft, January 1980.

1969-731977-781980-831986-891994-98

-6.50-4.31-3.35-4.99-4.00

19691978198219881995

Increased about 2 ft from below normal to normal, 1969-

81; decreased about 2 ft to below normal, 1982-88;increased about 4 ft to above normal, 1989-91;decreased about 2 ft to normal, 1991-94; remained nearnormal, 1995-2000.

1969-70, 1972,

1985-89, and1994-95.

1988. • Data missing from 1951 to 1968.

• ARIMA trend and exceedance values skewed by spar• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S1 8 1 0 (4 0 4 6 1 4 0 7 3 1 6 4 4 0 4 ). Screened 5 2 -5 5 ft below land surface in the upper glacial aquifer. Moderate-human-impact zone. Not useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +50.46 ft; minimum water level +43.30 ft, February 1967; maximum water level +56.28 ft, July 1984.

1951-521962-731977-781981-831985-891993-98

1999-2000

-2.91-7.16-1.50-2.31-3.60-4.38-1.94

1951196719771982198819962000

Increased about 3 ft from normal to above normal, 1951-

56; decreased about 1 ft to normal, 1957-61; decreasedabout 6 ft to well below normal, 1962-66; increasedabout 8 ft to above normal, 1967-79; decreased about 3ft to normal, 1980-82; remained in normal range, 1983-84; decreased about 3 ft to below normal, 1984-87;increased about 4 ft to above normal, 1988-90;decreased about 4 ft to below normal, 1991-95;increased about 2 ft to normal, 1996-97; remained nearnormal, 1998-2000.

1964-72, 1988,

and 1994-96.

1965-70. • Sharp decline from 1962 to 1967 attributed to effects 

since late-1970’s attributed to increased pumping and County and western Suffolk County.

• Long-term averages and exceedance values moderate

longer primary source of variability in record; not ususeful for monitoring human-induced changes.

Well S1 8 1 2 (4 0 4 9 5 8 0 7 3 0 8 5 0 0 1 ). Screened 4 6 -5 0 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +46.27 ft; minimum water level +40.09 ft, February 1967; maximum water level +51.97 ft, March 1979.

1951-521964-721977-781981-831985-891993-961997-98

1999-2000

-2.17-6.18-1.88-2.92-4.04-5.93-2.95-2.40

19511967197719811989199519981999

Increased about 2 ft from normal to above normal, 1951-

53; remained above normal, 1954-61; decreased about 6ft to well below normal, 1962-67; increased about 6 ftto above normal, 1968-79; decreased about 2 ft tonormal, 1980-84; decreased about 2 ft to below normal,1985-87; increased about 4 ft to above normal, 1988-90; decreased about 3 ft to below normal, 1991-95;increased about 2 ft to normal, 1996-2000.

1964-71, 1986-89,

1994-96, and1998.

1965-70 and

1987-88.

• Sharp decline from 1962 to 1967 attributed to effects 

decline since late-1970’s attributed to increased pumSuffolk County.

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S3 3 3 8 0 (4 0 4 9 3 2 0 7 3 0 5 5 9 0 2 ). Screened 8 4 0 -8 5 0 ft below land surface in the deep part of the Magothy aquifer. Low-human-impact zone. Useful for drought monitLong-term (33-year) mean water level (1968-2000) +49.06 ft; minimum water level +43.90 ft, August 1995; maximum water level +54.30 ft, April 1979.

1968-731977-781981-831986-891993-98

-3.86-1.76-2.42-3.74-5.16

19691977198219881995

Increased about 6 ft from below normal to above normal,

1968-79; decreased about 3 ft to normal, 1980-82;increased about 2 ft to above normal, 1983-84;decreased about 4 ft to below normal, 1985-87;increased about 4 ft to above normal, 1988-91;decreased about 4 ft to below normal, 1992-95;increased about 2 ft to normal, 1996-2000.

1969-71, 1987-88,

and 1994-96.

1969-70. • Data missing from 1951 to 1967.

• Very gradual decline since late-1970’s attributed to in

Suffolk County.

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

S fi igni cant periods of

below normal water level

in relation to annual mean

AR IMA trend at or below monthly

exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

exceedance

90%

exceedance

Table C. (Continued)Notable trends in ground-water levels at the 2 0 selected monitoring wells on Long Island, N.Y., 1 9 5 1 -2 0 0 0 or period of record
Well S3 5 1 3 (4 0 5 1 4 6 0 7 3 0 3 1 8 0 1 ). Screened 6 3 -6 5 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Useful for drought monitoring.

Long-term (50-year) mean water level (1951-2000) +63.88 ft; minimum water level +56.06 ft, March 1967; maximum water level +69.91 ft, May 1979.

1951-531964-731977-781981-831986-891995-98

-3.76-7.82-1.21-1.81-3.59-3.84

195119671978198219891996

Increased about 4 ft from below normal to above normal,

1951-56; remained above normal, 1957-61; decreasedabout 7 ft to well below normal, 1962-67; increasedabout 8 ft to well above normal, 1968-79; decreasedabout 3 ft to normal, 1980-82; increased about 2 ft toabove normal, 1983-84; decreased about 3 ft to belownormal, 1985-87; increased about 3 ft to above normal,1988-91; decreased about 2 ft to normal, 1992-95;remained near normal, 1996-2000.

1951-53, 1964-74,

1987-89, and1995-97.

1966-71. • Sharp decline from 1962 to 1967 attributed to effects 

since late-1970’s attributed to increased pumping in wCounty.

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S5 5 1 7 (4 0 5 1 4 9 0 7 2 5 3 2 2 0 1 ). Screened 8 5 -9 1 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +40.90 ft; minimum water level +33.34 ft, March 1967; maximum water level +47.43 ft, July 1998.

1951-521963-731975-781980-831985-891991-931994-96

-1.27-7.56-3.47-6.10-4.34-2.29-3.31

1951196719771982198919921996

Increased about 3 ft from normal to above normal, 1951-

55; remained above normal, 1956-61; decreased about 9ft to well below normal, 1962-67; increased about 5 ftto normal, 1968-79; decreased about 3 ft to belownormal, 1980-81; increased about 4 ft to normal, 1982-84; decreased about 4 ft to below normal, 1985-87;increased about 4 ft to normal, 1988-91; remained innormal range, 1992-94; increased about 3 ft to abovenormal, 1995-2000.

1965-73, 1976,

1980-83, and1986-88.

1966-69. • Sharp decline from 1962 to 1967 attributed to effects 

since early-1980’s attributed to a local decrease in puaquifer.

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S6 4 5 5 (4 0 5 2 2 3 0 7 2 5 2 3 4 0 2 ). Screened 9 5 2 -9 6 2 ft below land surface in the deep part of the Magothy aquifer. Low-human-impact zone. Useful for drought monitoLong-term (50-year) mean water level (1951-2000) +38.18 ft; minimum water level +33.82 ft, December 1966; maximum water level +42.50 ft, April 1979.

1951-521957-581963-721981-821986-891994-98

1999-2000

-2.31-1.15-4.36-2.42-2.88-3.12-1.25

1951195819671982198919962000

Increased about 2 ft from normal to above normal, 1951-

56; remained above normal, 1957-60; decreased about 4ft to well below normal, 1961-67; increased about 3 ftto normal, 1968-76; remained in the normal range,1976-88; increased about 2 ft to above normal, 1989-91; decreased about 2 ft to normal, 1992-94; remainedin normal range, 1995-2000.

1951, 1964-71,

1985, 1987-88,1994, and 1996.

1965-68. • Sharp decline from 1962 to 1967 attributed to effects 

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S4 2 7 1 (4 0 5 7 4 3 0 7 2 4 2 5 7 0 1 ). Screened 1 0 0 -1 0 5 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +10.89 ft; minimum water level +8.20 ft, August 1966; maximum water level +14.24 ft, August 1984.

1951-521953-541957-581961-721980-831985-891995-96

-1.68-1.18-1.74-2.69-2.19-1.48-1.58

1951195419571966198119861995

Increased about 1.5 ft from normal to above normal,

1951-59; decreased about 2.5 ft to well below normal,1960-66; increased about 2.5 ft to above normal, 1967-74; decreased about 0.5 ft to normal, 1975-76;increased about 0.5 ft to above normal, 1977-78;decreased about 1.5 ft to below normal, 1979-81;increased about 2 ft to above normal, 1982-83;decreased about 2 ft to below normal, 1984-86;increased about 2 ft to above normal, 1987-90;decreased about 0.5 ft to normal, 1991-94; increasedabout 0.5 ft to above normal, 1995-2000.

1951, 1963-71,

1981-82, and1987.

1965-68. • Sharp decline from 1960 to 1966 attributed to effects 

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

S fi igni cant periods of

below normal water level

in relation to annual mean

AR IMA trend at or below monthly

exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

exceedance

90%

exceedance

Table C. (Continued)Notable trends in ground-water levels at the 2 0 selected monitoring wells on Long Island, N.Y., 1 9 5 1 -2 0 0 0 or period of record
Well S3 5 4 3 (4 0 5 0 3 7 0 7 2 3 9 0 3 0 1 ). Screened 5 6 -5 8 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Useful for drought monitoring.

Long-term (50-year) mean water level (1951-2000) +18.24 ft; minimum water level +14.94 ft, November 1986; maximum water level +22.53 ft, July 1984.

1951-521957-581964-721980-821985-891992-96

1999-2000

-3.06-1.57-3.21-3.19-3.30-2.51-1.27

1951195719671982198719962000

Increased about 3.5 ft from below normal to above

normal, 1951-60; decreased about 3 ft to below normal,1961-66; increased about 3.5 ft to above normal, 1967-79; decreased about 3 ft to below normal, 1980-81;increased about 2 ft to above normal, 1982-83;decreased about 3 ft to below normal, 1984-87;increased about 3 ft to above normal, 1988-90;decreased about 1.5 ft to normal, 1991-93; remained innormal range, 1994-2000.

1951, 1964-70,

1981-82, 1986-88, 1993-96,and 1999.

1951, 1965-67,

and 1986-87.

• Sharp decline from 1962 to 1967 attributed to effects • Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S1 6 7 8 3 (4 1 0 6 3 4 0 7 2 2 2 3 6 0 1 ). Screened 2 0 -2 4 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Not useful for drought monitoring.Long-term (43-year) mean water level (1958-2000) +2.06 ft; minimum water level +0.89 ft, July 1964; maximum water level +4.11 ft, March 1998.

1962-71 -1.17 1964 Decreased about 1 ft from normal to below normal, 1958-

65; increased about 1 ft to normal, 1966-71; remainednear normal, 1972-84; increased about 0.5 ft to abovenormal, 1985-2000.

1959-74, 1976-81,

1983-85, and1987-88.

1959, 1961-70,

1978, and1984.

• Data missing from 1951 to 1957.• Sharp decline from 1961 to 1964 attributed to effects 

since mid-1980 s attributed to a local decrease in pumaquifer.

• Long-term averages and exceedance values slightly s

tidal fluctuations that tend to mask seasonal variationdrought-monitoring site.

Well S8 8 3 3 (4 0 5 7 5 6 0 7 2 1 7 3 5 0 1 ). Screened 1 0 -1 3 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Useful for drought monitoring.Long-term (50-year) mean water level (1951-2000) +16.08 ft; minimum water level +12.84 ft, March 1982; maximum water level +19.31 ft, June 1998.

1951-531957-581963-721980-831985-891992-96

1999-2000

-2.81-1.55-3.21-3.24-2.56-2.92

-2.11

1951195819671982198619962000

Increased about 3 ft from well below normal to normal,

1951-55; remained in normal range, 1956-2000.

1951-52, 1960-61,

1972, 1978-79,1981-85, 1987,1992-94, and1999-2000.

1951. • ARIMA trend and exceedance values skewed by spar

decline (not seen in trend) from 1962 to 1967 attribu

• Long-term averages and exceedance values slightly s

source of variability in record; useful as a drought-m

Well S4 8 5 7 9 (4 1 0 3 1 6 0 7 1 5 3 5 5 0 1 ). Screened 5 3 -5 6 ft below land surface in the upper glacial aquifer. Low-human-impact zone. Not useful for drought monitoring.Long-term (27-year) mean water level (1974-2000) +3.31 ft; minimum water level +2.46 ft, December 1976; maximum water level +4.30 ft, May 1998.

1980-82 -0.84 1980 Decreased about 0.5 ft from normal to below normal,

1974-80; increased about 0.5 ft to normal, 1981-84;remained near normal, 1985-93; increased about 0.5 ftto above normal, 1994-96; decreased about 0.5 ft tonormal, 1997-2000.

1974-84 and

1987.

1980-81. • Data missing from 1951 to 1973.

• Gradual rise since mid-1980’s attributed to a local de

glacial aquifer.

• Long-term averages and exceedance values slightly s

tidal fluctuations that tend to mask seasonal variationdrought-monitoring site.

S fi igni cant periods of

below normal water level

in relation to annual mean

AR IMA trend at or below monthly

exceedance percentiles

(part or all of each year)

P r e iod R emarks

Maximum

departure Year AR IMA trend in relation to annual mean

75%

e e xceedanc

90%

e e xceedanc

Table C. (Continued)Notable trends in ground-water levels at the 2 0 selected monitoring wells on Long Island, N.Y., 1 9 5 1 -2 0 0 0 or period of record
D. Water-table aquifer well N1614

47 

50 

53 

56 

59 

62 

65 

68 

71 

74 

A. Water-table aquifer well K1194 

-3 

-1 

1 

3 

5 

7 

9 

11

13 

C. Magothy aquifer well N7493 

2 

5 

8 

11

14 

17 

20 

23 

26 

29 

NO 

DATA 

B. Water-table aquifer well Q1249 

-7 

-2 

3 

8 

13 

18 

23 

28 

33

-3 

-1 

1 

3 

5 

7 

9 

11

13 

-7 

-2 

3 

8 

13 

18 

23 

28 

33

2 

5 

8 

11

14 

17 

20 

23 

26 

29 

47 

50 

53 

56 

59 

62 

65 

68 

71 

74 

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

1950

WATER YEAR 

Water Level

10 Percent Exceedance Percentile 25 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile 90 Percent Exceedance Percentile 18-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S 

48 50 48 48 49 48 49 50 49 46 49 46

28 30 26 31 29 27 28 28 31 30 30 30

17 13 16 17 19 24 16 15 14 13 12 16

22 15 28 16 19 31 20 17 29 17 15 35

Figure 12. Monthly water levels and exceedance statistics for 20 selected monitoring wells on Long Island, N.Y., 1951-2000 

or period of record. (Well locations are shown in figure 5.)

Selection of Drought-Monitoring Sites  35
H. Water-table aquifer well S1806 

45 

47 

49 

51 

53 

55

57 

59 

61 

63 

E. Water-table aquifer well N1616 

65 

67 

69 

71 

73 

75 

77

79 

81 

83 

85 

87 

G. Water-table aquifer well N1259

41 

43 

45 

47 

49 

51 

53 

55

57 

59 

F. Magothy aquifer well N2528

57 

59 

61 

63 

65 

67 

69 

71 

73 

75 

65 

67 

69 

71 

73 

75 

77

79 

81 

83 

85 

87 

57 

59 

61 

63 

65 

67 

69 

71 

73 

75 

41 

43 

45 

47 

49 

51 

53 

55

57 

59 

45 

47 

49 

51 

53 

55

57 

59 

61 

63 

1950

WATER YEAR 

Water Level

10 Percent Exceedance Percentile 25 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile 90 Percent Exceedance Percentile 18-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S 

49 49 48 48 50 48 47 49 49 47 49 49

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

50 49 49 47 50 47 50 49 49 48 49 50

28 31 36 31 33 40 30 29 41 28 30 43

50 50 49 48 50 48 50 50 49 48 50 50

Figure 12. (Continued) Monthly water levels and exceedance statistics for 20 selected monitoring wells on Long Island, 

N.Y., 1951-2000 or period of record. (Well locations are shown in figure 5.)

36  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
L. Magothy aquifer well S33380 

43 

45 

47 

49 

51 

53 

55 

NO 

DATA 

I. Magothy aquifer well S15622 

34 

36 

38 

40 

42 

44 

46 

48 

NO 

DATA 

K. Water-table aquifer well S1812 

39 

41 

43 

45 

47 

49 

51 

53 

J. Water-table aquifer well S1810 

43 

45 

47 

49 

51 

53 

55 

57 

34 

36 

38 

40 

42 

44

46 

48 

43 

45 

47 

49 

51 

53 

55

57 

39 

41 

43 

45 

47 

49 

51 

53 

43 

45 

47 

49 

51 

53 

55 29 26 27 30 29 31 30 29 29 27 29 31

1950

WATER YEAR 

Water Level

10 Percent Exceedance Percentile 25 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile 90 Percent Exceedance Percentile 18-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S 

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

38 39 41 36 32 42 35 38 43 38 38 43

49 49 48 45 48 45 49 49 49 47 50 50

12 16 19 12 15 21 12 15 20 15 13 26

Figure 12. (Continued) Monthly water levels and exceedance statistics for 20 selected monitoring wells on Long Island, 

N.Y., 1951-2000 or period of record. (Well locations are shown in figure 5.)

Selection of Drought-Monitoring Sites  37
P. Water-table aquifer well S4271

8 

9 

10 

11

12 

13 

14 

15 

M. Water-table aquifer well S3513

55 

57 

59 

61 

63 

65 

67 

69 

71 

O. Magothy aquifer well S6455

33

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

N. Water-table aquifer well S5517

33

35 

37 

39 

41 

43 

45 

47 

49 

55

57 

59 

61 

63 

65 

67 

69 

71

33

35 

37 

39 

41 

43 

45 

47 

49 

33

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

8 

9 

10 

11 

12 

13 

14 

15 46 47 44 48 46 48 45 48 48 45 47 49

1950

WATER YEAR 

Water Level

10 Percent Exceedance Percentile

25 Percent Exceedance Percentile

50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile

90 Percent Exceedance Percentile

18-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S 

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

31 29 39 31 28 47 30 32 42 30 31 41

45 44 46 42 46 46 44 44 47 44 47 48

45 43 45 43 40 47 44 44 47 48 45 45

Figure 12. (Continued) Monthly water levels and exceedance statistics for 20 selected monitoring wells on Long Island, 

N.Y., 1951-2000 or period of record. (Well locations are shown in figure 5.)

38  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
T. Water-table aquifer well S48579 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5 

NO 

DATA 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5 

Q. Water-table aquifer well S3543 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

S. Water-table aquifer well S8833 

12 

13 

14 

15 

16 

17 

18 

19 

20 

R. Water-table aquifer well S16783 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5 

NO 

DATA 

14 

15 

16 

17 

18 

19 

20 

21 

22

23 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5 

12 

13 

14 

15 

16 

17 

18 

19 

20 

15 13 20 14 19 19 14 18 22 17 18 24

1950

WATER YEAR 

Water Level

10 Percent Exceedance Percentile

25 Percent Exceedance Percentile

50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile

90 Percent Exceedance Percentile

18-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S 

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

29 11 34 15 18 33 21 15 37 17 16 34

23 26 37 23 21 38 24 26 36 25 29 41

42 43 43 42 43 48 42 41 44 43 42 47

Figure 12. (Continued) Monthly water levels and exceedance statistics for 20 selected monitoring wells on Long Island, 

N.Y., 1951-2000 or period of record. (Well locations are shown in figure 5.)

Selection of Drought-Monitoring Sites  39
at least five of the eight stations had data for that month. 

Also indicated on the hydrograph are the monthly means, 

the annual and monthly 10th, 25th, 50th (median), 75th, and 

90th exceedance percentiles, the ARIMA trend line, and the 

polynomial trend line.

The ARIMA trend line in the composite-average 

hydrograph (fig. 13) reflects many of the precipitation 

fluctuations depicted and tabulated earlier, including the 

1960’s drought as the major period of below-average 

precipitation and the above-average periods of 1971-79, 

1981-84, 1987-90, and 1995-98 (table 3C). The polynomial 

trend line for this composite shows that mean monthly 

precipitation since the 1960’s drought has been slightly above 

normal.

Stream Discharge

The long-term discharge records from the eight selected 

streams on Long Island (fig. 5) indicate that discharge of the 

four westernmost streams—Valley Stream, East Meadow 

Brook, Mill Neck Creek, and Massapequa Creek—showed 

large decreases in response to pumping and sewering. Valley 

Stream in the significant-human-impact zone, and the other 

three streams in the moderate-human-impact zone, no longer 

represent an aquifer under natural conditions. All four streams 

in Suffolk County (Connetquot River, Nissequogue River, 

Carmans River, and Peconic River) are in the low-humanimpact zone and reflect relatively natural conditions in the 

aquifer and, therefore, are useful as drought-monitoringindicator stations (table 6).

The 50-year composite-average hydrograph and boxplot 

of mean monthly stream discharge for three western index 

streams (moderate-human impact) are given in figure 14A; 

the composite-average hydrograph and boxplot for the four 

eastern streams (low-human impact) are given in figure 

14B. No composite-average hydrograph and boxplot was 

developed for the significant-human-impact zone because 

Valley Stream is the only station in this zone. Data for each 

month’s composite average were calculated only for months 

in which at least half of the streams for each index (moderate 

and low impact) had data for that month. Also indicated 

on the hydrographs are the monthly means, the annual and 

monthly 10th, 25th, 50th (median), 75th, and 90th exceedance 

percentiles, the ARIMA trend line, and the polynomial trend 

line.

The ARIMA trend lines in the two hydrographs (fig. 14) 

indicate that the two data sets were generally similar before 

the 1960’s drought and indicated above-average discharge; 

the trend lines also show the 1960’s drought (the major period 

of below-average discharge), and the above-average period 

through the 1970’s. Hydrograph A (streams in the moderatehuman-impact zone), shows the effects of increased pumpage 

and sewering generally from the late 1970’s to the end of the 

record (fig. 14A). The polynomial trend line for hydrograph 

A shows a sharp decline of average mean discharge that falls 

to near the 75-percent exceedance percentile by the end of the 

record. In contrast, hydrograph B (streams in the low-humanimpact zone), does not show this decline and the polynomial 

trend line generally remains above normal from the 1970’s to 

the end of the record (fig. 14B). This above-normal trend is 

also evident in the composite-average precipitation hydrograph 

described earlier (fig. 13), and indicates that this streamdischarge index (hydrograph B) is representative of natural 

conditions in the aquifer.

Ground-Water Levels

The long-term water-level records from the 15 selected 

water-table wells and the 5 Magothy wells (fig. 5) indicate 

wide variability across Long Island. Water levels at the 

three westernmost wells (those in the significant-humanimpact zone—K1194, Q1249, and N7493, fig. 15A) have 

been affected by human activity to the extent that natural 

fluctuations are no longer evident in the record. Water levels 

at the six wells in the moderate-human-impact zone (N1614, 

N1616, N2528, N1259, S1806, and S1810) have also been 

affected by human activity, but at a lesser extent than at the 

three westernmost wells. Natural fluctuations at these wells, 

although largely masked by human-induced effects, are still 

apparent in the record. Water levels at the 11 wells in the 

low-human-impact zone (S15622, S1812, S33380, S3513, 

S5517, S6455, S4271, S3543, S16783, S8833, and S48579) 

are relatively unaffected (as compared to wells in the other 

zones) by human activity and generally show only the natural 

fluctuations that result from changes in precipitation. Water 

levels at two of these wells (S16783 and S48579) are affected 

by tidal fluctuations that tend to mask seasonal variations in 

the record and produce unwanted variability in a composite 

index; therefore, these wells were omitted from the compositeaverage index. The remaining nine wells were included and 

are useful as drought-monitoring-indicator stations (table 6).

The 50-year composite-average hydrographs and 

boxplots of monthly water levels at the 18 selected wells 

are shown in figure 15. Composite-average hydrograph A 

(fig. 15A) shows the averaged monthly water-level data for the 

three wells in the significant-human-impact zone; compositeaverage hydrograph B (fig. 15B) shows those for the six wells 

in the moderate-human- impact zone; and composite-average 

hydrograph C (fig. 15C) shows those for the nine wells in the 

low-human-impact zone. Data for each month’s composite 

average were calculated only if more than half of the wells 

for each index had data for that month. Also indicated on 

the hydrographs are the monthly averages, the annual and 

monthly 10th, 25th, 50th (median), 75th, and 90th exceedance 

percentiles, the ARIMA trend line, and the polynomial trend 

line.

The ARIMA trend lines in the three hydrographs (fig. 15) 

indicate that the three data sets differ significantly. Compositeaverage hydrograph A (significant-human-impact zone, 

fig. 15A) is missing large amounts of data, especially before 

40 Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
and during the 1960’s drought; then, in the early 1980’s, a 

significant rise in water levels in this index to the well-abovenormal range is indicated and is attributed to a reduction 

in ground-water pumpage in southeastern Queens County. 

Composite-average hydrograph B (moderate-human-impact 

zone, fig. 15B) shows the effects of increased pumpage and 

sewering after the late 1970’s. The effect of the 1960’s drought 

appears exaggerated in this index because pumping and 

sewering in the western part of this zone had begun to lower 

water levels before the drought. The polynomial trend line for 

hydrograph B indicates a sharp decline of average water levels 

beginning in the mid-1980’s, and a decline to below the 90-

percent exceedance percentile by the late 1990’s. In contrast, 

composite-average hydrograph C (low-human-impact zone, 

fig. 15C), shows the effect of the 1960’s drought to have been 

of shorter duration than in the other zones; this hydrograph 

also does not show the decline beginning in the late 1970’s 

that is found in hydrograph B. The polynomial trend line for 

hydrograph C is generally above normal after the mid-1980’s. 

This above-normal trend is also evident in the compositeaverage precipitation and stream-discharge hydrographs 

described earlier (figs. 13 and 14) and indicates that this index 

is representative of natural conditions in the ground-water 

system.

Future Implementation of DroughtMonitoring Indices

The sites selected in this study to be part of the Long 

Island drought-monitoring network are the eight stations 

in the precipitation index (Central Park, Bensonhurst, 

LaGuardia Airport, Mineola, Setauket, Upton, Riverhead, 

and Bridgehampton, fig. 13), the four stations in the lowhuman-impact stream-discharge index (Connetquot River, 

Nissequogue River, Carmans River, and Peconic River, fig. 

14B, table 6), and the nine wells in the low-human-impact 

ground-water-level index (S15622, S1812, S33380, S3513, 

S5517, S6455, S4271, S3543, and S8833, fig. 15C, table 6). 

The three drought-monitoring indices developed in this study 

could be used by Federal, State, and local agencies to monitor 

monthly conditions in the ground-water system underlying 

Long Island during periods of below-normal precipitation and 

to determine whether ground-water levels are approaching 

A. Composite-average precipitation from the eight selected sites

0

2

4

6

8

10

12

Polynomial Trend Line

0

2

4

6

8

10

12

1950

WATER YEAR

Composite Monthly Precipitation

10 Percent Exceedance Percentile 25 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile 90 Percent Exceedance Percentile 12-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S

PRECIPITATION, IN INCHES

50 50 50 50 50 50 50 50 50 50 50 50

Figure 13. Composite-average monthly precipitation and exceedance statistics for eight selected stations on or near 

Long Island, N.Y., 1951-2000.

Future Implementation of Drought-Monitoring Indices  41
detrimental levels. This information could signal a need for 

prompt action by local water managers to help avoid water 

shortages and long-term adverse effects to the ground-water 

system.

The sites selected in this study that are not suitable to 

be part of a drought-monitoring network—Valley Stream 

and wells K1194, Q1249, and N7493, in the significanthuman-impact zone; East Meadow Brook, Mill Neck Creek, 

Massapequa Creek and wells N1614, N1616, N2528, N1259, 

S1806, and S1810, in the moderate-human-impact zone; 

and wells S16783 and S48579, in the low-human-impact 

zone—could be used by Federal, State, and local agencies 

to evaluate local or regional declines in the ground-water 

system associated with overpumping or other humaninduced changes (table 6). Indices from sites not selected for 

drought monitoring could be used in combination with the 

drought network to monitor ground-water pumpage in the 

heavily populated western parts of Long Island during severe 

droughts. This action would ensure that water levels do not 

decline sufficiently to cause permanent degradation of the 

ground-water system by saltwater intrusion, nor to lower water 

levels in streams and wetlands.

Data should be collected at each of the drought-index 

sites at least monthly to keep the averages current. These 

sites would be periodically reevaluated to assess whether 

human activity has affected the record such that it is no longer 

representative of conditions in that low-human-impact zone. 

Sites that are summarized in this report but not represented in 

the monthly composite averages also would be monitored at 

least monthly so that trends in those parts of the island could 

be identified.

Future studies are needed to determine (1) how to best 

implement the use of these indices to monitor the groundwater system, and (2) what exceedance percentiles should 

be used to implement water-use restrictions during a drought 

B. Composite-average discharge from four selected eastern

streams (low-human-impact zone)

15

25

35

45

55

65

75

Polynomial Trend Line

0

4

8

12

16

20

24

15

25

35

45

55

65

75

A. Composite-average discharge from three selected western

streams (moderate-human-impact zone)

0

4

8

12

16

20

24

1950

WATER YEAR

Composite Monthly Discharge

10 Percent Exceedance Percentile 25 Percent Exceedance Percentile 50 Percent Exceedance Percentile (median)

75 Percent Exceedance Percentile 90 Percent Exceedance Percentile 12-Month ARIMA Trend

EXPLANATION

MONTH

Maximum

10 Percent Exceedance

25 Percent Exceedance

Median

75 Percent Exceedance

90 Percent Exceedance

Minimum

Values at top of boxplot

indicate the number

of measurements 

used to calculate the

monthly statistics.

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S

DISCHARGE, IN CUBI

C

FEET PER SECOND

DISCHARGE, IN CUBIC

FEET PER SECOND

49 49 49 49 49 49 49 49 49 49 49 49

50 50 50 50 50 50 50 50 50 50 50 50

Figure 14. Composite-average monthly mean discharge and exceedance statistics for two stream-discharge index groups 

in the (A) moderate-human-impact zone, and (B) low-human-impact zone, on Long Island, N.Y., 1951-2000.

42  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
Table 6. Summary of degree of human impact and primary monitoring use of eight selected streamflow-gaging (discharge) 

stations and 20 selected monitoring wells on Long Island, N.Y., 1951–2000 or period of record.

[Refer to tables 4C and 5C for additional details. Locations are shown in figure 5.]

Gaging station or well name and station number Degree of human impact 1

Primary monitoring use

Natural conditions 

(drought) 2

Human-induced 

conditions 3

Streamflow-gaging (discharge) stations

Valley Stream at Valley Stream (01311500) Significant No Yes

East Meadow Brook at Freeport (01310500) Moderate No Yes

Mill Neck Creek at Mill Neck (01303000) Moderate No Yes

Massapequa Creek at Massapequa (01309500) Moderate No Yes

Connetquot River near Oakdale (01306500) Low Yes No

Nissequogue River near Smithtown (01304000) Low Yes No

Carmans River at Yaphank (01305000) Low Yes No

Peconic River at Riverhead (01304500) Low Yes No

Monitoring wells (ground-water levels)

Well K1194 (404059073520702) Significant No Yes

Well Q1249 (404240073443401) Significant No Yes

Well N7493 (404237073433701) Significant No Yes

Well N1614 (404446073392904) Moderate No Yes

Well N1616 (404554073351502) Moderate No Yes

Well N2528 (405101073343401) Moderate No Yes

Well N1259 (404317073291105) Moderate No Yes

Well S1806 (404442073240503) Moderate No Yes

Well S15622 (405250073180801) Low Yes No

Well S1810 (404614073164404) Moderate No Yes

Well S1812 (404958073085001) Low Yes No

Well S33380 (404932073055902) Low Yes No

Well S3513 (405146073031801) Low Yes No

Well S5517 (405149072532201) Low Yes No

Well S6455 (405223072523402) Low Yes No

Well S4271 (405743072425701) Low Yes No

Well S3543 (405037072390301) Low Yes No

Well S16783 (410634072223601) Low, but tidally affected 4 No No

Well S8833 (405756072173501) Low Yes No

Well S48579 (410316071535501) Low, but tidally affected 4 No No

1

 Refer to figure 7.

2

 Indicates that precipitation is the primary source of variability in the record (near-natural conditions). These stations are useful for drought monitoring and for monitoring the encroachment of human-induced stresses in the aquifer system.

3

 Indicates that precipitation is no longer the primary source of variability in the record, human-induced stresses such as pumping and sewering 

mask natural fluctuations. These stations are not useful for drought monitoring, but are useful for monitoring human-induced changes.

4

 Indicates that even though these stations represent near-natural conditions, fluctuations associated with tidal variations mask the natural changes in 

the aquifer system. These stations are not useful for drought monitoring, but are useful for monitoring long-term climatic and human-induced changes 

that may effect baseline averages and percentiles.

Future Implementation of Drought-Monitoring Indices  43
28 

30 

32 

34 

36 

38 

40 

42 

A. Composite-average water levels from three selected wells 

(significant-human-impact zone) 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

22 

24 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

22

24 

B. Composite-average water levels from six selected wells 

(moderate-human-impact zone) 

52 

54 

56 

58 

60 

62 

64 

66

68 

52 

54 

56 

58 

60 

62 

64 

66

68 

C. Composite-average water levels from nine selected wells 

(low-human-impact zone) 

28 

30 

32 

34 

36 

38 

40 

42 

Polynomial Trend Line 

1950

WATER YEAR 

Composite Monthly Discharge 

10 Percent Exceedance Percentile 

25 Percent Exceedance Percentile 

50 Percent Exceedance Percentile (median) 

75 Percent Exceedance Percentile 

90 Percent Exceedance Percentile 

18-Month ARIMA Trend 

EXPLANATION 

MONTH

Maximum 

10 Percent Exceedance 

25 Percent Exceedance 

Median

75 Percent Exceedance 

90 Percent Exceedance 

Minimum

Values at top of boxplot

indicate the number 

of measurements 

used to calculate the 

monthly statistics. 

EXPLANATION

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 O N D J F M A M J J A S 

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

50 50 50 50 50 50 50 50 50 50 50 50 

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

WATER LEVEL, IN FEET IN

REFERENCE TO NGVD 29

50 50 50 50 50 50 50 50 50 50 50 50 

32 28 31 28 30 29 29 27 27 25 25 30 

Figure 15. Composite-average water levels and exceedance statistics for three ground-water-level index groups in the 

(A) significant-human-impact zone, (B) moderate-human-impact zone, and (C) low-human-impact zone, on Long Island, 

N.Y., 1951-2000.

44  Statistical Analysis of Long-Term Hydrologic Records for Selection of Drought-Monitoring Sites on Long Island, New York
or other water emergency. Results obtained through these 

additional studies would give water managers a comprehensive 

and adaptable tool for drought monitoring and protection of 

the ground-water system underlying Long Island.

Summary and Conclusions

Aquifers underlying Long Island are the sole source of 

water supply for more than 3 million people on the island. 

About 1,000 public-supply wells together withdraw about 

400 Mgal/d from the ground-water system. Large-scale 

pumping projects have caused severe (greater than 15 ft) 

water-level declines that have led to saltwater intrusion and the 

curtailment of long-term pumping in some areas. The watertable recovery that followed this cessation of pumping has 

caused local flooding of basements and subways.

Installation of large-scale sanitary and storm-sewer 

systems, and the increase in impervious surfaces, such as roads 

and parking lots, has caused regional declines in ground-water 

levels; this, in turn, has caused a reduction in ground-water 

storage and a loss of freshwater habitat through the lowering 

of pond and lake levels and the reduction of stream length and 

discharge.

No method is currently available that can assess data 

from Long Island’s ground-water-monitoring network to 

provide Federal, State, and local agencies with timely warning 

of severe water-level declines. This report, describing the 

results of a study performed by the U.S. Geological Survey 

in cooperation with the New York State Department of 

Environmental Conservation, (1) quantifies past droughtand human-induced changes in the ground-water system 

underlying Long Island by applying statistical and graphical 

methods to long-term precipitation, stream-discharge, and 

ground-water-level data from selected monitoring sites; (2) 

evaluates the relation between water levels in the upper glacial 

aquifer under water-table conditions to those in the underlying 

Magothy aquifer; (3) defines trends in stream discharge and 

ground-water levels that might indicate the onset of drought 

conditions or the effects of excessive pumping; and (4) 

discusses the long-term records that were used to select sites 

for a Long Island drought-monitoring network.

The results from this report indicate that long-term 

hydrologic records on Long Island provide a sufficient 

basis from which to select sites for a drought-monitoring 

network. The data from 8 precipitation-monitoring stations, 

8 streamflow-gaging (discharge) stations, 15 monitoring 

wells screened in the upper glacial aquifer, and 5 monitoring 

wells screened in the underlying Magothy aquifer indicate 

that hydrologic conditions vary widely across Long Island— ground-water levels in parts of Kings and Queens Counties 

have risen markedly (greater than 15 ft) in response to a 

cessation in pumping, those in Nassau and western Suffolk 

Counties have declined sharply (greater than 10 ft) in response 

to pumping and sanitary- and storm-sewer systems, whereas, 

those in central and eastern Suffolk County remain relatively 

unaffected (as compared to other areas on the island), although 

the effects of human activity are discernible in the long-term 

record.

Four hydrologic-data sets were examined to produce 

three “human-impact zones” as a basis for evaluation of 

islandwide hydrologic conditions. These data sets were (1) 

population density, (2) locations of large-scale sanitary- and 

storm-sewer systems, (3) location and distribution of publicsupply wells screened in the upper glacial and underlying 

Magothy aquifers, and (4) long-term records of selected 

monitoring wells and streamflow-gaging (discharge) stations.

Fifty-year composite annual and monthly averages were 

calculated for precipitation, stream discharge, and groundwater levels. The precipitation composite represents eight 

stations on or near Long Island; the two stream-discharge 

composites represent a moderate-human-impact zone (three 

streams) and a low-human-impact zone (four streams); and 

the water-level composite represents all three human-impact 

zones—significant-human impact (three wells), moderatehuman impact (six wells), and low-human impact (nine wells).

The indices developed in this study could be used to 

monitor monthly conditions in the ground-water system 

underlying Long Island and be used in times of drought to 

predict whether the ground-water system is approaching 

detrimental levels that require prompt action to avoid longterm adverse effects and water shortages. Future studies 

are needed to determine how to best implement the use 

of these indices to monitor the ground-water system and 

what exceedance percentiles should be used to implement 

conservation during droughts.

Acknowledgments

Virtually all data on ground-water levels and stream 

discharge used in this study were collected through previous 

studies done by the USGS in cooperation with other Federal, 

State, and local agencies. Thanks are extended to all agencies 

and individuals who participated in this data-collection effort.

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