Long Island Precipitation and Recharge

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Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain. 

Water seeping down from the land surface and reaching the water table adds to the groundwater and is called groundwater recharge. Groundwater is recharged from rain water and snowmelt. Groundwater also can be recharged from other sources such as, water-supply system (pipeline) leaks, on-site septic systems, and when crops are irrigated with more water than the plants can use (Clark and Briar, 1993).

(Clark and Briar, 1993)


The first comprehensive precipitation study covering Long Island was done by. On Long Island, average monthly precipitation throughout the year is fairly constant (about 3 to 4.5 inches per month), but on an annual basis can vary by location (see figure below) from around 40 inches per year in southern Nassau County to about 50 inches per year in west-central Suffolk County (figure 8) (Miller and Fredrick, 1969).

graph of Mean precipitation on Long Island, N.Y. 1951-1965

Figure 8.  Mean precipitation on Long Island, N.Y. 1951-1965(Public domain.)


In the report, precipitation on Long Island averaged about 43 inches for the analysis period. (Peterson (1987), compares precipitation values given by (Miller and Frederick (1969) with more recent and extended long-term averages and calculated the average Long Island precipitation to be 45.2 inches per year. 

Much of the precipitation on Long Island never reaches the groundwater system because it is lost through evapotranspiration and as direct runoff. The remainder known as recharge, is the amount that infiltrates through the land surface and percolates downward to the water table, entering the groundwater-flow-system.  Natural recharge can be calculated from the following equation:

Recharge = Precipitation - Evapotranspiration - Direct Runoff

Recharge investigations across Long Island estimate that the average annual recharge is approximately 50 percent of Long Island’s annual precipitation. USGS studies on Long Island have shown that the construction of catchment or “recharge” basins can enhance recharge to the aquifer system by intercepting street runoff. Conversely, large scale sewering can direct water that was formerly recharged to the aquifer from individual septic systems to a central water treatment facility, where it is directed to tidewater and thus removed from the aquifer system.

The rate at which precipitation replenishes the groundwater system may affect future water supplies in some areas. Annual precipitation on Long Island averages 45.2 inches per year, but less than 23 inches (or 50 percent), recharges the groundwater system (Peterson, 1987).

graph of Mean long-term annual recharge in Nassau and Suffolk Counties

Figure 9. Mean long-term annual recharge in Nassau and Suffolk Counties(Public domain.)

The rate of recharge varies locally and ranges from 29 to 57 percent of precipitation (figure 9) depending on land use, season, and amount of storm sewering in the area. Recharge was calculated by subtracting evapotranspiration and direct runoff values from known precipitation values. Evapotranspiration was calculated by the Thornthwaite and Mather method. Direct runoff rates to streams were calculated from streamflow records and size of known storm-sewer service areas (Peterson, 1987).


Table of Contents

State of the Aquifer, Long Island, New York - Introduction

Location and Physical Setting


  1. Hydrolgeologic Units
  2. Fresh and Saltwater Relations/Interactions

State of the Aquifer System

  1. Precipitation
  2. NWIS - the USGS Data Archive 
  3. Surface Water - Streamflow
  4. Groundwater Levels
  5. Water Table and Surface Maps
  6. Water Use
  7. Groundwater Budget
  8. Inflow to the Groundwater System
  9. Outflow from the Groundwater System
  1. Case Studies

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