northern parts of the study area. Assuming the average annual precipitation 

 is approximately kS inches on the 19^0 square miles of land and 36 inches on 

 the Sound the Long Island Sound Region receives approximately 2,300 billion 

 gallons of water each year in the form of rain and snow. 



The precipitation that falls on the land under natural conditions 

 is either returned to the atmosphere by evaporation and transpiration (evapo- 

 transpi ration) or flows across the land and through subsurface materials to 

 stream channels, estuaries and the Sound. The average annual evapotranspi ra- 

 tion is approximately one half of the average annual precipitation. Almost 

 all evapotranspi rat ion occurs in the growing season (April through October) 

 when plants use a large amount of water, temperatures are above freezing and 

 hours of daylight are longest. 



The water that is not evapotranspi red either flows over the land 

 surface, to streams (direct runoff) or infiltrates downward through the 

 various earth materials eventually reaching the saturated zone. Most of the 

 subsurface water in the saturated zone, termed ground water, moves toward 

 and discharges into stream channels, estuaries and the ocean. The total 

 fresh water runoff into Long Island Sound is the sum of the direct runoff 

 and ground-water discharge from the entire Long Island Sound land drainage 

 area of approximately 16,500 square miles. The average annual total runoff is 

 approximately 6,200 billion gallons per year (bgy) or 5.5 cubic miles of 

 water as computed by the U.S. Geological Survey. 



The average annual runoff into Long Island Sound is almost three 

 times as large as the average annual precipitation within the study region. 

 Water quality, flood control and sedimentation are therefore significantly 

 affected by natural events and man's activities in the large 1U,500 square 

 mile drainage area north of the region boundary. 



Long Island contrasts sharply with the land on the north shore 

 of the Sound in respect to the pattern of water circulation. Long Island is 

 underlain by a wedge-shaped mass of unconsolidated sediments that rests on a 

 gently southeast-sloping surface of relatively impermeable bedrock. The 

 saturated zone beneath Long Island constitutes a large ground-water reservoir 

 containing in excess of 60 trillion gallons of fresh ground water (5).' The 

 fresh and salty ground water and fresh and salty surface water are hydrau- 

 lically connected and Long Island both within and out of the study region is 

 an integrated hydrologic system. 



Because Long Island is relatively flat and the surface materials 

 are permeable almost all the precipitation not consumed by evapotranspi ra- 

 tion percolates down to the saturated zone. Only 5 to 10 percent of the 

 streamflow consists of direct runoff and the ground water and surface drain- 

 age divides are not coincident. The ground water beneath Long Island is 

 constantly moving vertically downward and horizontally toward the shores 



1 - Underlined numerals in parentheses are references in Appendix A. 



