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Streams on Long Island have considerable esthetic, recreational, economic, and ecologic value. In relatively undeveloped areas, streams derive as much as 95 percent of the total flow from ground-water seepage (base flow). This report describes the application of a technique for estimating the average base flow of a stream that has only partial discharge records by relating the measured base flow of that stream to concurrent flows of a nearby stream having a continuous record. The stream's average base flow at the partial-record station is estimated from the average measured base flow for a given period at the continuous-record station and a regression equation. Data used in this analysis were from 1968-75, a period near hydrologic equilibrium on Long Island. The average base flow for this period was estimated for 20 streams that have partial-record stations. Analyses were considered acceptable if the regression coefficient was 85 percent significant or greater and the standard error of estimate was 0.5 (log units) or less. Average base flow of the nine streams that have a continuous record totaled 90 ft3/s (cubic feet per second). The predicted average base flow for the 20 streams with only a partial record was 73 ft^/s, with a 95-percent confidence interval of 63 to 84 ft3 /s. The total of 163 ft3 /s for all 29 streams represents most of the ground-water seepage to streams in the area and equals more than 25 percent of the area's recharge from precipitation. Results indicate that the method for estimating long-term average base flow is reliable for areas such as Long Island, where streams consist mostly of base flow and are geomorphically and hydrologically similar. INTRODUCTION Streams are a major component of Long Island's hydrologic system and play an important environmental and economic role. Many streams and ponds provide esthetic, ecologic, and recreational benefits, and several are the focal point of local parks, where they are used for boating and fishing and provide wildlife habitats. Two streams support freshwater fish hatcheries, and many contain spawning grounds for certain saltwater species. In addition, the continuous discharge of freshwater to the brackish bays surrounding Long Island (fig. 1), both as streamflow and as subsea discharge, maintains the delicate balance of salinity needed for the island's large shellfish industry. Thus, a primary concern in this densely populated area is the protection of these streams from contamination and depletion. Urbanization has had a marked effect on the hydrologic conditions in western Long Island (Buxton and others, 1981). Reduced ground-water recharge, which results from increases in impervious land surface and from the installation of storm sewers and sanitary sewers, has caused declines in ground-water levels, which, in turn, have reduced the base flow of streams. The hydrologic effects of urbanization have been observed in Nassau County and, to a lesser degree, in western Suffolk County (Sulam, 1979; Pluhowski and Spinello, 1978; Garber and Sulam, 1976; and Franke, 1968). Recent data indicate, however, that the hydrologic effects of urbanization are increasing eastward (Simmons and Reynolds, 1982, Prince, 1981, and Pluhowski and Spinello, 1978). The U.S. Geological Survey, in cooperation with Nassau County Department of Public Works and Suffolk County Department of Health Services, has investigated the effects of proposed sewering in Sewage Disposal District 3 (SDD-3) in southern Nassau County and Southwest Sewer District (SWSD) in southwest Suffolk County (fig. 2) on the ground-water and surface-water systems. Results indicate that the proposed sewering will decrease groundwater levels and ground-water seepage to streams (base flow) and surrounding bays (Reilly and others, 1983, Buxton and Reilly, 1985, and Reilly and Buxton, 1985). Estimation of the average base flow of streams in the area is an important step in defining the role of streams as a major discharge boundary of the Long Island ground-water system. The resulting information is needed to define initial conditions for future quantitative studies of the effects of sewers on the hydrologic system of Long Island.

Estimating average base flow at low-flow partial-record stations on the south shore of Long Island, New York