Estimation of nitrogen yields and loads from basins draining to Long Island Sound, 1988–98

Monitoring data on total nitrogen concentrations
and streamflow were used to estimate annual
nonpoint nitrogen loads for 1988?98 at 28 monitoring
sites and 26 unmonitored basins that drain
to Long Island Sound. The estimated total nitrogen
yields at monitoring sites were used with basin
characteristics and ancillary data to develop a
multiple-linear regression equation to estimate
nonpoint nitrogen yields from monitored and
unmonitored basins. The estimated nonpoint
nitrogen load to Long Island Sound from the
basins studied ranged from 21 million pounds in
water year 1995 to 50 million pounds in water year
1990.
Statistically significant regression variables
include time, population density, annual mean
runoff (minus wastewater return flow), pointsource
nitrogen yields, percentage of basin area
classified as urban/recreational grasses,
percentage of the basin classified as agricultural
land, and the ratio of deciduous to total forest area.
Nonpoint nitrogen loads from monitored and
unmonitored basins were computed using the
regression equation by setting the point-source
nitrogen yields and wastewater return variables to
zero, and incorporating streamflow information
from index stations in or near unmonitored basins.
Nonpoint nitrogen load information obtained
through use of this equation was summarized by
six Long Island Sound management zones.
Estimates of nonpoint nitrogen loads from
these basins can be improved by additional
sampling, and by developing data on nitrogen
loads from municipal wastewater-treatment facilities
outside of Connecticut, compiling information
on annual interbasin diversions of flow, studying
instream losses of nitrogen, and analyzing the
processing and storage of atmospheric nitrogen in
different forest types.