Assessment of nutrient trends and loads for the Mobile River Basin
Conference Proceedings
By Douglas Harned and J. Brian Atkins
Abstract
U.S. Geological Survey (USGS)
surface-water sampling sites in the Mobile River Basin were reviewed for
available nutrient data for the period 1970-97. Sites having sufficient
nutrient data were evaluated for long-term trends in nutrient concentrations
and nutrient transport was calculated for selected basins. The effort was
undertaken as part of the National Water-Quality Assessment Program.
USGS sampling sites having periods of continuous
streamflow and nutrient data were examined from the period 1970-97 for
long-term trends with the Seasonal Kendall trend test. Trends and loads were
estimated for total nitrogen at 15 sites and for total phosphorus at 14 sites.
The Seasonal Kendall trend test adjusts for seasonal variability using
nutrient concentrations adjusted for the effects of streamflow with residuals
from LOWESS (LOcally Weighted Sum of Squares) smoothed curves. Trends also
were determined for sites without continuous data using multivariate
regression analysis.
The Alabama River at Claiborne, Ala., and Tombigbee
River at Coffeeville, Ala., sites were used to represent the most downstream
locations in the Mobile River Basin. The trends identified included decreasing
concentrations of total nitrogen, which suggest that there has been an overall
reduction in the nitrogen contributions to Mobile Bay from the Mobile River
from the mid-1970s to the present. Total nitrogen concentrations have also
decreased (1980-95) in the Black Warrior River, one of the
major tributaries to the Tombigbee River. Total phosphorus concentrations have
increased from 1970 to 1996 at three stations on the Etowah River in Georgia.
Total nitrogen and total phosphorous loads were
calculated using multivariate linear regression with bias correction by a minimum variance unbiased estimation method (Cohn and others,
1989). Total nitrogen loads were estimated for the Tombigbee River at Coffeeville,
Ala. (66.2 tons per day average for 1988-96); and Alabama River at Claiborne,
Ala. (59.5 tons/day). The yield for the Tombigbee River site (1.31 tons/mi2/yr),
which drains a greater percentage of agricultural (row crop) land use, was
greater than that of the Alabama River site (0.99 ton/mi'/yr).
Two headwater tributaries to the Tombigbee River had
the highest estimated total nitrogen yields of the 15 sites evaluated: Tibbee
Creek (2.61 tons/mi2/year) and Luxapallila Creek (2.17 tons/mi2/year).
Other sites having nitrogen yields higher than 1.75 tons/mi2/yr
included the Cahaba River (1.98 tons/mi2/yr), and the Little Cahaba
River (1.85 tons/mi2/yr), both located in the Alabama River Basin.
Total phosphorous loads in the Tombigbee River at
Coffeeville were estimated at 8.8 tons/day (mean 1988-96) and 6.5 tons/day for
the Alabama River at Claiborne. The yield for the Tombigbee River at
Coffeeville (0.175 tonJmi2/yr) was greater than that of the Alabama
River at Claiborne (0.108 ton/mi2/yr). The phosphorus yield for the
Tombigbee River near Fulton, Miss. (0.164 ton/mi2/year) in the
headwaters of the basin was similar to that estimated at the most downstream
station at Coffeeville, indicating little variation in nutrient concentration
from the subbasin. The highest phosphorus yield was estimated for the Noxubee
River (0.265 ton/mi2/yr) located in an agricultural area of the
Tombigbee Basin. Tibbee Creek, another headwater tributary in an agricultural
area of the Tombigbee River, indicated a total phosphorus yield of 0.222 ton/mi2/yr.
Other yields higher than 0.16 tons/mi2/yr were estimated for the
Etowah River at Canton, Ga. (0.188 ton/mi2/ year), the Etowah River
at Rome, Ga. (0.163 ton/mi2/yr), and the Coosa River near Rome, Ga.
(0.241 ton/mi2/year). These three rivers drain developed areas around Rome,
Ga., and flow into Weiss Lake in Alabama-an impoundment of the Coosa River.
Weiss Lake has been classified as eutrophic.
Citation:
Harned, D.A., and Atkins, J.B., 1999, Assessment of nutrient trends and loads for the Mobile River Basin, in Hatcher, K.J., ed., Proceedings of the 1999 Georgia Water Resources Conference: Athens, Ga., Institute of Ecology, The University of Georgia, Mar. 30-31, 1999, p. 191.
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