Evaluating the Transport and Fate of Nutrients in Large Scale River Basins Using an Integrated Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 180

assess the effectiveness of different watershed management practices whether
directed toward nutrient source supply or abatement.
Keywords SWAT HEC-RAS Watershed model Riverine model Hydrology
Land use change Sediment Nutrient transport
10.1 Introduction
Nutrient pollution is a leading cause of water quality impairment in lakes and
estuaries and is also a significant issue in rivers (USEPA 2007 ). The Mississippi
River and tributary streams have been greatly impacted by excess nitrogen,
phosphorus, and sediment loadings from cropland and other sources. The impor-
tance of linkages between watershed processes, and the environmental and eco-
logical responses of aquatic resources are best exemplified by the formation of an
oxygen deprived area in Gulf of Mexico. The hypoxic zone in the Gulf varies in
size from year to year, in 2007, it was the third-largest hypoxic zone on record
(Devine et al. 2008 ). The hypoxic zone has become a serious threat to commercial
fishing, shrimping, and recreation industries. A number of factors contribute to the
size of the dead zone, but nutrient pollution substantially drives the problem. The
formation of hypoxic zone in the Gulf is a problem caused in large part by
excessive inputs of sediments and nutrients arising from industrial and agricultural
activities in the Mississippi River Basin (MRB) (Diaz and Rosenberg 2008 ). Soil
erosion that moves sediments and sediment-bound nutrients and pesticides into
waterways is another factor influencing water quality in the Gulf. In particular,
nitrogen is more abundant in dissolved forms, whereas phosphorus is largely
present in particulate forms (either adsorbed or as a constituent of inorganic and
organic particles). As a result, there is a strong correlation between suspended
sediment and total phosphorus concentrations, and changes to the river system that
alter the flow of water or sediment in the system are likely to cause a larger change
in the concentration and transport of phosphorus than of nitrogen (Wetzel 2001 ).
The evidence now suggests that both nitrogen and phosphorus affect the size of the
dead zone (Devine et al. 2008 ).
Nutrients enter the Mississippi River and the Gulf from a variety of sources,
including fertilizer runoff from farms, golf courses, and lawns; manure disposal;
discharge from sewage treatment plants and industrial facilities; nitrogen deposi-
tion from the atmosphere; and erosion of nutrient-rich soil. The data provided by
USGS shows that areas with significant agricultural uses are the largest contrib-
utors of nutrient pollution to the Gulf. The United States Geological Survey
(USGS) found that on average, from 2001 to 2005, the Upper Mississippi and
Ohio-Tennessee River basins represent about 31 % of the total land area within the
MRB, they contribute about 82 % of the nitrate-nitrogen flux, 69 % of the total
Kjedahl Nitrogen (TKN), and 58 % of the total phosphorus flux.