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 181

The amount of sediments and nutrients transported to the Gulf is not simply a
direct function of what is coming off the field but must also include what's lost
along the way as water moves through the drainage network (Alexander et al.
2000 ; Mueller and Spahr 2006 ). Studies conducted only at fields or small water-
sheds are not able to capture how sedimentation and nitrogen and phosphorus
concentration at multiple scales are influenced by various agricultural practices.
Effective management of water quality and ecosystem, whether directed toward
nutrient supply or abatement, requires both watershed and riverine models to
quantify the transport and fate of nutrients throughout the basin system.
The Soil and Water Assessment Tool (SWAT) model has been widely used to
quantify the flow, sediment and nutrient loadings in large river basins (Gassman
et al. 2007 ). The SWAT model can be used to identify the location and magnitude
of sediment and nutrient runoff hotspots associated with crop production and land
use changes. The flow-routing methods presently used in SWAT are hydrological
routing methods that are based on the continuity equation and on empirical rela-
tionships to replace the momentum equation. In SWAT, one-dimensional (1D)
hydrologic model is used to mathematically represent flow routing along a river
reach. In this case, simplified schemes, such as linear reservoirs, Muskingum-
Cunge methods may be applied. When dealing with large scale rivers, however,
backwater effect and floodplain inundation may become governing factors for
flood wave routing, and a 1D-hydraulic model is a more suitable method. The
weakness of SWAT models for dynamic flow routing in the large scale watershed
areas is well known. It is very important to perform the flow routing process
accurately because routed results affect other aspects such as sediment routing and
the in-stream nutrient process, both of which are strongly tied to water routing. To
address this problem, a 1D unsteady state flow model (UNET) developed for the
main stem of the Illinois River was coupled with the HSPF model to perform the
flow routing (Lian et al. 2007 ). SWAT was also coupled with 2D hydrodynamic
and water quality model (CE-QUAL-W2) in the Cedar Creek Reservoir study
(Debele et al. 2008 ). The results indicated that the two models are compatible and
can be used to assess and manage water resources in complex watersheds com-
prised of watershed and receiving waterbodies.
In this study, an integrated watershed and riverine modeling system was
developed. The water quality impacts of crop production and land use changes in
the Upper Mississippi River Basin (UMRB) were evaluated. The UMRB is the
main focus for future renewable bioenergy sources and provides a majority of
conventional and potential cellulosic feedstock for biofuel production (USDOE
2011 ). Results of the basin's SWAT model were used as the mainstem Hydrologic
Engineering Center-River Analysis System (HEC-RAS) model boundary condi-
tions and inputs for evaluating the transport and fate of sediments and nutrients in
the Upper Mississippi River. An integrated modeling system can be used to pre-
dict: (1) Spatial and temporal patterns of nutrient export in the watershed; (2)
Nutrient cycling and microbial and chemical reactions within the river; (3) Long-
term changes in riverine nutrient concentrations and their causes; (4) Effects of
watershed nutrient loading on the downstream water quality; and (5) Effectiveness