Environmental Engineering Reference
In-Depth Information
10.4 Application of the Integrated Modeling System
to the Upper Mississippi River Basin
The SWAT model inputs consist of topography, soil properties, land use/cover
type, weather/climate data, and land management practices. The study watershed
is divided into subbasins. The USGS divided the UMRB into 131 eight-digit-level
subbasins with an average drainage area of 3,755 km
2
(Fig.
10.8
). Each subbasin
is further divided into hydrological response units (HRU) based on topography,
land use, and soil.
We have calibrated and validated the SWAT outputs against observed data.
Those results showed that the SWAT model properly reproduced observed flow,
sediment, total nitrogen and phosphorus from the basin. We now have used those
calibrated outputs from the SWAT model as input into the HEC-RAS model and
calibrate the hydraulic and water quality processes in the UMR.
The dominant cause of nutrient flux to the UMR is agricultural activities in the
UMRB. Corn and soybean fields are the largest source of N input (52 %) followed
by atmospheric deposition (16 %). Among agricultural land uses, the largest P
input was from pasture and range lands (animal manure; 37 %) followed by corn
and soybeans (25 %). In contrast to N, urban sources (12 %) made a significant
contribution of P to the UMR (Alexander et al.
2000
). The SWAT model reveals a
consistent trend of heavy nitrogen and phosphorus exports from areas of intensive
agricultural production in the UMRB.
The SWAT model, in addition to runoff, outputs sediments and water quality
variables, such as total suspended solids (TSS), nitrogen species (organic nitrogen,
nitrate/nitrite, and ammonia/ammonium), phosphorus species (organic and mineral
phosphorus), chlorophyll-a, dissolved oxygen (DO), and carbonaceous biochemical
oxygen demand (CBOD). The HEC-RAS/NSM I model has the same water quality
state variables as the SWAT model and accepts inputs in terms of one-to-one
relationship. We used a separate interface tool that imports SWAT outputs into an
appropriate time-series dataset within the HEC-DSS for HEC-RAS. The interface
extracts daily SWAT outputs (runoff and its water quality constituents) at required
reach or subbasin outlets into HEC-DSS file that is acceptable to HEC-RAS.
Principal tributaries of the UMR are the Minnesota, St. Croix, Wisconsin, Rock,
Iowa, Des Moines, Illinois, and Missouri Rivers and several smaller rivers and
streams. In the 1,076 km of river between the first lock, Upper St. Anthony Falls,
and the last lock of the Channel Navigation Project, Lock 27, the UMR falls 128 m
with an average slope of approximately 9.5 cm/km. Average flow of the UMR
ranges from 280 m
3
s
-1
at St. Paul, Minnesota, to 4,955 m
3
s
-1
at St. Louis,
Missouri. Data input for HEC-RAS included geometric data to represent river
networks, channel cross-section data, and hydraulic structure data such as bridges
and culvert data. Cross-section data includes station-elevation data, main channel
bank stations, downstream reach lengths, roughness coefficients, and contraction
and expansion coefficients. Hydrologic events are represented by flow data. Time-
series predicted by SWAT for all of the required constituents and drainage sources
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