Environmental Engineering Reference
In-Depth Information
10.3 Integrated Watershed and Riverine Modeling System
10.3.1 SWAT Model
The SWAT is a lumped parameter watershed model developed and maintained by
the U.S. Department of Agriculture (USDA) Agriculture Research Service (ARS).
SWAT uses algorithms from a number of previous ARS models including
CREAMS, GLEAMS, EPIC, and SWRRB (Arnold et al.
1998
). The SWAT model
was developed to assess the impact of land management and climate patterns on
water, sediment, and agricultural chemical yields over long time periods in large
watersheds. The watershed is partitioned into a number of subbasins. Each subbasin
possesses a geographic position in the watershed and is spatially related to adjacent
subbasins. Each subbasin is further divided into hydrological response units (HRU)
based on topography, land use, and soil. HRUs are the smallest computational units
in SWAT with unique land use, soil type and slope within a subbasin. Thus, SWAT
can take two levels of the spatial heterogeneity into account. The first level (sub-
basin) supports the spatial heterogeneity associated with hydrology, and the second
level (HRU) incorporates the spatial heterogeneity associated with land use, soil
type and slope class. Within a subbasin, SWAT does not retain the spatial location
of each HRU. The loss of spatial information within the subbasin introduces a
measure of unrealism and requires caution in interpreting model results. In SWAT,
hydrologic, soil, water quality and other processes are modeled within the subbasins
through the use of HRUs. Flow generation, sediment yield, and pollutant loadings
are summed across all HRUs in a subbasin, and the resulting flow and loads are then
routed through channels, ponds, and/or reservoirs to the watershed outlet. SWAT
typically produces daily results for every subbasin outlet, each of which can be
summed to provide monthly and annual load estimates.
Major model components include climate, hydrology, nutrient cycle, pesticide,
plant growth, and land management. For climate, SWAT uses the data from the
station nearest to the centroid of each subbasin. The hydrology module simulates
major hydrologic components and their interactions as simple responses using
empirical relationships (Fig.
10.4
). Precipitation in the form of either rainfall or
snowfall is the major driving mechanism of the hydrologic cycle. SWAT calcu-
lates actual ET based on potential evapotranspiration (PET) from soils and plants
separately. PET can be estimated by three methods: Priestley-Taylor (Priestley and
Taylor
1972
), Hargreaves (Hargreaves and Samani
1985
), and Penman-Monteith
(Allen et al.
1989
). Surface runoff volume and infiltration are computed with the
modified curve number method or Green and Ampt equation. The peak rate
component uses Manning's formula to determine the watershed time of concen-
tration and considers both overland and channel flow. The soil profile is subdivided
into multiple layers that support soil water processes including infiltration, evap-
oration, plant uptake, lateral flow, and percolation to lower layer. A storage routing
technique is used to calculate redistribution of water between layers in the soil
profile. Lateral subsurface flow in the soil profile is calculated simultaneously with
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