Page 197 - Biosystems Engineering

GIS-Based W atershed Modeling Systems 175

5.2.5 Examples of Commonly Used Watershed Models
Currently, a number of NPS models exist that are designed to address
specific problem domains. Some of the more widely used watershed-
scale NPS models are described in this section.

SWAT: Soil and Water Assessment Tool
The SWAT model (Neitsch et al. 2002) is a modified version of the
simulator for water resources in rural basins (SWRRB) and routing
outputs to outlets (ROTO) models for application to large, complex
rural basins and uses new routing structures. Simulator for water
resources in rural basins—water quality (SWRRBWQ) was a continu-
ous simulation, daily time–step computer model developed to simu-
late hydrologic and nutrient-transport processes in rural basins. It
was designed to predict the effect of management decisions on water,
sediment, nutrients, and pesticide yields at the subbasin or basin out-
let. In SWRRBWQ a basin could be divided into a maximum of 10
subbasins to account for differences in soils, land use, crops, topogra-
phy, vegetation, or weather. SWRRBWQ also had a water quality
component that tracked the fate of pesticides and P from their initial
application on the land to their final deposition in a lake. SWRRBWQ
could be used to model the effect of farm-level management systems
such as crop rotations, tillage, planting date, irrigation scheduling,
and fertilizer and pesticide application rates and timing. The modi-
fied universal soil loss equation (MUSLE) was used to determine
sediment yield. Nutrient, pesticide, and sediment yields at the basin
outlet were determined after accounting for channel transmission
losses and deposition in the subbasins.
SWAT is an extended and improved version of SWRRB, running
simultaneously in several hundred subbasins to predict the effects of
management practices on sediment and chemical yields from large
river basins. SWAT has the ability to simulate surface flow, subsur-
face flow, sediment, nutrients, pesticides, and bacteria in addition to
various best management practices (BMPs): agricultural practices,
ponds, and tile drains, for example. Management practices are
handled within the MUSLE. Soil Conservation Service (SCS) curve
numbers can also be varied throughout the year to take variations in
management conditions into account. SWAT divides the watershed
into HRUs that have uniform properties. Edge-of-filter strips may be
defined in an HRU. The filter strip trapping efficiency for sediment is
calculated empirically as a function of the width of the filter strip.
When calculating sediment movement through a body of water,
SWAT assumes that the system is completely mixed. Settling occurs
only when the sediment concentration in the water body exceeds
the equilibrium sediment concentration specified by the user. The
sediment concentration at the end of a day is determined based on
an exponential decay function. SWAT also simulates the buildup
and washoff mechanisms similar to the Storm Water Management