Page 198 - Biosystems Engineering

176 Cha pte r F i v e

Model (SWMM). SWAT has its own GIS interface that uses the Envi-
ronmental Systems Research Institute’s (ESRI) ArcView 3.X and Arc-
GIS GIS softwares. SWAT is also linked to the water quality model
QUAL2E for in-stream nutrient processes.
AGNPS and AnnAGNPS Pollutant Loading Model
AnnAGNPS, developed by the USDA-ARS at the National Sedimen-
tation Laboratory, Oxford, Mississippi, is a batch-process continuous-
simulation, distributed parameter, watershed-scale pollutant–loading
computer model developed in ANSI Fortran 90. This model (Young
et al. 1987; Bingner and Theurer 2003) is mainly an expansion of the
capabilities in the AGNPS single-event model.
AGNPS, a watershed-scale, distributed parameter, event-based
model, was designed to simulate runoff, sediment, and nutrient
transport. Square cell representation was used in AGNPS to represent
field boundaries. AGNPS used the USDA SCS runoff curve number
and unit hydrograph method for calculating flow volume and peak
flow, respectively. The modified USLE was used for erosion, and a
simple correlation of extraction coefficients was used for nutrient and
sediment. Capabilities were also provided in AGNPS to simulate
gully, wastewater treatment plants, and feedlot point source pollu-
tion as well as a number of agricultural practices. Flow, sediment, and
nutrients could be calculated in every cell within the watershed.
AnnAGNPS divides a watershed into homogeneous land areas
with respect to soil type, land use, and land management. The areas
can be of any size and any shape, including hydrologically-based and
square grids. AnnAGNPS simulates movement of surface water, sed-
iment, N, P, and pesticides leaving the land areas as well as their
travel through the watershed. Portions of water, sediment, N, P, and
pesticides reach the watershed outlet while the remainder is depos-
ited within the watershed. Calculations are done on a daily time step
and pollutant loads generated from land areas are routed through the
stream systems on a daily basis.
AnnAGNPS is suitable for evaluating long-term NPS pollution
from agricultural watersheds. AnnAGNPS is also suitable for compar-
ing the effects of alternative cropping and tillage systems, fertilizer,
pesticides, and irrigation application rates and feedlot management
systems on levels of NPS pollutants. AnnAGNPS is also capable of
simulating point source loads. AnnAGNPS generates both event- and
source-accounting outputs. Source accounting indicates the fraction
of a pollutant loading at the watershed outlet or at a particular reach
coming from a user-identified source location. Outputs generated by
AnnAGNPS are water, sediment, nutrients, and pesticides at the
user-defined watershed source location (specific cell, reach, feedlots,
point sources, and gullies).
AnnAGNPS uses the SCS curve number method for determining
runoff volume and peak discharge is calculated using a procedure