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remaining at the cell outlet (USDA ARS 1995). Through the 
routing function, AGNPS links the upland erosion, sediment, 
and nutrients (N, P, and COD) with the downstream water 
quality. This feature allows the examination of amount of 
sediments and nutrients either for the entire watershed 
(measured at the watershed outlet) or on a cell by cell basis. By 
comparing runoff estimates from individual cells, problem areas 
within the watershed can be identified for targeting the best 
management practices (He et al. 1993). 
AGNPS requires 22 input parameters. These include: (1) cell 
number, (2) cell division, (3) receiving cell number, (4) receiving 
cell division, (5) flow direction, (6) SCS curve number, (7) land 
slope, (8) slope shape, (9) field slope length, (10) overland 
Manning’s roughness coefficient, (11) soil erodibility (K) factor, 
(12) cropping management (C) factor, (13) support practice (P) 
factor, (14) surface condition constant (adjustments for the time 
it takes channelization of overland runoff), (15) chemical oxygen 
demand, (16) soil texture, (17) fertilizer indicator, (18) pesticide 
indicator, (19) point source indicator, (20) additional erosion 
indicator, (21) impoundment indicator (number of ponds in the 
impoundment terrace system), and (22) channel indicator 
(indication of the number of channels in the cell) (USDA ARS 
1995). As AGNPS operates on a cell basis, each cell, when 
considered separately represents 22 layers of input data. 
Output of AGNPS includes estimates of surface runoff volume 
(inches), peak flow rate (in cfs), sediment yield (tons), mass of 
sediment attached and soluble N in runoff (Ibs/acre), mass of 
sediment attached and soluble P in runoff (Ibs/acre), and soluble 
chemical oxygen demand (Ibs/acre). These results can be 
viewed in either tabular or map format for examination of critical 
runoff, sediment, and nutrient loading areas. 
2.2 Development of ArcView-AGNPS Interface 
Analysis of nonpoint source pollution in an agricultural 
watershed by AGNPS involves providing 22 input parameters 
for each of the cells that represent the entire watershed, which 
is often a tedious and time-consuming task. To address this 
issue, a number of GIS-AGNPS interfaces have been developed 
such as GRASS-AGNPS (Engel et al., 1993; He et al., 1993; 
and Line et al., 1997) and Arc/Info-AGNPS (Liao and Tim, 
1997). Those interfaces operate in UNIX environment. In this 
study, we develop ArcView Nonpoint Source Modeling 
(AVNPSM), a WINDOWS-based interface to integrate the 
AGNPS with ArcView (Version 3.0a or later versions) Spatial 
Analyst and AGNPS using Avenue (a programming language for 
ArcView) scripts (He et al. 2001). 
The basic databases required for the AVNPSM include: soil 
database, digital elevation, land use/cover, water features such 
as watershed boundary and course of streamflow, climate, and 
crop management information. A soil database such as 
STATSGO (State Soil Geographic Data Base) is used to extract 
information on soil texture, hydrologic group, and soil erodibility 
factor (K). A digital elevation model (DEM) is used to derive 
slope, slope length, aspect, and other related parameters. Land 
use/cover file is used to determine SCS curve number and 
management factors such as crop management (C), 
fertilization , and support practice (P ), etc. The water feature 
database is used to help create the watershed coverage and 
process and edit the flow direction file. Climate data (storm 
events) are used to calculate surface runoff and soil erosion in 
the AGNPS model. Management information includes crop 
types and rotation, fertilization level, and tillage practices. These 
files need to be processed to either an Arc/Info coverage or 
ArcView shape format to be compatible with the format 
requirement of the AVNPSM interface. 
Once the input files are ready, the interface can generate the 
required AGNPS parameters (Parameter Generator), create an 
AGNPS input file (Input Processor), display the simulated 
AGNPS output (Output Visualizer), and conduct statistical 
analysis such as central tendency and analysis of variance 
(Statistical Analyzer). These components of the interface are 
discussed separately below: 
Parameter Generator. The AVNPSM, developed using ArcView 
Avenue scripts (ESRI, Inc., 1996), provides a pull-down menu 
to generate the required parameters. As shown in Fig. 1, a user 
first needs to set global variables, that is, giving the name and 
location of the basic GIS layers: soil, DEM, land use/cover, and 
water features (watershed boundary). The user can then list 
these global files to ensure they are set correctly. Once this is 
done, the user can follow the pull-down menu to generate each 
parameter sequentially. During the FISHNET (file name for 
dividing the study watershed into grids based on the watershed 
boundary database) creation (in the AGNPS Utility module), the 
user can determine the number of grids (cells) in a watershed 
either by grid size or by number of cells. The interface will 
create a grid file covering the entire watershed. For 
topographically related parameters (from Flow Direction to 
Slope Shape), the interface uses some of the ArcView Spatial 
Analyst built-in functions (Flow Accumulation, Flow Direction, 
Slope, Aspect) to extract those parameters. 
The Flow Direction, once created, needs to be carefully edited 
to remove any loops in the file, i.e. circles of flow involving 
several adjacent cells in the file. Those loops need to be 
removed. A separate pull-down menu is created to facilitate this 
process. The user can go to AGNPS Utility module to edit the 
flow direction either by one cell at a time or several cells at a 
time. 
The K-factor (soil erodibility) and soil texture are generated from 
the soil database such as the STATSGO or digitized soil maps. 
The soil texture includes 4 classes: sand, clay, loam, and peat 
or water. Values for the K factor are derived from STATSGO 
corresponding to the soil texture. 
The land use/cover related parameters such as SCS Curve 
Number, Manning's coefficients, C factor, fertilizer and 
pesticide application etc. can be entered either by land cover 
category or by block of cells. The information for these 
parameters comes from agricultural statistics, literature, and soil 
and water conservation district personnel 
Input File Processor. Once all the 22 parameters are 
generated, the user can go to the File Processor module to 
develop an input file for AGNPS model. The file is in ASCII 
format and compatible with AGNPS input format requirement. 
Model Executor. AGNPS model execution is done either within 
Windows or separately in the simulated DOS mode. Depending 
on the number of grids in the input file, model execution takes 
no more than a couple minutes. 
Output Visualizer. The simulated AGNPS results of hydrology, 
sediment, and nutrients can be viewed either in tabular or in 
map format. Users can select any variable from the output file 
and display it in ArcView for analysis of spatial pattern using the 
Output Visualizer. 
Statistical Analyzer. Many current GIS packages have limited 
statistical capabilities (Steyaert and Goodchild 1994). Although 
able to perform central tendency analysis such as mean and 
standard deviation, the ArcView GIS (Version 3.1) lacks other 
statistical functions. The AVNPSM interface adds the ANOVA 
(analysis of variance) function to the ArcView and enables a 
user to examine the relationships of land use/cover and 
simulated results of hydrology, sediment, and nutrients. 
Land Use Change Simulator. A land use change icon (P icon) 
in the AVNPSM interface allows a user to specify land use 
change scenario in a sub-basin or specific area based on the 
land use/cover file and evaluate the hydrologic impact of this 
change to the downstream area.