ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS’’, Bangkok, May 23-25, 2001
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2.3 Execution of AGNPS
The entire Dowagiac River Watershed is divided into 4803 cells,
with the size of each cell equal to 40 acres (16 ha). Digital
elevation data at a scale of 1: 250,000 were acquired from the
U.S. Geological Survey. State Soil Geographic Data Base
(STATSGO) was obtained from the U.S. Department of
Agriculture Natural Resource Conservation Service. The
watershed boundary was acquired from the Michigan
Department of Environmental Quality Hydrologic Studies Unit.
Land use/cover data of 1978-1981, the only data available for
the study area, were obtained from the Michigan Resource
Information System (MjRIS) for the entire Dowagiac River
Watershed.
The single storm event chosen in the model was a 24-hour
rainfall of 4.5 inches with an exceedance probability of 4
percent (recurrence interval of 25 years) from the Dowagiac
Weather Station (Sorrel and Hamilton 1990). In the Dowagiac
River Watershed, this type of storm usually occurs in the
summer months (June through August).
The AVNPSM was used to derive all the 22 input parameters
and generate an input file for the AGNPS model. AGNPS
Version 5.0 was run to produce estimates of surface runoff, soil
erosion and sediment, and nutrient loadings.
3.SIMULATION OF LAND USE CHANGE SCENARIO AND ITS
IMPACT ON WATER QUALITY
Residential and commercial developments are reportedly
considered as the two most important land use issues in the
Dowagiac River Watershed according to a joint report by the
Cass County Conservation District, Michigan Department of
Environmental Quality and Southwestern Michigan Commission
(1998). Several townships are reported to .likely have
population growth of more than 10 percent during the period of
2000-2010. Accompanying the projected population growth
comes increasing demands for residential housing and
commercial services. These demands will likely lead to
conversion of some agricultural, forest, and nonforest vegetation
(grassland) to residential and commercial lands. To help
resource planners and managers understand the potential
impacts of these developments on water quality in the Dowagiac
River, a land use change scenario is simulated and its water
quality effect is analyzed by using the AGNPS model.
The simulated land use change scenario is to convert the
northwest corner of the La Grange Township to residential and
commercial developments because it is adjacent to the City of
Dowagiac and very likely to face residential and commercial
expansion issues in the next decade. The selected area is about
1,400 acres (560 ha) and includes urban, agricultural, nonforest
vegetation, forest, and forested wetland. Conversion of this area
to urban land represents an expansion of the current urban land
by about 1240 acres. Assuming all other factors remaining
same, the AGNPS model was run to simulate the effect of the
changed land use on water quality and the result is summarized
in Table 1.
As shown in Table 1, conversion of about 1,240 acres of non-
urban land to urban land is simulated to cause little increase in
the peak flow rate and sediment yield at the mouth of the river
but has almost no impact on runoff volume and nutrient yields
(Fig.2). This is probably due to the fact hat the size of the
proposed land use change was too small to have any impact at
the watershed level. If more land in the watershed is converted
to urban uses, it might lead to greater changes in discharge and
sediment yield at the outlet of the watershed. In addition, the
hydrologic effects are related to the types of land use change
and the locations of the changes. The simulated land use
change is in the northwest corner of the La Grange Township,
adjacent to the City of Dowagiac. If the land use changes were
to occur in areas adjacent to a waterbody such as a lake or
river,
Table 1. Summary of the Impact of the Simulated Land Use
Change in La Grange Township on Runoff, Sediment, and
Nutrients at the Outlet of the Dowagiac River Watershed from a
Single Storm Event of 4.5 in.
Variable
Simulated
Result
Using the
Baseline
Data
Simulated Result Using
the Proposed Land Use
Scenario
25-Year Recurrence,
24-hour Storm (inches)
4.50
4.50
Surface Runoff (inches)
1.82
1.82
Peak Flow Rate (cfs)
3,231
3,235
Total Sediment Yield
(tons)
9,985
9.992
Total N in Sediment
(Ibs/acre)
0.85
0.85
Total P in Sediment
(Ibs/acre)
0.43
0.43
greater impact on water quality would have produced. In
simulating the effect of urban land expansion on water quality,
this study assumed the land cover is the only variable to be
changed and all other factors such as soil, topography, and
management practices are to remain unchanged. In reality,
urban development often causes changes in some related
factors such as topography and landscape irrigation. The
simulated magnitude of the land use change impact on water
quality should be evaluated using the field data in order to
support the informed land use decision making process.
Resource planners and decision makers should also realize that
the location of the land use change such as proximity to a
waterbody has significant impact on water quality and need to
consider this factor in dealing with the land use issues in the
Dowagiac River Watershed.
4.SUMMARY AND CONCLUSIONS
This study integrates AGNPS and digital databases of soil, land
cover/use, topography, water resource features and
management practices to simulate the impact of land use
change scenario on runoff, sediment, and nutrient yields based
on a 25-year, 24-hour period of single storm event of 4.5 inches
in the entire Dowagiac River Watershed. The simulation results
show that expansion of urban land will probably lead to an
increase in surface runoff, peak flow, and soil erosion. The
magnitude of the effect is related to the extent and proximity of
proposed land use change to water bodies in the watershed.
AGNPS requires 22 input parameters. Manual input of these
input parameters for each of the 4,803 cells in the Dowagiac
River Watershed would be time consuming, tedious, and
problematic. This study develops AVNPSM, an interface
between ArcView and AGNPS to derive, analyze, and visualize
the required model parameters and simulated results from the
databases of soil, topography, land cover, and water resource
features. The interface consists of parameter generator, input
file processor, model executor, output visualizer, statistical
analyzer, and land use change simulator. Application of the
interface to the study watershed indicates that it is user friendly,
and robust, and significantly improves the efficiency of the
nonpoint source pollution modeling process. With the interface,
land use change scenarios can be readily explored in the model
to help resource planners and decision makers develop
watershed management plan to minimize sedimentation and
nutrient loading to the receiving waters.
