Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 103

Table 7 Land use distributions in Dry Creek watershed in Georgia, Cedar Creek watershed in
Michigan, and Orestimba Creek watershed in California

Land use distribution (%)
Dry Creek watershed,
Georgia (12,322 ha)

Aggregated
land-use

Orestimba Creek watershed,
California (55,997 ha)
CPY cropland 45.75 22.72 22.16
Rangeland 1.46 0.88 29.63
Forests 16.31 36.97 14.88
Urban 3.99 6.75 3.58
Pasture / Hay 21.26 12.02 0.00
Water bodies 0.12 0.07 0.15
Wetlands 6.35 20.23 0.91
Grassland 2.73 0.36 27.95
Fallow soils 2.04 0.00 0.74
CPY cropland = crops eligible for receiving applications of CPY according to registered uses

Cedar Creek watershed,
Michigan (6,381 ha)

3.4 Watershed Scenarios

Model Setup . The three focus watersheds (Dry Creek in Georgia, Cedar Creek in
Michigan, and Orestimba Creek in California) were modeled using SWAT to simu-
late the daily hydrology and hydraulics and to route CPY sources in the watershed
to the basin outlet. CPY runoff mass within each watershed was simulated using
PRZM for Georgia and Michigan and winPRZM for California. Estimates of drift
onto water surfaces were calculated from AgDrift.
The Georgia and Michigan watersheds were delineated within the SWAT 2005
ArcView interface by using local 10 m digital elevation model (DEM) GIS data
from the National Elevation Dataset (NED) and hydrography datasets from the
area. These data were downloaded from the US Geological Survey available
through the National Map data server ( http://nationalmap.gov/viewer.html ) (Gesch
2007 ). Land cover/use for the locations was mapped within the model from the
USDA National Agriculture Statistics Service Cropland Data Layer (USDA 2012 ).
The watersheds were subdivided in sub-basins, primarily at stream confluences,
and an area threshold was used to include hydrologic response units (combinations
of soils and land uses) that accounted for more than 5% of the watershed area. This
threshold was used as a filter to aid in computational efficiency so that not every
land use and soil combination is simulated. Datasets of rainfall and max/min tem-
perature from local weather records were used to drive the hydrologic simulations.
Simulations for the Georgia and Michigan watersheds were simulated for the 30-yr
period of 1961-1990.
To provide for a conservative assessment, all cropland eligible for CPY applica-
tions according to product labels in the Dry Creek and Cedar Creek watersheds
were represented as “GA-pecan1” or “MI-cherries1”. For example, areas desig-
nated as pecan, cotton, sorghum, corn, peanut, and peaches in the Dry Creek water-
shed were represented as GA-pecan1, with respect to soil, crop, and CPY application