Page 199 - Biosystems Engineering

GIS-Based W atershed Modeling Systems 177

similar to the TR-55 method. The sheet and rill erosion model is based
on RUSLE. RUSLE is an erosion model that predicts longtime aver-
age annual loss of sediment resulting from raindrop splash and run-
off from specific field slopes in specified cropping and management
systems. HUSLE is used to determine the delivery ratio for the sheet
and rill erosion for each cell to its receiving reach. The delivery ratio
for the individual particle-size classes is proportioned according to
their respective fall velocities. The resulting sediment is called the
sediment yield to the stream system. Sediment reach routing by class
of particle size is based on a modified Einstein deposition equation
using the Bagnold suspended sediment formula for transport capacity.
If the upstream sediment load is greater than the transport capacity for
the respective particle-size class, then degradation is assumed.
Mass balance of N, P, and organic carbon is computed for each
cell on a daily basis. Uptake of N and P by plants, application of fer-
tilizers, residue decomposition, and downward movement of N and
P are the major components considered in AnnAGNPS. The output
from each field includes sediment N, dissolved N, sediment P, dis-
solved P, sediment organic carbon, and dissolved organic carbon.
AnnAGNPS partitions N and P into organic and mineral parts, and a
separate mass balance is computed for each. The N and organic car-
bon cycles represented in AnnAGNPS are simplifications that track
only major N transformations of mineralization from humified soil
organic matter and plant residues, crop residue decay, and fertilizer
and plant uptake. A simple crop-growth stage index is used to model
plant uptake of N and P.
Runoff, ET, and percolation are considered to affect soil moisture.
Runoff is calculated using the SCS curve number method, and poten-
tial ET is calculated using the Penman equation. Actual ET is estimated
as a function of potential ET and available soil moisture content.
Percolation is assumed to occur at the rate of the hydraulic conductiv-
ity corresponding to the soil moisture content using the Brooks–Corey
equation.
AnnAGNPS also includes winter routing for snow, snowmelt,
and frozen soil by maintaining a daily thermal energy balance to
track the soil and snowpack temperatures. Rainfall is supplemented
by snowmelt. The depth of the uppermost frozen soil layer is used to
adjust the curve number and soil erodibility for potential sheet and
rill erosion.
Several GIS interfaces using ArcView 3.X and ArcGIS have been
developed for the AGNPS and AnnAGNPS models.
Hydrologic Simulation Program—Fortran
Hydrologic Simulation Program—Fortran (HSPF) (Bicknell et al.
2001), supported by both USEPA and the U.S. Geological Survey
(USGS), performs long-term simulations of the hydrologic and asso-
ciated water-quality processes on pervious and impervious land