Introduction
to PRZM3
PRZM3 links two models, PRZM and VADOFT to predict
pesticide transport and transformation down through the
crop root and vadose (unsaturated) zone to the water
table. PRZM3 incorporates soil temperature simulation,
volatilization and vapor phase transport in soils,
irrigation simulation, and microbial transformation.
PRZM is a onedimensional finitedifference model which
uses a method of characteristics (MOC) algorithm to
eliminate numerical dispersion. VADOFT is a
onedimensional finiteelement code that solves
Richards' equation for flow in the unsaturated zone. The
user may make use of constituting relationships between
pressure, water content, and hydraulic conductivity to
solve the flow equation. PRZM3 is capable of simulating
multiple pesticides or parentdaughter relationships.
PRZM3 is also capable of estimating probabilities of
concentrations or fluxes in or from various media for
the purpose of performing exposure assessments. PRZM and
VADOFT are linked together with the aid of a flexible
execution supervisor that allows the user to build
models that are tailored to sitespecific situations.
Monte Carlo pre and postprocessors are provided in order
to perform probabilitybased exposure assessments.
PRZM3
Applications
PRZM3 is a U.S. EPA Model for
Predicting Pesticide Fate in the Crop Root and
Unsaturated Soil Zones. PRZM3 simulates the transport of
fieldapplied pesticides in the crop root zone and the
vadose zone taking into account the effects of
agricultural management practices. The model provides
estimates of probable exposure concentrations by taking
into account the variability in the natural systems and
the uncertainties in system properties and processes.
The program utilizes extended memory.
PRZM3 links
two models PRZM and VADOFT in order to predict pesticide
transport and transformation down through the crop root
and unsaturated zone. PRZM is a one dimensional,
finitedifference model that accounts for pesticide fate
in the crop root zone. PRZM3 incorporates several new
features, specifically:
 Soil temperature simulation
 Volatilization and vapor phase transport in
soils
 Irrigation simulation
 Microbial transformation
 A method of characteristics (MOC) algorithm to
eliminate numerical dispersion
PRZM is
capable of simulating transport and transformation of
the parent compound and as many as twodaughter species.
VADOFT is a onedimensional finiteelement code that
solves Richards' equation for flow in the unsaturated
zone. The user may make use of constitutive
relationships between pressure, water content, and
hydraulic conductivity to solve the flow equations.
VADOFT may also simulate the fate of two parent and two
daughter products. The PRZM and VADOFT codes are linked
together with the aid of a flexible execution supervisor
that allows the user to build loading models that are
tailored to sitespecific situations. In order to
perform probabilitybased exposure assessments, the code
is also equipped with a Monte Carlo preprocessor and
postprocessor.
PRZM3
Capability PRZM has the capability to simulate
multiple zones. This allows PRZM and VADOFT to combine
different root zone and vadose zone characteristics into
a single simulation. Zones can be visualized as multiple
land segments joined together in a horizontal manner.
There are three reasons a user may choose for
implementing multiple zones:
 to simulate heterogeneous PRZM root zones
with a homogeneous vadose zone
 to simulate a homogeneous root zone with
heterogeneous vadose zones
 to simulate multiple homogeneous root zones
with multiple homogeneous vadose zones
Another added feature is the ability to simulate as
many as three chemicals simultaneously as separate
compounds or as a parentdaughter relationship. This
gives the user the option to observe the effects of
multiple chemicals without making additional runs or the
ability to enter a mass transformation factor from a
parent chemical to one or two daughter
products.
Predictions are made on a daily basis.
Output can be summarized for a daily, monthly, or annual
period. Daily time series values of various fluxes or
storages can be written to sequential files during
program execution for subsequent analysis.
PRZM
Limitations
Hydrologic and hydraulic
computations are still performed in PRZM on a daily time
step even though, for some of the processes involved
(evaporation, runoff, erosion), finer time steps might
be used to ensure greater accuracy and realism. For
instance, simulation of erosion by runoff depends upon
the peak runoff rate which is in turn dependent upon the
time base of the runoff hydrograph. This depends to some
extent upon the duration of the precipitation event.
PRZM retains its daily time step primarily due to the
relative availability of daily versus shorter time step
meteorological data. This limitation has been mitigated,
in part, by enhanced parameter guidance.
In the
previous version of PRZM, the soil hydraulics were
simple  all drainage to field capacity water content
was assumed to occur within one day. (An option to make
drainage time dependent also was included, but there is
not much evidence to suggest that it was utilized by
model users to any great extent.) This had oneday
drainage assumption the effect, especially in deeper
soils, of inducing a greaterthananticipated movement
of chemical through the profile. While this
representation of soil hydraulics has been retained in
PRZM, the user has the option of coupling PRZM to
VADOFT. PRZM is then used to represent the root zone,
while VADOFT, with a more rigorous representation of
unsaturated flow, is used to simulate the thicker vadose
zone. For short distances from the soil surface to the
water table, PRZM can be used to represent the entire
vadose zone without invoking the use of VADOFT as long
as no layers that would restrict drainage are
present.
PRZM simulates only advective, downward
movement of water and does not account for diffusive
movement due to soil water gradients. This means that
PRZM is unable to simulate the upward movement of water
in response to gradients induced by evapotranspiration.
This process has been identified as an important one for
simulating the effects of volitization. However, the
process would seem less likely to impact the movement of
chemicals with high vapor pressures. For these
chemicals, vapor diffusion would be a major process for
renewing the chemical concentration in the surface
soil.
The final limitation is the use of
fieldaveraged water and chemical transport parameters
to represent spatiallyheterogeneous soils. Several
researchers have shown that this approach produces
slower breakthrough times than are observed using
stochastic approaches. This concern has been addressed
by adding the capability to run PRZM3 in a Monte Carlo
framework. Thus, distributional, rather than
fieldaveraged, values can be utilized as inputs that
will produce distributional outputs of the relevant
variables (e.g., flux to the water table).
VADOFT
Overview
VADOFT is a finiteelement code
for simulating moisture movement and solute transport in
the vadose zone. It is the second part of the
twocomponent PRZM3 model for predicting the movement of
pesticides within and below the plant root zone and
assessing subsequent groundwater contamination. The
VADOFT code simulates onedimensional, singlephase
moisture and solute transport in unconfined,
variablysaturated porous media. Transport processes
include hydrodynamic dispersion, advection, linear
equilibrium sorption, and firstorder decay. The code
predicts the infiltration or recharge rate and solute
mass flux entering the saturated zone.
The code,
which employs the Galerkin finiteelement technique to
approximate the governing equations for flow and
transport, allows for a wide range of nonlinear flow
conditions. Boundary conditions of the
variablysaturated flow problems may be specified in
terms of prescribed pressure head or prescribed
volumetric water flux per unit area. Boundary conditions
of the solute transport problem may be specified in
terms of prescribed concentration or prescribed solute
mass flux per unit area. All boundary conditions may be
time dependent. An important feature of the algorithm is
the use of constitutive relationships for soil water
characteristic curves based on soil texture.
VADOFT
Limitations
Major assumptions of the flow
model are that the flow of the fluid phase is
onedimensional, isothermal and governed by Darcy's law,
and that the fluid is slightly compressible and
homogeneous. Hysteresis effects in the constitutive
relationships of relative permeability versus water
saturation, and water saturation versus capillary
pressure head, are assumed to be negligible.
Major assumptions of the solute transport model
are that advection and dispersion are one dimensional
and that fluid properties are independent of contaminant
concentrations. Diffusive/dispersive transport in the
porousmedium system is governed by Fick's law. The
hydrodynamic dispersion coefficient is defined as the
sum of the coefficients of mechanical dispersion and
molecular diffusion. Adsorption and decay of the solute
is described by a linear equilibrium isotherm and a
lumped firstorder decay constant. Parent/daughter
chemical relationships may be simulated.
The code
handles only singlephase flow (i.e., water) and ignores
the presence of a second phase, i.e., air. The code does
not take into account sorption nonlinearity or kinetic
sorption effects that, in some instances, can be
important. The code considers only singleporosity
(granular) soil media. It does not simulate flow or
transport in fractured porous media or structured
soils.
Monte Carlo
Overview
Monte Carlo performs all the
functions necessary to execute a Monte Carlo simulation.
It reads special data for parameters to be varied (e.g.,
distribution types and moments) and output variables to
be observed; generates random numbers, correlates them
and performs transformations; exchanges these generated
values for PRZM3 parameters; performs statistical
analysis on the output variables; and writes out
statistical summaries for the output
variables.
The MCARLO module makes use of an
input and output file. Many of the parameters entered in
the MCARLO input file once designated as constants will
be used in lieu of that same parameter value entered in
the standard input file.
A small number of input
variables may be changed at random by invoking the Monte
Carlo routines. It is not difficult to add additional
variables, however.
PRZM3 can be run in a Monte
Carlo mode so that probabilistic estimates of pesticide
loadings to the saturated zone from the source area can
be made. The input preprocessor allows the user to
select distributions for key parameters from a variety
of distributions:
 The Johnson family (which includes the normal and
lognormal)
 Uniform
 Exponential
 Empirical
If the user selects distributions from the Johnson
family, he or she may also specify correlations between
the input parameters. The Monte Carlo processor reads
the standard deterministic input data sets for each
model and reads a Monte Carlo input file that specifies
which parameters are to be allowed to vary, their
distributions, the distribution parameters, and
correlation matrix. The model then executes a
prespecified number of runs.
The output processor
is capable of preparing statistics of the specified
output variables including mean, maximum values and
quantiles of the output distribution. The output
processor also can tabulate cumulative frequency
histograms of the output variables and send them to a
line printer for plotting.
PRZM3 includes the
source code, executable version, user's manual, and
technical support.
PRZM3 Requirements: PC 486 with 2
MB RAM and math coprocessor
