A Model for Predicting Pesticide in Runoff, Erosion, and Leachate:
                                 User Manual

                           USEPA/OPP 734F14002
                                 July 1, 2014
                              D.F. Young and M.M. Fry
                             Office of Pesticide Programs
                         U.S. Environmental Protection Agency
                              Washington, D.C. 20460
Based on previous PRZM manuals by R.F. Carousel, J.C. Imhoff, P.R. Hummel, J.M. Cheplick, A.S.
Donigian, Jr., L.A. Suarez

1   What's New in PRZM5	1
  1.1     Functions No Longer Supported in PRZM5	1
2   Input Files for PRZM5	4
  2.1     Meteorological Data File	5
  2.2     New PRZM5 Input File	5
3   Parameter Estimation	12
4   PRZM5 Theory	26
  4.1     Overview	26
  4.2     Description of the Algorithms	27
  4.3     Crop Growth	27
  4.4     Irrigation	28
  4.5     Precipitation & Snowmelt	29
  4.6     Runoff	29
  4.7     Canopy Water Interception	31
  4.8     Evaporation	32
  4.9     Leaching	35
  4.10   Erosion	36
  4.11   Soil Temperature	38
    4.11.1    Thermal Diffusivity	38
    4.11.2    Upper Boundary Temperature	40
    4.11.3    Temperature Dependent Degradation	41
  4.12   Chemical Application & Foliar Washoff	41
  4.13   Chemical Runoff & Vertical Transport in Soil	42
    4.13.1    Solution Method	42
    4.13.2    Runoff Extraction of Chemical	43
    4.13.3    Erosion Extraction of Chemical	45
  4.14   Chemical Volatilization	46
    4.14.1    Soil Vapor Phase and Volatilization Flux	46
5   Computer Implementation	50
6   References	51

1  What's New in PRZM5

    PRZM5 is an update of previous versions of PRZM, and for the most part PRZM5 performs much
like the previous versions. Many of the updates in PRZM5 are internal to the code, and typical users will
be unaware of any alterations, while those involved in development and maintenance should find PRZM5
easier to follow and work with than previous versions. Of special interest to users is the new input file,
which is easier to edit, better organized, and more intuitive.
       The PRZM5 code was converted to a more modern, free-format style consistent with Fortran
2003 (PRZM was coded previously in Fortran 77). The revised format has made the code much easier to
read and search.  All subroutines are now contained in modules and have improved names (for example,
"Read_Inputs" instead of "Rsinp2"). Some important variable names have changed to allow search
functions in code editors to populate more efficiently (e.g., "IY" was changed to "current_year").
       The new code has fixed all known bugs present in previous versions, including those well-known
and problematic bugs in the degradation, erosion, and application routines.  Additionally, many routines
that were not functioning or were not used have been eliminated. These improvements are documented in
the sections that follow.

1.1   Functions No Longer Supported in PRZM5

Many of the functions in the previous versions of PRZM did not function correctly or were unused. In
this regard, the following functions have been eliminated from PRZM5:
    •   Monte Carlo
    •   Nitrogen simulation
    •   Method of characteristics (MOC)
    •   Furrow irrigation
    •   Biphasic degradation
    •   Multiple zones
    •   VADOFT
    •   Hamon option for pan factor (did not work)
    •   Pesticide application according to moisture
    •   Pesticide degradation as a function of soil moisture (never worked)
    •   Restricted drainage or lateral flow
    •   Microbial growth kinetics
    •   Special actions
    •   Snap shots
    •   PRZM output (only time series is supported now)
    •   Echo levels
    •   Unnecessary text removed from output files
    •   Removed redundant Curve Number (CN) inputs
    •   Bulk density calculation (BD must be entered now)
    •   Water content automatic calculation by Rawls no longer supported (field capacity [FC] and
       wilting point [WP] must be entered now)
    •   There is no longer a run file


Additionally, the following items have been addressed:
    •  CN and erosion factors no longer have to begin with the emergence date, and order does not

    •  Irrigation 5 was updated. Even though it was stated to be over-canopy irrigation, it did not wash
       off pesticides from foliage.  Furthermore, because IRTYPE 5 and 7 violate the hydrologic (CN)
       conceptualization in PRZM, these two types have been eliminated.  IRTYPE 1, 3, 4, 6 are still
       supported. New variables are defined: under_canopy_irrig, over_canopy_irrig, and
       canopy_capture to help with code readability.

    •  Date accounting was updated so that future versions can more easily handle  four-digit year dates
       (year 2000 and beyond). PRZM5 uses a Julian-date referencing to 1/1/1900. This allows for the
       elimination of numerous inquiries (thousands) as to whether or not it is a leap year during the

    •  Curve numbers (CN) 1 and 3 are now directly determined from the official NEH-4 Curve
       Number tables. The previous method was based on an approximate algorithm that used integer
       math, resulting in a systematic underestimation of effective CNs.

    •  Calculation of runoff more closely follows the NEH-4 Curve Number method.  Initial abstraction
       is no longer manipulated by the crop canopy and is always calculated by NEH-4 as 0.2*S.
       Leftover water is distributed between the canopy and infiltration.

    •  The  erosion calculation for the time of concentration has been corrected. The sheet flow term had
       previously used runoff in the denominator, whereas precipitation is now used, as specified by TR-

    •  The  conversion of parent-to-degradate for sorbed-phase parameters in the previous versions of
       PRZM was not used. These input conversion ratios ran into dead-ends in the old PRZM code,
       and they were always equal to the aqueous phase conversion ratio. Thus, the requirement to enter
       those values has been eliminated.

    •  Volumetric heat capacity (vhtcap) is now only calculated by the program. Values cannot be
       entered  independently; this feature was not used previously.

    •  Pesticide incorporated into the soil during harvest is distributed uniformly to 4.0 cm as opposed to
       the being distributed according to the next manual pesticide application scheme. This fixes the
       situation of having undefined incorporation depths on the last harvest.
       With PRZM5, the meteorological file does not have to include only complete calendar years.

•   Previous PRZM versions could run if the minimum depth from which evapotranspiration is
    extracted (ANETD) was greater than the soil profile.  This error was not caught by the Lahey
    compiler, which was used to build previous PRZM versions. PRZM5 (built with the Intel
    compiler) will indicate an error for these undefined conditions. Thus, ANETD must be set to less
    than the profile depth.

•   Irrigation is prevented on days with precipitation.  The PRZM3 documentation erroneously states
    that irrigation can occur on precipitation days, but the PRZM3  code never allowed this to happen.
    PRZM5 documentation clarifies that this cannot occur in the model.

•   The number of soil compartments in a horizon now has the variable name Num_delx, instead of
    dpn (thickness of compartments in horizon).

•   Because PRZM uses spatial discretizations, depths are only approximations of specified target
    values. For these cases, PRZM5 selects the nearest node, whereas older PRZM versions selected
    the first node greater than the target.  This change  affects the critical depths for CN moisture
    adjustments, ANETD, root depth, horizons, and runoff extraction. In previous versions of
    PRZM, this could be problematic if a discretization change occurred at one of these  critical

•   Previous PRZM versions calculated the number of nodes in the top 10 cm three different times,
    twice in the same routine; PRZM5 now only does  it once.

    NOTE: When discretizing horizons, users should be aware of the depth of specific variables:
    Root Max, Runoff Extraction, Erosion  Extraction, as well as the fixed depths used internally in
    the code for the Soil Moisture Calculation for runoff (fixed at 10 cm). Discretization in the
    vicinity of these depths will affect the precision of the value that PRZM uses.

•   The time step (delt) is now a real number, not an integer.

•   All eroded soil  has the same Kd as the top  compartment; thus, the Kd of the eroding compartments
    (compartments down to erosion_depth  as defined in Record 29) should be the same.

•   Users now have control of the runoff and erosion extraction parameters and have the ability to
    revisit and calibrate these routines. In previous PRZM versions, pesticide extracted by erosion
    was taken only from the topmost compartment; this could be problematic because pesticide
    removal changed in direct proportion to the size of the top compartment. As a note, to simulate
    uniform extraction (runoff or erosion) in PRZM5,  the user should set the depth to the desired
    depth and the decline to le-5 (the error is less than 0.001% over 1 cm depth).

•   The drift input parameter has been removed. Drift never actually had a function in PRZM as it
    was simply used to carry over to external programs.  Because PRZM5 is not inherently linked to
    external programs like EXAMS, there are  better ways to deliver needed inputs to these other

    •   There is no longer a minimum value of 10~5 for the variable CONST (Previously in Record 46 in
       PRZM3, now in Record U2); now CONST (a multiplier for the output) can be any value.
       Previously PRZM3 would reset CONST to 1 if CONST was less than 10~5. This constraint and
       the resulting reset was undocumented in PRZM3 and thus users could unwittingly get results that
       were off by a factor 105.

    •   In previous versions of PRZM, the T-Band pesticide application routine contained a bug that
       manifested itself when PRZM generated EXAMS input files. This bug set T-band applications to
       zero in the top 2 cm (essentially all of the application available to runoff) for the first 2 years of
       the simulation. This problem does not occur in PRZM5.

    •   In PRZM3, plant uptake of pesticide from the first compartment was omitted. This is now fixed
       in PRZM5. (Note: in PRZM, the plant uptake routine does not actually store pesticide in the
       plant; it is only, in effect, a degradation process.)

    •   The irrigation dryness parameter PCDPL is now limited to a value from 0 to 1.0. In previous
       versions of PRZM, when PCDPL was set above 0.9, the program would reset the values to 0.5.

    •   PRZM5 allows user-specified irrigation depths. A user can now specify the depth of soil that
       should be checked for dryness.

    •   The PRZM3 variable  WTERM is now called soil_applied^vashoffto better reflect its use in the
       code. DIN is now potential'_canopy _holdup.
PRZM5 uses two input files:

    •   Meteorological data file (*.dvf).  This file contains the daily weather records used in the

    •   PRZM5 input file (*.inp). The PRZM5 input file differs from previous PRZM versions. First,
       absolute position requirements are no longer used; instead PRZM5 uses list-directed input, where
       inputs are delimited by commas or spaces.  Secondly, PRZM5 input order has been restructured
       for added flexibility; non-chemical inputs are separate from chemical inputs.  This change allows
       users to save physical scenarios files that are independent of chemical information. Thirdly, since
       PRZM5 no longer needs an execution supervisor file (, some of that information is
       now included in the *.inp file. Lastly, because some unused PRZM processes have been
       eliminated, the file  content has been greatly reduced.

PRZM5 requires the use of a meteorological file, specified in the PRZM5 input file (Section 2.2) and read
by Readlnputs.f90. Information on daily precipitation, pan evaporation, temperature, wind speed, and
solar radiation is included in each record of the meteorological file. Data format requirements for the
weather file are as follows:

Fortran-style format: 1X,3I2,5F10.0
Where the input data order is:
MM = meteorological month
MD = meteorological day
MY = meteorological year
PRECIP = precipitation (cm/day)
PEVP = pan evaporation data (cm/day)
TEMP = temperature (Celsius)
WIND = wind speed (cm/sec)
SOLRAD = solar radiation (Langley)

Example Partial Meteorological File
 010161   0.00    0.30    9.5   501.6   240.3
 010261   0.10    0.21    6.3   368.0   244.3
 010361   0.00    0.28    3.5   488.3   303.0
The new PRZM5 input file (PRZM5.inp) is much less sensitive to positioning of input data than previous
versions. Input values in the file are delimited by commas or spaces.  Like previous versions of PRZM,
the program recognizes a line starting with three asterisks (***) as a comment line. The word Records in
the text below refers to lines containing input data that are read by the program. PRZM5.inp is grouped
into three sections: 1) a physical scenario section where all land and crop descriptions are given; 2) a
chemical input section where the pesticide dependent characteristics are given; and 3) an output section
that specifies the output options.


RECORD Al:  Meteorology file
Full path and filename for the meteorological data file. If a filename is specified without a path, PRZM
will search the default directory for the file (Up to 300 characters).

RECORD A2: PRZM5 Time Series Output File
Full path and filename for the output time series file. If a filename is specified without a path, PRZM will
search the default directory for the file (Up to 300 characters).

RECORD A3: PRZM5 Advanced Options: OPT1, OPT2, OPTS, OPT4, OPTS, OPT6
These are available for diagnostics and should all be set to FALSE for normal users.
OPT1: Test Option (Boolean)
OPT2: Test Option (Boolean)
OPTS: Test Option (Boolean)
OPT4: Test Option (Boolean)
OPTS: Test Option (Boolean)
OPT6:  Read a diagnostics file (Boolean)

RECORD A4: Optional Calibration/Diagnostic File Name.
If OPT6 is TRUE in Record A3, then this record will be read; otherwise Record A4 should not be present.

PFAC:        Pan factor used to estimate daily evapotranspiration
SFAC:        Snowmelt factor in cm per °C above freezing

ANETD:     Minimum depth from which evaporation is extracted year-round (cm)
INICRP:      Index for the initial crop if the simulation date occurs before the emergence date of all
             cropping periods (see record 10). If value is 0, then there is no initial crop. If values are
             greater than 0, the value designates the index number of the crop to be used in the
             initialization (i.e., conditions present before the first emergence date in the first simulation
             period). INICRP must be equal to one of the values of ICNCN (Record 9) and INCROP
             (Record 11).

RECORD 2:   Erosion Flag
ERFLAG:      Flag to calculate erosion:
              ERFLAG = 0, No Erosion calculated
              ERFLAG = 2, MUSLE
              ERFLAG = 4, MUSS
USLEK:       Universal Soil Loss Equation (K) of soil credibility
USLELS:      Universal Soil Loss Equation (LS) topographic factor
USLEP:       Universal Soil Loss Equation (P) practice factor
AFIELD:      Area of field or plot in hectares
IREG:         Location of NRCS 24-hour hyetograph
SLP:          Land slope (%)
HL:           Hydraulic length (m)

RECORD 4:   Number of crops in simulation

Number of different crops in the simulation
Repeat this record up to NDC.
ICNCN:       Crop ID number
CINTCP:      Maximum interception storage of the crop (cm)
AMXDR:      Maximum rooting depth of the crop (cm)
COVMAX:    Maximum areal coverage of the canopy (percent)
WFMAX:      Maximum dry weight of the crop at full canopy (kg/m2); required
              if CAM = 3 (see record 16), else set to 0.0
 HTMAX:      Maximum canopy height at maturity date (cm) (see record 11)

Note: Repeat Records 6, 7, 7a, 8, 9,10 for each crop
RECORD 6:   CROPNO, NUSLEC, use_usleyears
CROPNO:     Crop number
NUSLEC:      Number of USLEC factors (1 to 52)
use_usleyears:  0=USLE numbers repeat each year; 1=USLE numbers are year-specific and require
              specification of the year (see record 7a)

RECORD 7:   4DayMon
4DayMon:     Four digit date combination (DDMM) is used. For example, March 24 is 2403, and Jan 1
              is 0101. These numbers represent the dates when a next erosion factor or curve number
              is initiated.
Note: These dates can be in any order. They do not even have to be sequential, but they must be ordered
in the same manner as the respective erosion factors or CNs. This may be a comma or space-delimited
list (e.g." 0101, 2502, 1206, 2210 ..."). Up to 100 values may be read (set by the variable
Num_hydro_factors in the Fortran code).

RECORD 7a:  Year
(Only if use_usleyears in Record 6 is set to 1)
USLE_Year:   The 4 digit year corresponding to the USLE dates in RECORD 7. Specification of
              these dates would normally be used for research applications rather than regulatory

RECORD 8:   Soil loss cover management factors, C value
USLEC:       Soil loss cover management factors (C value) are comma or space-delimited set of
              values corresponding to 4DayMon.

RECORD 9:   Manning's N
MNGN:       Manning's roughness coefficient, N, is a comma or space-delimited set of values
              corresponding to 4DayMon.

RECORD 10:  Runoff curve number

Runoff curve numbers of antecedent moisture conditions are comma or space-
delimited set of values corresponding to 4DayMon.
RECORD 11:  Number of applications that follow
col: 1-8        NCPDS:       Number of cropping periods (sum of NDC for all cropping dates).

Repeat this record up to NCPDS.
EMD:         integer day of crop emergence
EMM:         integer month of crop emergence
IYREM:       integer year of crop emergence
MAD:         integer day of crop maturation
MAM:        integer month of crop maturation
IYRMAT:     integer year of crop maturation
HAD:         integer day of crop harvest
HAM:         integer month of crop harvest
IYRHAR:     integer year of crop harvest
INCROP:      crop number associated with NDC (see record 8)

IRFLAG:      irrigation flag:  0 = no irrigation simulated, 1 = year round, 2 = during crop period only.
ITFLAG:      soil temperature simulation flag:  1 = yes, 0 = no.
IDFLAG:      thermal conductivity and heat capacity flag: 1= yes, 0 = no.

RECORD 14:  IRTYP, PLEACH, PCDEPL, RATEAP, AutoSpecifyDepth, irrig_depth
Only if IRFLAG = 1 or 2 (see record 13).
IRTYP:             type of irrigation:
                   1 = flood irrigation
                   2 = not used
                   3 = over canopy
                   4 = under canopy sprinkler
                   5 = over canopy without runoff generation
                   6 = over canopy, user-defined rates, with runoff generation
                   7 = over canopy, user-defined rates, without runoff generation

Auto SpecifyDepth
     leaching factor as a fraction of irrigation water depth
     fraction of available water capacity at which irrigation is applied;
     usually -0.45 - 0.55. PRZM accepts values between 0.0 and 0.9.
     maximum rate at which irrigation is applied (cm hr"1).  Note: Because PRZM is on
     daily time step, this value should be the 24-hour average.
     irrigate according to root depth (TRUE) or user specifies the depth (FALSE).
     user specified irrigation depth (cm) if AutoSpecfyDepth is FALSE, otherwise 0

Only if ITFLAG = 1 (see record 13)
ALBEDO:     monthly values of soil surface albedo (12 values)
EMMISS:     reflectivity of soil surface to longwave radiation (fraction)
ZWIND:      height of wind speed measurement above the soil surface (m)

Only if ITFLAG = 1 or 2 (see record 13).
BBT:          average monthly values of bottom boundary soil temperatures in °C (12 values)

RECORD 17:   QFAC(l), ..., QFAC(Nchem) TBASE(l), ... ,TBASE(Nchem)
Only if ITFLAG = 1 or 2 (see record 13)
QFAC:        factor for rate increase when temperature increases by 10°C.  If QFAC is set equal to
              zero, then PRZM will not simulate degradation change with temperature.
TBASE:       temperature during the test of biodegradation.
RECORD 18:  number of horizons
NHORIZ:      total number of horizons (minimum of 1)

              CLAY, THCOND, SPT
One line for each horizon (total lines = NHORIZ)
horizon number (used only for input file clarity)
thickness of the horizon (cm)
number of compartments in the horizon (new PRZM5 parameter)
pesticide hydrodynamic solute dispersion coefficient for each NCHEM
bulk density if BDFLAG=0 or mineral density if BDFLAG=1 (g/ cm3)
initial soil water content in the horizon (cm3/cm3)
water content that effectively can be held during the time step in the horizon (cm3/cm3)
lowest water content in the horizon (cm3/cm3)
organic carbon in the horizon (percent)
sand content in the horizon. Required if THFLAG = 1, else set to 0.0 (percent).
clay content in the horizon. Required if THFLAG = 1, else set to 0.0 (percent).
thermal conductivity of horizon (cm"1 day"1). Required if IDFLAG= 0, else set to 0.0
initial temperature of the horizon (°C)
RECORD 20:  New runoff extraction parameters: runoff_extract_depth, runoff_decline, runoff_effic
runoff_extract_depth:  soil depth over which runoff interacts with soil
runoff_decline:        exponential decline coefficient of runoff interaction (cm"1)
runoff_effic:          fraction of total runoff that interacts with soil

RECORD 21:  New erosion extraction parameters: erosion_depth, erosion_decline
erosion_depth:         soil depth over which erosion interacts with soil
erosion_decline:       exponential decline coefficient of erosion interaction (cm"1)


NAPS:        total number of pesticide applications occurring at different dates (1 to 800). Note: If two
              or more pesticides are applied on the same date, then NAPS = 1 for that day.
NCHEM:      number of pesticide(s) in the simulation

Repeat this record up to NAPS. Dates do not need to be in chronological order.
integer target application day
integer target application month
integer target application year
Chemical Application Method:
1 = soil applied, soil incorporation depth of 4 cm, linearly decreasing with depth
2 = interception based on crop canopy as a straight-line function of crop development;
    chemical reaching the soil surface is incorporated to 4 cm;
3 = interception based on crop canopy. The fraction captured increases exponentially as
    the crop develops; chemical reaching the soil surface is incorporated to 4 cm.
4 = soil applied, user-defined incorporation depth (DEPI), uniform with depth
5 = soil applied, user-defined incorporation depth (DEPI), linearly increasing with depth
6 = soil applied, user-defined incorporation depth (DEPI), linearly decreasing with depth
7 = T-Band application with total depth specified by DEPI and the fraction in the top
    2 cm specified by T-band_top
8 = soil applied, chemical incorporated entirely into depth specified by user (DEPI)
9 = linear foliar based on crop  canopy; chemical  reaching the soil surface incorporated to
    the depth given by DEPI
10 = nonlinear foliar using exponential filtration; chemical reaching the soil surface
    incorporated to the depth given by DEPI

depth of the pesticide(s) application (cm). Use with CAM=4-10, 7, 8. For CAM=2 or 3,
chemical not intercepted by the crop foliage is incorporated to 4 cm. The default
incorporation depth for CAM=2 or 3 can only be over-ridden by selecting CAM = 9 or 10
and entering a value >0.0 for DEPI. Should DEPI be zero, or a value less than the depth
of the top soil compartment, the chemical is distributed uniformly throughout the depth of
the top soil compartment.

target application rate of the pesticide(s) (kg ha"1)
APPEFF:       application efficiency (fraction); reduces target application by this fraction

T-band_top:    For T-band applications (CAM 7), this variable represents the fraction of chemical that
               is incorporated into the top 2 cm.


Repeat IPSCND and UPTKF for each chemical
FILTRA:      filtration parameter; set to O.OIPSCND:  condition for disposition of foliar pesticide after
              harvest. 1 = surface applied, 2 = complete removal, 3 = left alone. This number is
              required if CAM=2.

UPTKF:       plant uptake factor. Plant uptake flux = (UPTKF) x (evapotranspiration rate) x (dissolved
              phase concentration).

Only if CAM=2 or 3, repeat this record up to NCHEM
PLVKRT:     pesticide volatilization decay rate on plant foliage (days'1)
PLDKRT:     pesticide decay rate on plant foliage (days'1)
FEXTRC:     foliar extraction coefficient for pesticide washoff per centimeter of rainfall


Only if CAM=2 or 3, and NCHEM >1.
              PTRAN12:     foliar transformation rate for chemical 1-2
              PTRAN13:     foliar transformation rate for chemical 1 -3
              PTRAN23:     foliar transformation rate for chemical 2-3

DAIR:        diffusion coefficient for the pesticide(s) in the air (cm2 day"1). Only required if
              HENRYK is greater than 0, otherwise set to 0.0 for each NCHEM.
HENRYK:     Henry's law constant of the pesticide(s) for each NCHEM (dimensionless)
ENPY:        enthalpy of vaporization of the pesticide(s) for each NCHEM (kcal/mole)

RECORD C7: KD(1), KD(2), KD(3) for each horizon
One line for each horizon (total lines = NHORIZ)
KD:           pesticide(s) partition coefficient for each NCHEM.

One line for each horizon (total lines = NHORIZ)
DWRATE:     dissolved phase pesticide(s) decay rate for each NCHEM (day"1)
DSRATE:     adsorbed phase pesticide(s) decay rate for each NCHEM (day"1)
DGRATE:     vapor phase pesticide(s) decay rate for each NCHEM (day"1)

This record is used for parent-degradate relationships.  PRZM does not differentiate between sorbed and
aqueous-phase conversions. Enter 0 when degradates are not calculated.
One line for each horizon (total lines = NHORIZ)
DKRW12:     Molar ratio of parent (chemical 1) to degradate (chemical 2)
DKRW13:     Molar ratio of parent to degradate (chemical 3)

Molar ratio of degradate (chemical 2) to next sequential degradate (chemical 3).

NPLOTS:      number of times series plots (maximum of 12)

Only if NPLOTS > 0 (see record 45).  Repeat this record up to NPLOTS.
PLNAME:     name of plotting variable (see Table 4.1)
INDX:        index to identify which pesticide if applicable. 1 = first chemical, 2 =second chemical, 3
              = third chemical.
MODE:        plotting mode: TSER (daily), TCUM (cumulative), TAVE (daily average over multiple
              compartments), TSUM (daily sum over multiple compartments).
IARG:        argument value for PLNAME (see Table 4.1)
IARG2:        argument value for PLNAME (see Table 4.1) (If TSER or TCUM, enter same value as
CONST:       constant to multiply for unit conversion. Set to 1.0 if no conversion.

3   Parameter Estimation
Parameters used in the PRZM5 code are listed alphabetically for reference.

AFIELD - This parameter is the field area in hectares (ha).

ALBEDO - Soil surface albedo. To simulate soil temperatures, ALBEDO values must be specified for
each month. As the surface conditions change, the ALBEDO values change accordingly. Values for some
natural surface conditions are provided in Table 3.1.

AMXDR - Effective root depth, not necessarily the maximum depth. PRZM requires this parameter in
centimeters to estimate the measurement of root depth from the land surface. For ranges on specific root
depths, consult a current version of the USDA Usual Planting and Harvesting Dates or a local
Cooperative Extension Service.

Table 3.1 Albedo Factors of Natural Surfaces for Solar Radiation (Brutsaert, 1982; van Wijk, 1963)
Fresh Dry Snow
Clean, Stable Snow Cover
Old and Dirty Snow Cover
Dry Salt Cover
White Sand, Lime
Quartz Sand

Dark Clay, Wet
Dark Clay, Dry
Sand, Wet
Sand, Dry
Sand, Yellow
Bare Fields
Wet Plowed Field
Newly Plowed Field
Grass, Green
Grass, Dried
Grass, High Dense
Grass, High Dense
Prairie, Wet
Prairie, Dry
Stubble Fields
Grain Crops
Alfalfa, Lettuce, Beets, Potatoes
Coniferous Forest
Deciduous Forest
Forest with Melting Snow
Yellow Leaves (fall)
Desert, Dry Soils
Desert, Midday
Desert, Low Solar Altitude
ANETD - This value represents soil evaporation moisture loss during a fallow, dormant period. Values
for ANETD apply when there is no growing season, allowing a reduced level of moisture loss through
evaporation.  For soils with limited drainage, set ANETD to  10 cm. Values for free drainage soils are
shown in Figure 3.1.

APPEFF - Application efficiency of pesticide application (TAPP). TAPP will be multiplied by APPEFF
to calculate the effective rate of application.

Figure 3.1 Map of ANETD, soil evaporation loss.
BD - Soil bulk density. This value is required in the basic chemical transport equations of PRZM5 and is
also used to estimate moisture saturation values.  Values can be found in the USDA Soil Data Mart

BBT - Bottom boundary soil temperatures. BBT values for each month must be specified. The BBT for
shallow core depths will vary significantly with time throughout the year. For deep cores, BBT will be
relatively constant. The average temperature of shallow groundwater is displayed in Figure 3.2

Figure 3.2 Average shallow groundwater temperatures in the United States (from derived from Collins
CAM - Chemical application model flag. This flag specifies how the pesticide is applied to soil or
foliage. Several unused and/or unrealistic CAM schemes from PRZM3 were eliminated in PRZM5 (CAM
= 3, 5,  6, 9, 10).  The CAM schemes are:
       CAM = 1, which should be used for surface applied chemicals and results in a linearly decreasing
distribution in the soil to a depth of 4 cm.
       CAM = 2, which results in linear extraction by the crop foliage based on the degree of crop
canopy development.
       CAM = 3 results in nonlinear extraction by the crop foliage, i.e., the fraction captured by the
foliage increases exponentially as the crop matures.
       CAM = 4, which is used for uniform incorporation into the soil to a user-specified depth.
       CAM = 5 results in linearly increasing incorporation to a user defined depth. CAM = 6 results in
linearly decreasing incorporation to a depth specified by the user.
       CAM = 7, which approximates T-Band application to a user-defined incorporation depth.
Variable DRFT should be used to define the fraction of chemical to be applied in the top 2 cm. The
remainder of the chemical is uniformly incorporated between 2 cm and the user-defined depth.
       CAM = 8, which incorporates the chemical directly to the user-specified depth (modification of
CAM 1).
       CAM = 9 is a modification of CAM 2, allowing a user-specified depth (DEPI) of incorporation of
chemical not intercepted by the foliage.
       CAM = 10 is a modification of CAM 3, allowing a user-specified depth (DEPI) of incorporation
of chemical not intercepted by the foliage.


CINTCP - The maximum interception storage of the crop (cm). This parameter estimates the amount of
rainfall that is intercepted by a fully developed plant canopy and retained on the plant surface.

The PRZM 3.12 manual states that a z range of 0.1 to 0.3 for a dense crop canopy was reported by Knisel
(1980); however, reference to these values could not be found in Knisel (1980). The PRZM 3.12 manual
gives a table of values for CINTCP, but the source is unknown. Alternatively, CINTCP can be calculated
from a simple crop interception relationship cited by several authors (Dickinson, 1984; Brisson et al.
1998; Kozak et al, 2007; Giante et al., 2009), which assumes a 0.02 cm covering of the canopy according
                                       CINTCP  = 0.02 x LAI                              (3-1)

Where LAI is the leaf area index.

CN - Runoff curve numbers of antecedent runoff condition II, as defined by the Natural Resources
Conservation Service NEH-4 (NRCS, 2003).

COVMAX - This value is the maximum areal crop coverage. PRZM estimates crop ground cover to a
maximum value, COVMAX, by linear interpolation between emergence and maturity dates. As a crop
grows, its ground cover increases  and captures proportionally more pesticide from above  canopy
applications.  Similarly, rainfall storage capacity increases. For most crops, the maximum coverage will
be on the order of 80% to 100%.

DAIR - Vapor phase diffusion coefficient. When Henry's law constant (HENRYK) is greater than zero,
vapor phase diffusion is used to calculate equilibrium between vapor and solution phases. Jury et al.
(1983b) concluded that the  diffusion coefficient will not show significant variations for different
pesticides at a given temperature;  they recommend using a constant value of 0.43 m2/day  (4300 cm2/day)
for all pesticides.

DGRATE - Vapor phase degradation rate constant(s). Pesticides are degraded by different mechanisms,
and at different rates, depending on whether they are in vapor, liquid, or sorbed phase (Streile, 1984). A
lumped, first-order rate is assumed for DGRATE.

DISP - Dispersion coefficient of the pesticide(s). For root zone transport, this parameter  will be
dominated by hydrodynamic dispersion (not by molecular diffusion), and thus PRZM5 no longer groups
this parameter with chemical properties. Note that in most practical cases, the temporal and spatial
discretization scheme in PRZM5 will already create dispersion of the magnitudes observed in the field
(Young and Carleton, 2012); thus  DISP can be set to zero.

DKW112, DKW113, DKW123, DKS112, DKS113, DKS123 - Molar transformation ratio from a parent
chemical (1 or 2) to a degradate chemical (2 and/or 3) for dissolved (DKW) and adsorbed (DKS) phase

DSRATE - Sorbed phase degradation rate constant for parent and degradates.


DWRATE - Solution phase degradation rate constant for parent and degradates.

EMMISS - Infrared Emissivity (unitless). Most natural surfaces have an infrared emissivity between 0.9
and 0.99.  Values for all natural surfaces are not well known, but are usually close to unity. Specific
values of EMMISS for some natural surfaces are given in Table 3.2.

Table 3.2 Emissivity Values for Natural Surfaces at Normal Temperatures (van Wijk, 1963;
Brutsaert, 1982).
Natural Surface
Snow (old)
Snow (fresh)
ENPY - Enthalpy of vaporization. This parameter is used in the temperature correction equation for
Henry's Law constant.  See USEPA EPISUITE for estimates.

ERFLAG - Flag to designate which erosion routine should be used.

FEXTRC - Foliar washoff extraction coefficient in units of fraction removed per cm of rainfall. Washoff
from plant surfaces is modeled with consideration for rainfall, foliar mass of pesticide, and extraction
ability. FEXTRC is a required input for estimating the flux of pesticide washoff. Exact values are
variable and depend upon the crop, pesticide properties, and application method. Smith and Carsel (1984)
suggest that a value of 0.10 is suitable for most pesticides.

FILTRA - The filtration parameter of initial foliage to soil distribution. This parameter relates to the
equation for partitioning the applied pesticide between foliage and the ground. Lassey (1982) suggests
values in the range of 2.3 to 3.3 m2 kg"1. Miller (1979) suggests a value of 2.8 m2 kg"1 for pasture grasses.
Most of the variation appears to be due to the vegetation and not the aerosol. FILTRA only applies if

FLEACH - The leaching factor as a fraction of irrigation water depth. This factor is used to specify the
amount of water added by irrigation to leach salts from saline soil and is defined as a fraction of the
amount of water required to meet the soil water deficit. For instance, a value of 0.25 indicates that 25%
extra water is added to meet the soil water deficit.

HENRYK - Henry's law constant is a ratio of the pesticide equilibrium concentration in the air to its
concentration in water (unitless).

HTMAX - Maximum canopy height of the crop at maturation (cm). Canopy height increases during crop
growth resulting in pesticide flux changes in the plant compartment. Users should have site-specific
information on HTMAX since it varies with climate, crop species, and environmental conditions. General
ranges for different crops are listed in Table 3.3. This parameter is used in the volatilization routine.

Table 3.3 Maximum Canopy Height at Crop Maturation
Grain Sorghum
Height (cm)
90- 110
90- 110
Szeiczetal. (1969)
Smith etal. (1978)
Szeiczetal. (1969)
Szeiczetal. (1969)
Szeiczetal. (1969)
Smith etal. (1978)
Szeiczetal. (1969)
ICNCN - The crop number of the different crops. This value is calculated in relation to NDC (number of
different crops) and allows separate crop parameters to be specified for each different crop in a

IDFLAG - Thermal conductivity and volumetric heat capacity flag. This flag allows a user to simulate
soil temperature profiles. If IDFLAG = 0, the user must enter thermal conductivity (THCOND) and
volumetric heat capacity (VHTCAP). If IDFLAG = 1, the model automatically simulates soil temperature

INCROP - The crop identification number for a crop that emerges before the start of the simulation (if
this is the case).

IREG - NRCS rainfall distribution region. For time period May 1 to September 15, IREG will be used in
the time of concentration calculation of peak flow. For the rest of year, IREG=2. See Figure 3.3 for
appropriate region.
                                                                   Type I
                                                                   Type IA
                                                                I  I Type II
                                                                I  I Type III

Figure 3.3 Approximate geographic boundaries for NRCS rainfall distributions, from TR-55
(NRCS, 1986).

IRFLAG - Flag to simulate irrigation. If irrigation is desired, the user has a choice of applying water for
the whole year or during a cropping period whenever a specified deficit exists.

IRTYP - Specifies the type of irrigation used.

IPSCND - Flag indicating the disposition  of pesticide remaining on foliage after harvest. This flag only
applies if CAM = 2. If IPSCND = 1, the pesticide remaining on foliage is distributed in the soil to 4 cm
(this is an improvement over previous versions of PRZM in which the distribution of pesticide depended
on the future chemical application method)  If IPSCND = 2, the pesticide remaining on foliage is
completely removed after harvest. If IPSCND = 3, the  pesticide remaining on foliage is retained as
surface residue and continues to undergo decay and washoff

ITFLAG - Flag for soil temperature simulation. This flag allows a user to specify soil temperatures
(BBT) for shallow core depths. For deep cores (CORED), temperatures will remain relatively constant.

KD - Pesticide soil-water distribution coefficient. The user can enter KD directly if KDFLAG = 0 (see
PCMC and SOL) or allow the model to  calculate KD automatically (KDFLAG = 1).

MNGS - Manning's roughness coefficient (N) for the  field. Up to 32 values may be entered per year. A
value of 0.17 is recommended as a default value for typical row crop tillage. See Table 3.4 for values.

Table 3.4 Recommended Manning's Roughness Coefficients for Overland Flow
Cover of Treatment
Concrete or asphalt
Bare sand
Graveled surface
Bare clay-loam (eroded)
Fallow — no residue
Chisel plow



Moldboard plow (Fall)
Range (natural)
Range (clipped)
Grass (bluegrass sod)
Short grass prairie
Dense grass
Bermuda grass
Woods-Light underbrush
Woods-Dense underbrush
Residue Rate

1/4- 1
1/4- 1
1/4- 1

Coefficient Range
0.01 -0.13
0.02 - 0.24

NAPS - Number of pesticide applications. This is the total number of application dates specified during
the simulation.

NCHEM - Number of chemicals in the simulation. PRZM5 allows up to three chemicals to be specified.
Using more than one chemical (i.e., NCHEM=3) indicates either a parent-degradate relationship or
multiple separate chemicals.

NCPDS - Number of cropping periods. This is entered as a sum of all cropping dates from the beginning
simulation date to the ending simulation date.

NDC - The number of different crops in the simulation. This value determines how many separate crops
will be grown during a simulation. If only one type of crop is grown (i.e., corn), then NDC = 1. This value
includes the crop type of the initial crop (INICRP).

NHORIZ - Total number of horizons. PRZM5 allows the user to specify how many horizons are
simulated within the core depth (CORED). The horizon should serve as a distinct morphologic zone

generally described by layers (i.e., surface, subsurface, substratum) according to soil pedon descriptions
or soil interpretation records, if available.

NPLOTS - Number of time series plots. PRZM5 can report several output variables (PLNAME) to a time
series file. NPLOTS specifies how many are written in a single simulation.

Num_delx - This value is the number of compartments in a soil layer (or horizon).  The thickness of each
compartment within a horizon is calculated as THKNS/Num_delx. This compartment thickness affects
spatial resolution and numerical dispersion. As Num_delx increases, the compartment thickness
decreases, allowing a more precise placement of pesticide in the soil horizon and a higher resolution of
output. The compartment thickness will also directly affect the dispersion of the pesticide in the soil
profile. The dispersivity coefficient will be effectively as follows:

                                              a = ^ + T                                 (3-2)

where ot= dispersivity (cm)
       Ax = compartment thickness  (cm)
       T = dispersivity induced by the temporal discretization (cm)

The value of T is not under the user's control and is a function of the rainfall that occurs during a time
step (1 day). T is on the order of 2 cm for the typical U.S. rainfall in agricultural areas (Young and
Carleton, 2012).

Note: Num_delx is a new PRZM5 parameter. In previous PRZM versions, the number of compartments
was indirectly determined by a specified compartment thickness and horizon thickness.  When the
compartment thickness used by the program was not equal to the input compartment thickness,
unexpected program behavior resulted. Using Num_delx (an integer) fixes this problem.

OC - Percent of soil organic carbon (OC).  OC is conventionally related to soil organic matter as %OC =

PCDEPL - Fraction of available water capacity where irrigation is triggered. The moisture level where
irrigation is required is defined by the user as a fraction of the available water capacity. This fraction will
depend upon the  soil moisture holding characteristics, the type of crop planted, and regional agricultural
practices. In general, PCDEPL should range between 0.0 and 0.6, where a value of 0.0 indicates that
irrigation begins  when soil moisture drops  to the wilting point, and 0.6 indicates the more conservative
practice of irrigating at 60 percent of the available water capacity.  PRZM5 will accept values of PCDEPL
between 0.0 and  0.90. If the input value is outside this range, PRZM5 sets PCDEPL to 0.5 and issues a
warning message.

PFAC - The pan factor is a dimensionless number used to convert daily pan evaporation to daily potential
evapotranspiration (ET). Generally, pan factors range between 0.60 to 0.80. See Figure 3.4 for specific
regions of the United States.

Figure 3.4 Pan evaporation correction factors (from U.S. Weather Bureau).

PLDKRT - Foliage pesticide first-order decay rate. Pesticide degradation rates on plant leaf surfaces are
represented as a first-order process controlled by PLDKRT. The user must be consistent in specifying
PLDKRT and PLVKRT rates. If PLDKRT includes volatilization processes, then PLVKRT should be
zero. If PLVKRT is non-zero, then PLDKRT should include all attenuation processes except

PLNAME - Name of plotting variable. When creating a time series plot, PLNAME specifies the variable
in Table 3.5 for which that output data are written.

Table 3.5 Variable Designations for Plotting Files
Fortran Variable
Liable to
nitrate plant uptake
ammonia plant
nitrate plant uptake
ammonia plant
Plant return to
Litter return to
labile organic N
Litter return to
refractory organic
return to labile
organic N
return to refractory
organic N
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
kg ha"1 day"1
Required (IARG)
PLVKRT - Foliage pesticide first-order volatilization rate. Pesticide volatilization from plant leaf
surfaces is represented as a first-order process controlled by PLVKRT.

PTRN12, PTRN13, PTRN23 - lumped foliar transformation rate (days"1).

QFAC - Factor for rate increase when temperature increases by 10°C. Set to 2 for doubling of microbial
degradation rate every 10 degrees.

RATEAP - Maximum sprinkler application rate. RATEAP is used to limit sprinkler applications to
volumes that the sprinkler system is capable of delivering per time step.  Although this parameter is
defined as a depth (cm) of water delivered per hour; it should not be the maximum hourly delivery rate.
Because PRZM5 operates on a daily time step, the value should equal the daily amount of irrigation water
divided by 24, rather than the maximum hourly rate possible from a system.

SFAC - The snowmelt factor is used to calculate snowmelt rates in relation to temperature. PRZM5
considers snow to be any precipitation that falls when the air temperature is below 0°C. In areas where
climatology prevents snowfall, SFAC should be set to 0.0. Typical ranges for SFAC are provided in
Table 3.6.

Table 3.6. Typical Values of Snowmelt (SFAC) as Related to Forest Cover (Anderson, 1978).
Forest Cover
Coniferous - quite dense
Mixed forest - coniferous, deciduous, open
Predominantly deciduous forest
Open areas
Snowmelt Factor (cm/°C/day)
SLP - Slope of hydraulic flow path.

SPT - Initial soil temperature profile. To simulate the soil temperature profile, initial SPT values for each
soil horizon must be specified. Since PRZM5 is often used for long periods of simulation, the initial
temperature profile will not have any significant effect on the predicted temperature profile after a few
days or weeks of simulation unless the core depth (CORED) is deep. Lower horizons in the core should
be assigned values corresponding approximately to the bottom boundary temperature (BBT).

Tband_top - T-Band application (used with CAM = 7) to represent the fraction of the chemical
application incorporated into the top 2 cm.

TAPP - Target application rate for pesticide(s). For each pesticide and each application date, the amount
of pesticide is entered in kg active ingredient per ha. Typical rates are included on the product's
registration label. Actual rates used in the model are reduced by the application efficiency (APPEFF).

THCOND, VHTCAP - Thermal conductivity and volumetric heat capacity of soil horizon. If the user
chooses to have the model simulate the soil temperature profile and sets the IDFLAG flag to zero, then
the thermal conductivity (THCOND) and heat capacity (VHTCAP) must be specified. Representative
values for some soil types are given in Table 3.7. Note that the value of THCOND is entered in PRZM5
in units of cal cm"1 0C-1 day"1. Therefore, the values in Table 3.7 should be multiplied by 86,400 s/day. If
IDFLAG = 1, then THCOND and VHTCAP are calculated by the model from %Sand, %Clay, and %OC,
based on the method described in de Vries (1963).

 Table 3.7 Thermal Properties of Some Soil and Reference Materials (Rosenberg, 1974; Kilmer,
Dead Air
Hudson River
Podunk Fine
Sandy Loam
Water Content

Heat Capacity
(cal cm3/°C)
Thermal Cond.
(cal/cm/ °C/sec)

Leonardtown Silt
Muck Soil
Yolo Clay
Granite Sandy
Fine Calcareous
Granitic Sand
Barns Loam
Chester Loam
Herman Sandy
Kalkaska Loamy
Northway Silt
THEFC, THEWP - Field capacity and wilting point. These soil-water properties have been
characterized often and can be found in soil databases. If data are not available, several empirical
relations can be used, such as those of Rawls (1983) (See PRZM3 Manual).

THETO - Initial water content of the soil. This value provides the model with a starting calculation for
moisture. If site-specific data are not available, the field capacity value is recommended for THETO.

THKNS - Thickness of the horizon. This value is the depth (cm) of the horizon specified (HORIZN) in
relation to core depth (CORED).

UPTKF - Plant uptake efficiency factor for transpiration stream.  The product of UPTKF and the soil
pore water concentration represents the effective concentration of water that would flow into the plant at
the evapotranspiration flow rate. If UPTKF = 1.0, then uptake is estimated to equal the transpiration times
dissolved phase concentration. UPTKF is somewhat analogous to the short-term (24 to 48 hr)
transpiration stream concentration factor (TSCF) of Briggs et al. (1982). Briggs et al (1982) proposed an
empirical relationship as follows:
                         UPTKF « TSCF = 0.784 exp [-(log Kow - 1.78)2 / 2.44]
Briggs' laboratory results for UPTKF ranged from 0.11 to 0.94 for the 17 of the 18 pesticides that were
tested. One problem with the UPTKF concept in PRZM is that the effective uptake can continue
indefinitely, regardless of how much pesticide is implied to be held in the plant. Another consideration is
that PRZM does not distinguish evaporation from transpiration, so all plant uptake is based on the

evapotranspiration rate.  Caution should be used when implementing UPTKF for simulations that occur
for more than just a few days and where evaporation is a significant portion of the evapotranspiration rate.

USLEC - The universal soil loss cover management factor (C value). Values for USLEC are
dimensionless and range from 0.001 (well managed) to 1.0 (fallow or tilled condition). Up to 52 values
may be entered for an annual cycle, depending on crop growth and tillage operations. Specific values can
be calculated via Wischmeier and Smith  (1978). Generalized values are provided in the PRZM 3.12

USLEK - The universal soil loss equation (K) of soil credibility. This is a soil-specific parameter
developed by the USDA. Specific values can be obtained from the USDA Soil Data Mart.

USLELS - The universal soil loss equation (LS) topographic factor. This is a slope length and steepness
parameter developed by the USDA. The value is dimensionless and can be estimated from Wischmeier
and Smith (1978).

USLEP - The universal soil loss equation (P) practice factor. This value is developed by the USDA to
describe conservative agricultural practices. Values are dimensionless and range from 0.10 (extensive
practices) to 1.0 (no supporting practices). Specific values can be  estimated from Wischmeier and Smith
(1978) and Stewart (1975).

VHTCAP -  See THCOND for guidance.

ZWIND - Height of the wind speed measuring instrument. The wind speed anemometer is usually fixed
at 10 meters (30 feet) above the ground surface. This height may differ at some weather stations such as at
a class A station where the anemometer may be attached to the evaporation pan. The correct value can be
obtained from the meteorological data reports for the station whose data are in the simulation.
PRZM5 is a 1-D hydrology, heat and solute transport model, developed primarily for agricultural
pesticide simulations. The hydrologic component for calculating runoff and erosion is based on the Soil
Conservation Service CN method and the Universal Soil Loss Equation, respectively. Water balances are
maintained with consideration for runoff, evapotranspiration, irrigation, and precipitation. Daily
precipitation, temperature, wind speed, and pan evaporation are supplied as model inputs.

Vertical water movement is simulated by a capacity model (or tipping bucket) concept in which vertical
water movement is always downward and occurs when a soil compartment is filled to a maximum
capacity. Soil characteristics necessary for populating the capacity model are readily available and include
field capacity (surrogate for maximum water capacity) and wilting point (surrogate for minimum water

Dissolved, adsorbed, and vapor-phase concentrations in the soil are calculated by considering the
processes of pesticide runoff, erosion, degradation, volatilization, foliar wash off, removal by plant
uptake, leaching, dispersion, and sorption. The vertical transport is solved by a finite difference solution.
The time step is daily.
The current processes simulated by PRZM5 are:
    •   Crop Growth
    •   Irrigation
    •   Precipitation and Snowmelt
    •   Runoff
    •   Canopy Water Interception
    •   Evaporation
    •   Leaching
    •   Erosion
    •   Soil Temperature
    •   Chemical Application and Foliar Washoff
    •   Chemical Runoff and Vertical Transport in Soil
    •   Chemical Volatilization
Crop size is assumed to increase proportionally with time from its emergence date to its maturity date.
Crop size in PRZM5 refers to the canopy coverage and root depth. PRZM5 inputs are the maximum
fraction of areal coverage of the canopy and the maximum root depth.  Both of these parameters are the
values that would occur at the maturity of the crop. The canopy coverage and root depth are set to zero
on the harvest date.  This concept is depicted in Figure 4.1. Note that harvest does not necessarily refer
to harvest of the crop, and special consideration should be given to crops that do not lose their canopy at
harvest (e.g., apple trees whose PRZM-defmed "harvest" does not refer to the time of fruit harvest, but
instead refers to the  time of leaf fall).

  * I
  & 1
  *J 'v
  OC 2
  II  0.4
  I I
                                               MATURITY  HARVEST
Figure 4.1 Depiction of the crop growth routine in PRZM5. Crop size increases linearly from
emergence date to maturity date, where it remains at maximum size until the harvest date.

4.4  Irrigation

There are two types of irrigation available: over-canopy and under-canopy irrigation.  Over-canopy
irrigation is applied from above the canopy and may result in pesticide washoff if pesticide foliar
application is used. Under-canopy irrigation is applied to the ground and will not wash off for a foliar

Irrigation is controlled automatically, based on the soil moisture deficit and the precipitation in the
meteorological file. PRZM5 and previous versions of PRZM do not allow irrigation to occur on a day
with precipitation.  The moisture deficit is calculated for the soil profile from the  surface to a relevant
depth.  In PRZM5, this relevant depth can be set to the root depth or to a user-specified depth.  The soil
moisture deficit is calculated as follows:
D = total soil moisture deficit for the relevant soil depth (cm)
N = number of compartments for the relevant soil depth
6fc,i = field capacity for compartment /
61 = soil moisture in compartment /'
Az; = thickness of compartment /'

PRZM5 has four irrigation options that have been retained from previous PRZM versions:

   •  Type 1 :  This type of irrigation applies just enough water to satisfy the soil moisture deficit. The
       water is applied directly into the top surface layer and will not generate runoff.  In previous

       PRZM versions, this option was created to simulate flood irrigation.  There is no maximum limit
       to the amount of water that can be applied.

    •  Type 3: This type of irrigation applies water above the canopy.  Its effect on hydrology and
       pesticide transport is identical to precipitation. The amount of water supplied attempts to satisfy
       the soil moisture deficit, the canopy holdup, and any additional leaching requirements, but the
       amount is  limited by a user input (RATEAP).

    •  Type 4: This type of irrigation applies water under the canopy. Its effect on hydrology is
       identical to precipitation, except that the crop canopy has no impact on the water accounting. The
       amount of water supplied attempts to satisfy the soil moisture deficit and any additional leaching
       requirements, but the amount is limited by a user input (RATEAP).

    •  Type 6: This type of irrigation applies water above the canopy and applies a user-specified
       amount (RATEAP) that is not based on the soil moisture deficit. Its effect on hydrology and
       pesticide transport is identical to precipitation.
Incoming precipitation is first partitioned between snow and rain, depending upon temperature. Air
temperatures below 0°C produce snow and may result in the accumulation of a snowpack.  Snow
accumulation melts when temperatures are above 0°C. The daily rate of snowmelt is estimated by the
                                       {C T IP > C  T
                                       Lmi, zr > Lmi
                                       SP, SP  P+0.8S

Where Q = runoff (cm)
       P = precipitation (cm)
       S = potential maximum retention (cm)


S is related to the soil type, crop cover, and management practices and is calculated by tabulated values
for CN as follows:
                                                                                          ^   '
In this implementation of the CN method, rain, irrigation water, and snowmelt are treated as having the
same effect as precipitation. Thus, precipitation in the above equations is the sum of rain, snowmelt, and
irrigation.  The CN used for daily runoff calculations is adjusted on a daily basis according to the soil

Young and Carleton (2006) documented the method used in PRZM 3.12 and found it disagreed with the
methods described in previous PRZM manuals. The subsequent discussion here clarifies the way that
PRZM5 (as well as previous PRZM versions) calculate the curve number adjustments. In PRZM, the
user enters the CN, which represents average antecedent conditions (CNn).  PRZM then determines the
associated low and high CNs (CNi and CNm, respectively) from CN tables given in NRCS (2003).
PRZM then calculates the average soil moisture in the top 10 cm of soil for each day, and calculates an
adjusted CN based on this soil moisture.

As described in Young and Carleton (2006), PRZM uses the following definitions to make these
    •   CNi occurs when the average soil moisture content is zero in the top 10 cm of soil;
    •   CNn occurs when the soil moisture content in the top 10 cm of soil is equal to a representative
       halfway point between field capacity and wilting point;
    •   CNm occurs when soil moisture content rises to a value equal to the sum of field capacity plus
       wilting point.
When the PRZM-calculated soil moisture falls between these values, PRZM uses linear interpolation to
arrive at a CN.  The scheme is depicted in Figure 4.2. As shown, PRZM CNs are somewhat restricted in
variability and will never reach the CNi or CNm values.


1 UU
90 -
Curve Number
n O) -N| o
An -






Soil Moisture
Figure 4.2 PRZM scheme for CN adjustments. Solid lines reflect the possible range of CN
adjustments. WP is the wilting point; FC is the field capacity.  Dotted lines are used in the PRZM
calculations, but these CNs may not be attained in PRZM.

New Note for PRZM5: Unlike previous versions of PRZM, PRZM5 is more in line with the NRCS
(2003) method.  PRZM5 does not adjust the initial abstraction (the value 0.2S) or use throughfall instead
of precipitation in the runoff equation.  These values are already accounted for implicitly in the CN
method and making such adjustments would introduce false precision. Thus, PRZM5 maintains the
original intent and limitations of the NRCS CN method.
4.7  Canopy Water Interception

After the runoff is calculated, excess water (the difference between rain/overhead irrigation and runoff) is
used to satisfy the canopy capacity. Although in reality canopy capture would occur before runoff occurs,
the CN method already has canopy capture implicitly included, and thus runoff is predetermined for a
given precipitation event.  Canopy water holdup is given by:
=  min(P + / -
                                                   -i, Cmax)
Where Ct/2 = canopy water at end of precipitation event, but prior to evaporation (cm)
       Ct-i = canopy water from previous time step (cm)
       Cmax = maximum water that the canopy can hold (cm)
       P = precipitation (cm)
       I = overhead irrigation (cm)
       Q  = runoff (cm)

Daily potential evapotranspiration is calculated from the daily pan evaporation in the meteorological file
and the pan coefficient. This product, known as the free water surface evaporation, is a good
representation of potential evapotranspiration from an adequately watered natural surface such as soil or
vegetation (NOAA, 1982).

                                      ETp = KpEp                                        (4-6)
Where    ETP = potential evapotranspiration (cm)
          Kp = pan coefficient (unitless)
          Ep = pan evaporation (cm)

The potential evapotranspiration is used first on the plant canopy and then on the soil. The canopy water
content at the end of the time step (Ct) is equal to the amount of water added from equation (4-5) minus
the potential evaporation.

                            _  (Ct/2  - ETp   if Ct/2 -ETp>0
                            ~          0    ifCt/2-ETp<0                             V-'>
If the potential evaporation has not been satisfied by the canopy, then the remaining potential evaporation
is applied to the soil. The evapotranspiration is satisfied preferentially towards the surface in a
proportional manner, and it is also satisfied preferentially according to available water in a
proportional manner. This will roughly mimic the natural processes of plant uptake and root
structure (USDA, 1991).  This function of evapotranspiration with depth and available water is
as follows:
Where U is defined by a depth adjustment and a moisture adjustment:

Where Jmax(l) is the depth that evapotranspiration occurs on day i
       dx is the depth at position x.

Available water is defined as follows:

                            W^^=WMJ^-W%^}                              (4-10)
Where Wson,(0:x) = soil water in the spatial range 0:x


       WwA(0:x) = wilting point in the spatial range 0:x

As an example, Figure 4.3a shows a moisture distribution hypothetically taken as distributed in a
linearly increasing manner to the bottom of the evaporation zone (25 cm). Figure 4.3b shows the
contributions of depth and available water (the two parenthetical fractions in equation 4-9).
Figure 4.3c shows the combined effect of available water and depth on the distribution that the
model will use to extract water for evapotranspiration. Note that PRZM documentation seems to
state that evapotranspiration is satisfied by sequentially removing waters from the surface
downward; however the real mechanism that PRZM uses is the one presented below.
 "  2
 s  2
                            Depth (cm)
  £ 0.8
  c 0.6

...... .
- - Moisture Adjustment
                             Depth (cm)







                             Depth (cm)
Figure 4.3 a) Showing a hypothetical moisture distribution with depth; b) showing the
individual distributions of depth and moisture influence on et reduction; c) showing the
distribution of et with depth due to the combined effects of depth and availability of water.

The potential ET first consumes any available canopy held water. If the canopy water does not
satisfy the demand, then the water in the soil is used to attempt to satisfy the demand. The
remaining ET demand is distributed in a linearly decreasing manner through the surface soils to a
maximum specified depth and also is proportional to the amount of available water at any
location in the evaporation zone, with the distribution through the soil profile described by the
                                         " AW(x)dx
Where   Xet = the maximum depth for evapotranspiration action.
This type of distribution removes water from locations where it is most readily available. Water
is preferentially taken from the depths nearer the surface and preferentially from depths that
contain greater amounts of available water. The available water is not permitted to go below
zero. As with previous PRZM versions, PRZM5 applies an additional constraint: If the soil
moisture is less that 0.6 of the available water, then the available soil ET is reduced in proportion
down to WP (where it is zero).

If  _Wmaa  di   is less than 0.6, then the evapotranspiration is effectively reduced
Vertical water movement in PRZM5 is approximated by a capacity model also known as a "tipping
bucket" approach. Water content in any soil compartment is determined by continuity.  The soil water
content at any point in the soil column is first calculated from the amount of infiltrating water from the
above layer as:
                             9i,t+-L = Vi,t~ETiit£-i + 8iit                               (4-15)

Where 6^+1 = the soil water content of/' at the end of the current time step (cmVcm3)
       9i,t = the soil water content of/' at the start of the current time step  (cm3/cm3)
       vt = the water velocity entering compartment /' from the above compartment (cm/day)
       ETj = the evapotranspiration at depth /' (cm/day)
       At = time step (day)
       Az; = compartment /' thickness (cm)

If the soil water content exceeds the field capacity (THEFC), then the excess water is used as flow for the
next compartment and the flow into the next compartment is calculated as:

                              *i+i = (0i,t+i-
PRZM5 retains two of the three erosion options available from previous PRZM versions although the
remaining two options may still be problematic regarding quality assurance.  MUSLE (Williams, 1975)
has been challenged for not being mathematically sound (Kinnell, 2004), and MUSS lacks any published
documentation other than a conference abstract. (According to the abstract, MUSS is for "small
watersheds," but the definition of small is undefined). The third option, MUST, was eliminated because
of lack of use and documentation. Nevertheless, these models have been used for regulatory assessments
in the past. Thus MUSLE and MUSS have been retained in an effort to maintain continuity with previous
assessments.  MUSLE and MUSS options are formulated as follows:

MUSLE:                   Xe  =  1.586(yrqp)°'5V-12tf(LS)CP                             (4-19)

MUSS:                    Xe =  0.79(Vrqp~)°'65A0-009K(LS)CP                             (4-20)
Where Xe = the event soil loss (metric ton day"1)
       Vr = volume of event (daily) runoff (mm)
       qp = peak storm runoff (mm/h)
       A = field size (hA)
       K= soil erodability factor (dimensionless)
       LS = length-slope factor (dimensionless)
       C = soil cover factor (dimensionless)
       P = conservation practice factor (dimensionless)

The peak storm runoff value (qp) is calculated using the Graphical Peak Discharge Method as described in
NRCS (1986).
                                      qp  = quAQFp                                       (4-21)

Where qp = peak storm runoff (ft3/s)
       qu = unit peak discharge (csm/in)
       A = drainage area (mi2)
       Q = runoff (in)
       Fp = pond or swamp adjustment factor

For areas without swamps or ponds, Fp is equal to 1.0 (a value preset within PRZM).  The unit peak
discharge is calculated by NRCS (1986) as follows:

                        logfoj = C0 + C± log(rc) + C2 [log(rc)]2                           (4-22)

Where Tc = time of concentration (hr)
       Co, Ci, C2 = coefficients from NRCS (1986), given in Table 4.1 below

Rainfall intensity is assumed to occur according to design storm distributions (Type I, IA, II, and III) as
given by NRCS (1986).  Figure 3.3 shows a map of the rainfall type distributions in the U.S.

Table 4.1 Coefficients for the equation 4-19 (NRCS, 1986).
Rainfall Type
The time of concentration (TV) is defined as the time it takes water to flow from the furthest point in the
watershed to a point of interest within the watershed; Tc is a function of basin shape, topography, and
surface cover.  Tc is calculated by summing the travel time for various flow segments within the
watersheds (NRCS, 1986).  As with previous PRZM versions, PRZM5 is configured to have two flow
segments: 1) sheet flow for the first 100 meters, and 2) shallow concentrated flow (unpaved) for the
remaining portion of the hydraulic length.  Under the sheet flow segment, Tc is calculated as:
                                  _   0.007QVL)0-8
                               I c 	 CL  ,_^n ^ nA   I
                                       (P)0.5S0.4    3600 (16.1345s0-5)

Where s = slope (ft/ft)
       P = precipitation
       N= Manning's roughness coefficient for the watershed
       L = hydraulic flow length (m)
       P = daily precipitation (cm)
       s = slope of the hydraulic grade line (land slope, m/m)

       a,b= unit conversion factors

The equation for shallow concentrated flow is derived from Manning's equation, assuming a roughness
coefficient, N, of 0.05 and a hydraulic radius of 0.2 (Soil Conservation Service,  1986).  The average
velocities for estimating travel time for shallow concentrated flow are as follows:
                                      Unpaved V = 16.1345s05                              (4-24)
                                       Paved V = 20.3282 s°5                                (4-25)

Where V= average velocity (ft/s)
       s = slope of hydraulic grade line (watercourse slope, ft/ft)

These two equations are based on the solution of Manning's equation with different assumptions for N
(Manning's roughness coefficient) and r (hydraulic radius, ft). For unpaved areas, N is 0.05 and r is 0.4;
for paved areas, N is 0.025 and r is 0.2.
Soil temperature is modeled in PRZM5 to correct for temperature effects on volatilization and
degradation.  The objective of the soil temperature model is to provide a scientifically sound and usable
approach to predict with reasonable accuracy the daily average soil temperatures at the soil surface and
(in and below) the root zone, utilizing basic soil physical and thermal properties and daily climatic
measurements taken at weather stations.  PRZM5 calculates the soil temperature profile using inputs of
air temperature, solar radiation, surface albedo, wind velocity, evaporation, soil water content, and soil
physical properties, by the methods of de Vries (1963), Hanks et al. (1971), van Bavel and Hillel (1975),
and Thibodeaux( 1979, 1996).

Several models are available to predict soil temperatures under various soil surface conditions, but there
are restrictions to their general use as they either require large databases that are not available or are site
specific.  Existing soil temperature models form two general groups: (1) process-oriented models, which
require detailed information about soil and surface characteristics, initial and boundary conditions, and
inputs, and (2) semi- or non-process-oriented models, which often use weather station information and
soil temperature information at one depth to  develop empirical relationships. The  PRZM3 manual further
describes the key characteristics of the soil temperature models previously reviewed.

In PRZM5, daily bottom boundary temperature (BBT) and soil surface albedo (ABSOIL) values are first
calculated by interpolating between neighboring  BBT and albedo monthly values.  Surface albedo is
estimated from crop canopy albedo (COVER) and soil surface albedo, with an evaporation correction for
the canopy albedo.

If the user does not provide the thermal conductivity and heat capacity as inputs, the thermal diffusivity of
the soil compartment (quotient of thermal conductivity (A,) and heat capacity per unit volume (C), A/C) is
estimated using the methods  of de Vries (1963), where soil water content changes  with time and depth
(L).  For each soil compartment, PRZM5 estimates the corrected total volume (1-porosity, accounting  for


volume fractions of sand, clay, and organic carbon), and the volume fraction (Xvoi) of each soil
constituent from weight percentages (i.e., Sand(L), Clay(L), Organic Carbon(L)):
                   T/         _  _ ~    _ (A  ->
                   'corrected     Sand(L)    Ciay(L)   OC(L)*1.724       V ^~
                                 - T-H -- T-H -- T- *D£)(L)
                                L2.65cmJ 2.65cmJ  1.30g/cmJ J   v '
                                         2.65 g /cm3     corrected                             (4-2bJ

                              ,  n _ OC(L)*1.724*BD(L)
                         AVO((3,LJ-    130fl/cm3    * Corrected                           I4'27)

                      Xvol(2,L} =1-9(1}- Xvol(l,L} - Xvol(3,L}                       (4-28)
 Where 9(L) = porosity (unitless)
       BD(L) = bulk density (mass of soil /total volume [g/cm3])

The water and air content of the soil pores are also estimated. The water content (XVoL(4,L)) is equal to
the wilting point (THEWP) when the initial water content of the soil (THETO) is less than THEWP, or is
equal to the porosity (THETAS(L), 9) when the initial water content is greater than porosity. The air
content (XvoiX5,L)) is then calculated as the difference between the porosity (9) and the water content.

When the water content is greater than the field capacity of the soil (THEFC(L)), an additional parameter
(G), the depolarization factor of an ellipsoid, is estimated as:
                       G (5,1) = [0.333 - XV°eL^'L) ] (0.333 - 0.035)                         (4-29)
And the apparent thermal conductivity of the air-filled pores (ALAMDA, A,a), taking  into account
moisture movement, is calculated as the sum of the apparent thermal conductivities due to normal heat
conduction (AIRLMD) and vapor movement (VAPLMD).
                              A,a(5) = AIRLMD + VAPLMD                                (4-30)

Else, if the water content is less than the field capacity, G and A,a are calculated as:
      = 0 Q13 +    XVOLW      /      _ q(L)-Field Capacity^       _        _
               Field Capacity(L) LV                
                                           X0+ k-tX-t
Where ki represents the ratio of the average (by space) temperature gradient in the granules to the
corresponding gradient in the  medium:
                                      *• =  wm                                        <4-34'

  is calculated in PRZM5, assuming that the axes (a, b, c) of ellipsoidal granules are randomly oriented:

                                                         -1                               (4-35)
Volumetric heat capacity per unit volume (VHTCAP) of the soil layer (cal / cm3 °C) is then calculated as

           VHTCAP(L) = 0.46 ( Xvoi(l,L) + Xvoi(2,L) ) + 0.60 Xvoi(3,L) + THETO(L)             (4-36)

Where 0.46 cal/g °C represents the average specific heat for mineral soils at 10°C, and 0.60 cal/g °C is the
average specific heat for organic soils.

The diffusion coefficient (or diffusivity, DIFFCO, in cm2/d)  is finally calculated as follows:

                              DIFFCO = A,(L) / VHTCAP(L)                                (4-3 7)
In PRZM5, the upper boundary temperature is estimated using an energy balance at the air-soil interface.
From Thibodeaux (1996), the air density (AIRDEN), pair (g/cm3) is first calculated as:

                             pair = (-0.0042 T + 1.292) * le -3                               (4-38)

Then the heat transfer coefficient (HTC) at the air-surface interface (cm/d) is estimated as follows:
                                  __  vonKarman2 * WIND                                  ,. __,
                              H1C —-, WIND RefHelght-D . 7                                 (4-jy)
       Where vonKarman = von Karman constant used in boundary layer meteorology (0.39 or 0.40)
       WIND = Wind speed (cm/d)
       WIND Ref Height = Wind reference height (m)
       D = Zero displacement height (m)
       Z0 = Roughness length (m)

The Energy Balance equation at the air-soil interface is summarized as follows:

                                  qis = qiw + qe + qc + qs                                    (4-40)

       Where qis = Sensible heat flux into the soil column (cal/cm2 d)
       qiw = Net longwave  radiation flux (cal/cm2 d)
       qe = Evaporative (water) heat flux(cal/cm2 d)
       qc = Sensible heat flux (conduction) between surface and air (cal/cm2 d)
       qs = Shortwave solar radiation (cal/cm2 d)

These heat fluxes are calculated individually in PRZM5 as follows:


                                      qiw,atm = EMISS * 0.936e-5 * T2 * 11.7e-8              (4-41)
                                      qiw,soa = EMISS * 1 1 .7e-8                            (4-42)
                                      qe = 580.0 * EVAP * 1.0                             (4-43)
                                      qc = pair * 0.2402 * HTC                             (4-44)
                                      qs = (1- ABSOIL) *  SOLRAD                         (4-45)

The 4th order equation for upper boundary temperature (UBT) is then solved using the Newton-Raphson
method, in terms of soil surface temperature using the calculated heat fluxes.  The soil temperature profile
is then calculated, given the upper boundary, bottom boundary, and initial temperatures.

In PRZM5, there is also a correction for temperature dependent degradation based on the Qw equation
(similar to Arrhenius equation):
                              QIOFAC = QFAC(T-T™sE)/w                               (4.45)
Where QIOFAC = correction factor for biodegradation based on the actual temperature
       QF AC = factor for rate increase when temperature increases by 10°C
       T = actual soil temperature
       TBASE= temperature during the test of biodegradation
PRZM5 includes updated pesticide application methods. The previous PRZM versions had eight
application options, but several were unused or unrealistic; PRZM5 has eliminated these problematic
methods (CAM = 3, 5, 6, 9, 10).  The following CAM schemes have been retained in PRZM5:
    •   CAM=1 Ground or soil-surface application: residues are distributed to 4 cm, linearly decreasing
        with depth.
    •   CAM=2 Foliar or above canopy application: foliage captures pesticide in direct proportion to its
        areal coverage.
    •   CAM=4 Uniform incorporation: pesticide is evenly distributed in the soil down to a user-
        specified depth.
    •   CAM=7 T-Band:  a user-specified fraction of pesticide is applied in the top 2 cm, and the
        remainder of the chemical is applied uniformly between 2 cm and a user-specified depth.
    •   CAM=8 @Depth: pesticide is placed entirely into a single compartment at a user-specified depth.
Note: DEPI must be set to greater than 0.0 for CAM = 4, 7, and 8. If DEPI = 0, or DEPI < depth of the
first (top) surface soil layer, the chemical reaching the soil  surface is distributed into the first surface soil

Pesticide foliar washoff in PRZM5 are distributed in the soil uniformly to a depth of 2 cm. This is the
same way that PRZM3 handled washoff (although the PRZM3  manual incorrectly stated that the washoff
was applied in a linearly decreasing manner to 4 cm).

The two major components of PRZM5 are hydrology and chemical transport. Time-varying transport by
advection and dispersion in the dissolved phase or diffusion in the gas phase are represented in the model.
Dissolved, sorbed, and vapor-phase concentrations in the soil are estimated by simultaneously considering
the processes of pesticide uptake by plants, runoff, erosion, decay, volatilization, foliar washoff,
advection, dispersion, and retardation. The transport equations are solved in PRZM5 using a finite-
difference numerical solution (backwards-spatial difference for advection and fully temporally implicit
scheme). The hydrologic components that determine velocity were described previously.

       dmw _^_ dms _ _^ ^ ^  —kMC — q(z)C  — £Yz)C  — 7 d°w Az + DA3  °w Az          (4-47)
Where mw = solute mass in the water phase of the compartment (M)
       ms = solute mass in the sorbed phase of the compartment (M)
       kw = first order degradation coefficient for water phase of the compartment (T"1)
       ks = first order coefficient for sorbed phase of the compartment (T"1)
       Vw = volume of water in the compartment (L3)
       Ms = mass of soil in compartment (M)
       q(z) = runoff flow associated with depth z (L3/T)
       E(z) = erosion flow associated with depth z (M/T)
       I = infiltration flow (L3/T)
       Cw = concentration in water (M/L3)
       Cs = concentration on soil (M/M)
       D = dispersion coefficient (L2/T)
       A = area of effective dispersion (L2) = AT0
       z = vertical dimension (L)
       Az = compartment length in vertical dimension (L)
Substitutions and reducing the equation give:
                                                                 ar          A2r    \
(Vw+MsKd) 6CW
       VT     dt   VT\   w

And further substitutions give:
Where  Kd = sorption coefficient (L3/M)
        K'd = erosion-specific enhanced sorption coefficient (L3/M)
        VT = total volume of compartment (L3)
        pb = bulk density (M/L3)
        DR(Z) = area-normalized runoff intensity (q(z)/Afieid) associated with depth z, (L/T)
        EN(z) = area-normalized erosion intensity (E(z)/Afieid) associated with depth z, (M/L2/T)
        v = infiltration velocity based on full area (L/T)

Fully implicit, backwards spatial differencing:

(0 + PbKd)(Cf+1 - Cf) = [-kwQ- kspbkd - g) - g) kd] AtCft - ^ (Qt+1 - Qft1) +


Or generically:

                        A (Qft1) + B( cf+1) + Cfoft1) = F(cf)                         (4-47e)
             B =  0 + PbKd +     +     + ^6+ /cspb/cd +      +     fcd At              (4-47g)
                                     F = 0 + PhKd                                       (4-47i)
For the extraction of chemicals from the soil into runoff, runoff flow is conceptualized as partially
flowing through the soil. PRZM5 uses a more transparent method of accounting for runoff extraction
than PRZM3 .  (The PRZM3 code did not reflect its documentation, and the documentation including the
extraction equation itself was incorrect.)

In PRZM5, the runoff includes a portion that interacts with the soil and a portion that does not.  The
portion that interacts with the soil is conceptualized as having a flow profile that decreases exponentially
as depth increases.  This is mathematically the near equivalent of the PRZM3 conceptualization. PRZM3
invoked an abstract concept of partial concentrations that interacted with runoff in a nonequilibrium
manner, whereas PRZM5 maintains equilibrium conditions between the runoff and soil, in a manner
similar to Ahuja et al. (1981, 1983). The runoff flow distribution beneath the surface is represented by a
subsurface flow intensity distribution described by the following equation:

                                                 rz                                      (4-48)
Where q(z) = the runoff intensity at depth z (cm runoff/cm depth)
       qo = the hypothetical flow intensity at the surface (cm runoff/cm depth)


       Kr = the decline coefficient describing the decrease in flow with depth (cm"1)
       z = midpoint of the compartment depth (cm)

The flow is assumed to be constrained to a subsurface depth of D. In PRZM3, the value of D was 2 cm.
Accounting for the runoff distribution, the equation becomes:

                                  J0D q0e-Krzdz = FQr                                    (4-49)

       Where F = the fraction of runoff traveling through the soil (unitless)
       Qr = runoff (cm)
       D = maximum depth of runoff interaction (cm)

Thus, the surface runoff intensity is calculated as:
                                     9o  = (1!eQ-£0)                                       (4-50)

And the intensity at depth z can be estimated as:
The parameter F describes how much of the runoff flow interacts with the subsurface.  The amount (1-F)
can be thought of as bypass flow that does not contribute to the equilibrium extraction of the chemical in
runoff. PRZM3 effectively bypassed about 73.4% (F = 0. 266) of the runoff and used a decline
coefficient of 1.55 cm"1 (Figure 4.4).




                    Bypass Flow
Subsurface Flow Profile
                       0.1          0.2           03
                              Relative Flow Intensity (I/cm)
Figure 4.4 Depiction of the PRZM3 effective runoff extraction profile, using the PRZM5
conceptualization. In this case, 73% of the runoff flow does not interact with the subsurface
(bypass flow), and 27% of the runoff interacts in a manner that declines with depth. The
subsurface profile is equivalent to a flow profile with increasingly less flow as depth increases.
In PRZM5, the extraction of pesticide by eroded sediment is conceptualized differently than in PRZM3.
In PRZM3, the pesticide associated with erosion was taken from the topmost compartment regardless of
its size. This was undesirable, as pesticide transport off the  field became a strong function of the top
compartment size; thus, pesticide transport could be dramatically reduced by decreasing the compartment
size. To eliminate artificial relationships,  PRZM5 allows the user to specify the interaction in a manner
similar to the runoff extraction relationship.  In a derivation  analogous to the runoff extraction routine
above, the erosion intensity with depth is estimated as:
Where E(z) = the erosion intensity at depth z (kg /cm)
       Me = mass of eroding solids (kg)
       Ke = the decline coefficient describing the decrease in erosion with depth (cm"1)
       De = maximum depth erosion interaction (cm).

4.14 Chemical Volatilization
PRZM5 retains the PRZM3 chemical volatilization routine.  The theory behind the PRZM3 chemical
volatilization routine is described here, from section 6.3.6 of the PRZM3 Manual, including several key
processes (Figure 4.5):
    •  Vapor-phase movement of pesticide through soil profile
    •  Boundary layer transfer at soil-air interface
    •  Vertical diffusion of pesticide vapor within plant canopy
    •  Pesticide mass transfer between plant (leaves) and surrounding atmosphere
    •  Soil temperature effects on pesticide volatilization

                                                     " Height
                        Plant Compartment
                        Volatilization Flux
                                            A i i
                                    ...	__.£__     Canopy Height
          HORIZON 1
          HORIZON 2  -
          HORIZON 3

r^ v
' Volatilization '
^ From

-*r » Plant
From Soil |
F V/nnnr Riffi Rinn

                                                                Boundary Layer
       Figure 4.5 Pesticide vapor and volatilization processes.

   4.14.1 Soil Vapor Phase and Volatilization Flux Surface boundary condition
The initial volatilization rate after a pesticide is incorporated into the soil depends on the vapor pressure
of a pesticide at the surface. As the pesticide concentration changes at the surface, volatilization may also
become increasingly dependent on the rate of movement of the pesticide to the soil surface (Jury et al,
1983a, 1983b).

One boundary layer model used in PRZM3 determines pesticide volatilization based on molecular
diffusion through a stagnant surface boundary layer (Jury et al., 1983a,  1983b). If the diffusion rate
through the stagnant layer matches the upward flux to the soil surface, without the surface concentration
building up, then the stagnant layer does not act as a barrier to loss. Conversely, if the diffusion rate is
less than the flux to the surface, then the concentration at the surface will be greater than zero.
The pesticide volatilization flux from the soil profile is estimated as follows:

                                      A =  ~~T (Cg,l ~ Cg,d)

 Where Ji = volatilization flux from soil (g/day)
        Da = molecular diffusivity of the pesticide in air (cm2/day)
        A = cross-sectional area of soil column (cm2)
        d = height of stagnant air boundary layer (cm), assumed to be 0.5 cm (Wagenet and Biggar, 1987)
        Cg,i  = vapor phase concentration in surface soil layer (g/cm3)
        Cg,d = vapor phase concentration above the stagnant air boundary layer (g/cm3) (zero if soil
        surface bare; positive if plant canopy exists)
The pesticide volatilization flux through the plant canopy is calculated by Pick's First Law of diffusion:
                                       ;z(z)=  -tfzGO£                                   (4-54)
 Where Jz(z) = pesticide volatilization flux at height z (g m"2 s"1)
       dP/dz = pesticide concentration gradient (g m"2)
       Kz(z) = vertical diffusivity at height z (m2 s"1), as a function of meteorology

Based on Pick's First Law, pesticide concentrations at two  or more heights can be used to estimate the
pesticide gradient and subsequent flux.  To estimate vertical diffusivity, additional meteorological
information would be needed.  However, PRZM circumvents these data requirements by using a
relationship for Kz, which is a function of height within the canopy (Mehlenbacher and Whitfield, 1977).
Therfore only the pesticide concentration gradient is needed for estimating Jz(z). Kz is calculated at
various heights within the plant canopy as:
                                       Kz(z)= Kz(zch)e^-V                           (4-55)
                                       K  ,   N _  U" k(zch- D)
                                       Kz^zch) -  	^	                                (406)
                                       U* = 	^^	                            (4-57)
                                              ln[(zch-D)/z0]+ipmh = stability function for sensible heat
       Ym(<|>m) = integrated momentum stability parameter
       (|)m = stability function for momentum
       Uch = wind at canopy height (m s"1)

In agricultural applications, the canopy height (zch)  is used as the reference height for calculating U*.  The
user must input the wind speed and height at which the measurement was made. Since PRZM3 assumes
neutrally stable atmospheric conditions (where vj/m(<|>m) = 0), the wind speed at canopy height (UCh) is
calculated as follows:

                                       U   -  U  	z°,cfa J                                (4-58')
                                       uch    ur     [zr-Dr]                                 VH J°>
                                                  log -;—~
                                                     L zo,r
Where Ur = wind speed (m s"1) at zr, retrieved from meteorological file
       zch = top of canopy height (m)
       DCh = zero plane displacement height (m) associated with canopy
       z0,ch = roughness length (m) associated with canopy
       zr = reference height (m), assumed equal to  10.0 (for open flat terrain)
       Dr = zero plane displacement height (m) associated with measurement, assumed equal to 0.0 (for
       open flat terrain)
       z0,r = roughness length (m) associated with measurement, assumed equal to 0.03 (for open flat

Table 6.2 of the PRZM 3 Manual provides aerodynamic parameters (i.e., reference heights, zero plane
displacements, and roughness lengths) commonly used (Burns et al., 2005).  PRZM3 assumes the open
flat terrain conditions for its wind speed calculations, but the user may specify a reference height in the
PRZM input file.

For short crops (i.e. lawns), z0 adequately describes the total roughness length, and no zero plane
displacement is needed (D =  0).

For tall crops, z0 is related to canopy height (zcn) in the following equation:
                                       log z0  = 0.997 log(zch) - 0.883                     (4-59)

And D is calculated, as z0 (for tall crops) is not an adequate description of the total roughness length. For
a wider range of crops and heights (0.02 m < zcri < 25 m), the following equation is used (Stanhill, 1969):
                                       log D =  0.9793 log(zch) - 0.1536                   (4-60)

In PRZM3  when zcri < 5 cm, D = 0 and z0 is set to the value given by equation (4-48) evaluated at zcri =
0.05 m. Once z0 and D have  been estimated, U* can be calculated if the stability parameters (\|/m and (|)h)
are known.  These two stability parameters are closely related to the Richardson number (Ri), which is a
measure of the rate of conversion of convective turbulence to mechanical turbulence (or the relationship
between the temperature gradient and wind speed).  Based on Thibodeaux (1996), Ri (typically -2.0 < Ri
< 0.2) is calculated as follows:
                                       ^ = _S_(^ISyL = _S_ P-z-TiVzc/.                (4_61)
                                            Tmean (Sv/ox)   Tmean ((v2 V1)/zch)

Where g  = acceleration of gravity, 9.8 m/s2 (86400 s/d)2 = 7.32elO m/d2
                                                   S7"1 T
       Tmean = mean temperature at defined level (K) = ——h 273.15
       vi = wind speed at the soil surface (m/d)
       V2 = wind speed at the top of the canopy (m/d)
       TI = air temperature the soil surface (°C)
       T2 = ambient air temperature (°C)

The sign of Ri indicates the atmospheric condition and its magnitude reflects the degree of influence:
        For Ri > 0.003, stable conditions and little vertical mixing
        For Ri < 0.003, neutral stability conditions
        For Ri < -0.003, unstable conditions and convective mixing

To relate the atmospheric stability parameters to Ri, Arya (1988) proposed using the Ri to calculate a
dimensionless height (z):
                                               Ri    Ri <
                                         =  I   Ri
                                                    Ri >
The stability functions for momentum (c|)m) and sensible heat ($m) are then calculated as follows:

                                                     ~ 15z)~   Z < ° I                     (4-63)
                                                     1 + 5z     z > O/
The integrated momentum stability parameter (\|/m) is finally given by Thibodeux (1996):

                 *. W =  ff - 2 1"rl<*-> + '"" K^) (^)J1   Z < °
                           V                  -5z                     z>0
To calculate the volatilization flux from the soil, a resistance-type approach is used. For pre-plant
pesticides and time periods just after emergence and post-harvest, transport by volatilization from plant
surfaces is much less than vapor phase transport by other mechanisms. When plant leaves are not a
significant source or sink of pesticide vapor, the resistances for the whole plant compartment is estimated
as follows (Mehlenbacher and Whitfield 1977):
                                    T,R=Rbd + Rpc                                      (4-66)

                                        R™=                                            (4'66a)
        ER = total vertical transfer resistance (day cm"1)
        Rbd = boundary layer resistance (day cm"1)
        Rpc = plant canopy resistance (day cm"1)

The flux is then calculated as follows:
                                        JPc  = fj                                          (4-67)

Where Jpc = volatilization flux from plant canopy (g cm"2 day"1)
        Cg,i = pesticide vapor concentration in top soil layer (g cm"3)

If the plant canopy acts as a significant source or sink, another approach should be taken, as described in
the following section.


Based on Stamper et al. (1979), PRZM3 uses first-order kinetics to calculate volatilization flux from plant
leaf surfaces, where the user inputs a first-order rate constant for volatilization. The plant leaf
volatilization is estimated as follows:
 Where Jpi = volatilization flux from the leaf (g cm"2 day"1)
        M = foliar pesticide mass (g cm"2)
        Kf = first-order volatilization rate (day"1)

The average pesticide concentration in the plant canopy is also estimated:
                                 c; =  [;PC+;Pi]Zflo.5                                   (4-69)
Where Cg* = average concentration in air between ground surface and plant canopy height (g cm"3)
           5 = canopy resistance at one half of the canopy height to top of the canopy
PRZM5 is coded for the most part in the standards of Fortran 2003.  Related subroutines have been
grouped into modules with intuitive names (e.g., Erosion, Pesticide Application, and Irrigation). This
should greatly facilitate locating relevant routines during maintenance in comparison to previous PRZM
versions. The Main Program (PRZM5) is in the file PRZM5.f90. The main program calls the input file
read routines (PRZMRD_PRZM5) and then sets up a daily time loop. The daily time loop reads a line
from the meteorological file and then runs a day of the hydrology, plant growth, and transport routines.
Daily output is delivered to a * .zts file, where desired output is specified by the user in the input file.  A
file that records run status and errors is created as kecho.prn.  All files are delivered to the default
working directory.

To run PRZM5, the command line is as follows:

                                prompt> PRZM5.exe inputfilepath

Where the Inputfilepath argument is optional, \finputfilepath is specified, then it will be the default path
for the input file and where the output will be delivered.  If unspecified, then the default working
directory is used.  The input file  must be named "PRZMS.inp".

Example command: the following command from a command prompt (C:>) would run the PRZM5.exe
file that is in the C: \models\ folder and use the PRZMS.inp file that is in the C: \models\testrun\ directory:

                            C:> c:\models\Przm5.exe c:\models\testrun\

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