4/
7/
04
1
Introduction
to
Questions
to
the
Panel
Aquatic
Level
II
Model
4/
7/
04
2
1.
Varying
Volume
Water
Model
(
VVWM).

For
aquatic
risk
assessments,
OPP
currently
uses
a
water
body
fate
model
that
has
a
fixed
volume
and
does
not
consider
hydrologic
inputs
and
outputs.
The
SAP
2001
suggested
that
adding
volume
variations
and
overflow
to
the
Level
II
fate
model
would
improve
the
characterization
of
the
water
body
and
improve
estimates
of
aquatic
pesticide
concentrations.
4/
7/
04
3
Question
1
cont'd
In
response,
a
new
model
has
been
developed
that
allows
volume
variations
and
overflow
in
the
water
body.
The
new
model
also
allows
for
meteorologically
dependent
parameters,
such
as
temperature
and
wind
speed,
to
vary
on
a
daily
basis,
rather
than
a
monthly
basis,
to
better
capture
temporal
variability.
In
addition,
the
model
was
constructed
to
improve
runtime
because
of
the
potential
use
in
Monte
Carlo
simulations.
4/
7/
04
4
Question
1
cont'd
a.
Please
discuss
the
new
model's
capability
to
capture
the
most
salient
processes
influencing
the
variations
in
water
body
volume,
and
also
discuss
the
modification
allowing
daily
variations
in
meteorological
dependent
variables.

b.
Inputs
of
mass
on
a
given
day
are
assumed
to
occur
instantaneously.
Please
discuss
the
advantages
and
disadvantages
of
this
assumption
with
specific
consideration
for
the
trade
off
between
runtime,
accuracy
and
the
consideration
that
input
data
are
4/
7/
04
5
Question
1
cont'd
given
as
daily
values.
What,
if
any,
additional
approaches
regarding
modeling
input
mass
would
the
SAP
recommend,
please
provide
a
discussion
of
the
pros
and
cons
as
compared
to
the
current
method?

c.
What
additional
model
characterization
or
documentation
is
required
to
ensure
clarity
and
transparency?
4/
7/
04
6
2.
Exposure
Model
Testing
The
QA/
QC
testing
of
the
aquatic
Level
II
Version
2.0
exposure
model
demonstrated
that
the
refined
risk
assessment
shell
is
consistent
with
the
Level
II
Version
1.0
shell
(
PE4)
for
launching
PRZM
and
is
compatible
with
all
crop
scenarios
and
meteorological
files.
The
testing
also
showed
that
the
dissipation
algorithms
in
the
VVWM
are
consistent
with
EXAMS
and
that
the
4/
7/
04
7
Question
2
cont'd
volume
and
overflow
algorithms
are
correct.

Evaluation
of
the
VVWM
showed
the
potential
effect
that
a
varying
volume
water
body,
using
current
standard
field
size
and
water
body
volume
and
surface
area,
can
have
on
estimated
environmental
concentrations
due
to
dilution,
evaporation,
and
overflow.
4/
7/
04
8
Question
2
cont'd
a.
What
additional
testing,
evaluation
and/
or
sensitivity
analysis
can
the
SAP
recommend
to
ensure
that
the
aquatic
Level
II
exposure
model
meets
the
Agency
objectives
of
transparent
processes,
and
clear,
consistent
and
reasonable
products
suitable
for
risk
characterization?
4/
7/
04
9
Question
2
cont'd
b.
Based
on
the
evaluation
performed
using
the
VVWM
under
standard
field
(
10
ha)
and
standard
surface
water
scenario
conditions
(
1
ha
surface
area,
20000
m3
volume),
please
discuss
the
advantages
or
disadvantages
to
characterizing
risk
by
replacing
a
single
standard
with
multiple,
crop
scenario­
specific
standards
at
Level
II.
4/
7/
04
10
3.
Field
Drainage
Area
and
Water
Body
Size
Selection
At
Level
II,
the
risk
assessment
approach
is
aimed
at
addressing
the
risk
to
aquatic
species
in
high
exposure,
edge­
of­
field
situations.
The
surrogate
surface
water
used
for
Level
II
consists
of
a
small,
perennial
surface
water
body
at
the
edge
of
an
agricultural
field.
This
water
body
is
capable
of
being
supported
by
agricultural
field
runoff
alone,
and
of
supporting
an
aquatic
community.
Crop
scenario­
specific
4/
7/
04
11
Question
3
cont'd
input
values
for
field
size,
surface
water
volume,

surface
area,
and
depth
were
developed
and
systematically
explored
using
three
methods.

The
methods
used
readily
available
drainage
area
to
volume
capacity
(
DA/
VC)
ratios
and
associated
water
depth
guidance
for
construction
of
small
permanent
surface
waters
of
the
continental
U.
S.
4/
7/
04
12
Question
3
cont'd
a.
USDA's
(
1997)
DA/
VC
ratios
and
depth
guidelines
for
construction
of
small
permanent
water
supplies
(
e.
g.,
irrigation,
livestock,
fish
and
wildlife)
were
used
as
the
source
of
national
and
regional
DA/
VC
ratios
and
associated
water
depths.
What
additional
existing
sources
of
national
or
regional
DA/
VC
ratios
for
small,

permanent
surface
waters
(
e.
g.,
wetlands,
pools,

ponds)
should
be
considered?
4/
7/
04
13
Question
3
cont'd
b.
Please
describe
the
merits
or
limitations
to
the
approaches
and
assumptions
evaluated
for
using
the
U.
S.
Department
of
Agriculture's
(
1997)
guidelines
to
derive
field
size,
surface
water
volume,
and
surface
area
input
values
for
specific
crop
scenarios?
What,
if
any,
additional
approaches
and
assumptions
should
be
considered?
4/
7/
04
14
Question
3
cont'd
c.
A
default
minimum
depth
was
set
as
0.01
m.

What
minimum
depth
would
the
SAP
recommend
as
a
criterion
to
evaluate
the
biological
relevancy
of
the
scenario?

d.
Simulations
with
the
PRZM/
VVWM
were
performed
using
both
the
crop­
specific
surface
water
area
and
volume
and
the
historic
standard
values
(
DA/
VC
=
1.5
acres/
acre­
ft)
to
characterize
effect
on
exposure
outputs
for
a
relatively
arid
growing
region
(
DA/
VC
=
50
4/
7/
04
15
Question
3
cont'd
acres/
acre­
ft)
and
a
wetter
climate
(
DA/
VC
=
1
acre/
acre­
ft)
for
both
a
short­
lived
and
long­
lived
pesticide.
In
addition,
the
effect
on
volume
in
the
surface
water
body
was
characterized
for
all
crop­
specific
scenarios.
Please
discuss
what,
if
any,
additional
crop
scenario/
pesticide
evaluations
should
be
performed
to
further
characterize
the
impact
to
exposure
outputs,

and/
or
to
volume.
4/
7/
04
16
Question
3
cont'd
e.
What
are
the
advantages
or
disadvantages
to
characterizing
exposure
for
small,
perennial
surface
waters
at
the
edge
of
treated
fields
using
the
method
selected
for
setting
crop
scenariospecific
DA/
VC
ratio,
depth,
surface
area
and
volume
input
values?
What
adjustments
or
changes
to
the
method
does
the
SAP
recommend,
and
what
are
their
advantages
and
disadvantages?
4/
7/
04
17
Question
3
cont'd
g.
Simulations
with
PRZM/
EXAMS,
a
fixed
volume
surface
water
model,
will
be
performed
using
both
the
crop­
specific
DA/
VC
approach
and
the
historic
standard
values
to
characterize
effect
on
exposure
outputs
for
relatively
arid
growing
regions
(
DA/
VC
=
50
and
80)
and
a
wetter
climate
(
DA/
VC
=
1)
for
both
a
short­
lived
and
long­
lived
pesticide.
Please
discuss
what,
if
any,

additional
crop
scenario/
pesticide
evaluations
should
be
performed
to
further
characterize
the
impact
to
exposure
outputs
in
a
fixed
volume
situation.
4/
7/
04
18
Question
3
cont'd
h.
Please
discuss
sources
or
approaches
for
national
or
regional
DA/
VC
ratios
and
associated
water
depth
and
size
information
for
temporary
pool
and
pond
aquatic­
life
resources.
4/
7/
04
19
Question
3
cont'd
f.
Please
describe
the
weaknesses
and
strengths
of
using
simulated
exposure
concentrations
from
these
crop
scenario­
specific
water
bodies
as
a
surrogate
for
a
low­
order
stream
at
the
edge
of
a
field,
for
a
temporary
pool
or
pond,
and
for
a
small
tidal
creek
or
estuary.
4/
7/
04
20
4.
Curve
Number
The
SAP
2001
recommended
that
additional
characterizations
of
variability
should
be
given
to
those
parameters
in
the
exposure
model
that
have
a
major
impact
on
exposure
concentrations.
The
curve
number
is
perhaps
the
most
influential
parameter
in
PRZM,
and
it
has
been
interpreted
in
recent
literature
as
a
random
variable.
PRZM
currently
treats
the
curve
number
as
a
function
of
soil
moisture,

although
recent
literature
suggests
that
the
curve
number
may
more
appropriately
be
interpreted
as
a
random
variable.
4/
7/
04
21
Question
4
cont'd
a.
Please
discuss
the
pros
and
cons
of
assuming
strict
dependence
of
curve
number
on
calculated
soil
moisture
versus
treatment
as
a
random
variable
unrelated
to
soil
moisture
as
a
means
of
characterizing
runoff
variability?
Please
identify
and
discuss
alternative
methods.

b.
Since
the
curve
number
was
not
designed
for
use
in
continuous
modeling,
what
problems
may
arise
when
the
curve
number
is
used
in
this
manner?
Could
a
probabilistic
interpretation
address
some
of
these
issues?
If
so,
how?
4/
7/
04
22
Question
4
cont'd
c.
What
is
the
impact
on
interpretation
of
probabilistic­
simulated
exposure
values
when
the
curve
number
is
used
as
a
random
variable
and
autocorrelation
of
temporally
varying
physical
properties
that
may
impact
run
off
is
ignored?

d.
A
lognormal
distribution
is
being
investigated
to
characterize
variability
in
certain
curve
number
parameters.
Is
it
reasonable
to
assume
such
a
distribution
has
stationary
properties
(
constant
mean
and
variance)
for
all
rain
events
(
e.
g.,

large
and
small)?
Please
provide
rationale.
4/
7/
04
23
Question
4
cont'd
e.
Monte
Carlo
modeling
is
being
investigated
as
a
method
of
integrating
the
potential
variability
of
curve
numbers
into
exposure
modeling.
Can
the
SAP
recommend
other
methods
available
to
incorporate
variable
and
uncertain
curve
numbers
into
a
continuous
runoff
model.
Please
discuss
the
pros
and
cons
of
these
methods
versus
Monte
Carlo.
