United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-90/064 Sept. 1990
Project Summary
Progress Report: Comparison of
Precipitation Measurements by
Nipher-Shielded and Standard
Belfort Recording Rain Gages at
NADP/NTN Sites
Carol L. Simmons and David S. Bigelow
Persistent concern by the scientific
community that wet deposition was be-
ing incorrectly calculated due to under-
estimation of snow fall prompted EPA to
install devices on the field sampling
gagesto improve catch efficiency. Some
evidence indicated that snow fall was
being underestimated due, in part, to
windy conditions. A shield was installed
on the regular collection gage to improve
catch during snow and windy conditions.
The data collected during this study was
compared to data collected at the same
sites using the normal system.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
A widely recognized source of error in
precipitation estimates is incomplete collec-
tion of snow by precipitation gages, par-
ticularly under windy conditions. The Belfort
Universal recording precipitation gage, the
type used in most weekly monitoring net-
works in the United States, has been shown
to underestimate snowfall by as much as
50%, even when equipped with an Alter
shield to deflect the wind. In recent years, a
precipitation gage known as the Canadian
MSC Nipher-Shielded snow gage has re-
ceived considerable attention as an im-
proved gage for monitoring snowfall. This
gage has been shown to capture 90-100% of
"ground-true" snowfall under a wide range
of environmental conditions. The superior
performance of this gage has led to its
designation as the official snow gage in
Canada. Canadian scientists have adapted
the Nipher shield for use on standard Belfort
and Fisher and Porter recording precipitation
gages. In tests outside of Toronto, Canada,
the Nipher-modified Belfort gage collected
92-93% of the snowfall captured by the
MSC Nipher- shielded snow gage.
In late 1987, the US-EPA installed
Nipher-shielded Belfort recording precipi-
tation gages at nine NADP/NTN sites to
allow comparison of precipitation measure-
ments from the Nipher-shielded gages with
those from the standard Belfort recording
precipitation gages already in use at the
sites. A primary objective of the study was to
assess the relative performance of the two
gages under standard NADP/NTN operat-
ing protocols. A though the Nipher shield
was designed specifically to improve snow
capture, a second objective was to com-
pare measurements bythepairedgagesfor
various types of precipitation. This informa-
tion is important for networks that utilize a
single recording precipitation gage during
sampling intervals that contain both rain
and snow. If the existing gages are modified
for winter use by installing the Niphershield
at the onset of winter, there is a high prob-
ability that some rain will fall afterthe Nipher
shield is in place. This progress report
summarizes preliminary results from ap-
proximately the first year of the comparison
study.
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Discussion of Results and
Conclusions
Weekly precipitation values from col-
located Nipher-shielded and standard Belfort
recording rain gages were analyzed for
significant differences in volume. The effects
of site, precipitation magnitude and precipi-
tation type on the differences were evalu-
ated . In addition, the daily amounts from
two sites were analyzed and correlated
against concurrent wind speed and tem-
perature measurements in an attempt to
elucidate the nature of the variability be-
tween the two types of gages.
Study sites were selected to represent
a variety of snow collection conditions. The
Nipher-shielded rain gages are located
within two to 15 m of the standard rain
gages, and are operated and maintained by
the site personnel in the same manner as
are the standard NADP/NTN rain gages.
Maintenance includes charging the gages
with antifreeze at the onset of winter. At the
beginning of the study, the rain gages were
calibrated.
Weekly precipitation totals were calcu-
lated by summing the daily amounts. Daily
records of precipitation type and amount
were used to classify the weekly precipita-
tion as to type (snow, rain, mixed or un-
known). The weekly precipitation was
classified as "snow" if > 67 percent of the
total precipitation during the week occurred
as snow, or "rain" if > 67 percent occurred as
rain. Other combinations were classified as
"mixed", unless > 33 percent of the total
amount was "unknown", in which case the
precipitation for the entire week was clas-
sified as "unknown". Only those weeks for
which one or both gages indicated that
precipitation occurred, for which valid
measurements were available from both
gages, and for which the precipitation type
was known were used in the analyses of
weekly data. One site was excluded entirely
because of insufficient data.
At all sites the mean difference be-
tween the gages was positive (i.e., the Nipher
gage recorded more precipitation than the
standard gage), but the bias was significant
(P<0.05) at only five of the eight sites. The
magnitude of the total difference ranged
from 0.04 in. to 2.88 in. (water equivalent).
On a percentage basis, the Nipher-modified
gage measured up to 17% more precipita-
tion (in total) than the standard gage. There
was noclear relationship between thegages,
nor did the presence or absence of an Alter
shield surrounding the standard gage ap-
pear to play a critical role in determining
whether a significant difference between
gages was observed.
An objective of the current analysis
was to evaluate, on a site-by-site basis, the
effect of the precipitation type on the differ-
ence between the gages. This analysis was
constrained by small sample sizes for some
precipitation types at some sites; however,
mean differences between the gages were
found to be highly significant (paired t-test,
P<0.01) for snow at two sites, and for rain at
three sites. The largest percentage differ-
ence between the gages for snow occurred
where the Nipher gage recorded 37% more
snow (in total) than the standard gage. This
percentage difference represents an abso-
lute difference of only 1.46 in. (water
equivalent). The largest percentage differ-
ence for rain was 17%. The lack of signifi-
cance for some site-type combinations may
be attributable, in part, to small sample
sizes.
To circumvent the problem of small
sample sizes, the effects of precipitation
type were also evaluated using the combined
data from all sites. The difference between
the gages was found to be highly significant
for each of the three precipitation categories
analyzed (rain, snow, and mixed); however,
the mean differences for each of the three
types were not significantly different from
one another. This result is surprising, given
the fact that previous studies have shown
that error in precipitation gage measure-
ments is greater for solid than for liquid
precipitation. The Nipher-shielded gages
may have overestimated rainfall by capturing
droplets that had splashed off of the shield.
Although somefield observers reportedthat
rain did appear to be splashing off of the
shield into the gage, without "ground true"
information it cannot be determined whether
this phenomenon results in an overestimate
of the rainfall.
The examination of both weekly data
and daily data is important to the interpre-
tation of the differences (or lack of differ-
ences) between the gages. The daily data
permit fine--resolution in theclassification of
precipitation type, while the weekly data are
used by NADP/NTN to calculate deposition
and weighted-mean concentrations. Daily
data are more likely than weekly data to be
affected by the carry-over of snow from one
day to the next due to the tendency for wet
snow to stick in the gage orifice.
The lack of significant differences be-
tween the gages at some sites is puzzling,
given the fact that previous studies have
shown that the Nipher modification en-
hances snow capture. The age of some of
the standard rain gages in the current study
and the conditions under which NADP sites
are operated may contribute to greater
"noise" in the current study than has been
the case in previous studies, making it more
difficult to detect small differences between
the gages. Larger sample sizes should help
address this problem . It is also possible that
the environmental conditions at the study
sites are sufficiently different from those in
previous study areas to account for the
differences in the results.
Uncertainties in the measurements
made by the Belfort recording rain gages
used m this study and throughout the net-
work may be due to a number of factors,
with the relative importance of the factors
varying fro m site to site. First, at very windy
sites, the rain gage is subject to "wind
shake". Wind-induced vibrations cause the
precipitation pen to oscillate, thereby in-
creasing the uncertainty in the interpreta-
tion of the chart. The Nipher-shield appears
to increase wind shake under some condi-
tions, causing even greater uncertainty in
the interpretation of the Nipher gage chart
Second, although all gages used in the
study were calibrated atthe beginning of the
study, the gages tend to lose their calibra-
tion over time and most NADP site opera-
tors do not have the necessary skill to
calibrate the gages. Older gages may lose
their calibration more rapidly than newer
gages. Although calibration checks were
made during the period of the study, the
analysis of potential impacts of lack of
calibration (if any) on the differences be-
tween the gages was beyond the scope of
this preliminary analysis. Third, because
the cumbersome Nipher-shield must be
removed in order to access the catch bucket
operators may be more likely to operate the
Nipher-shieided gage on the back traverse
of the gage where at sites where the collec-
tor is installed on a platform that provides
little room for maneuvering, as is often the
case at sites with large amounts of snow
Finally, because the gages are maintained
weekly, rather than daily, there is limited
opportunity to detect and rectify problems
such as "capping over" of the Nipher gage
that can lead to inaccuracies in precipita-
tion measurement.
The preliminary results of this study do
not provide convincing evidence that the
adoption of the Nipher-shielded rain gage
for snow measurements would enhance
precipitation capture on a network-wide
basis; the effects of the shield appear to be
highly site-specific. The site-to-site differ-
ences are not clearly attributable to differ-
ences in amounts of snow and wind.
.S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20097
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Carol L Simmons and David S. Bigelow are with the Natural Resources Ecology
Laboratory, Colorado State University, Fort Collins, CO 80523
S.M. Bromberg is the EPA Project Officer (see below).
The complete report, entitled "Progress Report: Comparison of Precipitation Measure-
ments by Nipher-Shielded and Standard Belfort Recording Rain Gages at NADP/
NTN Sites," (Order No. PB90-261 538AS; Cost: $15.00 cost subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States Center for Environmental
Environmental Protection Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/064
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