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. ------- 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 ------- ------- 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 ------- |