Estimating Bias within the National
Atmospheric Deposition Program and the National Trends Network
Last Updated: September 28, 1995
Approach:
As there is no `standard' network against which the NADP/NTN
monitoring program can be measured, the network has attempted to estimate bias
by comparing NADP/NTN data to those of other similar networks. This comparison
is being accomplished through the collection and analysis of data from other
networks that have located there sampling equipment within 30 meters of NADP/NTN
sampling equipment. This type of collocation however, only identifies systematic
differences between the collocated networks. Collocated equipment precision
(random error) must also be known so that overall network measurement error can
be estimated as the sum of the systematic and random error. This approach
follows in part, the general framework of collaborative testing outlined by
Youden (1967).
Listing of Intercomparisons with other Networks
Chemical Analysis and Rain Gage Volumes:
Pooled estimates of bias are derived using the median differences found between
networks at each collocation site. The efficiency of the estimates are
proportional to the number of networks and sites participating in collocation
studies.
Bucket Volumes and Rain Gage Amounts:
Distributions of the differences between bucket volumes collected by the network
as compared to the network's standard gage are used to estimate regional and
seasonal biases.
A comparison of NADP/NTN standard gage depths to those of nearby National
Weather Service gage depths estimates differences between rain fall estimates
made by our national measurement standard and the network.
The classification of daily precipitation amounts by type show the distribution
of rain, snow and mixed events by region and season across the network.
Special Studies:
Special studies of collection devices both for rainfall amounts and chemical
composition have been conducted by the network to explore the nature and
magnitude of these biases.
- Nipher Rain Gage Study
Environmental Protection Agency. 1990. Progress Report: Comparison of Precipitation
Measurements by Nipher-Shielded and Standard Belfort Recording Rain Gages at NADP/NTN Sites. EPA
600-3-90-064.
Bigelow, D.S., A.S. Denning, and J. Baron. 1990. Differences Between Nipher and Alter Shielded
Universal Rain Gages at Two Colorado Deposition Sites. Environmental Science and Technology,
24, (5), pp 758-760.
- Leonard Mold and Die Collector Study
- Geotech Collector Study
Graham, R.C.; Obal, J. 1989. The Non-parametric Statistical Evaluation of
Precipitation Chemistry Sampler Intercomparison Data; Atmos. Environ., 23, 1117-1130.
Graham, R.C.; Robertson, J.K. 1990. An Assessment of Performance of Wet Atmospheric
Deposition Samples. U.S. Geological Survey; Reston, VA. Water-Resources
Investigations Report 90-4042.
Graham, R.C.; Robertson, J.K.; Obal, J. 1987. An Assessment of the Variability
in Performance of Wet Atmospheric Deposition Samples. U.S. Geological Survey; Denver, CO.
Water-Resources Investigations Report 87-4125.
Graham, R.C.; Robertson, J.K.; Schroder, L.J.; Lafemina, J. 1988. Atmospheric Deposition
Sampler Intercomparison; Water, Air, Soil Pollut. 37, 139-147.
