DESCRIPTION OF PARAMETERS INCLUDED IN NADP/NTN REPORT OF DEPOSITIONS
(Depositions Reported in Kilograms/Hectare)
Site ID
Alpha-numeric site identification code, first two characters of
which are the abbreviation of the state in which the site is
located. (For intercomparison sites this order is reversed.)
Summary Period
Summary period types are: annual, water,winter, spring, summer, fall,
jan, feb ... dec. Annual represents calendar years and Water
represents water years (Oct-Sep). Months represent calendar months.
Seasons are defined as follows:
winter=Dec-Feb,
spring=Mar-May,
summer=Jun-Aug,
fall=Sep-Nov.
Also note that because the seasons are offset relative to the calendar
year, the annual (calendar-year) depositions cannot be obtained by
summing the four seasonal totals.
Standard NADP operating procedures specify that weekly samples be
collected on Tuesday mornings. Beginning and ending dates of all
summary periods are adjusted to accommodate this sampling
schedule.
Year
Year of the summary period being reported. Note that the winter
summary period includes December of the previous year.
*Data Completeness Criteria
Criteria established by the NADP Technical Committee to provide a
measure of whether the data available are adequate to characterize
the summary period as a whole. DATA USERS ARE URGED TO USE THIS
INFORMATION TO ASSESS WHETHER THE DATA ARE ADEQUATE FOR AN
INTENDED APPLICATION. The NADP requires Completeness Criteria
values >= 74.5 for criteria 1, 3 and 4, and >= 89.5
for criterion 2, in its standard applications of deposition
data (e.g., published maps and data summary tables); however, data
users may wish to select different cut-off values, depending
upon the intended application of the data. Note that
depositions and precipitation totals are reported regardless of whether
the NADP Completeness Criteria were met. Following are brief
definitions of the four criteria. For a more complete explanation,
see the attached excerpt from the NADP/NTN Annual Data Summary
for 1991.
Criterion 1 - Percentage of the summary period for which there are
valid samples.
Criterion 2 - Percentage of the summary period for which precipitation
amounts are available either from the rain gage or
from the sample volume.
Criterion 3 - Percentage of the total measured precipitation associated with
valid samples. Criterion 4 - Collection efficiency. This is
the sum of the sample bucket depths (in centimeters) in the
summary period divided by the sum of the rain gage amounts
(in centimeters) for all valid samples where both values
are available. Criterion 4 is reported as a percentage.
*Depositions
Total depositions are reported in kilograms per
hectare for calcium, magnesium, potassium, sodium,
ammonium, nitrate, inorganic nitrogen (from ammonium and nitrate),
chloride, sulfate and hydrogen. (Hydrogen depositions calculated
from both laboratory and field pH measurements
are given). Depositions are computed by multiplying
the precipitation-weighted mean ion concentration (mg/L) for VALID samples by
the total precipitation amount in centimeters for the summary
period (Ppt, see description below) and dividing by
10. The precipitation-weighted mean concentrations used to
calculate all of the depositions EXCEPT FOR FIELD HYDROGEN,
H(field), were computed from measurements made on weekly samples at
the Network's Central Analytical Laboratory (CAL).
The precipitation-weighted mean concentrations used to calculate H(field) were
computed from pH measurements made on weekly samples by the site operator
in the field laboratory. See Valid Samp. below for
more complete descriptions of the samples included in the means.
H(field) values are not reported for periods prior to 1987. A set
of screening criteria have been approved by the NADP Technical
Committee for use in assessing the validity of individual field
chemistry measurements. Only those field chemistry measurements
which meet all of the screening criteria are selected for use in
calculating the field means and depositions. Individual field
measurements made before 1987 cannot be adequately screened
because sufficient measurements of a quality control check sample
were not required by the Network. For more information about the
field chemistry data and the screening criteria applied to them,
see the attached excerpt from the NADP/NTN Annual Data Summary for
1991.
For more information about the field chemistry data, see the
attached excerpt from the NADP/NTN Annual Data Summary for 1991.
*Svol
Total volume, in milliliters, of precipitation collected by the
sampler for VALID samples during the summary period.
*Ppt
Total amount of precipitation, in centimeters, measured by the
rain gage during the summary period. This value includes
precipitation amounts calculated from the sample volumes in cases
where the weekly rain gage measurements were unavailable.
*% Ppt Rep. by F Chem.
The percentage of the total measured precipitation (Ppt)
represented by the hydrogen deposition calculated from field pH
measurements. DATA USERS ARE URGED TO USE THIS VALUE TO ASSESS
WHETHER THE FIELD HYDROGEN DEPOSITION, H(FIELD), IS ADEQUATE FOR
AN INTENDED APPLICATION. The NADP requires this percentage to be
>= 49.5 in its standard applications of field deposition data
(e.g., published maps and data summary tables); however, data
users may wish to select a different cut-off value, depending upon
the intended application of the data. Note that H(field) is
reported regardless of whether this criterion is met.
In most cases, Data Completeness Criterion 3 is equal to the
percentage of precipitation represented by the depositions
calculated from laboratory (CAL) measurements. Precipitation
amounts from sampling intervals during which < 0.01 inch of
precipitation occurred are included in the Criterion 3 percentage,
if less than 10 mL of sample was collected. Complete laboratory
analyses are not performed on such samples. Therefore, the
percentage of precipitation represented by the laboratory means is
sometimes over-estimated by Criterion 3. Data users may contact
the Coordination Office for more information.
Valid Samp.
Numbers of samples used in calculating the precipitation-weighted
mean concentrations for the summary period. These are the samples
whose chemical measurements contribute to the deposition values.
L - Number of samples used in calculating the laboratory
means. Only VALID samples with complete, valid
LABORATORY (CAL) analyses and a valid measurement of
precipitation amount (from the rain gage or sample
volume) are used.
F - Number of samples used in calculating the field means.
Only VALID samples with complete, valid LABORATORY
AND FIELD analyses, and a valid measurement of
precipitation amount (from the rain gage or sample
volume) are used. The samples used in calculating the
field means are the same samples or a subset of the
samples used in calculating the laboratory means.
Days
Number of days included in the summary period.
Dates
The beginning date of the first sample included in the summary
period and the ending date of the last sample included in the
summary period, reported in Greenwich Mean Time (GMT), mmddyy.
*Missing values are indicated by --.
DATA SELECTION CRITERIA
A. Criteria for Determining a Valid Sample
Individual weekly samples are screened to determine their validity using
criteria based on the following parameters:
- Rain gage depth
- Sample volume
- Sampling interval
- Lab type
- Sample validation codes
- Rain gage depth is the precipitation depth measured by the rain gage
collocated with the wet/dry precipitation collector (see Bigelow 1984 for
details).
- Sample volume is the amount of precipitation contained in the
wet-side sample bucket as determined by weight in the field laboratory.
- Sampling interval is the time period over which the precipitation sample
was collected. The standard interval is one week, from 9:00 a.m. Tuesday
to 9:00 a.m. Tuesday of the following week. Samples are collected on this
schedule whether or not precipitation occurs.
- Lab type is a code assigned to samples received at the CAL, based on the
volume of the sample available for analysis. Each lab type keys a
different set of sample processing and measurement steps. A complete set
of laboratory measurements (pH, specific conductance, concentrations of
major cations and anions) is made only on samples with a lab type of wa or
w, which are samples of 10 mL volume or more. (Samples of lab type wa are
diluted in the laboratory to produce sufficient volume for analysis;
samples of lab type w are large enough to be analyzed without dilution.)
Lab type t samples are samples of less than 10 mL. Laboratory
measurements of pH and conductance are made on these samples if there is
sufficient undiluted sample volume. No other chemical constituents are
measured for lab type t samples. A lab type of da indicates that the
sample bucket was dry when it arrived in the laboratory. Chemical
analyses for lab type da samples are not included in this report.
- Sample validation codes (Bowersox 1985, Aubertin et al. 1990) are assigned
during routine NADP/NTN quality assurance evaluations to identify samples
that were not collected and/or processed according to NADP/NTN protocols,
or were contaminated. Samples are considered valid and are included in
calculations of ion concentration and deposition summary statistics if all
of the following sample validity criteria are met:
1. NADP/NTN criteria for site location, sample collection and
handling, and measurement protocols are satisfied (Bigelow 1984,
Bigelow and Dossett 1988).
2. The sample consists of "wet-only deposition," i.e., it was
not exposed to excess dry deposition (see Bowersox 1985).
3. The sample is not contaminated (see Bowersox 1985).
4. A complete set of chemical measurements (made at the CAL) is
reported for the sample.
5. The sampling interval is 6-8 days.
6. There is a rain gage depth or sample volume reported for the
sample.
In the evaluation of data completeness, two additional sample types are
considered valid:
1. All samples from sampling periods during which it was confirmed
that no precipitation occurred are considered valid. These samples
are generally of lab type da.
2. All samples from sampling periods during which a trace of precipitation
(<0.01 in.) occurred are considered valid if the sample volume was
less than 10 mL (lab type da or t).
When calculating data Completeness Criterion 1 values (see Section III.C)
for periods of one year or longer, all sampling intervals with <0.02 in.
of precipitation are treated as valid. (Note that this relaxation of the
above criteria applies only to the calculation of data Completeness
Criterion 1 and to summary periods of one year or longer.)
B. Criteria for Determining the Validity of Field Chemistry Measurements
Measurements of precipitation pH and conductance made by site
operators ("field pH" and "field conductance" measurements) are screened
to determine their validity based on the following information:
- Sample validation codes
- CAL measurements of pH and conductance
- Field measurements of the quality control check sample
- Field pH and conductance measurements of the precipitation sample
Sample validation codes are applied to exclude samples that fail to meet
sample validity criteria 1-3, described in Section III.A. Criterion 4,
the requirement for complete chemical analyses, is relaxed for field
chemistry measurements; it is required only that there be valid laboratory
pH and conductance measurements for the sample (see below). Criteria 5
and 6 are not applied to individual field chemistry measurements; however,
to be included in calculations of weighted-mean field pH and hydrogen ion
deposition for seasonal and annual summary periods, and in the statistical
summaries of field pH and hydrogen ion concentrations in Section VII.C,
samples must meet all six of the criteria.
CAL measurements of pH and conductance are used by the CAL to screen
samples for possible contamination. Valid laboratory measurements of
these parameters are required to ensure that the samples have been
subjected to this screening.
Field measurements of the quality control check sample are used to
evaluate the accuracy of the equipment and the site operator's measurement
technique. Prior to making field pH and conductance measurements, the
site operator measures and records the pH and conductance of a quality
control check sample (Bigelow and Dossett 1988). The CAL-certified values
of the quality control check sample are 22 microsiemens per centimeter
(S/cm) for conductance and 4.3 pH units for pH, and are known to the site
operator. For field precipitation chemistry measurements to be considered
valid, both of the quality control check sample measurements must have
been made, and the reported values must be 22.0 4.0 S/cm for conductance
and 4.30 0.10 pH unit for pH.
Field pH and conductance measurements of the precipitation sample are used
to check for inconsistencies between the reported field pH and conductance
values. For the field chemistry measurements to be considered valid, the
reported conductance must be equal to or greater than the calculated
conductance based entirely on the hydrogen ion concentration. The
calculated conductance is the product of the hydrogen ion concentration
(determined in the field laboratory) and the equivalent conductance of
hydrogen ion at 25 degrees Celsius (Franson 1985, Weast 1989). This check
ignores the potential contribution of other ions to the conductance, thus
only identifies certain cases where the hydrogen ion concentration is
likely to have been overestimated by the field pH measurement. While it
is also possible that the conductance measurement was inaccurate, this
risk is reduced by excluding field conductance measurements where the
quality control check sample conductance is reported as being outside the
acceptable range of 22.0 4.0 S/cm. Furthermore, field measurements of
conductance meet NADP/NTN accuracy goals more frequently than field
measurements of pH (Nilles et al. 1993).
C. Criteria for Including a Site in the Annual Isopleth Maps and
Seasonal Data Summary Tables
Four Completeness Criteria form the basis for the decision to include
the laboratory chemistry data from a site in the Annual Isopleth Maps
and Seasonal Data Summary Tables in Section VI. All four criteria must
be met. The criteria are listed below.
Criterion 1. There must be valid samples (as defined in Section
III.A) for at least 75 percent of the summary period.
Criterion 2. For at least 90 percent of the summary period there
must be precipitation amounts (including zero amounts)
either from the rain gage or from the sample volume.
Criterion 3. There must be valid samples (as defined in Section III.A)
for at least 75 percent of the total precipitation amount
reported for the summary period.
Criterion 4. For the entire summary period the total precipitation
as measured from the sample volume must be at least 75
percent of the total precipitation measured by the rain
gage for all valid samples where both values are available.
Criteria 1 and 2 ensure that measurements on valid wet deposition
samples and of precipitation amounts were reported for a minimum
acceptable fraction of the summary period. This requires a properly
operating wet/dry collector and rain gage. Criterion 3 ensures that there
are valid precipitation chemistry data to represent 75 percent of the
precipitation that was estimated to have occurred during the summary
period. Criterion 4 sets the minimum acceptable level for the collection
efficiency of the wet/dry collector relative to the rain gage. This
ensures some consistency in the operation of the rain gage and wet/dry
collector.
It is important to recognize that if all the criteria were satisfied
at the lower limit of acceptability, the summary could still account for
less than 75 percent of the actual precipitation at the site. This could
occur because the 90 percent precipitation coverage criterion (Criterion
2) is based on time and there may be no record of the amount of
precipitation missed during the unsampled 10 percent of the period. Where
the precipitation coverage is 100 percent, however, these criteria ensure
that at least 75 percent of the precipitation is represented by valid
samples.
The following example illustrates the interpretation of the
Completeness Criteria values for a hypothetical site. For the annual
summary period, the hypothetical values for this site are:
Criterion 1 (Percent valid samples) 91%
Criterion 2 (Percent precipitation coverage) 95%
Criterion 3 (Percent precipitation with valid samples) 65%
Criterion 4 (Percent collection efficiency) 98%
The values for Criteria 1 and 2 indicate that valid samples were obtained
for 91 percent of the summary period while valid precipitation
measurements were obtained for 95 percent of the summary period. (The
amount of precipitation that occurred during the remaining 5 percent of
the period is unknown.) Criterion 3 relates the amount of precipitation
represented by valid samples to the total measured precipitation. The
value for this criterion is calculated by summing the rain gage
measurements associated with valid samples (substituting the sample volume
where necessary), then dividing by the total measured precipitation. In
this case, although 91 percent of the summary period was represented by
valid samples, only 65 percent of the measured precipitation was
associated with valid samples. (This can occur when a few very large
precipitation events are not represented by valid samples.) Finally, the
value reported for Criterion 4 indicates that for valid samples, the
amount of precipitation captured by the wet-side bucket on the wet/dry
collector was 98 percent of that measured by the rain gage. It should be
noted that since Criterion 3 was not met for this hypothetical site,
annual NADP/NTN Completeness Criteria are not satisfied; therefore, this
site would not be included in annual isopleth maps in this report.
For each site, two sets of hydrogen ion values are included in this
report. One set is from pH measurements made at the CAL ("laboratory
pH"); the other is from measurements made in the field laboratory at the
site ("field pH"). While the CAL laboratory measurements are subject to
more rigorous quality assurance and offer greater standardization among
sites than the field measurements, the field measurements have the
advantage of providing information about the chemistry of the sample
shortly after the sample was collected. Comparisons of field and
laboratory pH measurements, as well as analyses of natural and simulated
precipitation samples processed according to the same protocols as
NADP/NTN samples, have shown that hydrogen ion concentrations tend to
decrease between the time the pH is measured in the field and when it is
measured again in the laboratory. This loss of hydrogen ion has been
variously attributed to microbial activity, degradation of organic acids,
dissolution of particulate matter, and ion exchange processes involving
the walls and/or lid of the shipping container (Bigelow et al. 1989). In
NADP/NTN samples, the annual median losses have ranged from 4
microequivalents per liter (eq/L) to 6 eq/L (Bigelow et al. 1989).
Because the pH scale is logarithmic, these losses have little effect on
sample pH values if the hydrogen ion concentrations are initially high.
However, where hydrogen ion concentrations are relatively low, as in the
western United States, these losses can result in changes of several
tenths of a pH unit. Given the magnitude of these changes, the NADP/NTN
has elected to display both the field-measured and laboratory-measured
hydrogen ion values in this report.
Precipitation-weighted means of field pH measurements are included in
the annual isopleth maps and seasonal data summary tables if: (a) all
four of the Completeness Criteria were met for the summary period, and (b)
at least 50 percent of the precipitation which occurred during the summary
period is represented by samples with complete, valid laboratory chemistry
data and valid field pH measurements. (Only these samples are included in
calculations of weighted-mean field pH and hydrogen ion deposition). The
required percentage of precipitation represented by valid field pH
measurements is lower than that for the laboratory-determined ion
concentrations because NADP/NTN protocols call for field chemistry
measurements to be omitted on samples of less than 70 mL.