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Subsections
As GREAT-ER introduces a new method to estimate environmental
concentrations (calculations are made on real-world data instead of
large-scale generic data sets) with frequency distributions, the calculation
of PEC values must be different from the known methods.
The PEC values calculated by GREAT-ER are currently under
discussion. For this reason not just one but a collection
of values are provided which are divided into two items:
PEC
initial and
PEC
catchment. The underlying theory and discussion would exceed
the limitations of this manual. We refer readers to the article in
Chemosphere (May 1999: in press).
is the spatial aggregation of concentrations in
the river directly after emission. This is comparable to PEClocal as
defined in the EU Technical Guidance Documents, although its calculation is
different. Based on the C
sim, start mean values,
the mean is calculated for
all stretches receiving treated or untreated discharges:
with
C
sim, start, i is the concentration at the beginning of
river stretch i, directly receiving treated or untreated waste
water emissions.
By definition, the PEC
initial is calculated on the base of the
C
sim, start mean values.
Selecting the item
PECinitial the calculation starts.
Please note that PEC calculations are not available if a subcatchment
is selected.
After the calculation a dialog displays the result alongside additional
information.
Figure 2.24: PEC
initial calculation results
is the aggregation of concentrations
representative
for the entire catchment. Although this is a new concept typical for
geo-referenced exposure assessment, it could be compared to the
PEC
regional in the EU Technical Guidance Documents.
As it is an average value,
C
sim, internal is the "most representative" value
for the concentration in the stretch. As PEC
catchment aims to provide
a measure for the "representative" concentration over the whole of
a catchment, it is based on the most representative local values.
As mentioned above, the PEC
catchment is currently under scientific
discussion. To demonstrate the variability of results based on the various
definitions, GREAT-ER computes four different PEC
catchment values.
These values differ in the set of stretches selected for the calculation
and in the weighting of concentrations.
All PEC
catchment values are defined as the mean of the weighted
concentrations of the selected stretches. Possible selections are (A) all
stretches within a catchment or (B) only polluted stretches. Option (A) is
more comparable to the current regional exposure assessment approach
(unit world models) in which all surface waters in a region are considered
for the dilution of the chemical mass loading. Clearly different, option (B)
considers only the loaded river stretches. The intention of this
approach in the meaning of risk assessment is to focus on the river
stretches potentially at risk: the loaded stretches.
Both approaches have to deal with different issues concerning scale
dependencies or data requirements. These items are discussed in the referred
article.
- 1.
- Weighting by stretch volume for both stretch selections.
The volume is calculated (assuming a rectangular cross section) by
flow, flow velocity and length. The weighting method focuses on
large rivers and therefore on high dilution situations where
exposure (and risk) levels may be lower than the representative
rate for the entire catchment.
- 2.
- Weighting by stretch length for both stretch selections.
The interpretation of the weighting method is that stretches with
equal length are of equal importance. Small rivers are considered
to be equally valuable as large rivers.
Methods 1 and 2 depend on the scale and detail of the underlying
digital river network, and the more (unpolluted) headwaters are included, the
lower the aggregated PEC
catchment will be. Conversely, in a
low detailed river network, small (and possibly more valuable) stretches
are neglected.
- 3.
- Weighting by flow increment considers all stretches within a
catchment.
The weighting of a stretch is calculated by the difference of flow in
relation to its upstream stretch (two stretches in the case of a
confluence). Thus the values related to stretches are similar
to those in
weighting by length. A slight accentuation might be given to
receiving stretches because, besides the natural flow increment,
these are artificially influenced by the waste water flow.
The initial stretch of a river is considerd with its complete flow
(there is no longer an upstream stretch and thus the increment is the
flow). If these stretches are unpolluted, the weighting method is
more or less independent of the scale of detail of the digital
river network: no matter how detailed rivers are digitised,
the flow of a given stretch can be considered as the sum of all its
upstream stretches and therefore the stretch representing the
headwaters is of the same value as all stretches regarded
separately.
The calculation of PEC
catchment can be based
on any of the previously derived percentiles of the model results.
When selecting PECcatchment the existence
of previously calculated
values is first checked because, depending on the catchment
extent and detail of the
digitised river network, the calculation will take several minutes.
Figure 2.25: PEC
catchment selection
A progress bar will index the progress of the calculation. After completion
the values mentioned above are listed in a small report window:
Figure 2.26: PEC
catchment calculation results
For additional analysis tools see the section below und Chapter 3
(Menu Structure).
Next: Additional display options
Up: Analysing model results
Previous: Analysing model results
F. Koormann & J.-O. Wagner, Institute of Environmental Systems Research, University of Osnabrück