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### PECcatchment

The PEC catchment 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.

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.

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.

A progress bar will index the progress of the calculation. After completion the values are listed in a small report window:

a)
  weighting by volume based on the subset of loaded stretches and on all stretches within the catchment
b)
  weighting by stretch length based on the subset of loaded stretches and on all stretches within the catchment
c)
  weighting by flow increment based on all stretches within the catchment.

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