Monitoring should preferably take place in hydrologically well defined small catchments, where the interaction between all the subprogrammes can be used at the catchment scale. Where such catchments can not be found other defined areas are acceptable provided input-output budgets can be made.
The following selection criteria should be met:
- The site must allow for input-output measurements. Input measurements mean that deposition is measured at the site. Output measurements mean that the drainage water flux can be quantified and its chemistry analysed.
- The site should be hydrologically well definable and as geologically homogeneous as possible.
- The site should not be less than a few tens of hectares and no more than a few square kilometers (range 10-1000 ha) and preferably buffered by a zone of similar land use.
- The ideal site is one in which there are no ongoing management activities. Otherwise, land use within the area should be controllable. This normally means that the area is protected in some way. If management activities take place they must be well documented.
- The site should be typical for the region.
- It is desirable that other scientific research related to environmental assessment/modelling is carried out at or close to the site.
- The closest significant point pollution source should be > 50 km away. Where the background level of pollutants is high, the distance to the pollution source can be less, but the distance should be greater when the background level is low.