Sampling in Lowland and Highland rivers is feasible between February and August, in the Alps and pre-Alp waters between February and May. The best suited sampling season is different for small rivers and larger rivers.
In favour rivers with a catchment area < 100 km² should be sampled in spring (February - April), rivers with a catchment area > 100 km² in early summer (Mai - July).
In general, sampling in extreme hydrological conditions should be avoided:
During or directly after a flood or drought period
During or directly after river maintenance work
Samples taken at these times are neither repeatable nor representative.