The application of throughfall measurements for atmospheric deposition monitoring

An overview of the technical requirements of throughfall, stemflow and precipitation measurements used for monitoring atmospheric deposition to forests is presented. A mechanistic basis is presented to link throughfall fluxes to atmospheric deposition. For homogeneous forest stands with a closed canopy, the overall uncertainty in annual mean soil loads can be as low as 10–15% if state-of-the-art measurement and analytical techniques are used in combination with a sufficiently large number of replicate samplers. The uncertainty in atmospheric deposition estimates, however, is estimated much larger, i.e. up to 30% for sulphur and 40% for nitrogen and base cations. This is mainly attributed to (i) uncertainties associated with the estimation of canopy exchange, and (ii) dry deposition to the forest floor and understorey vegetation which is usually not addressed in throughfall studies. Additional research on canopy exchange in relation to tree species, ecological setting and pollution climate is recommended, especially for nitrogen compounds and base cations, and may serve as a basis for improvement of current canopy budget models.