The contribution of occult precipitation to sulphur deposition in the Czech Republic

Abstract

We present a spatial estimation of the contribution of occult precipitation to the true atmospheric sulphur deposition across the territory of the Czech Republic. Our estimation is based on differences in two spatially explicit, data driven geo-statistical deposition models for forested areas: 1. a throughfall sulphur deposition model and 2. a sulphur deposition model calculated from dry plus wet-only deposition (often referred to as the “total” deposition). The sulphur deposition maps for Czech forested areas at 1x1 km resolution are presented and the dependence of the contribution of occult precipitation to sulphur deposition on altitude is assessed. All available data on precipitation chemistry at disposal, results of wet-only, bulk and throughfall measurements were compiled. Our preliminary results based on geo-statistical modelling indicate that the contribution of occult precipitation to sulphur deposition is significant, varies in a wide range, and is dependent on altitude. On 68% of the Czech forested area, the occult deposition in 2008 ranged between 0–0.5 g.m-2.year-1, on 25% between 0.5–1 g.m-2.year-1, on 3.5% between 1–1.5 g.m-2.year-1, and on 0.7% it was more than 1.5 g.m-2.year-1. In relative terms, in 2008 the occult precipitation contribution to sulphur deposition over the Czech forested area was up to 200% of wet plus dry deposition estimates in 99% of the grid cells, and in the remaining 1% of the grid cells it even ranged between 200–400%. Across ca. 50% of the Czech forested area 50–100% of sulphur deposition was contributed by occult precipitation. Our results indicate that when using the maps of the “total deposition” derived from wet and dry deposition without accounting for occult deposition (which is hitherto a common approach), we are likely to substantially underestimate the true atmospheric deposition and consequently its possible impacts on ecosystem health. The approach and the weaknesses that still remain are discussed. The presented results are rough estimations but can be considered the best available at the moment for the Czech Republic.