Two independent methods for mapping bulk deposition in France

Two independent methods—deterministic and geostatistical—were used to estimate the spatial distribution of bulk deposition for nine ions and the associated uncertainties for France.The deterministic models enable bulk deposition estimates using only a few auxiliary variables (precipitation, altitude and the period in the year). The models explain between 38% and 64% of the depositional variability depending on the ions. The compounds can be grouped according to their primary origin: (1) (S-SO42−, N-NO3−, N-NH4+, Ntot, H+) with high deposition levels in the case of heavy precipitation, and (2) (Na+, Cl−, Mg2+) with high deposition levels near the sea. Model validation for 1999, 2000 and 2001 generally shows a close relationship between calculated and measured bulk deposition. However, the models overestimate the values for smaller deposition quantities (S-SO42−, H+). Comparison with EMEP (European Monitoring and Evaluation Program) maps shows good correlation for averages at the country level, despite spatial differences mainly due to very different spatial scales .The geostatistical approach gave good results in spite of the small number (27) of plots in the CATAENAT sub-network. Spatially structured behaviour was identified and quantified for all the compounds. The contribution of one auxiliary variable (precipitation at 2614 “Météo-France” weather stations) appears to be significant for most of the interpolations. The maps produced were globally similar to those obtained with the deterministic approach. However, the deterministic approach slightly overestimated (7–11%) or underestimated (17–24%) bulk deposition depending on the ion. Finally, spatial characterisation of errors and a greater potential make the geostatistical approach more attractive than the deterministic method for monitoring levels above critical loads for acidity.The maps produced were compared with EMEP maps for Europe and critical loads are discussed.