4.4.! Wet deposition
4.43 Total deposition
The uncertainty in regional scaie total deposition estimates sfrongiy depends on the pollution climate and on landscape complexity of the area under study The uncertainty is determined by the uncertainty in wet, dry or cloud and fog deposifion. fog and cloud water deposition is not taken into account. Furthermore, deposition estimates yieid higher uncertainty in areas composed of compiex terrain and with strong horizontal concentration gradients. In order to quantify the totai uncertainty in the annual deposition estimates, an uncertainty analysis using Monte Carlo sampling and simulation is needed. Using this technique, probahility ranges can he attached to model parameters and input vaiues. Such an anaiysis wffl he carried out in the near future. Therefore, up to now only rough estimates of uncertainty can he given. It is thought that uncertainty in site-specific deposition estimates can he as large as 100%.
4.4.4 Throughfall measurements
The uncertainty in atmospheric deposition estimated from throughfall, stemflow and precipitation measurements is estimated to be 30% for suiphur and 40% for nitrogen and base cations, provided that state-of-the-art measurements and analytical techniques are used in combination with a sufficiently large number of replicate sampiers (Draaijers et al, 1996b). They found the uncertainty to a large
0
....,.,...,,...,,...,... The Rnnsh Envwonment 27extent to be attributed to:i)uncertainties associated with the estimation of canopy exchange, and ii) dry deposition to the forest fioor and understorey vegetation which is usually not addressed in throughfall studies.
4.5. Conclusions
in order to evaluate the EDACS deposition model, calculated deposition estimates for 1989 are compared to the available throughfall measurements at ICP IM sites and other throughfall sites in Europe for the period 1987-1994. A canopy budget model was used to estimate the impact of canopy leaching on throughfali and stemflow fluxes of Mg2, Ca2 and K. The EDACS model was run using base cation precipitation concentrations measured at the site and local information on the roughness length (z0) for each forest stand.
Significant relationships (p<O.O5) were found between modelled dry deposition and dry deposition estimated from throughfall and bulk precipitation measurements. However, especially for SO, and NO. the scatter was found considerable. To a large extent this is the resuit of the air concentrafions of acidifying components being based on the EMEP model which has a poor spatial resolution (l5Oxl5Okm), whereas air concentrations of base cahons are based on a simple scavenging model. To some extent the scatter can also be attributed to uncertainty in throughfall estimates. for NH, Ca2 and K the model seems to underestimate dry deposition at higher deposition leveis, probably because
‘background’ air concentrations are used while additional input by locai sources may occur.
For NH, Ca2 and K, the modelled dry deposition was on average not significantly different from the measured dry deposition. Significant differences found for SO,, NO. Na and Mg2 can to a large extent he attributed to the fact that in many cases estimates for different years are compared. As a resuit of their sea-sait origin, especially for Na and Mg2 a sfrong year-to year variation may be expected If only measurement data of 1989 were used for comparison no signrficant differences were found If onlyICP IMsites are considered, sigrnficant relationsh;ps between modelled and measured dry deposition are found for ali components except NH. On average both estimates differ significantly, SO, being the only exception.
Largest uncertainty in wet deposition was estimated in areas with low measurement density leading to relatively large interpolation errors (i.e. southern and eastern Europe). The uncertainty in the dry deposition velocffies is mainly the resuit of using the simple resistance formulation for a highly variable process, and, more specificaily, the surface resistance (Rc) parameterisations. Systematic errors may arise from neglecting complex terrain effects in the parameterisation of the deposition velocity Air concentrations have very large uncertainty because for acidifying components air concentrations are based on the EMEP model which has a poor spatial resolution (l5Oxl5Okm), whereas air concentrations of base cations are based on a simple scavenging model. For base cations, systematic errors may arise from using annuai mean air concentrations and deposition velocities for flux calculation, thereby neglecting temporal correiations.
Uncertainty in the total deposition estimates can be quantified using Monte Carlo sampiing and simulation. This technique wiil be applied in the near future.
Acknowledgements
Thanks go to Albert Bleeker for establishing the contact betweenRJVM and the Finnish Environment Institute.
The Finneh Envronment 27
0
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