Surface water acidification and critical loads: exploring the F-factor

摘要

As acid deposition decreases, uncertainties in methodsfor calculating critical loads become more important when judgements have tobe made about whether or not further emission reductions are needed. Animportant aspect of one type of model that has been used to calculatesurface water critical loads is the empirical F-factor which estimates thedegree to which acid deposition is neutralised before it reaches a lake atany particular point in time relative to the pre-industrial, steady-statewater chemistry conditions.In this paper we will examine how well the empirical F-functions are able toestimate pre-industrial lake chemistry as lake chemistry changes duringdifferent phases of acidification and recovery. To accomplish this, we usethe dynamic, process-oriented biogeochemical model SAFE to generate aplausible time series of annual runoff chemistry for ca. 140 Swedishcatchments between 1800 and 2100. These annual hydrochemistry data are thenused to generate empirical F-factors that are compared to the "actual"F-factor seen in the SAFE data for each lake and year in the time series.The dynamics of the F-factor as catchments acidify, and then recover are notwidely recognised.Our results suggest that the F-factor approach worked best during theacidification phase when soil processes buffer incoming acidity. However,the empirical functions for estimating F from contemporary lake chemistryare not well suited to the recovery phase when the F-factor turns negativedue to recovery processes in the soil. This happens when acid deposition hasdepleted the soil store of BC, and then acid deposition declines, reducingthe leaching of base cations to levels below those in the pre-industrialera. An estimate of critical load from water chemistry during recovery andempirical F functions would therefore result in critical loads that are toolow. Therefore, the empirical estimates of the F-factor are a significantsource of uncertainty in the estimate of surface water critical loads andrelated calculations for quantifying lake acidification status, especiallynow that acid deposition has declined across large areas of Europe and NorthAmerica.