Model application to study the effects of emission reduction scenarios on a forested catchment in Finland

P&s. Chem Earth (B), Vol. 24, No. 7, pp. 861-867, 1999 Perganon

0 1999 Elsevier Science Ltd All rights reserved 14641909/99/$-seetim&& PII: s1464-1909(99)ooo93-3

MWbl tion to Study the E&&s of Emission Scemm-bs bn a Forested Catchment in Finland J. Abonen and K. Rankinen Finnish Environment Institute, P.O. Box 140, Fin-00251 Helsinki, Finland Received 20 September 1998; accepted 24 November 1998

prevention. A Community Strategy to Combat Acidification (EC, 1997) is currently being developed by the European Commission (DG Xl). Based on the Council decision, the goal of this strategy is to reduce acidifying emissions to achieve critical loads for acidification in the European Union (EU) area. The strategy contains proposals for several different policy measures including the pqaration of a directive on national emission ceilings for the acidifying pollutants of SOz, NO, and NH, for each Member State. The Commission has consulted with the International InstiMe for Applii Systems Analysis (ILASA) to evaluate the moat cost-e&ctive stmtegies for combating the acidification problem, and the efftcts of several dif&rent scenarios on the exce&ne of critical loads (i.e. maximum tolerable values of suiphur and nitrogen deposition) have been assessed. An interim target for reaching the critical loads has been identified for the year 2010. The country-specific emissions of the acidifying pol&ants for each scenario are documented by IIASA (Amann et al., 19%). Both steady-state and dynamic models have been developed to predict the soil, lake, stream, and ground water acidification (e.g. Cosby et al. 1985, de Vries et al. 1989, Warfvinge and Sverdrup 1992). For mapping of critical loads and their exceedances. steady-state models are presently extensively used under the framework of the Convention on Long-Range Transboundary Air Pollution (LRTjQP) of the United Nation’s Economic Commission for Europe (UN/ECE, 1994). Dynamic models are used to predict the gradual chemical response of a receptor to changing depositions and land use practices. Forest soils and lakes are the most common receptors investigated with respect to acidification. Various but& and adsorption or desorption mechanisms are included in soil acidification models and the changes in finite element pools over time are

Abstract. SMARTZ, a revised version of the dynamic soil acidification model SMART, was tested at a forested catchment in Finland. A forest growth routine and a more detailed description of nitrogen processes in soil have been included in SMART2. The aim of the study was to calibrate the model with data set derived from the catchment and to study the model beihavior. A prediction of long-term effect of atmospheric deposition, originated from anthropogenic sources, on soil and runoff water was done, giving different scenarios of future deposition of sulphur and nitrogen. The emphasis was put on studying the acidifying effects of the deposition. Another aim of the study was to investigate the reprrsmtativity ofthe catchment corn* to a regional map, to which the critical loads of acidity have been calculated at European scale. Fe scenario runs showed a decline in soil base saturation an+l surface water pH in response to the rapid increase of acidif$ng deposition due to increased emissions starting in 1960’s. With the studied reduction strategies, environment deterioration can be stopped and the recovery will begin. The timing of the response depends on the stringency of the chosen abatement strategy. Comparisons of the regionally calculated critical loads of sulfur to the catchment value indicated that the catchment belongs to the most tolerant ecosystems of the grid cell. 0 1999 Else&r $cience Ltd. All rights reserved.

1 Introduction Acidification of soils and freshwater has remained a severe environmental threat in spite of efforts made for its

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Correspondence to: J. Ahonen 861

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J. Ahonen and K. Rank&n:

Effects of Emission Reduction Scenarios on a Forested Catchment in Finland

calculated. It is important to know the time development of acidification, in order to determine the timing of necessary measures for emission control, and to assess the dynamic response of potential critical load exceedance. The lint aim of the present study was to test SMART2 (Kros et al.,l995), a revised version of dynamic soil acidification model SMART (Simulation Model for Acidification’s Regional Trends) (Posch et al., 1993). that has been developed recently and has not been used widely. Here, SMART2 was used to predict the long-term acidification of soil and runoff water, giving different scenarios of future depositions ofsulfur and nitrogen. The model was applied and calibrated to a data set derived from a forested catchment of HietajZLrvi, located in Eastern Finland, that is one of the I1 sites belonging to the network of UN/ECE Integrated Monitoring (IM) Programme in Europe. Another aim of the study was to investigate the representativity of the catchment compared to a regional map, to which the critical loads of acidity are calculated at the European scale. For that purpose, the maximum critical load of sulfur at the site was calculated and the result was compared to the value presented in the national critical load map (Posch et al., 1995).

in Norway. Calculations were made with the DAIQUIRImodel (Syri et al., 1998). The matrices were derived with the average meteorology of I I years and were available for the gridsize of I50 km x I50 km. For the year 1990 and 20 IO, the depositions were calculated for gridsize of I50 km x I50 km. For 1992, deposition in 50 km x 50 km grids was available. The derivation of historical depositions for Europe is explained in Forsius et al. (1997). Site-specific historical and future scenarios of deposition to soil were derived using the measured deposition and biomass data of the site. The scenarios were then used as inputs to the dynamic acidification model SMART2. The model was calibrated to present measured soil and lake water chemistry data and to the estimates of present standing biomass volume and increment and the assumed maximal amount of biomass. The calibrated model was used for the assessment of future ecosystem responses. The maps of critical loads and their exceedances for the year 1990, 1992 and 2010 were produced using the calculated atmospheric depositions and the, so called, protection isolines of acidifying nitrogen and sulfur derived at CCE (Coordination Center for Effects) in RIVM research institute in the Netherlands (Posch et al., 1997a). 2.3 Deposition scenarios

2 Materials and Methods 2. I Site description The area of Hietajlltvi (63 o I ON, 30”43’E) forested catchment is about 540 hectares. Two lakes, Iso Hietajlrvi (83 ha) and Pieni Hietajiirvi (2.4 ha), are located in separate subcatchments. In the present study, only the subcatchment of Iso Hietaj&vi (464 ha) was included. The forests are mainly mature or old, but also some young cultivated stands are present. The dominant tree species is Scats pine (64% of the forest). Norway spruce and deciduous trees also exist. In Hietajtlrvi, the last major forest fire occurred 130-140 years ago. Precipitation chemistry is measured at the site and the stream water quality is measured at the outlet of the lake Iso Hietajfirvi (Bergstrom et al., 1995). A palaeolimnological study at the site (Simola et al., 1991) indicated that pH has been slowly decreasing but remained in the range of 6.4-6.8 during this century.

Future deposition was estimated for four emission scenarios (Amann et al., 1996 and Table 1): I) The Current Reduction Plan (CRP) Scenario for 2010: The CRP scenario is based on an inventory of officially declared national emission ceilings (UN/EC& 1994). This scenario gives tbe maximum deposition prediction for the future. 2) The Reference (REF) Scenario for 2010: The Reference (REF) scenario selects, for each country individually, the more stringent outcome of the CRP and the Current Legislation (CLE) scenarios.. 3) Scenario Bl: Reducing the Areas not Protected from Acidification by at least 50 Percent. 4) The Maximum Technically Feasible Reduction, Realistic (MFR) Scenario for 20 10. Table 1. Paecntagcsvrrngechangesinemissiwbythc yearZOlOin relation IO 1990 in Europe (Amann et al., 19%). The numbemmay vary from one countryto another. CRP

REF

BI

MFR -

2.2 Modeling concept A linked emission different with the

model system was used to estimate the effects of the scenarios. The country-specific emissions for the scenarios of the EU Acidification Strategy, obtained RAINS model (Amann et al., 1996), were used to calculate total deposition to grid cells throughout Europe using transport matrices provided by EMEPMSC-W institute _-__________________-_-_-

Correspondenceto: J. Ahonen

Europe(%): so2 NOX NH3

-40 -21 - lb

-58 -36 - lb

-67 -43 -22

-91 -n -46

The soil base saturation and surface water pH were chosen to demonstrate the effects of reduction scenarios on ecosystem

J. Ahonen and K. Rank&n: Effects of Emission Reduction Scenarios on a Forested Catchment in Finland response. Both base saturation and pH is considered as key variables regarding acidifying processes. Stream water pH reflects the balance. of the acid vs alkalinity generating processes in stream water. A decline in pH usually leads to deterioration of conditions for tish spawning and growth. Base saturation is a measure of the relative amount of base cations (Na, K, Ca, Mg) that are adsorbed to the exchange sites at the surfaces of the solid soil particles. A decline in base saturation may imply that the conditions for forest growth are getting poorer. When the critical load of soil is achieved, the aluminum - base cation ratio in soil water reaches the value of one. At the same time, the base saturation decreases. Values greater than 5% of base saturation are considered as ‘safe’ (Posch et al., 1997b). 2.4 SMART2 - Model description and calibration SMART2 is the revised version of the SMART model, developed to estimate long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. SMART (de Vries et al. 1989) is a simple one-compartment model that only includes geochemical buffer processes. For SMART2, separation of the soil compartment into organic and mineral layers has been made, a forest growth function has been added, and changes in descriptions of biocycling processes and hydtological processes have been made (Kros et al., 1995) . The model produces as output the soil base saturation (BS) and the concentrations of the major anions and cations in soil solution and runoff water. A simple lake module, describing retention of sulphate, nitrate and ammonia as well as inorganic carbon equilibria, is included. The model structure is based on the anion mobility concept by incorporating the charge balance principle (Reuss et al., 1987). SMART consists of a set of mass balance equations, describing the soil input-output relationships for the cations (Al’*, BC2(=Ca2++Mg2*),K+, Na : NH *f and strong acid anions (SO,” NO;, Cl), and a set of equilibrium equations, which describe the equilibrium soil processes. The soil solution chemistry depends solely on the net element input from the atmosphere and the geochemical interactions (weathering, cation exchange, HCO; -dissociation. sulphate adsorption) in the soil. The concentration of HCO; and Al” are determine.d by an equilibrium with H, the concentration of which is given by the charge balance equation. The various exchange reactions are described by Gaines-Thomas equations. Sulphate adsotption/desorption reactions are described by a Langmuir isotherm. The weathering rate of base cations‘ from silicates is independent of soil pH. Dissociation of organic anions is described as a function of pH. The model structure and the equations are explained in detail in de V&set al. (1989). In SMART2, upward seepage of water and lateral water flux ____________________-____ Correspondence

to: J. Ahonen

863

in the unsaturated zone caused by the seepage have been included. The impact of nutrient transport by the seepage water is taken into account in the calculation of soil water concentration. Transpiration and interception fractions of precipitation have also been included in mass balance equations. The humus ,layer and the mineral soil are treated separately. Total nutrient uptake is described as a demand function, which consists of maintenance uptake in leaves and growth uptake in stems. The growth uptake is proportional to the forest biomass accumulation that is cakxdated by a logistic growth rate function. Maintenance uptake is dependant on the direct leaf nutrient uptake or foliar exudation and the leaf nutrient content. Nutrients in litter fall and root decay are added to the mineralizing organic pool, where a distinction is made between the rapid decomposition of fresh litter (less than one year old) and slow decomposition of ok! litter (more than one year). Mineralization is regulated by the mineratization factor or a rate constant that is reduced by low pH, high C/N-ratio or high ground water level, Nitrification of NH,’ and denitrification of NO; are described as rate-limited equations that are reduced by low pH and the height of the water table. Immobilization of nitrogen is dependent on the soil CM ratio. The additions made to SMART2 enable a wider use of the model, previously focused on the acidification studies, for example to study the relations between soil nitrogen availability and forest growth. They also improve the acidification studies with more precise description of nitrogen, one of the acidifying components in the soil. The equations and the parametets inchrded in the SMART2 model are presented in Ahonen et al. (1998). The parameters describing the forest growth were first fixed to give the estimated present forest biomass (78 ton ha’) and the annual growth (3.5 m’ ha” a-‘). Then, the model was calibrated to produce present-day ( 1988-94) soil and stream water chemistry with yearly time step. The criterion in accepting the calibration was that the modeled annual pH, Gran alkalinity, and ion concentrations in runoff water would fall between the measured annual minimum and maximum values. Then, the modeled pH in stream water was examined against the measured yearly mean values and the modeled soil base saturation was studied against the measured value averaged over the cat&tent from the mearuremen tsoffour different plots, each of them containing fourreplicates. Model parameterization, calibration and the calibration results are presented more precisely in Ahonen et al. (1998). 2.5 Calculation of critical loads

_

Critical loads of sulfur and nitrogen in steady-state were derived with the Simple Mass Balance method (Posch et al., 1995). To link the critical loads to the present deposition, the so-called protection isolines were determined. The protection

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J. Ahonen and K. Ranlcinen: Effects of Emission Reduction Scenarios on a Forested Cat&rent

isolines describe combinations of sulfiu and nitrogen depositions at which protection against acidification and/or eutrophication is obtained for certain percentile of the ecosystem areas in the grid cell. The derivation of critical loads and the protection isolines was realized in CCEIRJVMinstitute according to Posch et al. (1995) with the national data. The maximum critical load of sulfur, assuming no nitrogen deposition, is given in the isolines determined for each of the grid cells in the EMEP coordination system. In the EMEP system the whole of Europe is devided into calculation matrices with the grid cell size of 150 km x 150 km or 50 km x5okm. The Hietajarvi catchment is located in EMEP 50 km x 50 km coordinates in the grid cell (58,84), and in EMEP 150 km x 150 km coordinates in the grid cell (20,28). To investigate the representativity of the catchrnent as typical forest site within the grid cell, the maximum critical load of sulfur for Hietajarvi catchment was calculated also using the measurements according to Posch et al. (I 995): CL,,,(S) = BC,

- Cl,

+ BC, - BC, - Alk,,,,,

b) 0.1,

in Finland ca+lAo(e.q

m” ,

I

Fig. I Simulatedconcentrationin streamwater of a) nitrateand b) divalent basecations,and simulatedforestc) growth nitrogenuptakeand d) biomass in HietajtQvi.

(1)

where BC,, is the base cation deposition, Cl, is the chloride deposition, BC, is the base cation weathering, BC, is the base cation net uptake by forest growth and Alk,,,, is the critical leaching of alkalinity in steady-state. In the calculation, for the cation uptake, the mean of 128 years modeled growth uptake was used. The base cation weathering was taken from Lindroos et al. (1996), a study where results of three different methods of estimating cation weathering rates were presented for Hietajtbvi catchment. Calculations were made for the estimated minimum and maximum weathering rates and for the mean of them. The rates were, correspondingly, 0.0 I eqm’ ‘aa-‘,0.038 eqm’*a“, and 0.024 eqm-‘a-‘.

3 Results and discussion

Fig. 2 Historicaland t%turcdepositionscenariosfor Hietajlrvi catchment.

3.1 Model behavior The calibration criterion was met by the stream water pH and alkalinity, and the concentrations of ammonium, nitrate, sulphate, chloride, and monovalent base cations. The model also fitted well to the mean of the measured base cations. During the whole simulation period from I860 to 2050, the NO,-concentration reflected the effect of the simulated forest nitrogen uptake (Fig. la,c ). This can be seen for instance as a decrease after the beginning of the assumed rapid forest growth in the 1920’s. Also, the BC2 concentration made a slight fall during the rapid uptake period (Fig. 1b). Nitrogen concentrations in both soil and stream water showed responses _________________________ Correspondence

to: J. Ahonen

to changes in nitrogen deposition (Fig. 2). A decrease of deposition after the year 1990 was reflected in all nitrogen related processes: the mineralization flux, btter fall flux, nitrogen root uptake, and the soil and stream water concentrations (Fig. 3). Future depositions of both sulphate and nitrogen oxide are . clearly lower for all the reduction strategies compared to the current reduction plan (CRP) (Fig. 2). The MFR strategy gives clearly the biggest reductions, being about 75% for sulphate and 53% for nitrate oxides by the year 2050. Ammonium is remarkably reduced only with the MFR strategy. The model results show a start of a decline in soil base saturation and

J. Ahonen and K. Rank&n:

Effects of Emission Reduction Scenarios on a Forested Catclunent in Finland

865

Fig. 3 Simulated a) nitrogen mineralization,b) nitrogen in litter fall, c) nitrogencontentin leaves, and c) total nitrogenmot uptake fluxes.

stream water pH in 1960’s when the acidifying deposition starts to increase rapidly (Fig. 4). SMART2 gave the present value of pH which was a little lower than the pH estimated in the palaeolimnological study. The decline of both pH and BS continues through the years of peak deposition but the base saturation does not reach the critical level of 5% (0.05 in Fig. 4). According to the SMART2 model, the ecosystem starts to recover with all reduction strategies. The recovering starts with the shortest delay, round year 2020, if the MFR strategy is chosen. At 2050, both pH and BS are increasing with all the strategies in SMART;! simulations. 3.2 Deposition scenario analysis If the ecosystem response is studied during an extended time period, assuming that atmospheric deposition and nutrient uptake will remain constant, one can see that pH and base saturation will be stabilized (Fig. 4). With the MFR strategy, they are returned to the initial level round the year 2 120. With B I, REF and CRP strategies, the ecosystem deterioration will be permanent to a certain extent. The CRP strategy gives the lowest level of recovery, stabilization occurring around the year 2200. 3.3 Connection between dynamic and steady state features The critical loads for acidity are quite low in Finland, which

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Fig. 4 Simulatedhistoricaland future a) streamwater pH and b) soil base saturationby SMARTZ.

means that Finnish forest soils are sensitive. The exceedances of critical loads are, however, still low (Fig. 5). The highest values are found in Southern Finland, where the impact of Central European emissions can be seen. With the CRP reduction strategy, half of the Finnish surface area would be protected, and with MFR strategy, no exceedances would occur. The maximum critical loads of sulfur in Hiemj&rvi catchment, calculated from the isolines and the measurements are presented in Table 2. In the critical load studies, the less tolerant fifth percentile of the ecosystems within a grid cell has been chosen to determine the acceptable load level in that grid cell. Comparison of the values calculated from the Eq. I for Hietaj&vi and from the steady state isolines for the grid cell suggests that Hietaj&vi is among the most tolerant ecosystems in the grid cell. The critical load of sulfur by Eq. 1 stays, with all the weathering rates used. between the minimum and the maximum defined by the isolines but does not include to the most sensitive five percent. In Hietajikvi, the acidifying deposition in a steady state situation (with CRP strategy 30 meq m”a-‘) exceeds the maximum critical load of sulfur calculated for the EMEPSO and EMEPISO grid cell (20.0 and 28.6 meq m-’ a-‘, Table 2). However. the modeling result indicates that the soil base saturation does not reach the critical level of 5%. This agrees with the conclusion that the

866

J. Ahonen and K. Rankinen: Effects of Emission Reduction !knarios on a Fomted Catchment in Finland

HietajPrvi catchment would not include to the most sensitive ecosystems presented in the map. Better tolerance of Hietajiirvi catchment is probably due to the high weathering rate at the site. Table 2. Maximum critical loadsof sulfur basedon the protectionisolincs (EMEPSO and EMEPISO) and on the Equation I. CLmax(S) meqm-’

EMEPSO

EMEPISO

measurements (Eq. 1)

We thank the Finnish Fores1Rrsearxh Institute (ME’f1.A) for providing unpublished data for the model calibration.The modelingwas carried out as a partof the project‘Developmentof Assessmentof Monitoringlhchniqucs at IntegratedMonitoringSitesin Euro@, linanccdby the FinancialInstrument for the Environment(LIFE) of theElJ (project LIFE9YFIN/AI I/El’f/387). The load map xas producedas part of the projec:‘CouplingofCORINAIR data to cost-effective emission reduction strategies based on critical thresholds’.This ongoing project is also financed by ELI/LIFE (projecl LIFE97/ENVIFIN/336).

9.0 8.5 41.6 min. References 20.0 28.6 5th % 58.8 85.2 83.8 (mean DC..) median Ahoncn J., Rankinen K., Holmberg M.. Syri S. And Forsius M., 1998. 105.2 226.8 112.0 mitY _________________________________________________~~____~~______~~~_~_~~~~____ SMART2 model application to an Integrated Monitoring Catchment in Finland: Eflects of emission reductionsscenarios. Submitted to Boreal EnvironmentResearch3 (in press). a) Cribcal foad of sulfur (CL(S))

b) Exckdmeb n 1QQtl

CL(S)

C) Exceedancaof CL(S) in 2010 wifh CRP

Amann, M., Bcrtok. I.. Cofala, J., Gyarfas, F., Heyes, C.. Klimont, 2. and sCh6pp.W., 1996. Cost-effectiveControlOfAcidificationand Ground-Level Ozone.SecondInterimReportto the EuropeanCommission,DG Xl. IIASA. December 19%. I I2 p. BergstromI., MllkelBK.. Starr M., 1995. IntegratedMonitoringPtugramme in Finland.FirstNationalRepwr.Ministryofthe Environment,Environmental Policy Department,Helsinki. 138 p. Cosby,B.J., Homlxrger, GM. andGalloway,J.N.. 1985. Modelingtheeffects ofacid deposition: Assessment ofa lumpedparametermodelofsoil and water acidilication.Water ResourcesResearch21.51-63.

Fig. 5 a) Critical loadof sulfurin Finlandin steadystate.b) The exceedQ~s ofcritical loadofsulfurin 19%andc)tbeprcdicted exceedancesin2OlOwith the CRP emissionreductionstrategy.Hietaj8rvi catchmcntis locatedin the boundedcell.

4 Conclusions The desired accuracy in the calibration ofthe SMART2 model was achieved. It encourages the wider use of SMART2 in the firture especially if detailed information on soil nitrogen state or relation between soil acidity and the growth of vegetation are of interest. In Hietajiirvi cat&men& SMART2 model results show a decline in base saturation and stream water pH, i.e. the ecosystem starts to acidify, in response to the rapid increase of acidifying deposition starting in 1960’s. The decline is, however, stopped and turned to its inverse by the year 2010 assuming any of the reduction plans. The final level of BS and pH depends on the chosen strategy. The more ambitious the strategy is. the faster and more complete is the recovery. In spite of started decline in soil base saturation, the critical level of 5% is not achieved according to the model thus confirming that HietajBrvi catchment belongs to the tolerant ecosystems within the EMEP grid cell.

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60: J. Ahonen

De Vries, W., Porch. M. and Kgmari. J., 1989. Simulationof the long-tern, soilresponseto acid depositionin variousbuffer ranges.Water, Air and Soil Pollution48.215-246. EC, 1997. Draft Communicationto the Council and Parliament on a: Community Strategy to Combat Acidification. COM(97)88/4 European Commission,Brussels. Forsius M., Alveteg M., Bali J.. Guardans R., Homlberg M., Jenkins A.. Johansson M., KleemolaS.. RankinenK., RenshawM., SvcrdrupH. and Syri S., 1997.Assessment ofthe Effectsofthe EU AcidificationStmtegy:Dynamic Modeling on Integrated Monitoring Sites. Finnish Environment Institute, Helsinki. Finland.40~. Kros J.. Reinds G.J.,de Vries W., Latour J.B. and Bollen M.J.S., 1995. Modelingofacidity and niaogenavailabilityin naturalewsyscrmsin response to changesin aciddepositionand hydrology.Report95. DLO WinandStaring Centrc, Wageningen,The Netherlands.88~. LindroosA.-J., Starr M.. TarvainenT.. Tan&men H., 19%. Weatheringrates in the H&j&vi IntegratedMonitoringcatchmentarea. In:KohonenT. and Lindbcrg 8. (eds.), 1995. Abstracts.The 22nd Nordic Geological Winter Meeting, 8-1 I January1996, Turku, Finland.AboAkademi. Abe. p. 120. PoschM., ReindsG.J. and de Vries W.. 1993. SMART- A simulationmodel for acidification’sregional trends: Model descriptionand user manual. MimeographSeriesofthe NationalBoardof Watersandthe Environment477. Helsinki, Finland.43 p. Porh M.. de Smet P.A.M., HettelinghJ.-P. And Downing R.J. (eds.). 1995: Calculcationand mappingofcriticalthresholdsin Europe.Statusreport 1957. RIVM RcponNo.259101004.pp. I-41.

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