Sensitivity Analysis of a Regional Scale Soil Acidification Model

XOT FOR QUOTATION WITHOUT PERMISSION OF THE AUTHOR

SENSITMTY ANALYSIS OF A REGIONAL SCALE SOIL ACIDIFICATION MODEL

M. Posch, L. Kauppi, J. Kamari

November

1

9 8 5 CP-85-45

C o l l a b o r a t i v e R z p e r s r e p o r t work which h a s not been performed solely

at

t h e International Institute f o r Applied Systems Analysis and which h a s received only limited review. Views

or

opinions expressed h e r e i n do not necessarily r e p r e s e n t those of t h e Insti- t u t e , i t s National Member Organizations, o r o t h e r organizations supporting t h e work.

INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS 2361 Laxenburg, Austria

AUTHORS

Yaximilian Posch is a r e s e a r c h s c h o l a r

at

t h e International Institute f o r Applied Systems Analysis, Laxenburg, Austria.

Lea Kauppi and J u h a Kamari

are

former r e s e a r c h s c h o l a r s of t h e International Institute f o r Applied Systems Analysis, Laxenburg, Austria.

They have r e t u r n e d t o t h e

Water

R e s e a r c h Institute of t h e National Board

of

Waters in Helsinki. Finland.

-

ⁱⁱⁱ

-

PREFACE

The IIASA "Acid Rain" p r o j e c t s t a r t e d in 1983 in o r d e r

to

provide t h e European decision makers with

a

tool which c a n be used

to

evaluate policies for controlling acid rain. This modeling e f f o r t i s p a r t of t h e official cooperation between IIASA and t h e U N Economic Commission f o r E u r o p e (ECE). The IIASA m o d e l c u r r e n t l y contains t h r e e linked compartments: Pol- lution Generation, Atmospheric P r o c e s s e s and Environmental Impact. Each of t h e s e compartments c a n b e filled by d i f f e r e n t substitutable submodels.

The soil acidification submodel i s p a r t of t h e Environmental Impact

com-

partment.

A m o d e l which i s intended f o r use in decision making, d e s e r v e s a vigorous testing program

to

s t r e n g t h e n t h e confidence of model u s e r s in i t s estimates. Such a program i s c u r r e n t l y underway

at

IIASA

to test

t h e m o d e l system. P a r t of t h e a p p r o a c h involves conventional model validation and verification. A less conventional a p p r o a c h i s a l s o being t a k e n by ack- nowledging t h a t model uncertainty exists and t h a t i t should b e i n c o r p o r a t e d explicitly in t h e m o d e l . This p a p e r d e s c r i b e s r e s u l t s of sensitivity

tests

on t h e soil acidification submodel.

Leen Hordijk P r o j e c t Leader

The authors a r e indebted to the many individuals who have supported this study in many ways. Our special thanks a r e due t o L. Hordijk, P . Kauppi, J. Alcamo and E. Matzner.

-

vii

-

ABSTRACT

A dynamic model h a s been introduced f o r describing t h e acidification of f o r e s t soils. In one-year time s t e p s t h e model calculates t h e soil pH as a function of t h e acid

stress

and t h e b u f f e r mechanisms of t h e soil. Acid

stress

is defined

as

t h e hydrogen ion input into t h e top soil. The b u f f e r mechanisms c o u n t e r a c t acidification by providing

a

sink f o r hydrogen ions.

The concepts b u f f e r

rate

and b u f f e r capacity

are

used t o quantify t h e b u f f e r mechanisms. The model compares (i) t h e

rate

^{of acid}

stress

(annual amount) t o t h e buffer r a t e , and (ii) t h e accumulated acid

stress

( o v e r s e v e r a l y e a r s )

to

t h e b u f f e r capacity. These two types of comparisons pro- duce a n estimate of t h e soil pH.

The model h a s been i n c o r p o r a t e d into t h e RAINS model system of t h e International Institute f o r Applied Systems Analysis f o r analyzing t h e acidic deposition problem in Europe. The d a t a on acid s t r e s s , e n t e r i n g t h e soils, i s obtained from t h e o t h e r submodels. Data on b u f f e r

rate

and b u f f e r c a p a c i t y h a s been collected from soil maps and geological maps.

The sensitivity of t h e model t o t h e forcing functions, p a r a m e t e r values and initialization of t h e soil v a r i a b l e s i s evaluated in t h i s p a p e r . The model's sensitivity t o initial b a s e s a t u r a t i o n a p p e a r s

to

b e crucial. Base s a t u r a t i o n v a r i e s widely in f o r e s t soils, while t h e variation of, e.g., t o t a l cation exchange c a p a c i t y i s normally not more t h a n i 50% of t h e a v e r a g e . Whenever possible, r e c e n t measurements about t h e s t a t u s of t h e soil should b e used.

The difference of acid

stress

and t h e b u f f e r

rate

of silicates d e t e r - mines whether t h e soil alkalinizes o r acidifies. The sensitivity of t h e model t o t h a t difference v a r i e s in time and s p a c e , being highest in areas where t h e deposition

rate

n e a r l y equals t h e s i l i c a t e buffer r a t e , e.g.

at

p r e s e n t in

Scandinavia.

TABLE OF CONTENTS

1. Introduction 2. Soil Acidification 3. The Model

4. Screening

4.1 Dominant Soil Types 4.2 Soil P a r a m e t e r s

4.3 Atmospheric P a r a m e t e r s 4.4 Critical pH

5. Sensitivity Analysis 5.1 Soil P a r a m e t e r s

5.2 Atmospheric P a r a m e t e r s

5.3 Precipitation and Evapotranspiration 5.4 Critical pH

6. Conclusions Figures

References

SENSITIVLTY ANALYSIS OF A REGIONAL SCALE SOIL ACIDIFICATION HODEL

M. Posch, L. Kauppi, J. K6miri

1. Introduction

Soil acidification i s considered

as

one important link between a i r pollu- tion and f o r e s t damage. The ability of t h e soil t o b u f f e r acid deposition i s also

a

key f a c t o r in controlling t h e s u r f a c e

water

a n d groundwater acidifi- cation. T h e r e f o r e soil acidification

was

considered

a

suitable s t a r t i n g point f o r IIASA's Acid Rain P r o j e c t f o r evaluating environmental impacts of acid precipitation in Europe. The o v e r a l l objective of t h e p r o j e c t i s

to

develop

a

framework, which would a s s i s t in comparing t h e c o s t and effectiveness of d i f f e r e n t pollution c o n t r o l s t r a t e g i e s (Alcamo

et

^al., 1985).

The aim of t h i s p a p e r i s t o

test

t h e sensitivity of t h e soil acidification model. The modeling of soil acidification in t h e IIASA RAINS (Regional Aci- dification Information a n d Sfmulation) model system i s based on t h e descrip- tion of p r o t o n consumption r e a c t i o n s presented by Ulrich (1981, 1983). The uncertainty in t h e model s t r u c t u r e , i.e. in t h e underlying t h e o r y , i s not considered in t h i s p a p e r . W e r e s t r i c t ourselves t o t h e evaluation of t h e sen- sitivity in t h e forcing functions, p a r a m e t e r values and initialization of t h e soil variables.

2. S o i l A c i d i f i c a t i o n

Soil acidification h a s been defined

as a

d e c r e a s e in t h e a c i d neutraliza- tion capacity (van Breemen

et

al., 1984). The acidification i s caused by acid s t r e s s , which i s defined

as

t h e input of hydrogen ions into t h e t o p soil. The acid

stress

d u e t o a i r pollution can r e s u l t from t h e d i r e c t deposition of hydrogen ions o r from t h e indirect e f f e c t of a c i d producing substances, such as t h e d r y deposition of sulfur compounds.

A consecutive s e r i e s of chemical r e a c t i o n s h a s been documented in soils, in which acidification proceeds. Information regarding t h e dominant r e a c t i o n s counteracting acid s t r e s s h a s been used f o r defining c a t e g o r i e s , called buffer ranges. Buffering in e a c h r a n g e c a n b e described using two v a r i a b l e s , b u f f e r capaclty (BC, kmol ha ^-I), t h e g r o s s potential, and b u f f e r

rate

( b r , kmol ha ^-l yr -I) f o r t h e

rate

of t h e r e a c t i o n . They c a n b e quanti- fied f o r any volume of t h e soil. In t h e following p a r a g r a p h s w e briefly d e s c r i b e t h e different b u f f e r ranges. The original description c a n b e found in Ulrich (1981, 1983).

Calcareous soils are classified into t h e c a r b o n a t e buffer r a n g e (pH 2

6.2). I t s buffer capacity i s proportional t o t h e amount of CaC03 in t h e soil.

The buffer r a t e , i.e. t h e dissolution

rate

of CaC03, i s high enough

to

b u f f e r any o c c u r r i n g

rate

of acid

stress.

If t h e r e i s no CaCO in t h e fine e a r t h f r a c t i o n a n d t h e carbonic acid i s t h e only acid being p r d u c e d in t h e soil, t h e soil i s classified into t h e sili-

cate

b u f f e r r a n g e (6.2

>

^{pH 2} 5.0). Buffering i s based on weathering of sili-

cates.

The b u f f e r capacity i s high (practically infinite considering

a

time horizon of hundreds of y e a r s ) , but t h e b u f f e r

rate

i s quite low. The weather- ing of silicates o c c u r s throughout all b u f f e r r a n g e s . The switch

to

lower b u f f e r r a n g e s implies, t h a t t h e weathering r a t e of silicates i s not sufficient t o b u f f e r a l l t h e incoming

stress.

When t h e cation exchange r e a c t i o n s play t h e major r o l e in t h e acid buffering, t h e soils

are

classified into t h e cation exchange b u f f e r r a n g e (5.0

<

^{pH r} 4.2). The acid

stress

not b f f e r e d by t h e silicate b u f f e r r a n g e i s a d s o r b e d in t h e form of H+- o r AIY+-ions

at

t h e exchange s i t e s , t h u s displacing t h e b a s e cations. The b u f f e r

rate

(=

rate

of t h e cation exchange r e a c t i o n s ) i s high, effectively counteracting any o c c u r r i n g acid

stress.

The b u f f e r capacity, CECtOt, i s generally r a t h e r low, depending mainly on t h e soil t e x t u r e . The remaining buffer capacity

at

any given time i s e x p r e s s e d by b a s e s a t u r a t i o n , t h e p e r c e n t a g e of b a s e cations of t h e t o t a l cation exchange capacity.

When b a s e s a t u r a t i o n d e c r e a s e s below 5-10X. t h e soils

are

classified into t h e aluminum b u f f e r r a n g e (4.2

<

^pH

<

3.8). H+-ion

are

consumed by releasing aluminum, mainly from clay minerals. High A?+-concentrations c h a r a c t e r i z e t h e soil solution and may c a u s e t o x i c e f f e c t s

to

b a c t e r i a and plant roots. The b u f f e r c a p a c i t y i s almost infinite d u e t o t h e abundance of aluminum compounds in t h e soil. The d e c r e a s e of pH below 3.8 implies increasing solubility of i r o n oxides and t h e soil i s classified into t h e i r o n b u f f e r r a n g e , although in quantitative terms aluminum may still

act

as t h e dominant b u f f e r compound.

3. TheYodel

The model d e s c r i b e s soil acidification in t e r m s of a sequence of b u f f e r r a n g e s . The model compares (i) t h e amount of a c i d

stress

accumulated o v e r t h e c o u r s e of time t o t h e b u f f e r capacity, a n d (ii) t h e

stress rate,

t h e time d e r i v a t i v e of t h e amount of s t r e s s ,

to

t h e b u f f e r

rate.

A s t h e buffer capa- city of silicates i s v e r y l a r g e , only t h e b u f f e r

rate

i s compared in t h a t r a n g e . The b u f f e r

rates

of c a r b o n a t e and cation exchange r a n g e

are

always high enough t o c o u n t e r a c t any o c c u r r i n g

stress rate.

Thus, only t h e capaci- t i e s of t h e s e r a n g e s h a v e t o b e considered.

Within one time s t e p t h e capacity of t h e cation exchange buffer system,

EL,

is depleted by t h e difference of t h e acid

stress rate, a s t ,

and t h e buffer r a t e of silicates, b r a (Eq.1). A t pH-values between 5.6 and 4.0 a non- linear relationship i s assumed between base saturation and soil-pH within t h e silicate, cation exchange and t h e upper aluminum buffer range, a s long a s BC& 2 0 (Eq.2)

BC& = -

^(as

' -

^{bra ) (1)}

The shape of t h e pH

-

base saturation relationship has been adopted from results of an equilibrium model by Reuss (1983).

If

BCL

^=0, equilibrium with gibbsite i s assumed. A s precipitation infil-

trates

into t h e soil and mixes with t h e soil solution. disequilibrium concen- trations [A1 ^{3 + ] s} and [ H C ] , a r e obtained

where

Vf

is t h e volume of soil solution a t field capacity and P and

E

mean annual precipitation and evapotranspiration, respectively. The soil solu- tion volume is simply defined by

The soil thickness, z , i s fixed t o 50

c m

and t h e volumetric

water

con- t e n t value a t field capacity,

ef,

is estimated separately for e a c h soil type based on the grain size distribution in t h e soil. Aluminum is dissolved o r precipitated until t h e gibbsite equilibrium s t a t e is r e a c h e d (Eq.6). This pro-

cess

involves a change from disequilibrium concentrations

as

defined in Eq.7

3+ t

3 [ [ A I ~ ~ ] ,

-

[AI ]

] ⁼

[ H I ] (

-

[ H I ] ,

Combining Eqs.6 and 7 yields a t h i r d o r d e r equation which h a s a single r e a l r o o t

The main c h a r a c t e r i s t i c s of t h e model are summarized in t h e flow c h a r t given in Figure 1 a n d described as w e l l

as

demonstrated in more detail in Kauppi

et

al. (1985a.b).

4. Screening

In this section w e will s c r e e n all t h e input variables, p a r a m e t e r s and forcing functions in o r d e r t o find out, which of them should b e looked

at

in more detail.

4.1. Dominant Soil Types

IIASA's soil acidification model deals with f o r e s t soils only. To focus t h e sensitivity analysis o n t h e most important soil types, t h e soils were ranked according t o t h e i r c o v e r a g e of t h e

total

f o r e s t

area

in Europe T h r e e soil t y p e s

-

Orthic Podzol (Po), E u t r i c Podzoluvisol (De) and Orthic Luvisol (Lo)

-

^{are estimated}

^to

comprise o v e r 50% of t h e t o t a l f o r e s t e d area of Europe (see Table 1 ) . These t h r e e soil t y p e s will b e used f o r testing t h e sensitivity of t h e model t o varying p a r a m e t e r values and forcing functions.

4.2. Soil Parameters

The m o d e l r e q u i r e s initial values f o r t h e following soil parameters:

c a r b o n a t e buffer capacity. BCCa, silicate b u f f e r r a t e , bra, t o t a l cation exchange capacity, CECtot, b a s e saturation,

8 ,

and volumetric

water

con- t e n t

at

field c a p a c i t y ,

Bf.

Since all t h e t h r e e dominant soil t y p e s (Po, De and Lo) are non-calcareous, BCCa c a n b e neglected.

Bf

i s used in t h e m o d e l only when calculating equilibrium concentrations in t h e aluminum buffer range. Testing t h e sensitivity of t h e H+-concentrations (given by Eqs.3-8) f o r a r a n g e of

Bf

-values of 0.05-0.30 i t

was

found t h a t t h e e f f e c t of varying

Bf

on t h e r e s u l t i s negligible. The soil p a r a m e t e r s

to

b e looked

at

a r e t h e r e f o r e brsl, CECtot, and

8 .

4 -3. Atmospheric Parameters

The model i s d r i v e n by two forcing functions: acid deposition and n e t precipitation. The above mentioned t h r e e main soil t y p e s (Po, De and Lo) o c c u r in quite d i f f e r e n t p a r t s of Europe. Orthic Podzols dominate in Scandi- navia, but

are

almost a b s e n t elsewhere, while E u t r i c Podzoluvisols and Orthic Luvisols

are

typical f o r e s t soils in C e n t r a l E u r o p e (Figures 2.3 and 4). Because acid deposition in Scandinavia is generally lower t h a n in Cen- tral Europe t h e typical acid

stress

on Orthic Podzols is lower t h a n on t h e two o t h e r soil t y p e s (Figure 5). Thus f o r P o 2 kmot ha 'l yr

w a s

used

as a

high

stress

r a t e , while 4 kmot h a - l y r - l

w a s

used f o r De and 6 kmol ha'lyr f o r Lo. The low values used were 0.5 kmot ha-I yr-I f o r Orthic Podzol and 1 kmot ha yr f o r E u t r i c Podzoluvisol and Orthic

Table 1. Dominant f o r e s t soil t y p e s in Europe. The soils

are

r a n k e d a c - cording

to

t h e i r coverage of t h e t o t a l f o r e s t a r e a in Europe.

Soil Symbol

(

^X

Orthic Podzol E u t r i c Podzoluvisol

Orthic Luvisol Dystric Cambisol

Dystric Histosol Gleyic Luvisol

Leptic Podzol Dystric Podzoluvisol

E u t r i c Cambisol Haplic Chernozem

Humic Cambisol Chromic Cambisol

Calcic Cambisol Chromic Luvisol Humic Podzol Gleyic Podzol Haplic Kastanozem

Lithosol-.

. .

Vertic Cambisol

. .

.-Gelic Regosol Rendzina E u t r i c Regosol Luvic Chernozem

Ranker Mollic Gleysol Calcaric Regosol

Luvic Phaeozem

Calcaric Fluvisol

Luvisol.

Also precipitation, P, and e v a p o t r a n s p i r a t i o n ,

E,

which e n t e r

as

a driving function in t h e aluminum buffer r a n g e , v a r y significantly o v e r Europe. In o u r d a t a base, which was derived from

a

³⁰ y e a r climatic mean s t a t i s t i c s of 253 stations in Europe, t h e Sovjet Union and Northern Africa (Miiller, 1982), t h e annual n e t precipitation, P -E received by t h e soil t y p e P o , r a n g e s from 9 5 m m

to

^{1950 m m} (mean: 430 m m ), while f o r De t h e r a n g e is 127-365 m m (mean: 263 m m ) and f o r Lo 67-1590 m m (mean: 285 mm). In t h e sensitivity

tests

^{300 m m} w a s considered

a

typical n e t precipitation f o r Po, and t h e r a n g e used w a s 100-700 m m ; f o r De 270 m m (range: 100-400 m m ) and f o r Lo 200 m m (range: 100-600 m m ) were chosen as typical values (Figure 6).

The local stress r a t e in f o r e s t s resulting from

a

given regional mean of sulfur deposition may v a r y significantly. Forests a r e known t o a b s o r b a i r pollutants more effectively t h a n open land, and estimates of t h i s filtering f a c t o r , rp, v a r y from 1.1 to 4.0 (Table 2). Secondly, p a r t of t h e acid stress deposited i s accompanied by a i r b o r n e dust, and o t h e r impurities which con- tain significant amounts of b a s e cations. Depositing b a s e cations contribute t o t h e exchangeable b a s e cations in t h e soil and t h e r e f o r e t h e estimated b a s e cation equivalents have t o b e s u b t r a c t e d from t h e calculated sulfate equivalents. This phenomenon is especially important in areas where d r y deposition comprises

a

significant p a r t of t h e t o t a l s u l f u r deposition.

According t o t h e l i t e r a t u r e t h e acid

stress

p a r a m e t e r , u, expressing t h e fraction of acid s t r e s s t h a t i s not counteracted by b a s e cation deposition, v a r i e s between 0.56 and 0.78 (Table 3).

Table 2. *values calculated from local observations on bulk deposition and t o t a l deposition

to

f o r e s t f l o o r measured

as

throughflow+stemflow. Bulk deposition is assumed t o r e p r e s e n t deposition

to

open field.

Species Quercus-Carya

Fagus-Acer Quercus Quercus Quercus-Betula

Pinus Pinus Pinus P i c e a a b i e s P i c e a sithcensis

-!'-

-"-

-"- -"-

-!'-

Location R e f e r e n c e

Walker Branch, USA Hubbard Brook, USA

Solling, FRG F r a n c e Poland Xetherlands Netherlands

Sweden J a d r a a s , Sweden

Solling, FRG Kilmichaer, UK Leanachan, UK S t r a t h y r e , UK

Kershope, UK Elibank, UK Fetteresso. UK

S c h r i n e r & Henderson (1978) Likens

et

al. (1977)

Ulrich (1984) Rapp (1973) Karkanis (1976) van Breemen

et

al. (1982)

-1'-

Bringmark (1977) Andersson et al. (1980)

Ulrich (1984) Miller & Miller (1980)

-I1-

-IP- -1'-

-'I-

-!'-

The atmospheric t r a n s p o r t models provide t h e a v e r a g e t o t a l sulfur deposition, d t o t , in e a c h grid

as

input t o t h e soil model. The deposition on f o r e s t s within one g r i d s q u a r e , d f , i s assumed t o b e rp times l a r g e r than t h e deposition on open land, d o , i.e.

Table 3. a-values calculated from local observations on c ~ ~ + + M ~ ~ + - deposition and SO:--deposition (see t e x t f o r f u r t h e r details).

I

^{Location Q Reference}

I

Birkenes, Norway Fyresdal-Nissedal, Norway

Langtjern. Norway Fillef jell, Norway Beech, Solling, FRG S p r u c e , Solling, FRG

Oak, Solling, FRG Pine, Solling, FRG Heath, Solling, FRG

J a d r a a s , Sweden

Wright & Johannessen (1980) Johannessen & J o r a n g e r (1976)

Henriksen (1976) Dovland (1976) Matzner (1983)

-I1- -1'-

-I1- -I1-

Andersson et al. (1980)

where f i s t h e f r a c t i o n of f o r e s t s within t h e grid. From t h i s w e g e t

The acid s t r e s s , as, on t h e f o r e s t s within t h e grid i s then given by

The sensitivity of t h e model t o t h e p a r a m e t e r s cp and a was t e s t e d by looking at t h e changes in t h e area of soils below

a

c r i t i c a l pH-value in Europe.

4.4. Critical pH

The concept "critical pH" r e f e r s t o a n increased r i s k f o r f o r e s t dam- a g e due t o changes in soil chemistry. The value 4.2 has been used in t h e model application t o a European scale. This i s t h e value, which

-

^according

t o Ulrich (1981, 1983)

-

implies t h e change from t h e cation exchange r a n g e t o t h e aluminum r a n g e . The connection between f o r e s t damage and increased dissolved aluminum-ion concentrations in t h e soil solution i s not, however, straightforward. I t does not mean t h e r e f o r e , t h a t t h e r e would b e no r i s k above t h e c r i t i c a l pH, n o r t h a t t h e r e definitely o c c u r s damage below it. Some c r i t e r i a h a v e been proposed by Ulrich

et

al. (1984) f o r t h e evaluation of r i s k s caused by soil acidity (Table 4). This information c a n a l s o a s s i s t in i n t e r p r e t i n g r e s u l t s from o u r model. Concerning t h e Euro- pean application t h e e f f e c t of varying t h e c r i t i c a l pH-value on t h e estimate of f o r e s t area under r i s k w a s tested.

Table 4. C r i t e r i a f o r relating r i s k of f o r e s t damage t o some chemical c h a r a c t e r i s t i c s of soils (cf. Ulrich et al., 1984).

1

Increasing r i s k High r i s k Very high r i s k

I

I ^pH',fi ' ¹

2 0 . 0 5 ^{2 4.2 4.0-4.2}

<

0.05

^<

0.0 ^4.0 [ ~ l " ] peq / 1

<

80 80-320

>

320

C a / A1

>

0.4 0.1-0.4

<

0 . 1

5. Sensitivity Analysis

5.1. Soil

Parametera

The values f o r t h e b u f f e r capacities and b u f f e r

rates were

initialized based on t h e International Geological Map of Europe and t h e Mediterranean Region (UNESCO, 1972) and t h e Soil Map of t h e World (FAO-UNESCO, 1974).

The Geological Map provided information about t h e p a r e n t material of t h e soils and t h e Soil Map a b o u t t h e dominant soil types. These s o u r c e s , how- e v e r , do not give t o o much d i r e c t information about t h e buffering p r o p e r - t i e s of t h e soils. S o t h e silicate b u f f e r

rate

was r e l a t e d t o t h e Ca+Mg con- t e n t of t h e p a r e n t material following t h e b u f f e r

rate

values given by Ulrich (1981). The estimation of t h e t o t a l cation exchange capacity a s w e l l

as

t h e base s a t u r a t i o n of

a

c e r t a i n soil t y p e was based o n (i) information given by t h e definition of t h e soil t y p e (FAO-UNESCO, 1974) and (ii) descriptions and r e s u l t s of analyses of typical soil profiles given in t h e Appendix of t h e Soil Map.

According

to

Ulrich (1983), t h e silicate b u f f e r rate may vary from 0.1

to

1.0 kmol ha yr f o r

a

50 cm soil layer. This r a n g e

was

also used in t h e sensitivity r u n s (see Table 5 f o r t h e values and r a n g e s used in t h e sensi- tivity runs). The acid

stress rate, as

(in kmol ha yr -I), which i s com- p a r e d t o b r a , v a r i e s

at

p r e s e n t between z e r o in some remote

areas

and o v e r t e n in some p a r t s of Europe. If as and b r a are

at

t h e same level, t h e model i s highly sensitive t o changes in bra (see Figures 7a-10a). In

a

c a s e of

a

high

stress

rate, on t h e c o n t r a r y ,

a

change of bra from 0.1 t o 1.0 kmol ha yr h a s only a marginal e f f e c t on t h e results. because in any c a s e i t c a n b u f f e r only

a

minor p a r t of t h e

stress

(Figures l l a , 1 2 a ) . Due t o t h e temporal and s p a t i a l variation of t h e acid

stress

t h e sensitivity of t h e model t o t h e b u f f e r

rate

of silicates v a r i e s a l s o in time and space. A t p r e s e n t t h e model i s sensitive t o bra only in remote

areas

like Scandinavia.

If, however, emissions a r e going t o d e c r e a s e considerably in t h e f u t u r e , new

areas

will o c c u r , where t h e value of bra i s important.

The e f f e c t of t h e t o t a l cation exchange capacity i s quite straightfor- ward: t h e h i g h e r t h e capacity of t h e soil, t h e longer i t t a k e s t o consume i t f o r t h e incoming p r o t o n flux. Doubling CECtot r e s u l t s in doubling t h e time needed t o e x h a u s t i t , when o t h e r p a r a m e t e r s

are

k e p t constant (Figures 7b-12b). CECtot, however, i s quite strongly r e l a t e d t o t h e soil type. i.e.

CECtot of

a

c e r t a i n soil h a s only

a

limited r a n g e of variation, typically not