Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 657

Surface Energy Exchange and Hydrology of a Poor Sphagnum Mire

BY

ERIK KELLNER

ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2001

Dissertation for the Degree of Doctor of Philosophy in Hydrology presented at UppsalaUniversity in 2001

ABSTRACT

Kellner, E., 2001. Surface Energy Exchange and Hydrology of a Poor Sphagnum Mire. ActaUniversitatis Upsaliensis, Comprehensive Summaries of Uppsala Dissertations from theFaculty of Science and Technology 657. 38 pp. Uppsala. ISBN 91-554-5121-7.

Mires surface energy and water budgets govern the conditions for climatic, hydrological,ecological and carbon balance processes. The components of the water and surface energybudgets were quantified over two growing seasons for an open boreal mire. Themeasurements of fluxes were complemented with data on the spatial variation of watercontent and temperature in different micro-relief elements (hummocks and hollows). Sincemeasurements on mires are scarce, special investigations of aerodynamic properties weredone as well as a calibration of TDR function for peat. The partitioning of available energy atthe surface depended mainly on air temperature and relative humidity. There was a trend offalling Bowen ratio both during the day and during the season from May (monthly value 0.9)to September (0.6). The bulk surface resistance (rs) to evapotranspiration was considerableand varied little (mean rs = 160 s m-1). The cause of its relatively large value could be foundin a great aerodynamic resistance within the canopy layer, and the peat wetness variationinfluenced little. In the scale of the whole mire, the water storage were similar over thecentral, open areas. On a smaller scale, the presence of a pronounced micro-topographycaused a variation of the surface wetness. This was also reflected in the spatial variation ofsoil temperatures. The heat storage in hummock was largely influenced by lateral heat fluxes.There were considerable effects of peat elasticity and approximately 40 % of the changes inwater storage was caused by swelling/shrinking of the whole peat mound. This effect shouldbe incorporated in future models of mire-water dynamics.

Keywords: bog, mire, energy budget, surface resistance, Sphagnum, water storage, peat, TDR.

Erik Kellner, Department of Earth Sciences, Hydrology, Uppsala University,Villavägen 16, SE-752 36 Uppsala, Sweden

© Erik Kellner 2001

ISSN 1104-232XISBN 91-554-5121-7

Printed in Sweden by Kopieringshuset AB, Uppsala 2001

Avhandling för filosofie doktorsexamen i hydrologi framlagd vid Uppsala universitet 2001

REFERAT

Kellner, E., 2001. En fattigmyrs ytenergiutbyte och hydrologi. Acta Universitatis Upsaliensis.Sammanläggningsavhandlingar från Uppsala universitets teknisk-naturvetenskapliga fakultet657. 38 pp. Uppsala. ISBN 91-554-5121-7.

Myrars energi- och vattenbudget ger förutsättningarna för deras klimatiska, hydrologiska,ekologiska och kolutbytesprocesser. En öppen fattigmyrs vatten- och energibudgetskomponenter kvantifierades för två växtsäsonger. Flödesmätningar kompletterades medmätningar av den rumsliga variationen av temperatur och vattenhalt mellan olika mikro-topografiska element (tuvor och höljor). Eftersom få fysikaliska mätningar har gjorts imyrmark, gjordes särskilda undersökningar av de aerodynamiska egenskaperna samt enkalibreringsfunktion för fuktighetsmätning med TDR i torv. Fördelningen av ytanstillgängliga energi mellan latent och sensibelt värme berodde huvudsakligen på luftenstemperatur och relativa fuktighet. Det fanns en trend med minskande Bowenförhållande underdagen men också under säsongen från maj (månatligt värde 0.9) till september (0.6).Ytresistansen mot avdunstning (rs) var betydande (medelvärde rs = 160 s m-1) och varieradelitet. Det höga värdet berodde på en stor aerodynamisk resistans i växtskiktet medanvariationen av fuktighet i torven hade liten betydelse. Sett över hela myren, var lagringen avvatten likformig i olika delar. I mindre skala var det dock stor skillnad i fuktighet mellanhöljor och tuvor. Detta återspeglades också i en rumslig variation av marktemperaturer.Tuvornas värmelagring påverkades också stort av laterala värmeflöden. Torvens elasticitetinnebar också stora effekter när det gäller variationen av vattenhalter och omkring 40 % avlagringsförändringarna skedde i form av svällning/krympning i myrens torv. När manutvecklar modeller för myrens hydrologiska system måste man ta hänsyn till torvenselasticitet.

Indexord: myr, energibudget, ytresistans, Sphagnum, vitmossa, hydrologiska modeller, TDR.

Erik Kellner, Institutionen för geovetenskaper, Hydrologi, Uppsala universitet,Villavägen 16, 752 36 Uppsala

© Erik Kellner 2001

ISSN 1104-232XISBN 91-554-5121-7

Tryckt på Kopieringshuset AB, Uppsala 2001

TABLE OF CONTENTS

TABLE OF CONTENTS................................................................................................ 4PREFACE....................................................................................................................... 5INTRODUCTION .......................................................................................................... 7BACKGROUND ............................................................................................................ 8

Mires, definitions and features.................................................................................... 8Aspects of climate, regional hydrology and biological processes .............................. 8Mire hydrology; achievements and gaps..................................................................... 9

MATERIALS AND METHODS.................................................................................. 11Site description.......................................................................................................... 11Methods..................................................................................................................... 13Evaluation of surface properties (III, IV).................................................................. 14Evaluation by model simulation (V) ......................................................................... 15

RESULTS ..................................................................................................................... 16Peat properties and TDR calibration (I) .................................................................... 16Water budget, water storage and water-content variations (II)................................. 17Heat storage, temperatures, heat-flux estimations (IV, III) ...................................... 21Aerodynamic properties (III, IV) .............................................................................. 22Energy partition and Bowen ratio (IV) ..................................................................... 24Simulations (V) ......................................................................................................... 26

DISCUSSION............................................................................................................... 29Energy partition in comparison with other studies ................................................... 29The regulation of surface energy exchange .............................................................. 29The hummock-hollow concept.................................................................................. 31Physical properties of peat ........................................................................................ 32Water storage............................................................................................................. 33

CONCLUSIONS........................................................................................................... 34ACKNOWLEDGEMENTS.......................................................................................... 35REFERENCES ............................................................................................................. 36

5

PREFACE

This thesis is based on the following papers which are referred to in the text by theirRoman numerals:

I Kellner, E., Lundin, L.-C. 2001. Calibration of time domain reflectometry forwater content in peat soil. Nordic Hydrology, accepted to be published in volume32, part 4.

II Kellner, E., Halldin, S. 2001. Water budget and surface-layer water storage in aSphagnum bog in central Sweden. Hydrological Processes, 15 : in press.

III Mölder, M., Kellner, E. 2001. Energy balance closure and excess resistance oftwo bog surfaces in central Sweden. Agricultural and Forest Meteorology,submitted in August 2001.

IV Kellner, E. 2001. Surface energy fluxes and control of evapotranspiration from aSwedish Sphagnum mire. Agricultural and Forest Meteorology, revised versionre-submitted in August 2001.

V Kellner, E. 2001. Governing processes for a Sphagnum mire surface water andheat exchange. Hydrological Processes, submitted in August 2001.

Nordic Hydrology (I) and John Wiley & Sons (II) kindly gave permission to reprintthe articles for this thesis.

In papers I and II, I made the measurements and the analysis, but shared the writing.In (III), the senior author made most measurements, analysis and writing, whereas Itook part in sampling data and in the writing.

7

INTRODUCTION

Mires cover large areas in the regions of the northern boreal biome and lower tundraand their climatic and hydrological processes are important from several points ofview. The interaction between land-surface and atmosphere is an important componentin the climate processes which gives impact on the regional as well as on globalclimate systems (Hutjes et al., 1998). In regions with a certain mire coverage, theregional runoff production largely depends on their properties (Roulet, 1990). Thewater in the peat surface layers also governs many processes in the mire ecosystems,such as photosynthesis and competition among species (Clymo and Hayward, 1982;Rydin, 1985), partitioning of the surface energy and water balances (Kim and Verma,1996), and the heat flux and temperature within the peat (den Hartog et al., 1994).Since wetness and temperature govern most biological processes also below thesurface, they are also strongly connected to emissions of greenhouse gases such asmethane and carbon dioxide (Bubier and Moore, 1994; Granberg et al., 1997). It isthus necessary to describe the surface-layer processes and the hydrological systems ofmires. In spite of this there is a lack of knowledge of mire hydrology, which restrainsdevelopment of physically-based models. One reason for this lack of knowledge is theshortage of comprehensive measurements. There are many measured time-series ofrunoff, water-table variation, climatic variables and also of surface energy balance.Nevertheless, coexisting measurements of water-budget flows and energy-budget,together with surface peat wetness, have been lacking until now, and for this reasonphysically-based analysis has not been viable.

In this thesis, I present and analyse results from concurrent field measurements at anopen, undisturbed mire. The measurements cover surface energy-balance components,water-budget components, and spatial and temporal variation of peat water content andtemperatures.

The overall objective with this study was to increase our understanding of theenergy exchange at the mire surface and the climatic and hydrological functions of amire. Within the study the more specific aims were to:

- quantify the components of the surface energy balance and mire water budget;- examine the temporal and spatial variation of heat and water storage within the

surface layers;- explore the dynamics of water storage and water-table movements in relation to

surface layer wetness;- relate properties of the surface layer to different processes such as the control of

evaporation and the significance of various wetness regimes of different micro-relief elements;

- identify the processes which govern the surface energy partitioning, and parametersthat can be used in physically-based descriptions of different functions;

- estimate the aerodynamic properties of the surface;- evaluate the use of TDR in peat soil and to test calibration functions;- estimate soil-water-retention capacities in surface layers of natural peat;

8

BACKGROUND

Mires, definitions and features

The term mire can have different meanings and the definition depends on the contextor the approach to the study of mires. It is used here as defined by Heathwaite et al.(1993) as a common expression for wet areas with a minimum peat depth of 30 cm.The further subdivision of mires can likewise be made in various manners dependingon the aims. However, the most explicit difference between different types is thesurface appearance, i.e. the vegetation, wetness, presence of free water or surfacetopography variation. Many of the properties of the surface appearance are coupled towater supply. The presence and density of different vegetation species, for example,often depend on mean position and fluctuation range of water table level, which in turndepend on the water budget terms. Apart from the amount of water supply, thepresence or absence of different plant species is further dependent on the acidity andnutrients content of the supplying water, which determine the trophic state of the mire.A rudimentary division can be made between fens and bogs, where the fens get at leastsome of their water supply from lateral inflow from surrounding mineral soil areas,whereas bogs are only supplied by precipitation. Depending on the geologic settings ofthe surroundings and the proportions of lateral water supply compared to precipitation,the fens can be divided into rich or poor. In practice, the vegetation type is oftenchosen as indicator between rich and poor fens since the correlation between water pH,mineral content and vegetation is not completely straightforward (Sjörs, 1956).

At poorer fens and at bogs, the soil layer is covered by peat mosses (Sphagnumspp.). There are many species of Sphagnum, specialised to diverse conditions. Unlikevascular plants, they do not have roots or internal water conducting tissues, but dependon capillary transport from underlying layers. Some species can retain and transportmore water up to their top than others. They are therefore more resistant to lowering ofthe water table, whereas others are more competitive in wet conditions. In manynorthern mires, the surface is heterogeneous (patterned) with a small-scale topographyconstituted by hummocks and hollows, ridges and pools. These micro-elements (ormicroforms in Ivanov, 1975) are inhabited and, in fact, made up by different species ofSphagnum mosses. The wetness varies with distance to the water table, and thehummocks and hollows are also inhabited by different vascular plants.

Aspects of climate, regional hydrology and biological processes

Studies of global and regional climates have identified that understanding of theexchange processes at the interface between land surfaces and the atmosphere is ofutmost importance for the accuracy in climate-change predictions (Garratt, 1993).Climate models are very sensitive to changes in surface properties in terms of thepartitioning of available energy at the surface into evaporation and sensible heat flux.These properties can not be assumed constant in time but very much depend on currentsoil surface moisture and transpiration capacity of plants. During the last decades,there has also been a thriving development of knowledge and methods within the

9

research on hydrology and land-surface processes. The possibilities to make moredetailed studies have increased with the enhanced technology of instruments andcomputers. The descriptions of natural systems have advanced with the increasingaccess to data and considerable research effort is heading towards models that aredescribing processes physically, with the ability to compare the inherent variables andparameters with measurable entities. This development has further improved thepossibilities to study processes more in detail. Better knowledge about the processes ofthe hydrological cycle and the interaction between different land surfaces and theatmosphere will lead to a better understanding and prediction of climate variability.Moreover, we will get better tools to solve problems with local and regional water-resource aspects, such as storage and runoff estimations for hydropower, dam-safety,erosion, water supply and water quality. However, despite many studies withindifferent ecosystems, the knowledge of how governing processes should beparameterised is still incomplete. Among the least studied and understood systemsherein, are the wetlands within the northern boreal and subarctic climate zones (Rouletet al., 1998; Verseghy, 2000).

Besides the climatic and hydrological aspects, the land-surface physical processesand properties of mires are crucial for the microclimate at the surface and within upperlayers of peat. Storage and fluxes of water and heat sets the environmental conditionsfor the biological processes. Plant functions are thus depending on the moisture andtemperature of the surface layers and as are the photosynthesis and respiration of thesurface mosses, probably to an even larger extent. Many ecological studies point outthe water-table level and moss water-retention capacities as major factors for thecompetition between plant species. The subsurface biological processes have alsoattracted great attention in last years, mainly because of the potential importance ofpeatlands as greenhouse gas emitters. The linkage between climate, soil climate andCO2 balance is close (Scanlon and Moore, 2000) and the methane production withinpeat is largely dependent on soil wetness and temperature in surface layers (e.g.Granberg et al., 1997).

It is therefore important to quantify the components of both water and energybalance, to understand how different factors influence these terms and how they can berepresented by physically-based models. Since the energy balance is tightly bound tothe evaporation, both storage and fluxes of water and heat at the mire surface layersshould be studied at the same time.

Mire hydrology; achievements and gaps

Besides from utilisation aspects, the research on natural mires has long been primarilyconcerned with botanical and ecological issues. In connection to these topics theimportance of hydrology and hydrochemistry has led to studies of water-tablevariations and water flow paths associated with different habitats. Classical mirehydrology includes studies of peat hydraulic properties, such as saturated conductivity,storage coefficients and water holding characteristics, and water level – runoffrelations (Romanov, 1968a; Boelter, 1969; Päivänen, 1973; Ivanov, 1975; Rycroft etal., 1975). Evaporation measured by lysimeters or estimated by water balance studies

10

has given information for deriving empirical functions for water-budget estimations,based on general climatic information and classification of mires (Romanov, 1968b;Ingram, 1983). In latest years, increased interest in protecting the ecosystems fromdrainage and to restore mires has led to further studies of peat hydraulic properties(e.g. Baird, 1997; van der Schaaf, 1999). Increasing insight in the need to treat themire as an element among other land-surface types within the regional hydrologycalled for further studies. Time series of runoff measurements were recorded andanalysed (e.g. Bay, 1969; Brandesten, 1987; Verry et al., 1988) and conceptual modelswere developed specifically for peatland areas (Ahti, 1987; Guertin et al., 1987).However, data are still scarce and the storage-runoff processes of patterned mires areintricate and not easily described (Price and Maloney, 1994; Quinton and Roulet,1998). In recent years, seasonal or part-seasonal atmospheric measurements havefacilitated evaluations on evaporation and surface energy balance (Lafleur and Roulet,1992; Price, 1991; den Hartog et al., 1994, Moore et al., 1994; Kim and Verma, 1996;Lafleur et al., 1997, Phersson and Pettersson, 1997). However, concurrentmeasurements of peat moisture and temperature, atmospheric fluxes, water tablevariation and runoff have been lacking.

The scarcity of data has made it difficult to identify the governing processes in theenergy partitioning at soil surface, which has been difficult to simulate, especially forsparsely vegetated mires (Comer et al., 2000). If there is a dense canopy of vascularplants, it dominates the exchange processes with the atmosphere and the energypartition largely depends on its transpiration capacity. Since the plants also areequipped with extensive root systems, they are seldom restricted by water supply(Lafleur, 1990). In many poor mires, the vascular plant cover is sparse and theproperties of the underlying Sphagnum moss layer is important for the energypartitioning of surface available energy. Their evaporative capacity should hencegreatly depend on peat wetness. Very little is known about the variation of peatwetness and its influence on the total evapotranspiration, though it seems to be crucialfor some types of mires (Price, 1991). It is also probable that the impact of surfacewetness varies during the seasonal development of vascular plants (Kim and Verma,1996). The heterogeneity of patterned mire surfaces with hollows and hummocksmakes it more complicated because of a spatial wetness variation and it is likely thatthis variation is further complicated by dissimilar temporal dynamics among thedifferent micro-relief element types. It is thus crucial to monitor the peat wetnessthrough the growing season in order to further our understanding. In order tophysically describe and model the wetness, the physical properties of the peat need tobe examined. The micro-relief and the sparse canopy may also produce aerodynamicproperties being unlike other surfaces. The influence of the soil surface appearance onthe aerodynamic properties needs to be explored and parameterised in order toestimate the atmospheric fluxes.

For hydrological modelling, the relations between peat wetness, water storage andwater-table level need to be properly described. To get an accurate description ofwetness variations in the surface layer, it is crucial to have the lower boundaryconditions properly reproduced, since the unsaturated zone is small. The traditionalmeasure of water storage and peat wetness is the water table of a single well. Peat-water storage is commonly calculated from a groundwater level and an estimated

11

storativity (Ivanov, 1975). Moisture distribution in the unsaturated zone is thenimplicitly assumed to depend only on the groundwater level. This assumption furtherrests on assumptions of a small spatial variability of the unsaturated water content andof a similarly varying water table over the whole mire. These assumptions are validwhen the surface is homogeneous and when there are no obstacles to redistributionbetween different micro-relief elements and subareas. However, there are indicationsof different water budget in different micro-relief elements (Price and Maloney, 1994).Differences in water-table levels may appear if water redistribution between the micro-relief elements is limited by a sharply decreasing hydraulic conductivity when thewater table drops to lower layers (Ingram, 1983). Wetting and drying of theunsaturated zone may also show a hysteresis effect, partly because of the ink-bottleeffect (Hayward and Clymo, 1982) and there are several reports of mire breathing, i.e.the surface moves in parallel with the water table during drying or wetting cycles(Ingram, 1983; Almendinger et al., 1986; Roulet, 1991; Schlotzhauer and Price, 1999).

MATERIALS AND METHODS

Site description

Measurements were carried out at the Stormossen mire at 60°07’N, 17°05’E, 45 mabove sea level (Figure 1), in central Sweden, 50 km north of Uppsala and 100 kmfrom the Baltic Sea. At Films kyrkby, 50 km from the site, the average (1961-1990)annual temperature is 5 °C, with January and July temperatures of –5 and 16 °C,respectively, and average annual precipitation is 696 mm (SMHI, 1996). The totalmire area is about 2 km2 and the main, open parts are undisturbed. The peat depth ismore than 3 m in large parts of the mire and at the centre of the western mound thepeat depth is 5 m. The underlying mineral soil is mainly constituted of glacial clay ofunknown thickness. At the western part, the surface topography is typically patternedwith ridges and hollows regularly following the surface and groundwater moundcontours, whereas the eastern part is more irregular. Hollows are dominated bySphagnum balticum, and Eriophorum vaginatum (Hare's-tail cottongrass) dominatesthe cover of vascular plants. Ridges and hummocks are dominated by Sphagnumfuscum and covered by low Ericaceae shrub dominated by Calluna vulgaris (Heather)and Empetrum nigrum (Crowberry). Individual hummocks have a typical width of 0.5– 1 m and are normally not higher than 20 cm. They occur mostly in more or lesscoherent complexes of varying sizes, sometimes as elongated ridges, where the heightof individual hummocks can reach 50 cm above the hollow bottom. The hollow andhummock/ridge vegetation types were evenly represented, when sampled in August2000. Mean dry weight of green biomass was 22 g m-2 of Ericaceae shrub and 33 g m-

2 for Eriophorum v. Total dead and non-transpiring biomass was 77 g m-2 (dry weight)and total (green) leaf area index was 0.3. The amounts of both green and non-transpiring biomass were greater on the hummock than in hollow, whereas the leafarea indexes were similar. The outer parts of the mire are covered with stunted Pinussylvestris with an understorey of Ledum palustre and Vaccinium uliginosum shrub.

12

The mire is bordered by forest in all directions but the surrounding landscape ispatchy, characterised by small agricultural fields and forest stands of various sizes.

In (III), parts of the measurements were also made at an adjacent, similar mire,Ryggmossen, (60°00’N, 17°15’E, 58 m above sea level). The surface appearance issimilar to Stormossen. Measurements were made there during the growing season of1994.

Figure 1. Aerial view of the mire. The tower for flux measurements is located at the origin ofthe coordinate system. “P” marks the location of ground measurements. “x” marks location ofrunoff measurements. Indicated are also calculated daytime (06-18 hours) distance for 90 %of the flux source area in each direction and the border to the forested areas. Principally,ridges and hummocks make up the dark parts of the pattern. The “stripes” in the north-western corner denote old peat cuttings (Copyright © National Land Survey of Sweden).

13

Methods

The energy balance for the bog surface is here taken as:Rn = H + LE + G (1)where Rn is the net radiation, H and LE are the sensible and latent heat fluxes,respectively, and G is the ground heat flux from the surface into the soil. In thisequation, the fluxes are treated as mean values for the total surface. The sensible andlatent heat fluxes were obtained by the Bowen-ratio-energy-balance method, usingmeasurements of Rn, G and profiles of air temperature and humidity. The evaporationE (this term cover both surface evaporation and plant transpiration) is then derivedfrom the latent heat flux term.

Measurements took place during two growing seasons, from 25 May to 4 October,1996 and from 13 May to 4 October, 1997. The fluxes of H and LE were calculatedevery 10 minutes and hourly averaged values were used in the analysis. Hourlyaverages of soil temperatures were used for the calculation of ground heat flux.Volumetric water contents were recorded once a day. The spatial variability of thegroundwater table was measured manually in a grid net of groundwater tubes twice aweek during 1996. In addition, groundwater table was monitored at a recording well.Precipitation and runoff were recorded every 10 minutes.

Air temperature and humidity were measured at 1-, 2- and 3-m heights (above meansurface level) with ventilated dry and wet thermometers on a temperature-interchangesystem, TIS (Lindroth and Halldin, 1990), mounted on a ladder. Values were omittedwhen the wind came from a northern sector, in which the forested border was tooclose. In the other directions, using a flux-source model proposed by Schuepp et al.(1990), the distance to areas with tree canopies was considered to be enough to getrepresentative flux values during daytime. The wind speed was measured at the sameheights as the temperature.

The components of net radiation were measured at a place representative for thesurface along a ridge-hollow transect at the centre of the mire (Figure 2). Surfacetemperature was estimated by radiation measurements. Ground measurements weremade in profiles along this transect (Figure 2). The temperature was monitored in soilprofiles down to 40-cm depths at the ridge and 20-cm depths at the hollow. Peat-watercontent was monitored with time-domain-reflectometry (TDR) technique in soilprofiles located along the transect at the depths of 5, 10, 20 and 40 cm in two profilesat the ridge and at 5, 15 and 25 cm depth at one profile beneath the hollow surface.The precipitation was measured at the ridge-hollow transect and runoff was recordedin a ditch at the southern end of the mire.

Since peat soils differ from mineral soil in several respects, the mineral-soil-calibration functions for TDR may not be applicable in peat soils. Therefore, sampleswere taken into laboratory (I) for analysis of relationships between water content (θ)and apparent dielectric constant Ka. The samples were taken both at hollow andhummock, they were 45 cm deep and their horizontal dimensions were 45 cm × 20 cm.The samples were also subjected to evaluation of the water-holding capacity of thepeat.

14

Ground heat flux was calculated from peat temperature gradients and moisturemeasurements (IV), using a thermal conductivity, which varied linearly with watercontent. An alternative method was used for some days in (III) to extrapolate thesurface ground heat flux from temperature measurements. Ground heat flux was alsomonitored with two pairs of heat flux plates, inserted 3 cm below surface. One pairwas placed in a hollow, the second pair in a hummock. Ground heat flux plates haveshown to give erroneous flux values in peat soils (e.g. Halliwell and Rouse, 1987) andwere used as complements to ground profile measurements. A comparison was madeof results achieved by the soil-profile evaluation and from the heat flux plates.

Figure 2. Topography and location of sensors along the ridge-hollow transect at site P in(Figure 1). Measurements included peat water content (×) and temperature (●) in profiles atthe ridge (RP1, RP2), ridge margin (RM) and in the hollow (HP), and groundwater levels atridge (WTR) and hollow (WTH). Vertical water-content probes and temporary tensiometers(not shown) were also installed. The datum was selected to coincide with the surface of thehollow when the WTH tube was installed. Groundwater levels were measured relative to thetop of the tube. (from II).

Evaluation of surface properties (III, IV)

A standard tool to estimate evaporation E is the Penman-Monteith (P-M) model(Penman, 1953; Monteith, 1965), which describes the surface as a single evaporatinglayer:

))((

))(()(

rr1∆L

r

eTeCGR

E

a

s

a

spn

++

−+−

=γ

ρ∆

(2)

where ∆ = rate of change of saturation vapour pressure with temperature, ρCp = airheat capacity, T = air temperature, e is actual vapour pressure, and es(T) is thesaturated vapour pressure at 2 m height, L = latent heat of evaporation, γ =psychrometric constant, rs = canopy (and surface) resistance, ra = resistance to transferthrough the atmosphere. This model was chosen to evaluate the factors limitingevaporation. The surface resistance rs was calculated by solving Eq. (2), using themeasured value of E. The bulk surface resistance rs is the result of resistance to

-40

0

40

80

2 4 6 8 10 12 [m]

[cm

]

RP1RP2

HP

WTRWTH

RM

15

transpiration exercised by plant stomata and to direct evaporation by mosses andplants. It is used here to quantify how the mire surface controls the evapotranspiration.

Estimation of ra is made by the sum of ram, which is the resistance for momentumtransfer, adjusted for surface layer stability, and rb, the excess resistance for mass andheat transfer (Stewart and Thom, 1973):

Bukuz

dz

rrr 1mm0mv

bamm

va

−+

−

=+=**

)ln(Φ

ΦΦ

ΦΦ (3)

where k = von Kármán constant, u* = friction velocity (uncorrected for stability), z =

reference height above surface (here 2 m), z0m = roughness length for momentum andd = displacement height. Φv and Φm are stability functions for vapour and momentumexchange, estimated by formulae recommended by Högström (1996). The transfer ofmomentum from a surface is enhanced by bluff-body effects, which do not affect thetransfer of mass or heat. To estimate the aerodynamic resistances for heat and watervapour from the aerodynamic resistance for momentum, an additional resistance has tobe introduced (Thom, 1972). The additional resistance involves a dimensionless factorB-1, which in the form kB-1 was estimated for Stormossen and Ryggmossen in (III).

)ln(H0

m01zz

kB =−(4)

where z0H = roughness length for heat (or mass) transfer. The variation of rs wasevaluated, especially its relations with peat wetness and atmospheric conditions. Themeasured E was also compared with the potential evaporation, Ep, of a wet surface asgiven by a version of the Penman (1948) equation.

The equilibrium evaporation Eeq, given by the first part of Eq. (2),Eeq = ∆ (Rn-G)/L(∆ + γ), represents evaporation from a sufficiently large, wet surfaceinto an air mass that has come into equilibrium with the surface. The Priestly andTaylor (1972) model, E = α × Eeq, was also evaluated in (IV).

Evaluation by model simulation (V)

A one-dimensional soil-vegetation-atmosphere-transfer (SVAT) model was used tosimulate the system of surface layers in order to test descriptions of soil and surfaceproperties derived from literature and measurements and to identify the processesgoverning the surface energy partitioning. The COUP-model (Jansson and Karlberg,2001) is a coupled model of both soil physical and biological processes, whose abioticparts are based on the SOIL-model (Jansson and Halldin, 1979). The model simulatesone-dimensional water and heat flows in the soil-vegetation-atmosphere system byusing physically-based algorithms. Soil profile descriptions were derived fromliterature and laboratory results. The formulations given by van Genuchten (1980)yielded the parameters for the pore-size distribution and water-content-tensionrelation. Calculation of the unsaturated hydraulic conductivity k(θ)w was made by theexpression given by Mualem (1976), which relates k(θ)w to pore-size distribution. Soilheat flux is calculated as the sum of conduction and convection by liquid-water and

16

vapour flux. The same thermal-conductivity function was used here as in measurementevaluations. For the surface energy exchange, the model solves the energy-balanceequation for separate layers, one ground layer and (if needed) several canopy layers.There is no interaction between layers and the fluxes to and from the layers go inparallel.

The model was run with a simplified water storage-runoff function, where themeasured level of water table was chosen to be a driving variable. The chosen soil-water parameter settings were tested by comparing simulated water contents withmeasurements. They were then adjusted if needed. Estimation of parameter valuesdescribing soil surface layer, leaf area index and aerodynamic resistance properties,were made by using the root of mean squared errors of soil temperature at 5 cm depthas criterion. Two sets of driving variables were prepared, one from 15 June to 4October 1996 and one from 1 June to 4 October 1997. Both sets consisted of hourlyvalues of air temperature, wet-bulb air temperature, wind speed, global radiation, netradiation, and precipitation.

RESULTS

Peat properties and TDR calibration (I)

The water-retention characteristics found in (I) conformed to results found by otherauthors (Figure 3), even if the variation between samples could be significant. In thesurface layer, with living mosses, the moss species composition was important, but inlower layers, the degree of humification and bulk density seemed to be dominatingfactors. There were also signs of pressure-induced compression effects. The surfacelevel of the hollow samples dropped on average 2 cm when the water table waslowered 25 cm. With an original sample height of 40 cm this means a compression of5 %. However, the surface levels in the hummock samples did not change notably.

The dielectric properties of the peat turned out to be significantly different frommineral soil and also deviated from other published Ka(θ) relations for peat by havinggenerally lower Ka values for the same θ values (I). There were also largedissimilarities in the Ka(θ) relation between different samples and there were signs ofinfluence by the peat’s physical properties as the less decomposed samples yieldedlarger values of Ka at the same water content. The samples were then put in differentclasses depending on degree of humification.

An empirical polynomial model was easier to fit to humification-classified groupsof data than theoretical models. However the two types almost equivalentlyreproduced pooled data. The ability to include physically-based temperatureadjustments gives an advantage for theoretical mixing models over polynomial, forfield measurements with substantial temperature fluctuations. A three-phase mixingmodel (Roth et al., 1990) was thus chosen to be used in the evaluation of field andlaboratory measurements.

17

Figure 3. Water holding characteristics for laboratory peat samples and from literature. Alldata are established during a drying sequence starting with completely saturated samples.(from I).

Water budget, water storage and water-content variations (II)

The evaporation ranged from 1.0 to 3.5 mm d-1 and averaged 2.1 mm d-1 during June-August 1996. The evaporation rate for June-August 1997 averaged 2.4 mm d-1. Forboth years, evaporation rates increased from May to July and decreased thereafter untilearly October, when measurements were halted. The summer discharge was lowexcept for flows after large rains. Runoff ceased totally during long dry periods.Accumulated seasonal (24 May to 4 October) rainfall, evaporation, and runoff were200, 256, and 43 mm in 1996, respectively, and 310, 286, and 74 mm in 1997.Consequently, the water budgets were negative in each year, -99 mm in 1996 and -50mm in 1997. The highest summer deficits were 109 mm on 25 September 1996 and107 mm on 16 August 1997. Within the tree-covered areas at the border next to theditch with runoff measurements, the larger surface roughness and greater leaf areaindex would result in larger interception losses and more transpiration. The borderareas may then have acted as sinks for the outflow from the open area, which would beunderestimated. This error was estimated to cause an average increase of the open-area-storage deficit of 20 % during a dry period.

The groundwater level varied similarly over most of the bog. The manual dip-wellsoundings showed that the water-table movements were homogeneous over the openbog area throughout the season. The main exceptions from this uniform motionoccurred at the mire borders and in the vicinity of drainage ditches, mainly during dryperiods. The largest amplitude (from min to max) was about 30-40 cm and occurred atthe drained sites. The effect of the drainage was minimal 80 m from the ditch. This

0.1

1

10

100

1000

10000

Volumetric water content

Hummock 5 cm depth

Hummock 10 cm depth

Hummock 15-25 cmdepthPäivänen, 1973

Hollow 5-10 cm depth

Hayward & Clymo, 1982(Sphagnum capillifolium)Hayward & Clymo, 1982(Sphagnum papillosum)

0 20 40 60 80 100[%]

18

implied that the area close to the ditch could influence the short-term storage, but notsignificantly affect the long-term total water storage of the bog.

Lowering of groundwater levels at the ridge and in the hollow was similar.Probably, there were small lateral flows, which evened out differences in water tableheight. However, the response to rain was more pronounced in the hollow, with asharp rise of water level, followed by a decline.

There were differences in the temporal variation of water content among hollow andridge profiles. The ridge profile had very small changes in water content in upperlayers and the largest variations took place just above the water table, while the hollowvolumetric water content at 5 cm depth varied from saturated down to 50 % (Figure 4).The water content at 5 cm in hollow was always greater than it was in the upper 20 cmof the ridge. However, there were signs of sharp water content gradients and somevisual observations of hollow mosses drying out occasionally. The temporal variationsof water table and surface peat-water content from TDR measurements were generallymatching, but there were several situations when they were not. The relation betweenwater content and groundwater level was thus hysteretic, most pronounced in the peatlayers just above the water table and least in the ridge surface layers (Figure 5).

Figure 4. a) Water table levels and b) hollow and ridge (RP1) profiles volumetric peat watercontents at the Stormossen mire in 1996, (dots or crosses) and 1997 (solid lines). Water tablelevels are related to a fixed level equal to the surface of the hollow at the time of installation.(from IV).

19

Heat storage, temperatures, heat-flux estimations (IV, III)

There were great differences in the peat microclimate among the different micro-reliefelements. The diurnal soil temperature variation was generally less in the hollow thanin the ridge at corresponding depths (IV). This probably depended on better contactbetween the hollow surface and saturated peat layers with large thermal capacity.There was a notable difference between surface temperature and air temperatureduring daytime. The difference between surface and air temperatures could not becorrelated to the variation of measured peat wetness. However, there was a spatialvariability with up to 10 °C found (III) at the Ryggmossen bog surface, although thespatial differences were not complemented with any surface description ormeasurements of surface wetness.

Figure 5. Simultaneous water table levels and volumetric water contents in the ridge (RP2;top) and hollow profiles (HP; bottom) at bog Stormossen in 1996 (×) and 1997 (●). Arrowspoint out the course of change during drying (leftwards) or wetting (rightwards). Water-tablelevels are related to a fixed level equal to the hollow surface at time of installation. (from II).

30

40

50

60

70

80

90

100

-250 -200 -150 -100 -50 0 5050

60

70

80

90

100

20 cm depth

30 cm depth

5 cm depth

Ridge

40 cm depth

5 cm depth

Hollow

20

The daily averaged temperatures at the 5-cm and 10-cm depths were similar in allprofiles, the main exception was a faster warming of the hollow in early summer. Indeeper layers, the temperature regimes were more disparate (Figure 6). The hummockprofile cooled down more during winter and remained cold or frozen substantiallylonger in the spring than the saturated hollow profile. The greater cooling of hummockcould have been caused by unevenly distributed snow i.e. the hummocks are lessinsulated. However, the lateral heat flux to and from the hummocks was probably amain contributing factor. In mid-May 1997, there was a substantial rise in temperatureat 40-cm depth in the ridge while the 20-cm depth was still frozen. The increase oftemperature was probably caused by lateral heat flux. The horizontal gradientcorresponded to a considerable lateral heat flux, with a daily mean of the same order ofmagnitude as the surface heat flux. There was also a possible contribution ofconvective flux due to water flow but the size could not be determined.

The measured heat flux in the ridge was lower than in the hollow. The highesthourly values were 110 W m-2 for the hollow site and 60 W m-2 for the ridge site. Bothoccurred in May 1997 when surface temperature gradients were largest. At both sites,the estimated daily mean ground heat fluxes were at a maximum in May and decreasedthrough the summer to a negative value by mid-September. The calculated profilefluxes were larger than the values acquired from the heat-flux plates (HFP). To getsimilar values of fluxes, the ordinary heat-flux-plate calibration coefficients had to bemultiplied by 1.31 and 1.73 for the hollow and the ridge respectively. No distinctlinkage was found between groundwater level, or water content at 5 cm, and the sizeof heat-flux-plate underestimation. The underestimation by heat-flux plates wascomparable to the results by Halliwell and Rouse (1987), who found the quotientbetween calculated fluxes and heat-flux plate readings varying from 1.4 to 2. Thealternative calculation method of the ground heat flux at this site (III) by making aFourier-series fit of temperatures, gave about 3 times and 2 times higher values thanthe heat-flux plates at the hollow and the ridge, respectively. The surface-temperaturevariation, which is important in that method, is possibly overestimated by using data ofupward long-wave radiation, whose source is partly made up by the plant cover. Thedaytime flux calculated by this alternative method was larger than that obtained byordinary calculations but agreed with the simulations. However, the largest deviationsoccurred for night-time flux (Figure 7).

Aerodynamic properties (III, IV)

In the analysis of wind profiles in neutral conditions, the zero displacement height d was notfound to be significantly different from zero. This may be due to the low and sparse canopyand that the amplitude of the ridge-hollow topography was not sufficiently large to give adisplacement effect. The roughness lengths for momentum z0m were lower for the eastern andwestern sectors (2.1±1.0 cm) than for the southern sector (3.2 ± 1.0 cm). There should not beany fetch-related problem in profile measurements and the roughness length estimates basedon the measured profiles were assumed correct. The kB-1 factor took the form: kB-

1=1.58Reo0.25-3.4 (III). Omitting the dependence on Reynolds number, Reo, gave a constant

value of 3.2 for kB-1. The relationship with Reo lays in-between the bluff-rough andvegetated-

21

Figure 6. Peat temperatures at three depths in the ridge profile and at two depths in the hollowduring the growing seasons of 1996 and 1997. (from IV).

Figure 7. Ground heat fluxes on 20-21 August 1996 at Stormossen measured with heat-fluxplates (HFP) (III and IV), calculated with traditional method from profile measurements (IV)(profile), calculated with the novel method used in (III) (novel method), and simulated (V);(a) in hollow; (b) in hummock.

0

10

20

0

10

20

5 cm20 cm40 cm

0

10

20Tem

pera

ture

s [°C

]

0

10

20

May Jun Jul Aug Sep May Jun Jul Aug Sep

Ridge 1996 Ridge 1997

Hollow 1996 Hollow 1997

22

surface cases. No significant changes were found in roughness during the season, thuschanges of ra only depended on the atmospheric conditions (IV). During daytimehours, the average value of ra was about 60 s m-1, with more than 80 % of daytimevalues below 80 s m-1. The aerodynamic resistance, calculated by ignoring the bluff-body effect and stability corrections, had little correlation with the values of raachieved by considering these effects (r2=0.26).

Energy partition and Bowen ratio (IV)

The available energy from Rn was largest in early summer but varied from day to daydepending on cloudiness. The relation between sensible and latent heat, i.e., theBowen ratio, decreased from about 0.9 in May to about 0.6 in July, whereas theevaporative fraction (= LE / (LE+H)) increased during the season (Table 1; Figure 8).The trend was similar in both years but the Bowen ratio was a little higher in July andSeptember 1997 than in the corresponding months in 1996. The net radiation had asimilar diurnal variation in early summer as during midsummer, but there was adissimilar pattern of energy partition between sensible and latent heat (Figure 9). Theearly summer forenoon sensible heat flux was particularly high whereas the latent heatwas clearly larger from noon onwards at midsummer. There was no increase ofsensible heat during dry periods, but the Bowen ratio decreased slightly withdecreasing water content.

Figure 8. Daily mean fluxes of net radiation (Rn), latent heat (LE), sensible heat (H) andground heat flux (G). (from IV).

0

100

200

0

100

200

Flux

es [W

m-2

]

RnLEHG

1996

1997

Jun Jul Aug SepMay

23

Figure 9. Mean diurnal variation of net radiation (Rn), latent heat (LE), sensible heat (H) andground heat flux (G) during two periods (13-31 May and 1-31 July 1997) and of Bowen ratioduring four different periods. (from IV).

The surface resistance for E, rs, had a small variation and when Rn-G > 100 W m-2, itwas normally within the range 100-300 s m-1 with mean value of 160 s m-1. The day-time surface resistance in May 1996 and 1997 was lower than in the wet period of July1996, whereas it was higher during the dry spell in August 1996. However, thecorrelation between rs and peat wetness or water-table level was low. There was astrong trend of rising rs with increasing water-vapour deficit, and the resistance wasgenerally larger on days with previous large deficits. The surface resistance often hadits diurnal maximum in the late afternoon or evening (Figure 10) when vapour deficithad passed its daily maximum. In spite of these hysteresis effects, the relationshipbetween rs and vapour deficit still dominated over the influence of other factors. Thesurface resistance did not increase with increasing available energy. Instead there wasa slight negative relationship between Rn-G and rs.

-100

0

100

200

300

400

500

0 4 8 12 16 20 24

Flux

es [W

m-2

]Rn May 1997LE May 1997H May 1997G May 1997Rn July 1997LE July 1997H July 1997G July 1997

0

0.5

1

1.5

2

0 4 8 12 16 20 24Local (solar) time of day

Bow

en ra

tio

13 May - 1 June 19975 - 25 July 1997

Mid September199728 Sept - 3 Oct 1997

24

The evaporation was steadily lower than Penman potential evaporation Ep. The ratioE/Ep varied little between different periods (Figure 10), and mean values increasedfrom 0.61 in May to 0.77 in July, 0.74 in August and 0.72 in September 1996 (basedon hourly values when Ep > 4 mm day-1). This ratio did not vary with the variation ofwetness and was close to 1 only when both available energy and vapour deficit werelow.

There was a strong linear relationship between equilibrium evaporation (Eeq) andmeasured E (Figure 10). The mean value of the ratio E/Eeq, corresponding to the termα in the Priestly-Taylor model, was 0.8, slightly increasing through the season butnotably stable. Moreover, the α value was stable through the day.

Table 1Monthly mean values of different fluxes (W m-2). The energy balance equation may not tallydue to the rounded values. (from IV)

Month Groundheat flux,

hollow

Groundheat flux,

ridge

Netradiation

Evapotrans-piration

Sensibleheat

Bowenratio

Evaporativefraction

June 1996 12.7 6.6 122 62 51 0.82 0.55July 1996 6.1 3.0 107 65 39 0.60 0.62

August 1996 4.9 1.5 102 61 38 0.62 0.62September 1996 -3.4 -2.1 51 33 20 0.61 0.62

May 1997 11.0 13.6 116 55 49 0.89 0.53June 1997 10.9 6.9 108 58 47 0.81 0.55July 1997 7.3 2.5 134 75 58 0.77 0.56

August 1997 4.5 1.2 106 63 41 0.65 0.61September 1997 -3.9 -2.3 45 34 14 0.40 0.71

Simulations (V)

The initially used soil parameters, derived from literature and laboratory studies (I),resulted in too low water contents in most hummock layers and had to be adjusted.

The moss surface had to be simulated as continuously moist and evaporating duringboth wet and dry periods to avoid overestimation of peat temperature. This was trueespecially for the hollow profile, while the hummock could optionally have a slightlydryer surface, but then needed to be combined with a smaller aerodynamic resistance.Otherwise, both profiles needed a continuous large resistance to avoid too greatdifferences between calm and windy situations. The above-surface descriptions weresimilar for the simulated hollow and hummock profiles. The aerodynamic resistancefor heat transport from the soil surface had to be large, about 250 sm-1, whereasresistance from the canopy was about 100 sm-1. The simulated partition of availableenergy at the soil surface into sensible and latent heat varied with the soil surfacewetness. In all simulations with a constantly moist surface, the evaporation LE (i.e.,the latent heat) was clearly greater than the sensible heat flux, H, during both wet anddry periods (Figure 11) whereas a slightly dryer hummock version described a morevarying partition depending on the surface wetness.

25

Figure 10. a) Mean diurnal variation of surface resistance rs to evaporation (± SD) duringdifferent periods, b) ratio of hourly values of measured evaporation, E, and Penmanevaporation, Ep, against values of Ep, and c) measured evaporation E against equilibriumevaporation Eeq. (from IV).

a) 0

200

400

600

800

4 8 12 16 20Local (solar) time of day

Surfa

ce re

sist

ance

[s m

-1]

May 1996 and 1997July 1996August 1996September 1996

b)0

0.4

0.8

1.2

0 4 8 12E p [mm day-1]

Rat

io o

f E to

Ep

May 1996June 1996July 1996August 1996September 1996

c)-2

2

6

10

0 4 8 12E eq [mm day-1]

E [m

m d

ay-1

]

May 1996June 1996July 1996August 1996September 19961:1 Line

26

For the canopy layer, on the other hand, H dominated over LE in all simulations. TheBowen ratio, B = H/LE, was higher and varied more for the dryer profiles than for thewet surfaces. Both surfaces agreed with the measured during wet periods, whereasmeasured B was lower than for the dry surface and higher than for the wet during dryperiods. Even if the scatter was large, the tendencies were constant through theseasons, i.e., there were no signs of changing properties of vegetation with time.

The simulated soil-surface temperatures were similar to the measured surfacetemperature, which included the canopy temperature as well. However, the variation insimulated canopy temperature was greater with up to a 10 °C difference from themeasured.

Even though the wetness was well described with the adjusted soil parametervalues, the hummock subsoil temperature variation was poorly described by using thebest-fitting parameter-value set for 5 cm temperature. Much larger soil heat fluxeswere needed to match the measured temperatures at 20 and 40 cm depth, and none ofthe used parameterisations could simulate the subsoil variations properly.

The description of hollow soil-water content variation was generally good and sowas the description of temperature, although differences between wet and dry periodswere overestimated. The simulated ground heat fluxes, G, were similar to thoseestimated from measurements, the largest deviations occurred during nights(Figure 7).

Figure 11. Simulated fluxes of LE and H from soil and canopy, respectively, and in total (soil+ canopy). Mean values from the period 11-20 July 1997. Hollow profile (from V).

-50

0

50

100

150

200

0 4 8 12 16 20 24

Time on day

Sim

ulat

ed fl

ux (W

m-2

) Total LETotal HSoil LESoil HCanopy LECanopy H

27

DISCUSSION

Energy partition in comparison with other studies

In comparisons with other surfaces, the evaporation from this mire was considerablyrestricted. Kelliher et al. (1995) stated a typical value of bulk surface resistance forordinary non-wetland, natural surfaces to be 50 s m-1. In comparison with other openboreal and arctic systems, the mire of this study gives Bowen ratios in the higher endof the spectrum (B = 0.2-0.7) of boreal wetlands (Eugster et al., 2000). The finding ofa restricted E agrees with other studies of poor-mire systems where the surface is notsaturated and the canopy is sparse (Price, 1991; Lafleur and Roulet, 1992; den Hartoget al., 1994; Phersson and Pettersson, 1997). In studies of mires with denser canopies,Bowen ratios are typically found to be about 0.5 when leaves are developed, otherwiseranging between 0.2 and 1, depending on the surface wetness and climatic conditions(Lafleur and Rouse, 1988; Moore et al., 1994; Lafleur et al., 1997). The few availablevalues of mire bulk surface resistance are generally lower; Kim and Verma (1996)measured average rs = 100 s m-1 at a Sphagnum fen (leaf area index was 0.4 - 0.7);Lafleur and Rouse (1988) measured rs = 90 s m-1 at a subarctic marsh. The compositionand density of vascular plants in this study are similar to those in open tundraecosystems (upland moist, non-shrub tundra according to Eugster et al., 2000) withtussocks of Eriophorum vaginatum and sparse growing Carex spp., Betula nana, Salixand Ericaceae shrubs (McFadden et al., 1998; Vourlitis and Oechel, 1999). The largestdifference in energy partition between these and the present study is the colder climateof the tundra. The strong heat sink of permafrost gives a larger ground heat flux in thetundra systems (G/Rn = 0.12-0.24, Eugster et al., 2000), and the Bowen ratio isgenerally also higher (B = 0.8-1), probably depending on colder air. However, therelationships to equilibrium evaporation, given by the ratio E/Eeq, are similar to thosein this study, and the bulk canopy resistances rs = 100-180 s m-1 are also similar(McFadden et al., 1998; Vourlitis and Oechel, 1999; Eugster et al., 2000).

The regulation of surface energy exchange

The partitioning of available energy was relatively constant over the seasons. Thechanges in Bowen ratio were caused rather by atmospheric properties than by soilmoisture. The surface resistance, calculated by the P-M model (Eq. 2), increasedduring dry periods but its variation was largely linked to the vapour-pressure deficitVPD (VPD = es(T)- e). There was a diurnal hysteresis with larger rs in afternoons whenVPD decreased from its daily maximum. There can be various contributing processesbehind this relation. Despite their abundant access to water, the vascular plants closetheir stomata when exposed to increasing vapour-pressure deficits (Blanken andRouse, 1996; Takagi et al., 1998). Takagi et al. (1998) showed that diurnal hysteresisoccurred for all of their surveyed vascular wetland species and that the rs / VPDrelation varied with the size of daily maximum deficits. The surface mosses can alsobe a source of increasing rs with increasing evaporative forcing, if their capillarytransport capacity is too low. Hence, the moss surface may become drier in afternoons

28

during days with large evaporation. Since the resistance did not change much betweendry and wet periods, the evaporation may be attributed to the vascular plants.However, when the plant transpiration was estimated by assuming reasonable valuesof stomatal conductance (IV), it was clear that a considerable part (55-70 %) had tocome from the moss surface. The small difference between wet and dry periods is thenpeculiar. There can be two different explanations to these measurement results. One isthat the soil-surface resistance to evaporation is relatively constant through time. Inother words, the surface moisture has to be kept sufficiently high to withstand aconstant evaporation rate even through dry periods. Another explanation is that thesurface moisture can vary significantly between wet and dry periods, but the total fluxfrom soil and canopy would be constant in time because of small-scale advectioneffects. During dry periods, the dryer surface would get heated and then supply thevascular plants with extra heat, which would boost their transpiration (Kim andVerma, 1996). The relation between total sensible and latent heat fluxes would then berelatively unchanged. Although the surface moisture was not measured, there werevisual observations of some hollow mosses drying out during dry spells and it isprobable that they did under these circumstances (Clymo and Hayward, 1982; Ingram,1983; Schipperges and Rydin, 1998). This fact supports the latter hypothesis.

However, in the simulations, the soil surface of both hollow and hummock had tobe kept continuously moist (or at least almost freely evaporating) to avoid overheatingof the surface peat layers. This rather supports the first hypothesis. The simulationscould also help to reveal the anomaly of the relatively great bulk surface resistance toevaporation despite the (constantly or occasionally) wet surface. The soil surfaceexperiences a great aerodynamic resistance to the ambient air mass. Despite thesimulated evaporation totally dominated the partitioning of surface available energy,the resistance to vapour transport from the surface through the canopy layer produceda net effect of a large bulk surface resistance when the single-source model was usedfor analysis. The model also attributed the sensible heat flux to the canopy layer. Evenif the canopy was sparse, its smaller aerodynamic resistance made the contributions ofenergy flux from the soil and canopy almost equally large.

Since the model did not simulate any interaction between soil and canopy layer, thevalidity of this model solution is uncertain and the small-scale-advection theory maystill be valid. In the survey made in (III), the spatial variation in surface temperaturemay indicate differences in surface wetness. Rydin (1984) concluded that the mosssurface water content was the most significant factor for surface temperature. On theother hand, the measured peat temperatures called for an evaporating surface,otherwise they would have been higher.

There is thus a large anomaly between indicated variations in soil surface wetnessand the apparent need of a constantly moist surface. The results from the analysis ofmeasurements and simulations concerning the surface energy partitioning correspondvery well to earlier failures of simulations (Comer et al., 2000), where differencesbetween wet and dry periods have been overestimated. The physical conception ofmosses and their functions that we have today seems to be erroneous and there is aneed for more detailed studies. Test simulations in (V) were made with a dry surfacebut with enhanced soil vapour transfer. It turned out that if vapour transfer wasenhanced to be five times greater than had it been subjected to only diffusion, the soil

29

temperatures agreed with measurements. It is not known if such large fluxenhancements are likely to exist but there could be various processes to cause sucheffects (Alvenäs, 1999). There could be other phenomena. The mosses could berewetted on the outside of their stems and leaves by dew every night sufficiently to beable to create a capillary continuum from lower layers to the surface until theafternoon, in agreement with the afternoon rise in surface resistance. There could alsobe effects of aerodynamic processes that were not considered in the analysis, likedifferent stability phenomena in the layer close to the ground or effects of small-scaleadvection not only between surface and canopy but also between different micro-elements.

The hummock-hollow concept

There was a great difference in the peat microclimate among the different micro-reliefelements, but the impact on surface energy partition does not seem to be great. Thesimulated processes were also very similar in terms of energy partitioning and above-surface parameterisation. On the other hand, the influences of topography andhummock-ridge dimensions on heat and water exchange are important issues, whichneed to be further explored. Since the horizontal and vertical dimensions of thehummock are of similar order of magnitude, the energy (and presumably also water)exchange of hummocks can be assumed to be largely influenced by lateral fluxes. Thesoil heat flux at the hummock surface was lower than at the adjacent, flat hollow, butbecause of lateral fluxes the net energy storage seemed to be as great as in the hollow.There was a bad agreement in simulated hummock temperatures, which most likelydepended on significant fluxes of heat going through the sides of the hummock. Theuse of a one-dimensional model to simulate hummock peat temperatures was thus notsuitable. Test simulations with increased thermal conductivity indicated that groundheat flux had to be increased by about 70 % to reach the measured temperatures. If wecould better describe the effects of topography and hummock dimensions, the energystorage could be more properly described and the problems of a non-closing energybalance (III, Lafleur et al., 1997) could perhaps be solved. In addition, peattemperature, peat wetness and quality of peat are crucial for biological processes.Therefore, the distribution of micro-relief elements has to be considered for a correctdescription of greenhouse-gas production and carbon balance.

The water-table variations were similar all over the open area of the mire at this site,among both hummocks/ridges and hollows, in contrast with the results of Price andMaloney (1994). The micro-relief of the mire in their study was also moredistinguished. Anyway, the processes behind the even water-table movements are notclear. There is probably a redistribution going on more or less continuously to even outdifferences in water level, caused by differing water budgets and storativites, amongmicro-elements. The sudden rise and subsequent drop in water table after rainstorms atthe hollow but not in the hummock is in contrast with findings in other studies (e.g.Sjörs, 1948; Ingram, 1983). The conclusions from these other studies were that thewater table at a hummock should rise more sharply after a rain than at a hollowbecause of a smaller storativity. However, when the water level is in its lower range,the storativity of the hollow peat may be as small as in the hummock. The sharp fall in

30

water table just after the rise may then depend on absorption by the peat, turning freewater into bound water. The greater unsaturated zone of the hummock may dampenthis effect. Ahti (1987) found the same effect with a sharply rising groundwater levelin recording wells after rainstorms followed by a decline. He proposed that it was anartefact by the method since there were reports that this effect could not be seen intensiometer measurements. However, in this study, tensiometers situated closer to therecording well showed similar water-table dynamics as in the well.

Physical properties of peat

A higher humification means more fine particles and, consequently, more water at agiven tension. The wetness close to the fluctuating water table is a favourableenvironment for decomposition. This is in line with the increase in retention strength atthe hollow 5 – 10-cm depth, below the layer of living mosses, and also in the lowerhummock layers. Weiss et al. (1998) suggested that the level relative to the surfaceshould be a significant factor for water-retention properties, but the level relative to themean water table level seems to be more important for surface layers.

Since there are few studies on water-retention capacities of surface layers, thedeviations between the laboratory-study results, literature values and adjusted valuesneeded for simulations can be within normal variability. The presence of peat elasticityphenomena is one partial explanation for the simulated water content beingunderestimated. The hydraulic conductivity, derived from literature, also had to beincreased in order to give acceptable simulations of soil-moisture variation. TheMualem (1976) function, which was used here, may be a less useful description forundecomposed mosses, since it is developed for granular or aggregated soils. Themosses are transporting the water in continuous films along the outside of their stems,functioning like wicks. The thickness of water films varies with varying water contentbut paths are the same, which may give a smaller decrease of hydraulic conductivitywith drying than the one given by the Mualem function.

The results from the TDR calibration confirm that there are large differencesbetween peat soils and mineral soils in dielectric properties, but also indicate thatdiverse peat properties can cause different Ka(θ) relations. Differences in amount ofbound water and structural orientation are probable causes to the diversities in theKa(θ) relation among the different samples. Useful parameters expressing these entitiescould be humification, combined with bulk density, structure and orientation.However, apart from bulk density, there is a lack of readily used methods to measureand quantify these properties.

Further studies on physical properties of peat soil are needed in order to usetheoretically deduced functions properly. This is true both for describing soil hydraulicproperties and attaining a better description of dielectric properties. In addition, due tothe heterogeneity of the peat, you cannot tell the soil properties from its location only,but you have to make a destructive analysis.

The measured water content varied during the seasons in deeper layers, that werecontinuously saturated (II). These anomalies could not be explained by shrinking-swelling effects. Probably, there were effects of temperature changes during early-summer and fall periods. The temperature dependence of dielectric value of peat soils

31

could deviate from the theoretical estimations (Pepin et al., 1995) and there arefindings of temperature-induced changes in volume of methane bubbles (Fechner-Levy and Hemond, 1996). In connection with the lower water table in July-August,pressure induced changes in bubble volumes could also cause a lower water content(Reynolds et al., 1992; Fechner-Levy and Hemond, 1996). However, there was nopossibility to quantify the effect of methane bubbles.

Water storage

There was a considerable hysteresis in the surface-layer water-content / water-tablerelation (II). The hysteresis can be explained both by pore-throat limitations and byelasticity/compression effects. The elasticity of the peat can change the relationbetween water table and peat-water content in two ways. First of all, bulk density maychange with tension, i.e. in the unsaturated zone, the bulk density may change withwater-table movements while the water content remains constant. A second effect ofthe elasticity is compression within the groundwater mound, “mire-breathing”. Thecompression causes the surface layer to move up and down with the water table, withonly small changes in the surface-water content.

Apart from pore-throat limitations, the hysteresis in the relation betweengroundwater level and water content was probably caused by compression both in thesurface layers and in the underlying saturated layers. Whereas the compression withinthe surface layers could not be determined, the amount of water storage in the surfacelayer could. The mean surface-layer storage was about 60 % of the net sum of waterfluxes. For a typical summer deficit of 100 mm, 60 mm would be depleted from thesurface layer and 40 mm from compressive storage in the deeper layers.

The consequences are that these effects have to be considered when water-contentvariations in the surface layer should be described and that there is no one-to-onerelation between groundwater level and water content. This is important for thedescription of surface-layer water content and for the description of the storage-runoffrelation. There is also a hysteresis effect in the swelling and shrinking of the peatmatrix under variations of pressure (Schlotzhauer and Price, 1999). The mire surfacemovements always are, hence, coming later than the water table, causing larger runoffduring wetting sequences than during drying for the same water-table level. This effectcan be remarkable since the relation between storage and runoff is delicate with just afew cm in groundwater level between seasonal maximum and total cease of waterflow, corresponding to a sharp decrease of hydraulic conductivity with depth.Meanwhile the hydraulic conductivity may as well vary because of the shrinkage andswelling within the porous medium. In addition to pore size changes, there are findingsof increased clogging of pores by swelling methane bubbles during pressure decrease(Baird and Gaffney, 1995; Beckwith and Baird, 2001), which would decrease thehydraulic conductivity even further. To get a better knowledge about the processesinvolved in the mire breathing and their consequences, the distribution of compactionthrough the peat layers during water-table variations has to be explored and incombination with measurements of hydraulic conductivity.

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CONCLUSIONS

The physical properties of peat conformed to results found by other authors. The mossspecies is an important factor in the surface layer and the degree of humification andbulk density are dominating factors in lower layers. The distance to the mean waterlevel is more important than the distance from the surface in the subsurface layers.

Deviations between different samples within this study indicate dissimilarities indielectric properties between peats with different degrees of humification. Connectionsto physical properties such as amount of bound water and structural orientation arelikely to exist. There is, however, a lack of methods to measure and quantifyparameters expressing these properties. Therefore, until further studies on physicalproperties are accomplished, empirical or semi-empirical calibration curves for TDRare preferable to more physically based functions.

The surface micro-relief with hummocks and hollows has to be considered in manyaspects. There are great differences in water-content and temperature variation amongthe hummocks and hollows, being of major importance for biological processes, bothat the surface and within peat. These temperature differences indicate that thetopography has to be considered for surface energy-exchange estimation, although theeffect in partition of available energy at the surface was uncertain. The greaterexposure of hummocks are indicated by large lateral heat flows and should also bereflected in the fluxes of heat from surface to atmosphere. The water storage within thesurface layers was clearly different between the micro-relief elements.

The spatial variation of water storage was small at the scale of the mire. Thesimilarity of water-table fluctuations over large areas of the mire indicates a possibilityto model the mire hydrology as a one-dimensional system. Such a simplified modelmust account for the effect of micro-relief, which in some way should beparameterised. However, there are both traditional hysteresis effects, caused by air-entry pressures, and effects from swelling and shrinking of the peat material. Theseeffects have to be considered in the design of such a model.

The non-rigidity of peat allows the mire to store water by changing the peat volumeinstead of the water content of the unsaturated zone. The variable peat volume is also amajor reason for the highly hysteretic relation between the water content (bothsaturated and unsaturated) and groundwater level in a mire, even if there is equilibriumbetween water tension in the unsaturated layers and the underlying groundwater.Future studies should be designed such that changes in peat volume and position ofindividual sensors can be monitored continuously.

The energy partitioning of available energy at the surface is slightly changing with afalling Bowen ratio over the season but it is not dependent on peat wetness. Thesurface evaporation is considerably restricted, even during wet conditions, by a largeaerodynamic resistance within the canopy layer. Compared to other surface types, bothwetlands and non-wetlands, the resistance to evaporation was large. However, thesurface properties were similar to those described in studies of sparsely vegetatedmires and subarctic tussock tundra surfaces.

The governing processes for the behaviour of the surface are unclear. In thesimulations of the surface energy exchange, the peat temperature was overestimated assoon as the simulated moss surface became dry. Consequently the simulated surface

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had to be made continuously wet and evaporating to avoid overheating. This is inagreement with earlier failures of simulating sparsely vegetated peatlands butcontradicts several reports about surface mosses drying out during dry spells. Thisanomaly remains unexplained but there may be pore vapour flow enhancements orother phenomena within the moss layer, which is not considered by existing models.

From the results in this thesis, some questions have been answered but several newquestions have been raised and it is evident that a lot of work remains for mirehydrologists in order to understand the functions of the system. On the other hand, wecan look into the possible applications from the results achieved here. From theclimate modeller’s point of view, the behaviour of the surface is very stable throughthe growing season and the evaporation is very well described by a simple radiationmodel of the Priestly and Taylor (1972) type. However, since all processes behind thissimple relationship are not yet recognised, further research is needed to reveal whichsurface properties should be considered for parameter-value derivations. For thehydrologist with more interest in local runoff production, the evaporation is of equalinterest but the requirement of time resolution may be smaller and the peat elasticitymay be of greater importance. The storage-groundwater-table relation with peatvolume and distribution of hydraulic conductivity is indicated here as a major factor toconsider and a lot of work needs to be done to get further. The differences in surface-layer water storage between different micro-relief elements is also of importance whena runoff model should be parameterised, but it is uncertain how detailed it has to bedone. From the biologist’s and biogeochemist’s point of view, the heterogeneity of themire surface must be considered. The micro-climate variation is considerable andwhereas the hollow carpet subsurface layers may offer a straightforward one-dimensional system, the hummocks present a more complicated. Not only the partialcoverage of hummocks but also the dimensions of individual hummocks are ofsignificant importance. These different applications all are tied together in the samesystem by the hydrology. Further knowledge on one part of the system will be helpful,and sometimes also necessary for successive work on another part of the system.

ACKNOWLEDGEMENTS

I am grateful to my supervisors Sven Halldin and Lars-Christer Lundin for theirwillingness to help, their guidance along the paths into science and for their supportand patience. I would also like to thank all other colleges I have met at the departmentand at other places. The studies at Stormossen were supported by Swedish NaturalScience Research Council (grants I-GB 01923-321 and I-AA/GB 01923-320).Professor Anders Lindroth kindly let me use the TIS equipment he had set up atStormossen. Landowners around Stormossen have been very helpful and pleasant. Lastbut not least I would like to thank my relatives and friends for being there during theseyears.

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