Modelling meteorological and substrate influences on peatlandhydraulic gradient reversais
Dennis ColauttiDepartment ofGeography
McGill UniversityMontréal, Québec
August 2001
A thesis submitted ta the Faculty ofGraduate Studies and Research inpartial fulfillment ofthe requirements ofthe degree ofMasters of
Science
© Dennis Colautti 2001
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Table of Contents
ABSTRACT 1
RÉSUMÉ 11
ACKNOWLEDGEMENTS III
LIST OF FIGURES IV
LIST OF TABLES IV
CHAPTER 1.0 INTRODUCT10N l
CHAPTER 2.0 LITERATURE REVIEW AND RESEARCH OBJECTIVES 2
2.1 GENERAL INTRODUCTION TD PEATLAND HYDROLOGY AND GROUNDWATER MODELLlNG 32.1.1 Peatlands and their hydrology 32.1.2 Contemporary numerical modelling and ils application to peatlalld subsurface flow.........................................................................................................................................102.13 Contemporary studies ofsubsurjace flow reversais in peatlands 14
2.2 RESEARCH OBJECTfVES 19
CHAPTER 3.0 RESEARCH METHODOLOGY 20
3.1 MODELDOt'vlAfN DESCRIPTIONS 213.2 MODEL METEOROWGICAL FORCING AND SUBSTRATE ALTERATION 223.3 MODELRUNCONFlGURATlONS 24
CIIAPTER 4.0 RESULTS 25
4.1 [.RET SIMULATIONS 264.1.1 F~treme drought conditions 264.1.2 Drought severity variatwll .324.13 Catotell'll aUeration ; .414.1.4 Discussion ofLRET simulations ..42
4.2 KBT SIA.fULATlONS 474.2.1 Extreme drougllt conditions .474.2.2 Hydraulic conductivity .484.2.3 Drough.t severity variation 494.2.4 Porosity 504.2.5 Discussion ofKET sùnulations 51
CHAPTER 5.0 SUMMARY AND CONCLUSION : 54
REFERENCES 57
Abstract
A hydrological modelling effort using MODFLOW wasundertaken in order ta determine the relative importance ofsorne of the factors influencing hydraulic gradient reversaIs inpeatlands. Model domains were of two types, large raised bogtype (LRBT) and kettle bog type (KBT), and were made toundergo various levels of meteorological forcing (waterdeficit). Substrate, too, was varied in order to determine itsimportance on reversaIs. Domain-wide reversaIs weresuccessfully sitnulated in LRBT systems, but not in KBTsystems. Although simulated flow patterns matched fieIdobserved patterns, both pre- and post- drought, simulatedreversaIs occurred more quickly than in the field. This Inay bedue to insufficientIy distributed parameters, such as hydraulicconductivity. ReversaIs were easily terminated by simulatingnon-drought conditions. In the LRBT system, reversaI durationdecreased, and time-to-reversal increased, with a decrease indrought severity. Increasing drought severity in KBT systemshad the opposite effect on the duration of semi-reversed flowpatterns, suggesting a possibly different/additionalmechanism for flow reversaIs in KET systems. Hydraulicconductivity had an appreciable effect on flow reversaIevolution, though neither changing porosity, nor differencesin catotelm Iayering had a great effect.
i
Résumé
Une étude hydrologique a été effectuée, en utilisant le logicielMODFLOW, afin de déterminer l'importance relative descertains des facteurs qui peuvent influencer les renversementsde gradient hydraulique dans deux types fondamentaux (LRBTet KBT) de tourbière. Les deux types ont été subis à dediverses conditions météorologiques (déficit d'eau). Aussivarié était le substrat, afin de déterminer son importance. Desrenversements ont été fructueusement simulés partout dansles systèmes LBRT, mais pas dans les systèmes KBT. Bien queles simulations de flux ressemblaient, en forme, desrenversements observés, les renversements simuléesurvenaient beaucoup plus vite que dans la nature, peut être àcause de l'utilisation des paramètres insuffisammentdistribués, tel conductivité hydraulique. Les renversementsétaient terminés aisément avec la simulation des conditions denon-sécheresse. La durée de renversement diminuait, et letemps requis pour effectuer le renversement accroissait, avecune diminution dans la sévérité de sécheresse dans lessystemes LRBT. Inopinément, la durée des semi-renversementsdans les systemes KBT diminuait avec les plus grandessécheresses. Ceci suggére un mécanisme alternative/différentpour effectuer des renversements dans les systèmes KBT.Quant à l'évolution des renversements, conductivitéhydraulique était plus important que les différences dans lesconfigurations variables de catotelm et aussi la porosité.
ii
Acknowledgements
1 thank Nigel Roulet for his supervision, advice, and
patience -- especially the patience.
Financial assistance was provided by a NSERC PGS-A (two
year postgraduate scholarship).
Support from family and friends back home was invaluable,
and 1am very grateful for having received it.
Hi
List of Figures
4.1.1 Selected time steps of Simulation 4.1.14.1.2 Selected time steps of Simulation 4.1.24.1.3 Selected time steps of Simulation 4.1.34.1.4 Selected time steps of Simulation 4.1.44.1.5 Selected time steps of Simulation 4.1.54.1.6 Selected time steps of Simulation 4.1.64.1.7 Selected time steps of Simulation 4.1.74.1.8 Selected time steps of Simulation 4.1.84.2.1 Selected Ume steps of Simulation 4.2.14.2.2 Selected time steps of Simulation 4.2.24.2.3 Selected time steps of Simulation 4.2.34.2.4 Selected time steps of Simulation 4.2.5
List of Tables
4.1 LRBT configurations and parameters4.2 LRBT surface boundary conditions4.3 KBT configurations and parameters4.4 KBT surface boundary conditions
iv
Chapter 1.0 Introduction
Over the past few years, a peculiar peatland groundwater
flow phenomenon has been observed in the field, with possible
consequences for peatland biota on the local scale, and, on a
much larger scale, for global dimate change. The
phenomenon in question involves the periodic reversaI of the
usual hydraulic gradients within an unconfined aquifer of
certain peatlands, which reverses the usual groundwater flow
direction. Because such reversaIs have been observed
particularly after the onset of periods of water deficit, it has
been hypothesized that they result from a time lag in pressure
transmission down the peat column, due, in turn, ta the law
hydraulic conductivity of peat.
The aim of the present study is ta numerically madel
variaus peatland settings in arder to elucidate haw various
levels of drought influence the short-term evolution of
gradient reversaIs, and how the peat substrate itself might
influence said evalutian. The appraach taken is not one in
1
which real-world peatlands are modelled exaetly. Instead, the
simulations contained herein are the resuit of generai
parameter input, since the goal is to bring to light the relative
effects that various forcing agents (meteorology, substrate
characteristics) have on the development of hydraulic
gradient reversaIs, which may give insight into other
phenomena, such as peatland vegetation change and methane
outgasing.
Chapter 2.0 Literature review and research
objectives
This purpose of this chapter is three-fold: (1) to introduce
peatlands, general peatiand hydrology (peatland subsurface
flow reversaIs in particular), and the more salient aspects of
groundwater modelling to the reader; (2) to discuss flow
reversaIs, a specifie aspect of peatland hydrology, in the
eontext of the relevant scientific literature; and (3) to outline
the modelling objectives as they relate to groundwater flow
2
reversaIs in several differing hypothetical peatland (bog)
settings.
2.1 General introduction to peatland hydrology
and groundwater modelling
2.1.1 Peatlands and their hydrology
Hydrology plays a leading role in peatland development,
chemistry, and biology (Glaser et al., 1981: Mitsch and
Gosselink, 1993; Waddington and Roulet, 1997). A good
understanding of peatlands, therefore, relies on a good
understanding of peatland hydrology, but before focussing on
some of the specifies of peatland hydrology, a distinction
between the different types of peatlands is usefuI.
In general, two basi€ types of peatlands exist: fens and
bogs (Glaser et al., 1981; Ingram, 1983). Fens tend to have fiat
or concave surfaces which are not higher than the
surrounding landscape. Bogs, however, tend to have a higher
surface than their surroundings, and very often are domed.
3
Fen waters are characterized by pH values greater than 4.2
and by concentrations of inorganic solutes which are higher
than those of bogs (e.g., fen calcium concentrations typically
are greater than 2.0 ppm, while bog concentrations typically
are lower than 2.0 ppm); bog waters typically have pH values
lower than 4.2 (Reeve et a1., 2000). Because the type of
vegetation growing in a particular wetland depends on surface
water chemistry, it is not surprising, then, that the biota of
fens and bogs differs significantly, with the former typically
being dominated by calciphilic flora -- especially sedges, such
as the Carex species -- and the latter by mosses, such as the
Sphagnum species.
The aforementioned morphology, biology, and chemistry of
peatlands have led many researchers to make certain
inferences about the origin of water that sustains these
landforms. For instance, it is thought that differences in
chemistry result from fens being supplied, in large part, by
groundwater (i. e., water flowing upwards into the fen,
through underlying mineraI sediments, and derived possibly
from regional groundwater flow) , and from bogs being
4
supplied almost exclusively by direct meteoric (Le., solute
poor) water input. This, in turn, is thought to arise largely
from the difference in landform morphology between these
two peatland types. That is, because bogs tend to have convex
surfaces, water table mounds form under their surface, and
this leads to almost exclusively recharge conditions. This is
thought to prevent sub-bog groundwater from discharging
into the bog; thus, mineral-poor, directly-derived meteoric
water constitutes, by far, most of the bog's water supply.
Contrastingly, the concave surface of a fen is thought to lead
to recharge conditions which are so strong that they prevent
sub-fen, mineral-rich groundwater from discharging into the
fen.
Further to the differences between the two basic types of
peatlands, there are many different types of bogs. Two are
studied in this thesis: the blanket bog, and the kettle bog.
The differences between these two types arise from their
topographie setting. The blanket type is characterized by a
gently sloping surface everywhere except for the steeply
sloping margins. AIso, blanket bogs are much more expansive
5
than are kettle bogs, and often are underlain by a rather
impermeable, fiat mineraI substrate layer. Their greater areal
expanse is due to their ability to expand laterally over rather
fiat terrain. Kettle bogs, on the other hand, form in
depressions often left behind by glacial activity, and thus are
characterized by a rather pronounced concave lower
boundary. Their upper surfaces often are domed, and slope
gently from their central regions up to and including their
margins. The margins coincide approximately with the rim of
the depression, which greatly confines lateral expansion of the
kettle bog. Thus, the blanket bog is able to grow in more of a
horizontal manner, whereas the kettle bog has to grow in a
more vertical manner. In the case of both types, there often is
a water conduit, or 'lag', found at the bog margin, which
serves to drain and channel water away from the bog. Also
common to both bog types is the presence of two distinct peat
layers -- the acrotelm and the catotelm (Ingram, 1983). The
former term refers to the uppermost peat layer, which often is
about 50 cm in thickness (e.g., Devito et aL, 1997; Fraser,
1999), is composed mainly of living plants and slightly
6
decomposed peat, and thus is characterized by hydraulic
conductivity (K) values of about 10-2 ms-! (Ivanov, 1981; Roag
and Priee, 1995). The underlying catotelm, which rests
directly on the mineraI substrate, is composed of much more
humified peat, and thus has a much lower overaIl K value of
about 10-5 ms-! to 10-8 ms-! (Chason and Siegel, 1986). Because
of this, many (Ingram, 1983; Ingram and Bragg, 1984, for
example) have hypothesized that the acrotelm may channel
water quickly to the margins' lags, while the catotelm may be
relatively inactive with respect to groundwater flow, and may
not even interact significantly with the underlying, mineraI
substrate. The assumption that most of the drainage of bogs is
effected by only a thin (-50 cm) surface layer is one reason
that deeper bog peat has not been studied to the extent that
the more superficial bog peat has been studied.
Whether or not deeper bog peat plays a significant role in
bog drainage, we can point to sorne general groundwater flow
features of peat bogs. For instance, flow in bogs is said to be
Darcian, as opPOsed to laminar, in nature (Hemond and
Goldman, 1985). Renee, the general continuity equation,
7
based on Darcy's Law, may be used ta describe saturated
subsurface flow in bag settings:
(1)
where: Kx, Ky, and ~ are camponents of the hydraulic
conductivity tensor; h is hydraulic head; S5 is specifie storage;
W is a general sink/source term, and where the three spatial
dimensions are represented by x, y, and z (Freeze and Cherry,
1979).
Another general feature of bog subsurface flow is the
steepening of hydraulic gradients as one goes from a bog's
centre to its margins (Ingram, 1982; Gafni and Brooks, 1990),
and yet another is the general tendency for subsurface water
to flow from a bog's centre, downwards to sorne extent, and
outwards to its margins. However, of increasing interest, due
to its contradiction of the last-l'downwards and outwards" flow
feature, is the field-observed phenomenon of groundwater
flow reversaIs in bogs (Fraser, 1999; Devito et al., 1997;
8
Romanowicz et al., 1993). This phenomenon has been
obsetved to occur soon after the onset of significant drought
conditions. It is thought to arise from a time lag in the vertical
transmission of hydraulic pressure change as water is
removed, by the drought, from the uppermost region of the
peat column. That is, the near-surface pressure is thought to
change rapidly in response to the removal of near-surface
water, while deeper peat water experiences the drought
induced pressure change much later. This time lag is thought
to come about due to the very low hydraulic conductivity
values of peat -- especially deeper peat, which almost always is
orders of magnitude less conductive than is the acrotelm's
peat. Until the deeper peat water undergoes a pressure
decrease, the water pressure in the vicinity of the water table
(where water is being removed from the system) may
temporarily drop below that of deeper peat water, thus
effecting flow more upwards than usual (Fraser, 1999).
9
2.1.2 Contemporary numerical modelling
and its application to peatland subsurface
flow
In view of the important effects that hydrology has on
various peatiand processes, it is not surprising that there have
been many studies of peatland hydrology. The complexity of
peatland systems, as is the case with many non-peatland
groundwater systems, makes the numerical modeUing
approach especially attractive. For instance, the present
study, which seeks to elucidate the importance of various
hydrogeologicai factors in bog groundwater flow reversaIs,
does so by varying a single factor (e.g., precipitation level)
from one numerical simulation to the next, with aU factors
remaining equal. It would be impossible to find real-world
bogs which varied (which could he controlled) in such a
fashion. Unfortunately, only a few studies to date (Reeve et
al., 2000; McNamara et al.,1992; McKenzie, 1997; Siegel, 1981,
1983) have made use of the numerical modeUing approach in
studying peatland subsurface water flow. McNamara et al.
10
(1992) modelled the linked evolution of peatiand morphology
and hydrology, while McKenzie (1997) modelled this and
varying hydrogeological configurations' effects on peatland
groundwater flow. McNamara et aI. (1992) observed a
permanent reversaI in flow during the fen-to-bog evolution
which was the focus of their study. They credit the slightly
minerotrophic nature of their study bog vegetation to periodic
reversaIs of hydraulic gradients, which result in the discharge
of mineral-rich groundwater to the bog. However, both of
these studies focused on long-term peatland hydrology
change, thus negiecting changes of a temporary nature,
including temporary flow reversaIs, which have been observed
in field studies.
Much more modelling has been done for the non-peatland
setting, beginning with Toth (1963) and Freeze and
Witherspoon (1967) who investigated the effects of various
hypothetical hydrogeological configurations on generailocal
to-regional-scale groundwater flow in small basins. Winter
(1978) and Cheng and Anderson (1994) did the same with
respect to lake/groundwater systems. Anderson and Munter
Il
(1981) looked at the effects of various configurations on
groundwater flow reversaIs around lakes. But, contrary to
Ingram's (1991) assertion that bogs are virtually lakes, they
are essentially mounds of water, with Uttle solid matter and
therefore are characterized by internaI hydraulic gradients.
Because of these bog-Iake differences and the lack of
modelling studies with respect to short-term, transient
hydrologicai change in bogs, there is a need to model the
occurrence, frequency, and magnitude of groundwater of flow
reversaIs within bogs (see next section for introduction to flow
reversaIs) .
Briefly, numerical modelling involves discretizing of a
domain (the bog, in the present study) into cells, in one, two,
or three dimensions, and assigning equations (see Equation 1)
to each individual cel!. Here, the solution of each cell's
equation results in a hydraulic head value, which is assigned
to the appropriate cell. From the pattern of hydraulic head
values , one may determine how groundwater flows
throughout the modelled domain. As with any such modeI,
the validity of the results is highly dependent upon the
12
validity of the input data and the numerical scheme used.
Thus, while the present study presents numerous flow
patterns, it should not be forgotten that these patterns are
dependent upon input data sets which are incomplete. For
example, within a bog, hydraulic conductivity is highly
variable even within distances of a few centimeters (Fraser,
1999). Because of this, it is impossible to sample every single
Kvalue within a peatland without destroying the system,
which in turn makes it impossible to create a numerical model
which is completely accurate in its description of a bog in
terms of K. The same can be said about describing the bog's
lower boundary condition, or about porosity or storativity
values. Hence, it becomes necessary to extrapolate the values
that we do have; we do no discretize a bog dOlnain in such a
wayas to account for every real-world Kvalue. Instead, a
single K value may be assigned to a cell covering tens of
meters spatially (as opposed to a few centimeters), and bog
boundaries are made to be smooth and symmetrical. Of
course, this isn't the case in the field, but in the present study
the purpose is not to exactly replicate real-world bogs; instead,
13
the approach is ta simulate bogs to an extent which allows us
to make very general conclusions about how their subsurface
flow patterns might react in response to changes in forcing
conditions -- especially drought severity.
2.1.3 Contemporary studies of subsurface
flow reversaIs in peatIallds
Siegel (1988) investigated the recharge-discharge function
of Alaskan wetlands -- a function used to determine the
importance of wetlands to the environment. Recharge
wetlands contribute a certain proportion of their groundwater
to underlying mineraI sediments via downward flow, thus
possibly helping to repienish aquifers. Discharge wetlands
receive upward-moving groundwater from underlying mineraI
sediments, which can keep water tables high and import
nutrients to these wetlands, thereby affecting their ecology.
However, McNamara et. al. (1992) point out that the above
delineations are merely the results of inductive Inference .
Periodic reversaIs in hydraulic gradient and, thus, in
14
groundwater flow direction have been observed in the field
(Fraser, 1999; Devito et al., 1997; Romanowicz et al., 1993),
illustrating that sorne wetlands are recharged at certain times,
and discharge groundwater at other times. Thus, a particular
bog may not necessarily faIl into the recharge categoI}' on a
permanent basis. Wetlands such as bogs that are thought by
many ta be removed from intermediate and regional flow
systems may receive upwelling, possibly nutrient-Iaden
groundwater from these sub-systems (or at least from
groundwater system(s) beyond the bog perimeter) from Ume
ta time, thus having important effects on wetland chemistI}'
and biota. Furthermore, such upwelling can play an important
role in the pattern of release of greenhouse gases such as
methane (Romanowicz et al., 1993).
More recently, Fraser (1999) observed a reversaI in
subsurface bog water flow at Mer Bleue in 1998 -- a blanket
bog situated near Ottawa, Canada. Throughout the period
from mid-July ta roughly the end of August, water table draw
down occurred, due to evapotranspiration greatly exceeding
precipitation. As the bog's water table elevation decreased by
15
up ta 20 cm, the head values of deeper peat water decreased
by only 10 cm. Since the difference in hydraulic head from
the surface to the base of the peatland was less than 10 cm, a
situation arose where the levels at depth were greater than
those at the surface. The head values of deeper peat water did
gradually decrease, approaching those of surface head values,
but only after six weeks. The flow reversaI came to an end
when the water table elevation increased due to a rainfall at
the end of August, 1998. The surface head values became
higher than those at depth, and the bog's 'normal' recharge
condition was restored. Although the storm caused deeper
head values to increase marginally, surface values increased
by much more. Fraser attributed his flow reversaI to drought
which was driven in large part by measured
evapotranspiration. He discounted the possibility of regional
flow since Mer Bleue is isolated from regional flow by means of
an underlying, low-K stratum.
Devito et al. (1997) came to a similar conclusion: intra-bog
flow reversaIs can occur even when the bog is isolated from
regional flow; and such reversaIs may arise due to the onset of
16
water deficit. These researchers observed such behaviour in
two bogs in Ontario, Canada, and in one bog in Sweden. AlI
three bogs were isolated from any groundwater flow beyond
their perimeters and bases. Upward flow was observed after
the onset of drought, leading the researchers ta conclude that
groundwater forcing from outside the bog proper is not at aH
necessary ta effect flow reversaIs. At the Dorset bog (southem
Ontario), usual downward flow changed ta upward flow (2
mm/day) in response to the water table dropping to 60 cm
below the peat surface). At the other Canadian site
(Experimental Lakes Area, northwestern Ontario), a water table
drop ta 20 cm below the average elevation resulted in a
change of flow from the usual 9 mm/day to the reversed DA
mm/day. Similarly, the Swedish bog (northern Sweden, near
Umeà), after experiencing a water table drop of greater than
15 cm, displayed a discharge pattern, which ended with a
precipitation event. These authors go so far as to state that
bogs typical of Canada's Precambrian Shield region (Le., bogs
which experience ephemeral groundwater input due to their
being disconnected from non-local sources of groundwater)
17
are more, not less, likely to experience groundwater flow
reversaIs than are bogs that are connected to larger
groundwater systems.
Romanowicz et al. (1993) describe the causes of flow
reversaIs in the Glacial Lake Agassiz peatlands of Minnesota.
However, they note interaction with intermediate-Ievel
groundwater flow to be an important driving factor behind the
flow reversaI. The intrusion of extra-bog groundwater into the
bog from below that changes portions of the intra-bog flow
from downwards to upwards becomes possible when water
table draw-down (magnitude of 50 cm to 200 cm) occurs.
Upward, vertical hydraulic head gradients as high as 0.04 were
measured. Gnly when precipitation increased, thus increasing
water table elevation, did the flow pattern return to its usual
recharge pattern. The authors attribute the return to 'normal'
flow to the re-establishment of the bog's water table mound,
which Bmits the amount of intermediate-Ievel groundwater
flow that can penetrate the bog's lower boundary. Thus, they
do nat attribute the flow reversaI to the aforementioned time
lag effect (see Devito et al., 1997).
18
In any case, it seems obvious that drought conditions are
often, if not aiways, necessary precursors to bog flow reversaIs.
Still, several other factors may play significant roles. The
magnitude of the drought, the position of the bog in the
regionallandscape, the locaVregional topography, the depth
to the water table, the porosity, storativity, and layering of the
bog's peat, and the depth of the combined
Iocal/intermediate/regional groundwater system are a fewof
these factors. In order to elucidate the roles that these factors
play, a numerical modelling approach holds the greatest
promise.
2.2 Research objectives
The purpose of my research is to model groundwater flow
in a variety of hypothetical hydrogeological configurations to
determine if and how these factors influence the frequency,
magnitude, and duration of hydraulic gradient reversaIs in
bogs. This will be accomplished using a numerical model of
transient groundwater flow in a series of hypothetical (though
19
reality-based), situations. The flnite difference groundwater
modelling computer program used is MODFLOW (McDonald
and Harbaugh, 1988). The height of the water table boundary
condition will be variably altered in each setting ta attempt ta
induce flow reversaIs. This will be accomplished by
increasing/decreasing precipitation and evapotranspiration
levels. At the same time, the internaI characteristics of the
model which are most important in affecting groundwater flow
(e.g., hydraulic conductivity, storativity, topography) will he
changed from setting to setting. A deterministic approach is
favoured for hydrogeological parameter input.
Chapter 3.0 Research Methodology
Numerical modelling of several bog configurations was
undertaken using the groundwater modelling software
package Visual MODFLOW. From one configuration to the
next, meteorological factors (precipitation and
20
evapotranspiration) and substrate factors (hydraulic
eonduetivity, specifie yield, specifie storage) were altered in
such a wayas to make eaeh model domain unique. In addition
to this, the overall model domain (bog)morphology was
configured in one of two basic ways, each of whieh reflect two
common bog morphologies found in nature: the kettle bog,
and the expansive, raised bog.
3.1 Model domain descriptions
Two basic, two-dimensional peatland morphologies were
modelled. One, the large, raised bog type, hereafter referred
to as LRBT, was characterized by a length of 4 km, a height of
6 m, a very graduaI central slope ("'0.00005), and very steeply
sloping ("'0.014) margins. The second basic peatland
morphology modelled was that of a 'typical' kettle bog,
hereafter referred to as KBT. The KBT lower boundary was
shaped like a bowl. The modelled KBT was different from the
modelled LRBT in two main ways: the former was much
smaller in lateral extent (about 600 m from margin to margin),
21
and had a gentle slope (---0.004) throughout this extent,
including the margins. The KBT heights/depths, though, were
similar to those of the LRBT: "" 6 m. Both KBT and LRBT
models were given either two or three peat layers with
differing hydraulic conductivity and storativity (see next
section).
3.2 Model nleteorological forcing and substrate
alteration
AU simulations were run for at least one thousand days (""
three years), and some were run for as much as two thousand
days ("" six years); run length depended on how long it took ta
bring the system into 'realistic' hydrological equilibrium. That
is, it depended on input values of hydraulic conductivity,
storativity, domain morphology, and
recharge/evapotranspiration levels.
Hydraulic conductivity values in aH directions (Kx , Ky, and
Kz ) ranged from 10-2 ms-l ta 10-9 ms-l, based on K ranges found
in the literature for glacial tills, clay, silt, sUty sand, ands sand
,22
(Freeze and Cherry, 1979; Fetter, 1994), and for peat (Chason
and Siegel, 1986). Highest K values were assigned to the
acrotelm, and lowest values were assigned usually to the
deepest peat and to mineraI substrates. Horizontal isotropy
was assumed, and vertical anisotropy (Kh/~) ranged from 10
to 1000 within each layer. The specifie yield (Sy) of humified
(Le., sub-acrotelm) peat was assigned values ranging around
0.05 (Siegel, 1988), and that of less humified (Le., acrotelm
and near-surface) peat was assigned values ranging around
0.5. Mineral substrates were assigned values of either 0.25 or
0.03, reflecting typical values of sand and silty clay,
respectively (Davis and DeWeist, 1966,).
Evapotranspiration extinction depth was set to two meters
(settings of 0.5 m and 1.0 m provided identical results).
Precipitation (P) and evapotranspiration (ET) levels were
adjusted to levels which allowed the model domains to de
water properly. Thus, model P and ET levels do not always
match levels found in nature; instead, it was considered more
important to allow the modelled systems to de-water, and to
23
eoneentrate on the relative differences between P and ET
within any one simulation.
3.3 Model run configurations
AlI simulations were transient and were solved using the
W.H.S. (Waterloo Hydrogeologie Software) solver. The
maximum number of outer and inner Iterations were fifty and
500, respectively The maximum head change criterion for
convergence was 0.01 m.. The uppermost layer was set to
'lunconfined", while each underlying layer was set to
"confined/unconfined" (Le., having variable storativity and
transmissivity). Cell re-wetting was used, and recharge was
applied to the highest active grid ceU in each vertical column,
as opposed to the top grid layer as a whole.
24
Chapter 4.0 Results
This chapter begins by examining the importance of
extreme drought conditions on hydraulic gradient reversaIs,
followed by a look at the minimum drought conditions needed
for the onset of hydraulic gradient reversaI in a LRBT. This is
fol1owed by an examination of how a moisture surplus, and
then changes in layer hydraulic properties, affect flow pattern.
Tables 4.1 to 4.4list model configurations which describe the
change in forcing conditions and hydraulic properties from
model to model.
Finally, the results are briefly discussed, including a
rationalization of the models used. Section 4.2 of this chapter
will fol1ow along similar Hnes, though it will deal with the KBT
case.
25
Table 4.1. LRBT configurations and parameters
Uppermost Layer Middle Layer Lowest LayerKx,y (mis.) Kz (mis) 55 and Sy n Kx,y (mis) Kz (mis) S5 and Sy n Kx,y (mis) Kz (mis) S5 and Sy n
Simulation4.1.1 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-05 2.00E-06 0.15 0.904.1.2 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-05 2.00E-06 0.15 0.904.1.3 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-OS 2.00E-06 0.15 0.904.1.4 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-05 0.20 0.90 2.00E-OS 2.00E-06 0.15 0.904.1.5 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-05 0.20 0.90 2.00E-OS 2.00E-06 0.15 0.904.1.6 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-05 2.00E-06 0.15 0.904.1.7 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-05 2.00E-06 0.15 0.904.1.8 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-04 2.00E-OS 0.20 0.90
where Kx,y = hydraulic conductivity in the x,y dimensions,
Kz = hydraulic conductivity in the vertical dimension, 55 = specifie storage (m' I),
Sy = specifie yield, and n = porosity.
Table 4.2. LRBT surface boundary conditions
Days
Simulation 100 to 400 400to 500 500 to 1200 1200 to 1500 1200 to 2000
4.1.1 Recharge 1200 600 1200 600ET 1200 1600 1200 1600
4.1.2 Recharge 1200 600 1200 600 1200ET 1200 1600 1200 1600 1200
4.1.3 Recharge 1200 600 1200 800ET 1200 1600 1200 1400
4.1.4 Recharge 1200 600 1200 1100ET 1200 1GOO 1200 1300
4.1.5 Recharge 1200 600 1200 11 saET 1200 1600 1200 1250
4.1.6 Recharge 1200 600 1200 1185ET 1200 1600 1200 1215
4.1.7 Recharge 1200 GOa 1200 1190ET 1200 1600 1200 1210
4.1.8 Recharge 1200 GaO 1200 1175ET 1200 1600 1200 1225
ET refers to evapotranspiration. Both ET and recharge are in mm/yr. The meteorological forcingof Days 400 to 500 is meant to stabilize the system (Le., to rid the system of overflow). Cellre-wetting, from below and from the sides of ceUs, was used in ail LRBT simulations, andconductance of the two drains was set to 10,000 square meters per day.
Table 4.3. KBT configurations and parameters
Uppermost Layer Middle Layer Lowest LayerKx.y (mis) Kz (mis) 55 and Sy n Kx,y (mis) Kz (mis) 5s and Sy n Kx•y (mis) Kz (mis) 5s and 5y n
Simulation4.2.1 2.00E-02 2.00E-03 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-04 2.00E-OS 0.20 0.904.2.2 2.00E-03 2.00E-04 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-04 2.00E-OS 0.20 0.904.2.3 2.00E-03 2.00E-04 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-04 2.00E-OS 0.20 0.904.2.4 2.00E-03 2.00E-04 0.45 0.90 2.00E-04 2.00E-OS 0.20 0.90 2.00E-04 2.00E-OS 0.20 0.904.2.5 2.00E-03 2.00E-04 0.45 0.95 2.00E-04 2.00E-OS 0.20 0.85 2.00E-04 2.00E-OS 0.20 0.85
where Kx,y = hydraulic conductivity in the x,y dimensions,
Kz =hydraulic conductivity in the vertical dimension, 55 =specifie storage (m'I),
Sy =specifie yield, and n =porosity.
Table 4.4. KBT surface boundary conditions
Days
Simulation 100 to 400 400 to 500 500 to 1000 1000 to 2000
4.2.1 Recharge 100 600 1000 600ET 100 600 1000 1600
4.2.2 Recharge 100 600 1000 600ET 100 600 1000 1600
4.2.3 Recharge 100 600 1000 800ET 100 GOO 1000 1400
4.2.4 Recharge 100 600 1000 920ET 100 600 1000 1080
4.2.5 Recharge 100 600 1000 600ET 100 600 1000 1600
ET refers to evapotranspiration. Both ET and recharge are in mm/yr. The meteorologicalforcing of Days 400 to 500 is meant to stabilize the system (I.e., to rid the system ofoverflow). Cell re-wetting, from only below celfs, was used in ail KBT simulations, andconductance of the two drains was set to 1000 square meters per day.
4.1 LRBT Simulations
4.1.1 Extreme drought conditions
Figure 4.1.1 shows the model domain of a LRBT bog
(Simulation 4.1.1) comprising a 50 cm-thick, relatively high-K
acrotelm layer (Table 4.1), and a 5.3-to-5.5 cm-deep catotelm.
The catotelm had twenty-one layers and two K zones: the
upper catotelm zone being nine layers (2.3 m to 2.5 m) deep
and the lower zone being twelve layers (3.0 m) deep. Within
each K zone, Kx, KY' and ~ values are constant from ceU to ceU,
and ~ is one order of magnitude lower than both Kx and Ky.
After sorne initial adjustment of P and ET early in the
simulation (Table 4.2), the bog achieved a stable state (ôS/ô t
= 0), long before Day 1200. (The adjustment consisted of
lowering P and raising E in order to de-water the system,
whlch had been overflowing from the beginning.
Subsequently, P and E were made to match each other in
magnitude -- a situation which does not occur in peatlands
most of the year, but which does occur during the late spring-
26
1 <" / 1 1 l' J " \ \ \,(0
\ " " ....'Id" " lit .... ,; Id1
,~ \
~1 " \ î\ ' l \co l
~,. "~' .~j ~ i j j j ~ \ \ '\ ... \. \ '\\.a) Day 1200.001
..... ~' '0 / if'..' i i l.. ~~ ,u) /" ..../t?0"/ /.. i' l
.' ,/ / ,.... l l \,
",01
' . ....... .......·~·Id··... - 1Sl'" -.'. -.... Id ",,. l 1 1 1
~1 \ '\
~ I~., \. l \(l) l • • ln~ " j " i l ! \ " ,
... " \ \ '.', '1Sl '. 1 ; \ \ \
b) Day 1200.006
................ --._.-..... U) '0 1 if'.. i , , 1 ~ 1 1 1 J l• \&. .-ID./' /~ / / " J' ~ ( " ( 1 1. . . . .• • 1 1; i
--- ~ - .•- : -- ~....- Id-~ - Id- '.
1 \ \ '\ ~ ~ ~ ~ ~I~ ~ ~ ~
f) Day 1200.136
Figure 4.1.1: Selected time steps soon after the onset of a 'drought' whichbegins on Day 1200 of Simulation 4.1 .1. Ali storage values and other domainparameters can be found in Tables 4.1 and 4.3. Dimensions are in meters.Vertical exaggeration is 70 X. The water table obscures the appearance ofarrowheads for sorne flow arrows. See continuation of this figure forfurther time steps of this simulation.
·1
i~;i/i.~ i i ~.....~)~·~~·l-7.i~~u'"ro:~;--,'---lo....,ro~---:::...J..-.;'-rr-''--'----'~I
g) Day 1200.199
1 t t r"t t u:i
'W-'-'--.,,.----.,r----"!,---,,-----.,r-~-- .~., -- -·k~---'---'-.....,!~ ~~
h) Day 1200.604
i) Day 1200.872
j) Day 1211.263
k) Day 1300.470
---'~r------'ll,r---' "1500 2000 2500
1) Day 1500.000
Figure 4.1.1 continued: Note that in ail figures, solid black region representsunsaturated zone, and hydraulic head equpotentials have units of meters.See continuation for final two featured time steps of Simulation 4.1.1 .
1 ~ 1Irll t 1
"='{,..'----'---"'---,F"""'-..o.=:...t---,~~_'__,;LiJlir-t-J...~--t,r----'-.J,,-..:;;....L:::.:-.J..i~r-~'---~t~"f4X~ t4.Ü):~J:~)
m) Day 1518.780
n) Day 1522..536
Figure 4.1.1 continued: Final two featured time steps of Simulation 4.1.1.
early summer months in eastem North America, when
hydraulic gradient reversaIs are likely to occur.) A
meteorological deficit of 1000 mm/yr was then simulated, via
the lowering of precipitation from the pre-drought value of
1200 mm/yr to a value of 600 mm/yr, and via the raising of
evapotranspiration from the pre-drought value of 1200
mm/yr to a value of 1600 mm/yr.
Figure 4.1.1-a is still early enough in the drought that it
shows the system's pre-drought flow pattern, which generally
is downwards and outwards, from the centre of the bog to its
margins. Note that the difference in hydraulic head between
the uppermost cell in column 57 (horizontal centre of the bog)
and its lowermost cell of the same column is only 0.00023 m
at this point in time. Aiso note that the water table eievation
at the bog's centre is about 5.8 m. Between seven and eight
minutes later (see Figure 4.1.1-c and section 4.1.4 for a
discussion of why such a rapid reversaI occurs in the model,
but not in the field), however, a reversaI of the usual
'downwards and outwards' flow pattern occurs throughout
much of the domain. More specifically, Figure 4.1.1 shows this
27
reversaI ta be limited ta the acratelm and ta the very
uppermost portion of the uppermost catotelm Kzone. AIso,
except for the very center of the bog, the reversaI occurs from
one end of the bog to the other. (Note that the focus
throughout this thesis will be away from the very ends of the
various bogs, in an simulations, since the drains are Iocated
here and they likely interfere with flow patterns in their
Immediate surroundings.) At the very center, flow still is
downwards, as in a) and b) of Figure 4.1.1 , but there is a
noticeable Iaterai component to the flow vectors. By Day
1200.006 (Figure 4.1.1-d), which is about only ten minutes
Iater, flow even at the bog's center (2000 meters from its
edges) is very clearly upwards, and the upward flow shown in
Figure 4.1.1-c is even stronger (Le., more vertical) from one
end of the bog to the other. While hydraulic head at the top
of the bog is decreasing at this point, note that head at the
bottom of the bog has not changed yet, and likely is the reason
for the flow reversaI. However, the reversaI still hasn't made
its way down the peat profile. Figure 4.1.1-e shows even
stronger upward flow, along with no noticeable downward
28
progression of the flow reversaI. On the other hand, Figure
4.1.1-f, which is 199 minutes into the 'drought', indeed shows
such a downward progression, with the reversaI reaching the
very uppermost portion of the lower catotelm zone, though
not yet around the 2000 meter horizontal mark of this zone.
At this point, flow near the bog's surface is almost aH vertical,
while flow deeper down still has sorne horizontal tendency. At
the very bottom of the bog, flow doesn't appear to be any
different than that of Figure 4.1.1-a, which is to say that it is
downwards and outwards. About ninety minutes later (Le.,
about 286 minutes into the 'drought'), Figure 4.1.1-g shows
that flow halfway down the peat profile has 10st its horizontal
tendency, and now is almost compIeteIy vertical. By this
point, hydraulic head in the uppermost ceH of column 57 has
decreased from its "original" value by only 0.00022 m, while
the lawermast ceH in the same column has remained the same.
By Day 1200.604 (Figure 4.1.1-h), which is about 864 minutes
inta the 'drought', upwards flow finally can be observed at the
very bottom of the peat profile. In fact, at the 2000 meter
mark, flow is more upward-tending than is the flow ta its sides
29
(e.g., at the 1000 m and 3000 m marks). Flow all along the
lower boundary of the domain continues to be significantly
horizontal. Figure 4.1.1-1 ,shows that flow on Day 1200.872 is
almost completely vertical throughout the bog, including its
lowermost reaches, where flow now is only slightly horizontal,
and where hydraulic head finally has begun to decrease. Ten
days later (Figure 4.1.1-j), aU flowappears to be upward,
which remains true up until 300 days into the 'drought' (see
Figures 4.1.1-k and -1). Note that, during these 300 days, head
throughout the system continues to drop, and the water table
therefore predictably flattens. By Day 1518.780 (Figure 4.1.1
m), the head value in the uppermost cell of column 57 nears,
though is stilliess than, that of its 10wermost cell -- the
difference is 0.00529 m. This changes by the next time step
(Day 1522.536, Figure 4.Ll-a), when flow at and about the
horizontal center (2000 meter mark) of the bog finally
becomes downward again; the head difference between the
uppermost cell and lowermost cell in column 57 now is only
0.00282 m, which is in the range of differences observed on
Day 1200, just after the onset of 'drought'. Note that the
30
water table now is completely fiat, and that the highest
hydraulic head difference (between the uppermost and
lowermost ceUs in column 57) occurred roughly after Day
1210 but before Day 1300; during this time, this difference
was in the range of only 0.7 cm.
Simulation 4.1.2 (Figure 4.1.2) is identical to Simulation
4.1.1, except that the former was subjected to (normal' (Le.,
non-defictt, or P = ET) meteorological conditions on Day 1500,
untU the end of the simulation on Day 2000. Figure 4.1.2-a,
(Day 1500.408) shows the re-establishment of downward flow
at and around the horizontal center (1000 meter-to-3000
meter mark) of the bog, in the acrotelm and the uppermost
portion of the immediately underlying catotelm layer.
Hydraulic head through the domain is now rising, whereas
head feU during the (drought'. One day later (Figure 4.1.2-b),
fiow clearly has much less of a horizontal component to it, and
occurs from one side of the bog to the other, though
downward f10w still is limited to the upper portion of the bog.
Another day later (Figure 4.1.2-c), downward flow occurs as
far down as the upper portion of the lowermost catotelm layer.
31
a) Day 1500.408
b) Day 1501.462
c) Day 1502.527
d) Day 1503.640
e) Day 1504.368
Figure 4.1.2: Five featured time steps of Simulation 4.1.2. Note that thissimulation's 'drought' time steps are featured in Figure 4.1.1 (a to 1).Dimensions are in meters. Vertical exaggeration is 70 X.
The next day (Figure 4.1.2-d), the hydraulic head value in the
lowermost ceIl of column 57 finally becomes lower (by
0.00102 m) than that of its uppermost ceH, and downward
flow prevails throughout the bog, except for a couple of zones
at the very bottom of the domain; Figure 4.1.2-d, shows the
disappearance of these zones. Note that the water table is not
yet flat (though it has been lowered by about 43 cm at the
2000 meter mark to an elevation of 5.42 m), and yet there is a
return to a 'non-deficit' (P = ET) flow pattern; in Simulation
4.1.1, this return occurred much (about twenty days) later,
when the water table was basically flattened after being
lowered at the 2000 meter mark by 45 cm, to an elevation of
5.40 m.
4.1.2 Drought severity variation
The simulations included in this section examine whether
or not changing magnitudes of drought affect hydraulic
32
gradient reversaIs differently. Table 4.2 shows how the
magnitude of drought is decreased from one simulation to the
next. Another aim is to determine whether or not there exists
a definite minimum drought magnitude requirement for the
onset of a hydraulic gradient reversaI, or rather a range of
magnitudes providing various reversaI 'strengths.'
Figure 4.1.3, shows the flow patterns resulting from a
simulation which matches Simulation 4.1.1 , in every way
except for the drought magnitude, which now has been set to
600 mm/yr (800 mm/yr of precipitation, 1400 mm/yr of
evapotranspiration) instead of 1000 mm/yr. (This 600 mm/yr
deficit approximates the fifty-day 1998 meteorological balance
of 504 mm/yr observed at Fraser's (1999) Mer Bleue field site.
(Of the fifty days, only the Iast forty were characteristic of
drought conditions, thus making 504 mm/yr an underestimate
during this true drought period.) ET being larger than P
(during drought) in aIl of the simulations is consistent with
observed ET and P during summer months in peatland areas.
Pre-drought modelIed equivalence of ET and P, though not
observed in the field during non-summer months, can occur
33
l1 1
1/ ;;, /
~/ "'.... .1/ '" l.l' 1 ;'
t
a) Day 1200.006
b) Day 1200.014
..., " , ''"'~. ",... ./
t;:;, IT~'l
1 l" i i i l ~ j ~i
~ \ '1'0 co !
,~~Ld' .ou) /' /ID/ l " / ID 1 r 1 1Il ~ '1 " JL " '. \ \ ", l',. ' ...__ n:- ~1 • 1 • i 1 \ "
. li . _il . . Je _IJJJ 'é1l!;sJ tM.ww 1!'é1S§ '~1 ~~1 e:.2.®.!!J ~.2.\
c) Day 1200.017
d) Day 1200.418
e) Day 1200.726
\ \. \
(l) ,
ljJt'!"' 1 t~4l""t :J4LO t .Il' Il'~/' ID
.t ,1 l.f .••..• //. /.l''''_~
=?f-'----.,,-~--.,Hr-~---,Hr-----,u'-----,( -, ~
1000 1500 2000 2500 3000 3500 4000
f) Day 1200.872
Figure 4.1.3: Selected time steps of Simulation 4.1.3. Dimensions are inmeters. Vertical exaggeration is 70 X. See continuation for further timesteps of this simulation.
g) Day 1248.447
h) Day 1300.466
11 t 1 t 1 t t t t 1 ~ t 1 ~ t tiit t 1 IÔt .
r-_~\_}:,,~~.~~,~t__\_~_g~""~.~.F"t;_)_,~~1.-~~~!~~]-~_t_~~l __-1/-1 t 1~~J~~i) Day 1500.000
DDay 1916.667
Figure 4.1 .3 continued
lt
•tt 1t 1
•1l
Figure 4.1.4: Final time step (Day 2000.000) of Simulation 4.1.4. Dimensionsare in meters. Vertical exaggeration is 70 X.
for a short period of time leading up ta a drought.) Like
Simulation 4.1.1 , a flow reversaI occurs on the tirst day of
deficit (see Figure 4.1.3-b), and again is limited ta the acrotelm
and to the sides of the horizontal center Hne (2000 m mark) of
the bog. Within about 24 minutes of the onset of 'drought',
the reversaI occurs even at the bog's center Une (see Figure
4.1.3-c). This same pattern occurred less than ten minutes
Îllto the 'drought' of Simulation 4.1.1. Figure 4.1.3-d shows
the flow reversaI making its way down the peat profile, to the
upper portion of the lowermost catotelm layer. The next
figure shows that the reversaI has reached the bottom of that
layer (Le., the bottom of the bog), though flow there still has a
significant horizontal direction. The upward flow in Figure
4.1.3-f has much more of a vettical direction, and this is even
more so in the next three figures. The last figure of Simulation
4.1.3 (Day 1916.667) finally shows a return to downward flow
at the 2000 meter mark of the bog, though this occurs later
than it did in Simulation 4.1.1 (and thus later than it did in
Simulation 4.1.2). Note that the water table isn't quite flat
yet, and that the maximum upper-Iower bog head difference in
34
this simulation was 0.5 cm, around Day 1250. This difference
is 0.3 cm less than that simulated in Simulation 4.1.1, when
the meteorological deficit was 400 mm/yr larger.
Figure 4.1.4 shows the flow patterns resulting from a
simulation which matches Simulation 4.1.1 in every way
except for the drought magnitude, which now has been set to
200 mm/yr (1100 mmlyr of precipitation, 1300 mm/yr of
evapotranspiration) instead of 1000 mm/yr. This 200 mm/yr
balance is now lower than the 504 mm/yr balance observed by
Fraser (1999). Like Simulation 4.1.1 and Simulation 4.1.2, a
flow reversaI occurs early on in the present simulation, but
this reversaI begins later (58 minutes into Day 1200) and
continues to the end of the simulation (Day 2000). Note that
the water table has not been lowered to the extent that it was
lowered in Simulations 4.1.1 and 4.1.3; in Simulation 4.1.4, on
Day 2000, the water table is observed to have dropped by only
about 15 cm, down to roughly 5.7 m elevation. Thus, the
water table at this time still is clearly mounded. Note also that
the hydraulic head difference, between the uppermost and
lowermost cells in column 57, at this time is only 0.05 cm,
35
indicating that a retum ta downward flow (Le., a hydraulic
gradient reversaI) isn't far off.
Lowering the 'drought' severity again in Simulation 4.1.5,
to only 100 mm/yr (1150 mm/yr precipitation, 1250 mm/yr
evapotranspiration) gave similar results (see Figure 4.1.5) ta
thase of Simulation 4.1.5. Specifically, a flow reversaI occurs
from the first day of 'drought', and continues until the end of
the simulation on Day 2000, when flow is strongly upwards.
Also similar is the hydraulic head difference between the
uppermost and lowermost ceUs in column 57 at this time,
which is only 0.02 cm, again indicating the soon return of
downward flow. The water table never drops below an
elevation of 5.78 m.
In Simulation 4.1.6 , 'drought' precipitation was raised to
1185 mm/yr, and 'drought' evapotranspiration was lowered to
1215 mm/yr, giving a deficit of only 30 mm/yr. While a flow
reversaI did occur, it did not occur as quickly after the onset
of 'droughf as was observed in Simulations 4.1.1 to 4.1.5.
Specifically, after twenty-one hours have elapsed (see Figure
4.1.6-a), flow throughout the bog clearly is downwards,
36
Figure 4.1.5: Final time step (Day 2000.000) of Simulation 4.1.5. Dimensionsare in meters. Vertical exaggeration is 70 X.
a) Day 1200.872
b) Day 1202.615
-- ...- -- ...- J"'".- ....... -- ---"'lo -- --
.-,- - -- .....~ _l~
c) Day 1204.523
d) Day 1205.429
e) Day 1206.516
. -. " \.... " ... t.
-... __ .....- __ .~W" .-.. _ _ ._.- _ ......
1 / ,/ / /LIS'l../ ,.." ./ .,J>ll:.... ,," lSi... ."\fI'l) ca \. /.,. 'O.
o q-i/-J-lt)..;...._-_ld -_-..,;flr-_-ln~....._-_-_-_-..,~,_-_-~1rl~"'_-·_~.,t~'_--~----"'_···---'u,-·--···_·-----,·~ _.... ~- -- -- --,...; -- .. -"d :u
o 500 1000 1500 2000 2500 3000 3500 4000
f) Day 1207.820
Figure 4.1.6: Selected time steps soon after the onset of 'drought' inSimulation 4.1 .6. Dimensions are in meters. Vertical exaggeration is 70 X.See continuation of this figure for further time steps.
g) Day 1216.221
- - - .-\ ... \'" \ \ 1 • .1 ,1 /'1 1 1
'. '-, \ (/ ./..... .••. \ .. •••4l'
~'D:@J1j) .'" ... .:.P'- ._~
h) Day 1258.138
i) Day 1344.673
-_-~____ ".., { '} ,-~_-..f».-~-_~_-
1 ~..... -" -' --- ....... -. .....- ...~ ...... "" "" " l ..--- ~J /" "---~I~"'" .- .'''' -~ -- -- ...... 'an
. t:5· _.~~ - ~ -- -- -- -~- -. -. "'-. \.'. ---- .."'.- .,~ .- - -~ -~ -- ~ ~ ~:t- 1do -t, - -. -' - ..·--1.,---·----11--- -----.---,:- -- ----------u--- ----- -1.~-------'T--- - -.----.r---~-'-
o 500 1000 1500 2000 2500 3000 3500 4000
j) Day 2000.000
Figure 4.1.6 continued
though with almost horizontal flow in the acrotelm to the sides
of the horizontal center region (2000 m mark). The hydraulic
head difference between the uppermost and low€rmost ceUs in
the center column (column 57) has decreased to only 0.015
cm, which suggests that a reversaI may soon come about.
Figure 4.1.6-b shows a flow pattern, more than 2.5 days into
the 'drought', which differs only slightly from that of Figure
4.1.6-a: where flow in the acrotelm had been almost
horizontal, it now is completely so. Note that the head
difference between the uppermost cell and lowermost cell in
the center column now is down to only 0.006 cm, and that the
lower head value has risen, though by only 0.002 cm;
meanwhile, head in the uppermost ceH has dropped by 0.004
cm since Day 1200.872. Figure 4.1.6-c shows the flow pattern
two days later: flow in the center region of the bog is
downward at aIl depths, while flow just to the sides actuaIly is
slightly upward in the acrotelm, horizontal in the layer
immediately underlying the acrotelm, and slightly downwards
in the lowermost catotelm layer. Beyond these side zones, flow
is more or less horizontal at aIl depths. FinaIly, about S.S days
37
into the drought (see Figure 4.1.6-<1), a clear flow reversaI
occurs, though it differs from the reversaIs seen in previous
simulations in that this one begins below the acrotelm. (Up
until now, reversaIs always had begun within the acrotelm.)
Note that there now is no head difference between the
uppermost and lowermost ceUs of the center column; the
upper value has decreased while the lower value has increased
to match it. The head value at the stagnation point, which is
found near the boundary between the two catotelm layers, is
only 0.001 cm greater than the upper- and lower head values.
About 6.5 days into the 'drought' (see Figure 4.1.6-e), the flow
reversaI has made its way down to the bottom of the bog, and
upward flow occurs throughout the domain, except at the very
top-center, where flow is horizontal. The stagnation point has
disappeared, and the head at the top of the bog has decreased
by another 0.001 cm, while head at the bottam has remained
constant. By Day 1207.820 (Figure 4.1.6-f), the top head value
has decreased by another 0.002 cm, while the bottom value
has decreased by haIf as much. The resulting flow pattern
shows flow throughout the domain to be more strongly
38
upward than in part e). This flow pattern is not unlike that
obsenred by Fraser (1999), though the present, simulated
'drought' is more than an order of magnitude less severe than
that which occurred at his field site in 1998. Sîxteen days into
the 'drought' (see Figure 4.1.6-g), upward flow is even more
pronounced, and the head difference between the top and
bottom of the bog now has doubled to 0.004 cm; while both
head values have continued to decrease since Day 1207, the
upper value decreased by 0.009 cm as the lower value
decreased by 0.007 cm. The flow reversal's strength begins to
wane by Day 1258 (see Figure 4.1.6-h), as evidenced by the
more lateral flow at the top-center of the bog, and by the fact
that the deep head value changed slightly faster (dropped by
0.045 cm) than did the shallow head value, which decreased
by 0.044 cm The shallow-deep head difference reflects this,
as it now has decreased by 0.001 cm, down to only 0.003 cm.
This head difference, along with the head gradient, remains
the same by Day 1344 (see Figure 4.1.6-i), though a re
establishment of downward flow now can be seen at the
shallow center of the bog, and a re-establishment of lateral
39
flow now occurs just to the sides of center. Although there
still is upward flow throughout much of the deeper peat
profile, it is weakened as the shallow~deephead difference
reverses to 0.001 cm by Day 2000, and downward flow pattern
prevails throughout the shallow portion of the bog. Note that,
since the beginning of this simulation's 'drought', the water
table has lowered by only 0.5 cm.
Simulation 4.1.7 (see Figure 4.1.7), which features a
'drought' deficit of only 20 mm/yr, yields no flow reversaI at
aIl. This is the first time, since we began Iowering the deficit
successively from one simulation to the next, that a flow
reversaI has not occurred. From Day 1200.034 ta Day 1223,
the flow pattern throughout the bog acquires a more lateraI
tendency, and the same is true on Day 2000. The end result
(see Figure 4.1.7~c) is a pattern in which flow still is
predominantly downwards. It is important to note, however,
that, despite the meteorological deficit, the water table
actually rose slightly (by 0.5 cm) since the beginning of this
simulation's 'drought'.
40
a) Day 1200.034
" . ,.... ;" ,.J / , " \, "'... ~'" '.., " "... '- "- ~'ld~'" '- Irl- '0 m l'
._....::.:.-----,~ ,,.....-ID~:-. -;-;g-'i=%I~"'-~):"""'::_: ...:.,~_.:~~""'~~-~:D""'~ _~'~~_'~_~O:H;_::~)_~~_O' -~~:_;-,\;]';"'-t-~:_' _:-'~~~ t~b) Day 1223.361
6""~: t"/.. -"01~... ~.~: I-.o~ ~OO~ -_~ ~·.00 t -..~ ~~ ~.:. ;/.. 1 \ '-', '0'00 ~-- '- '~ '- -... '- 0---'.--. -- ~- n~w . ~." - l'.' ~ .' (0 .~ ,,' ". i \ '\. ..... ...... 'Lo ....... "- '.., '00.' .oo~ "'''' ~. 1
", . -- --ur- -- -- __ " , , 0 • • • "', ••-- ,; -- _ -~ -- -- -~.-- _-- ~_ .0
Oc;".~" lfJ .....1$) ~ !J" lj -- lf' -,,-- : .-- --,... ;' _...~\- 'J J '
o 500 1000 1500 2000 2500 3000 3500 4000
c) Day 2000.000
Figure 4.1.7: Selected time steps fram Simulation 4.1.7. Dimensions arein meters. Vertical exaggeration is 70 X.
4.1.3 Catotelm alteration
Figure 4.1.8 shows the flow patterns resulting from the
merging of the two previously distinct catotelm layers into one
layer, with aIl storage properties (see Table 4.1) being constant
throughout. The meteorological deficit is only 50 mm/yr.
Figures 4.1.8-a and 4.1.8-b show how the flow pattern changes
from one which is mostly downward to one which has a clearly
more lateral aspect to it. However, it's not until the second
day (see Figure 4.1.8-c) of the 'drought' that a reversaI takes
place -- one which is limited to two zones, each a few hundred
meters long, just to either side of the 2000 m mark. While
flow at the 2000 m mark still is downward, it is much more
horizontal than on Day 1200. This changes back to more
downward flow, and the two zones of upward flow disappear
by Day 1500 (see Figure 4.1.8-d), and this new pattern persists
for the rest of the simulation (see Figure 4.1.8-e). By the end
of the simulation, the water table ended up being lowered by
only 0.8 cm.
41
III ,c- 1It"~; i'lO"'- /" ",/ / /" ./ ~ ./ / / 1:J "ur -uï- -.rr-- .-.- -- ~........- J.-' /~ 11 U
b :lUCI il :JU:1 i\ "iO
t1 ~
111
\
:~:.Jt:.Ii
a) Day 1200.017
1 \1 \/ .....3ITJll'l
b) Day 1200.872
------ .......L" lit" ID '0 '" 00
"-ut --aô- -!n-- - ~- - .".1- ~ ~- - - -'-
c) Day 1202.178
... ~-- ~ ---- - -- ....- .....- .......co~ -- .-- .,-r
.~ ID '0 '" "~ "-ut -I{)-"-'.- uf - - - ~- - -
D UQ] ::t:::Œ ::uhrr
d) Day 1500.000
e) Day 1847.222
Figure 4.1.8: Selected time steps tram Simulation 4.1.8. Dimensions arein meters. Vertical exaggeration is 70 X.
When the deficit was lowered to 0111y 20 mm/yr
(Simulation 4.1.9), no reversaI occurred, but note that the
water table indeed did faH with time, which wasn't the case in
Simulation 4.1.7.
When the deficit was raised from 50 mm/yr to 600 mm/yr,
the result was a progression of flow patterns almost identicaI
to that of Simulation 4.1.3, which also has a deficit of 600
mm/yr.
4.1.4 Discussion of LRBT simulations
The first two simulations, 4.1.1 and 4.1.2, when considered
together, clearly show that a precipitation event can bring
back, as occurred in Fraser (1999), (downwards and outwards'
flow more quickly than if a system is aHowed to continue
without such drought relief. The reason for this return is the
hydraulic head contribution, made by precipitation, to the
upper portions of the peat column. Head in this zone
increases ta a level which is greater than deep head, thus
restoring non-drought flow conditions. The implications of
42
this on peatland hydrology are important, as it i5 rare for
peatland regions to go without rain for months on end. Thus,
reversed groundwater flow does not persist for long, and one
would expect the vegetation of a peatiand to be only
minimally influenced by discharging groundwater, despite this
water likely being much more nutrient-Iaden than the
recharging precipitation which dominates throughout most of
the year. Still, it was deemed instructive to simulate hundreds
of days of continuous drought, in order to gain an idea of how
much time it might take for reversaIs to disappear without
being forced to do sa by changing meteorological conditions.
This would appear to be relevant, considering that climate
change may cause peatland regions to experience longer
drought periods in the future. In turn, this wouid expose
peatland vegetation to longer periods of discharging, possibly
nutrient-Iaden groundwater.
As expected, Simulations 4.1.3 to 4.1.7 showed that less
severe droughts lead to more time required for a gradient
reversaI ta begin, and that the duratian of the reversaI
decreases with less severe drought. The reason for this is that
43
more severe droughts induced greater drops in hydraulic head
in the upper portions of the peat column, thus necessitating
less time for these heads ta drop below the head values of
deep peat. Along similar Unes, the longer length of reversaIs
induced by the more severe droughts has to do with their
greater (reversed) head gradients; that is, surface heads in
systems with less severe droughts didn't have to recover as
much head, relative ta deep heads, as did surface heads in
systems with more severe drought.
Aiso unsurprisingly, there exists a drought magnitude
which can be considered to be the minimum required for a
hydraulic gradient reversaI to occur. However, this minimum
magnitude (only 20 mm/yr) hardly can be considered a
drought. One possible reason for this low cut-off point is the
overall low hydraulic conductivity of the systems, combined
with the great closeness in value of shallow heads and deep
heads (even at the center of the domain) just before the
beginning of (drought' conditions; in the field, it probably is
somewhat uncommon to encounter such similar shallow/ deep
head values at a bog's domed center, though such close values
44
are commonplace between a bog's dome and its margins.
Regardless, a eut-off point was reached, which demonstrates
that a deficit is required for the onset of a reversaI.
Equally unexpectedly, reversaIs were brought on much
more quickly than in the field. Simulated reversaIs developed
within minutes, for example, while observed reversaIs take
days to develop. Again, this likely is due to the hydraulic
conductivity assigned to the modelled domains. While
simulated Kwas made to match observed K values, it is
entirely possible that effective K in the field differs
significantly from individual, averaged values. Effective K
might be influenced by factors such as pipes and/or layering
of peat which could not be simulated due to complexity. A
stochastic approach, which is beyond the scope of the present
study, rather than the deterministic approach used, might
have led to more realistically distributed Kvalues. In fact,
simulated K could be altered only so much in the present
study, due to system overflow and/or lack of convergence
when choosing certain desired K values. That is, stable
solutions were possible only with a limited set of Kvalues.
4S
While this is somewhat disconcerting7 the reversed flow
patterns simulated closely match those observed (see Fraser,
1999). Furthennore, the simulations provide us with a sense
of the rela tive importance of one meteorologicai setting over
another.
Merging of the two catotelm zones resulted again in flow
reversaIs, at drought Ievels of both 50 mm/yr and 600 mm/yr,
which mimicked corresponding simulations of bi-catotelmic
systems. The possibility exists, then, that the upper catotelm
layer of previous simulations played a more important raIe
(than the lower catotelm layer) in influencing reversaIs, seeing
as the adoption of the upper catotelm layer's peat parameter
values by the lower catotelm layer did not produce observably
different flow. Or, it may be the case that hydraulic
conductivity in both layers, merged or not, was so low as to
negate the possibility of not producing a flow reversaI. Again,
due to overflow and convergence considerations, KcouId not
be varied by rnuch.
46
4.2 KBT simulations
In this section, aIl simulations are for a kettle bog made up
of 100 columns, which cover the 1000 m length of the bog,
and 19 layers that cover the 6.2-to-S.2 m height of the bog. As
in the previous section, K, along with specifie yield and
specifie storage, are constant from cell to cell within any lK
zone', and Kz always is one arder of magnitude lower than Ky,z
in any one cella Again, horizontal isotropy is assumed, and full
parameterization descriptions can be found in Table 4.3.
4.2.1 Extreme drought conditions
Figure 4.2.1 shows a simulation in which only one catotelm
layer, with a maximum depth of 6 m, is present. The acrotelm
is a constant 50 cm deep. After the system is allowed to reach
a stable state over the first 1000 days of the simulation, a
meteorological deficit of 1000 mm/yr is brought about, with
no apparent change in flow pattern for the rest of the
simulation (see Figure 4.2.1), even though hydraulic head at
47
\1 \
a) Day 1000.000
1 \ "1 \
b) Day 1000.566
c) Day 1007.381
J.r 1 ~
8.2::
200 400
i \
l "
600 800 1000
d) Day 1022.310
Figure 4.2.1: Selected time steps from Simulation 4.2.1. Demensions arein meters. Vertical exaggeration is 15 X.
the horizontal center of the bog (500 m mark) is changing
(decreasing) faster in the shallow part of the peat profile than
it is in the deep part. By Day 2000, the water table has been
lowered by about nineteen cm.
4.2.2 Hydraulic conductivity
Figure 4.2.2 shows flow patterns of a simulation which is
identical to Simulation 4.2.1, except that the former is
characterized by acrotelm K which is one order of magnitude
lower than acrotelm K in the latter. The results are quite
noticeable, as flow just to the sides of the horizontal center
(columns 50, 51) of the bog changes from being more or less
lateraion Day 1000.000 (see Figure 4.2.2-a) to much more
upwards on Day 1003. This flow pattern persists up until
about Day 1115, at which time flow changes back to being
more or less horizontal (see Figure 4.2.2-e). From Day 1200
on, this new flow pattern doesn't really change from that
featured in Figure 4.2.2-e (Day 2000). The end water table
height (at the center of the bog) on Day 2000 is 7.49 m, which
48
a) Day 1000.000
b) Day 1003.489
c) Day 1010.689
; \1 \.
d) Day 1115.683
o 200 400 600 800 1000
e) Day 2000.000
Figure 4.2.2: Selected time steps trom Simulation 4.2.2. Dimensions arein meters. Vertical exaggeration is 15 X.
translates into a drop of about 22 cm since the beginning of
the ldrought.'
4.2.3 Drought severity variation
Figure 4.2.3 shows what happens when the precipitation
and evapotranspiration levels of Simulation 4.2.2's 'drought'
period are lowered to 800 mm/yr and 1400 mm/yr,
respectively. The resulting 600 mm/yr deficit results in the
same sort of flow patterns observed in Simulation 4.2.2 (see
Figures 4.2.2 and 4.2.3). Hydraulic head throughout the
system declines throughout the ldrought' stress period. (The
end water table elevation was 7.6 m, which is a drop of about
Il cm from the elevation at the beginning of the ldrought'.)
As head drops, horizontal flow just to the sides of column 51
changes to much more upward flow, as early as Day 1000.203
(see Figure 4.2.3-b). This pattern remains the same up until
about Day 1662, at which time flow here becomes horizontal
again.
49
a) Day 1000.000
-~-~ -1- i :-:r-l- / s, \S " -f' __ r- F-\- ~_f·1-'- -..0_ ~ - ""':' - -'D- /' 1 \" ~-- ~ "':' - ~ ~...- .J,
b) Day 1000.203
c) Day 1662.926
o 200 400 600 800 1000
d) Day 1755.528
Figure 4.2.3: Selected time steps trom Simulation 4.2.3. Dimensions arein meters. Vertical exaggeration is 15 X.
In Simulation 4.2.4, a water table drop of about only 2 cm
was effected (by setting precipitation to 920 mm/yr and
evapotranspiration to 1080 mm/yr) over the course of the
(drought' period. The resulting flow was indistinguishable
from that of Simulation 4.2.3 in terms of observable flow
pattern and flow pattern evolution. For this reason,
Simulation 4.2.4 does not have a fIgure.
4.2.4 Porosity
In Simulation 4.2.5, porosity of the 50 cm acrotelm was
raised from 90 % to 95 %, and the porosity of the catotelm was
lowered from 90 % to 85 %. The meteorological deficit was
1000 mm/yr (600 mm/yr of precipitation minus 1600 mm/yr
of evapotranspiration). Although the flow 'reversaI' seems to
have begun slightly earlier than it did in Simulation 4.2.2, the
resulting flow patterns for the first 1423 days matched those
of the first 1423 days of Simulation 4.2.2, which had the same
meteorological deficit, but a porosity value of 90 % throughout
the entire domain. After Day 1423, however, Simulation 4.2.5
50
a) Day 1000.000
b) Day 1000.099
c) Day 1002.381
1 \1 \
d) Day 1423.202
o 200 400 600 800 1000
e) Day 1467.860
Figure 4.2.4: Selected time steps from Simulation 4.2.5. Dimensions arein meters. Vertical exaggeration is 15 X. Note that Simulation 4.2.4 doesnot have a figure.
differs in that flow returns to a more horizontal pattern (see
Figure 4.2.4-d). The water table drawdown ended up being
about 22 cm which matches the drawdown of Simulation 4.2.2.
Note that hydraulic head in the uppermost cell of column 51
increases slightly between Days 1423 and 1467, while head in
the column's lowermost cell decreases, thus showing the time
lag in pressure change transmission down the peat profile
which aIl simulations feature.
4.2.5 Discussion of KBT simulations
Because no KBT flow reversaI could be effected via
meteorological forcing of even an extreme nature (see next
paragraph), more emphasis was placed on determining how
altering substrate properties might influence, and perhaps
even bring about, flow reversais.
Even though there existed a Ume lag (see Simulation 4.2.1)
in head change between upper and Iower sections of the
domain -- the supposed driving factor behind gradient
reversais, also seen in the LRBT simulations -- no reversai
51
developed in any KBT simulation. The fact that reversaIs have
been observed in field KBTs (Devito et al., 1997) but not in the
simulations suggests that, as with the LRBT simulations,
parameterization was not sufficiently mimicked from the field.
As stated previously, it is possible that the distribution of
certain parameters such as hydraulic conductivity did not
match, in any simulation, real-world distribution of K.
Another possibility is that the morphology/topography of the
present study's KBT domains did not sufficiently match that of
Devito et al. (1997), for example.
Although no reversaIs could be obtained for KBT
simulations, a switch to more upwards-directed flow when
going from Simulation 4.2.1 to Simulation 4.2.2 suggests that
acrotelm hydraulic conductivity plays a role in KBT reversaIs.
The more upward-directed flow of Simulation 4.2.2 is similar
in pattern to the reversed flaw observed at Fraser's (1999) Mer
Bleue site, even though the former is a KBT and the latter a
LRBT. While Simulation 4.2.2 did nat praduce upward flow at
its horizontal center, it shauld be nated that the reversaI
52
observed at Mer Bleue, tao, did not occur at its horizontal
center.
Unexpectedly, lessening drought severity led ta longer
lasting (reversais' (Le., upward-tending flow) -- likely a result
of the water table not dropping as far in the less severe
drought simulations (4.2.3,4.2.4). However, one might expect
more severe droughts to bring about longer lasting changes in
flow pattern, as occurred in the LRBT simulations. This brings
up the possibility that the KBT 'reversaIs' had nothing to do
with the time lag effect produced in both KBT and LRBT
simulations. The possibility exists that the bowl shape of KBT
systems has more to do with these switches to more upward
tending flow than does the time lag effect. The testing of
various bowl shapes is beyond the scope of the present study.
While changing the porosity of both the acrotelm and the
catotelm (Simulation 4.2.5) seemed to have Httle effect on flow
pattern, it is reasonable to suggest that the quicker return to
horizontal flow exhibited by Simulation 4.2.5, as compared to
4.2.4, may be due to the acrotelm's slightly increased ability to
drain away water. The hypothesis that acrotelm K and
S3
porosity are more important, as regards flow reversaIs, than
corresponding catotelm properties is one that probably
favours K, as was shown previously.
Chapter 5.0 Summary and Conclusion
As observed in the field, hydraulic gradient reversaIs can
be significant in altering the chemistry of peatlands -
particularly around the water table -- even when low hydraulic
gradients are involved (e.g., see Fraser, 1999, for an example
of dissoived organic carbon movement). Thus, aithough the
hydraulic gradients of the present study are even lower than
field-observed gradients, the importance of reversaIs should
not be overlooked. However, the effects of reversaIs on
peatland water storage and mass movement of water appear to
be limited, due to both the low hydraulic gradients involved
and the limited time during which reversaIs exist. Because of
this, their effects on both short-term peatland vegetation
change and long-term peatland evolution, for example, may be
limited. Still, the phenomenon of methane outgasing from
54
peatlands is of importance in the context of global climate
change, and appears ta require no more than a slight change
in flow pattern in arder ta occur. Thus, the dramatic reversals
produced in many of this study's simulations, and the
differences in their evolution, would appear ta be of some
significance in at least the dimate change context, even
though the timing of the simulated reversals differed from
those observed in the field. This thesis has shown that
meteorological deficit is required for the onset of hydraulic
gradient reversaIs, and it appears equally safe ta say that
larger droughts should be associated with longer-lasting flow
reversaIs in peatlands, and, therefore, possibly with larger
methane outgasing events. This thesis has shown that
hydraulic conductivity, too, plays a key role, though the
influence of catotelm division may not be as important ta
reversaIs (and outgasing) as once thought. It is hoped that,
with further work involving better parameterization -- perhaps
of a stochastic nature -- both simulated and observed flow
change (in response to meteorological and substrate forcing)
will come to match each other more closely. As the present
55
study was aimed more at discovering relative differences
between the effects of various forcing agents, rather than
matching field conditions exactly, its results should be viewed
accordingly.
S6
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60