Modelling meteorologicalandsubstrate influences meteorologicalandsubstrate influences onpeatland hydraulicgradientreversais

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  • Modelling meteorological and substrate influences on peatlandhydraulic gradient reversais

    Dennis ColauttiDepartment ofGeography

    McGill UniversityMontral, Qubec

    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

    RSUM 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 snulations 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 fieId-observed 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

  • Rsum

    Une tude hydrologique a t effectue, en utilisant le logicielMODFLOW, afin de dterminer l'importance relative descertains des facteurs qui peuvent influencer les renversementsde gradient hydraulique dans deux types fondamentaux (LRBTet KBT) de tourbire. Les deux types ont t subis dediverses conditions mtorologiques (dficit d'eau). Aussivari tait le substrat, afin de dterminer son importance. Desrenversements ont t fructueusement simuls partout dansles systmes LBRT, mais pas dans les systmes KBT. Bien queles simulations de flux ressemblaient, en forme, desrenversements observs, les renversements simulesurvenaient beaucoup plus vite que dans la nature, peut tre cause de l'utilisation des paramtres insuffisammentdistribus, tel conductivit hydraulique. Les renversementstaient termins aisment avec la simulation des conditions denon-scheresse. La dure de renversement diminuait, et letemps requis pour effectuer le renversement accroissait, avecune diminution dans la svrit de scheresse dans lessystemes LRBT. Inopinment, la dure des semi-renversementsdans les systemes KBT diminuait avec les plus grandesscheresses. Ceci suggre un mcanisme alternative/diffrentpour effectuer des renversements dans les systmes KBT.Quant l'volution des renversements, conductivithydraulique tait plus important que les diffrences dans lesconfigurations variables de catotelm et aussi la porosit.

    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