Hydrology and hydrological modelling of acidic mires in central France

517 Pages·2015·40.64 MB·English

H H M
YDROLOGY AND YDROLOGICAL ODELLING
A M C F
OF CIDIC IRES IN ENTRAL RANCE

ARNAUD J. DURANEL

Thesis submitted for the degree of
Doctor of Philosophy (PhD)

June 2015
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Declaration

I, Arnaud DURANEL, confirm that the work presented in this thesis is my own. Where information
has been derived from other sources, I confirm that this has been indicated in the thesis.

15/06/2015

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Abstract
This thesis identifies, quantifies and models water fluxes within the Dauges National Nature
Reserve, an acidic valley mire in the French Massif Central. A range of techniques were used to
investigate the nature and geometry of granite weathering formations and of peat deposits.
Rainfall, reference evapotranspiration, stream discharge, stream stage, groundwater table
depths and piezometric heads were monitored over a three-year period. The distributed,
physics-based hydrological model MIKE SHE / MIKE 11 was used to model water flow within the
mire and its catchment. It was shown that the mire is mostly fed by groundwater flowing within
the densely fissured granite zone and upwelling through the peat deposits. Upwelling to the peat
layer and seepage to overland flow were highest along the mire boundaries. However
hydrological functioning differs from this general conceptual model in some locations due to the
high variability of the peat hydraulic characteristics, the presence of highly permeable alluvial
deposits or past human interference including drainage. The equivalent porous medium
approach used to model groundwater flow within the fissured granite zone gave satisfactory
results: the model was able to reproduce discharge at several locations within the high-relief
catchment and groundwater table depth in most monitoring points. Sensitivity analyses showed
that the specific yield and horizontal hydraulic conductivity of the fissured zone are the
parameters to which simulated stream discharge and groundwater table depth, including in peat,
are most sensitive. The model was forced with new vegetation parameters to assess the
potential impacts of changes in catchment landuse on the mire hydrological conditions.
Replacement of the broadleaf woodlands that currently cover most of the catchment with
conifer plantations would lead to a substantial reduction in surface and groundwater inflows to
the mire and to a substantial drop in summer groundwater table depths, particularly along the
mire margins.
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Table of contents
Table of contents

Declaration ..................................................................................................................................... 3
Abstract .......................................................................................................................................... 5
Table of contents ............................................................................................................................ 7
List of figures ................................................................................................................................ 13
List of tables ................................................................................................................................. 23
Acknowledgements ...................................................................................................................... 27
Copyright permissions.................................................................................................................. 31
Acronyms and abbreviations ........................................................................................................ 33
Chapter 1. Hydrology and hydrological modelling of mires ......................................................... 35
1.1. Introduction ...................................................................................................................... 35
1.2. Peat, peatlands and mires ................................................................................................. 35
1.2.1. Definitions .................................................................................................................. 35
1.2.2. Peatland distribution .................................................................................................. 37
1.2.3. Mire classification ...................................................................................................... 38
1.2.4. Vegetation and environmental gradients in mires ..................................................... 41
1.2.5. Values and environmental services ............................................................................ 44
1.2.6. Threats to mires ......................................................................................................... 48
1.3. Peatland hydrology ........................................................................................................... 51
1.3.1. Water and water flow in peat soils ............................................................................ 51
1.3.2. Mire water balance .................................................................................................... 58
1.4. Peatland hydrological modelling ....................................................................................... 63
1.4.1. Hydrological modelling .............................................................................................. 63
1.4.2. Use of hydrological modelling in peatland research and conservation ..................... 68
Chapter 2. Mires of the Massif Central and the Dauges catchment ............................................ 79
2.1. Introduction ...................................................................................................................... 79
2.2. Peatlands of the French Massif Central ............................................................................ 79
2.2.1. Physical context .......................................................................................................... 79
2.2.2. Statutory designations ............................................................................................... 80
2.2.3. Changes in landuse in the Massif Central over the last century ................................ 82
2.3. Review of available research on peatland hydrology within the Massif Central .............. 83
2.4. Thesis aims ........................................................................................................................ 84
2.5. Research objectives ........................................................................................................... 84
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2.6. Research site: the Dauges National Nature Reserve ........................................................ 85
2.6.1. Location and general context .................................................................................... 85
2.6.2. Geology ...................................................................................................................... 89
2.6.3. Landuse ...................................................................................................................... 91
2.6.4. Rationale for the choice of the Dauges catchment as a research site ....................... 92
2.7. Research design and thesis outline .................................................................................. 93
Chapter 3. Geological model of the Dauges catchment .............................................................. 97
3.1. Introduction ...................................................................................................................... 97
3.2. Surface topography........................................................................................................... 97
3.2.1. Methods .................................................................................................................... 97
3.2.2. Results ..................................................................................................................... 100
3.3. Development of a 3D model of granite weathering formations .................................... 103
3.3.1. Current knowledge on granite weathering and peri-glacial formations within the
research site ...................................................................................................................... 103
3.3.2. Methods .................................................................................................................. 104
3.3.3. Results and discussion ............................................................................................. 112
3.3.4. Conclusion on granite weathering formations and periglacial formations within the
Dauges catchment ............................................................................................................. 136
3.4. Development of a 3D hydrogeological model of peat and alluvial deposit .................... 139
3.4.1. Methods .................................................................................................................. 139
3.4.2. Results and discussion ............................................................................................. 142
3.5. Hydraulic conductivity of peat and alluvial sediments ................................................... 150
3.5.1. Methods .................................................................................................................. 150
3.5.2. Results and discussion ............................................................................................. 153
3.6. Pedological survey of mineral soils ................................................................................. 160
3.7. Conclusion ...................................................................................................................... 161
Chapter 4. Hydrology: data acquisition and qualitative analysis ............................................... 165
4.1. Introduction .................................................................................................................... 165
4.2. Stream stage and discharge monitoring ......................................................................... 165
4.2.1. Methods .................................................................................................................. 165
4.2.2. Results and discussion ............................................................................................. 169
4.3. Piezometry ...................................................................................................................... 179
4.3.1. Methods .................................................................................................................. 179
4.3.2. Results and discussion ............................................................................................. 184
4.4. Conclusion: conceptual hydrological and hydrogeological model ................................. 208
Chapter 5. MIKE SHE / MIKE 11 model development ............................................................... 211
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5.1. Introduction .................................................................................................................... 211
5.2. Model objectives and choice of modelling environment ............................................... 211
5.3. The MIKE SHE/MIKE 11 modelling environment ............................................................ 212
5.3.1. General description .................................................................................................. 212
5.3.2. Use of MIKE SHE in wetland hydrological modelling ............................................... 219
5.4. Model design and initial parametrisation ....................................................................... 225
5.4.1. Precipitation ............................................................................................................. 225
5.4.2. Evapotranspiration and unsaturated flow ............................................................... 225
5.4.3. Land use ................................................................................................................... 231
5.4.4. Hydrographic network and hydrodynamic model ................................................... 244
5.4.5. Overland flow ........................................................................................................... 247
5.4.6. Saturated flow .......................................................................................................... 247
5.4.7. Summary of initial parameters ................................................................................ 250
5.4.8. Model grid size ......................................................................................................... 251
5.5. Conclusion ....................................................................................................................... 256
Chapter 6. Model calibration, validation & sensitivity analysis ................................................. 259
6.1. Introduction .................................................................................................................... 259
6.2. Model calibration and validation .................................................................................... 259
6.2.1. Calibration and validation against observed time-series ......................................... 259
6.2.2. Validation of the calibrated model against wetland vegetation distribution .......... 276
6.3. Model sensitivity ............................................................................................................. 279
6.3.1. Sensitivity of the initial model to varying depth and shape of the fissured zone .... 279
6.3.2. Systematic sensitivity analysis of the calibrated model ........................................... 283
6.3.3. Sensitivity of the calibrated model to model resolution ......................................... 288
6.3.4. Impact of spatial variation in potential evapotranspiration and peat
characteristics .................................................................................................................... 293
6.3.5. Sensitivity to the length of the warm-up period and the issue of the overland flow
component convergence ................................................................................................... 298
6.4. Model performance and sensitivity: general discussion and recommendations ........... 301
6.4.1. The issue of microtopography ................................................................................. 301
6.4.2. Channel flow ............................................................................................................ 302
6.4.3. Parametrisation of the unsaturated and saturated peat ......................................... 303
6.4.4. Parametrisation of the unsaturated zone on mineral soils ...................................... 304
6.4.5. Parametrisation of the granite fissured zone ........................................................... 305
6.5. Conclusion ....................................................................................................................... 307
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Chapter 7. Simulated water balance and hydrological fluxes between the mire and its catchment
................................................................................................................................................... 309
7.1. Introduction .................................................................................................................... 309
7.2. Spatial characterisation of the mire hydrology .............................................................. 309
7.2.1. Methods .................................................................................................................. 309
7.2.2. Results and discussion ............................................................................................. 310
7.3. Water balance ................................................................................................................. 316
7.3.1. Methods .................................................................................................................. 316
7.3.2. Results and discussion ............................................................................................. 318
7.4. Conclusion ...................................................................................................................... 328
Chapter 8. Impacts of catchment landuse on wetland hydrology ............................................. 329
8.1. Introduction .................................................................................................................... 329
8.2. Impacts of catchment landuse on the hydrology of mires ............................................. 329
8.3. Methods ......................................................................................................................... 331
8.4. Results and discussion .................................................................................................... 333
8.5. Conclusion ...................................................................................................................... 357
Chapter 9. Hydrology and hydrological modelling of acidic mires in the French Massif Central:
conclusion and recommendations ............................................................................................ 361
9.1. Concluding review .......................................................................................................... 361
9.1.1. Development of a three-dimensional geological model of the mire and its catchment
........................................................................................................................................... 361
9.1.2. Hydrometeorological monitoring ............................................................................ 363
9.1.3. Conceptual hydrological model ............................................................................... 364
9.1.4. Numerical hydrological model ................................................................................. 365
9.1.5. Sensitivity analyses .................................................................................................. 366
9.1.6. Simulated water balance and hydrological fluxes ................................................... 367
9.1.7. Hydrological response to changes in catchment landuse ....................................... 368
9.2. Recommendations for further research ......................................................................... 369
9.2.1. Improving the hydrological understanding and modelling of the Dauges site ........ 369
9.2.2. Validating research findings using other methodological approaches .................... 377
9.2.3. Broadening research questions: the impact of climate change .............................. 379
9.3. Recommendations for conservation management ........................................................ 382
9.3.1. Management of minerotrophic mires in basement regions ................................... 382
9.3.2. Management of the Dauges National Nature Reserve and Special Area of
Conservation ...................................................................................................................... 384
9.4. Conclusion ...................................................................................................................... 386
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