Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure   总被引：1，自引：0，他引：1  

Maricris Marinas  James W. Roy  James E. Smith 《Ground water》2013,51(1):41-50

Water table fluctuations continuously introduce entrapped air bubbles into the otherwise saturated capillary fringe and groundwater zone, which reduces the effective (quasi‐saturated) hydraulic conductivity, K_quasi, thus impacting groundwater flow, aquifer recharge and solute and contaminant transport. These entrapped gases will be susceptible to compression or expansion with changes in water pressure, as would be expected with water table (and barometric pressure) fluctuations. Here we undertake laboratory experiments using sand‐packed columns to quantify the effect of water table changes of up to 250 cm on the entrapped gas content and the quasi‐saturated hydraulic conductivity, and discuss our ability to account for these mechanisms in ground water models. Initial entrapped air contents ranged between 0.080 and 0.158, with a corresponding K_quasi ranging between 2 and 6 times lower compared to the K_s value. The application of 250 cm of water pressure caused an 18% to 26% reduction in the entrapped air content, resulting in an increase in K_quasi by 1.16 to 1.57 times compared to its initial (0 cm water pressure) value. The change in entrapped air content measured at pressure step intervals of 50 cm, was essentially linear, and could be modeled according to the ideal gas law. Meanwhile, the changes in K_quasi with compression–expansion of the bubbles because of pressure changes could be adequately captured with several current hydraulic conductivity models.  相似文献   

Thorough wetting and drainage of a peat lysimeter in a climate change scenario     

Maurizio Previati  Davide Canone  Edoardo Iurato  Davide Gisolo  Stefano Ferrari  Pietro Teatini  Mario Putti  Stefano Ferraris 《水文研究》2020,34(5):1269-1284

A peat deposit (Zennare basin, Venice coastland, Italy) was monitored in previous field studies to investigate the hydrological response of organic soil to meteorological dynamics. Field tests and modelling predictions highlighted the risk of the complete loss of this peat layer during the next 50 years, due to oxidation enhanced by the increased frequency of warmer periods. Unfortunately, despite the considerable impacts that are expected to affect peat bogs (in this area and worldwide), only a few experimental studies have been carried out to assess the hydrologic response of peat to severe water scarcity. Because of that, an undisturbed 0.7 m³ peat monolith was collected, transferred to the laboratory and instrumented. The total weight (representative of the water content dynamics of the peat monolith as a whole), and two vertical profiles of matric potentials and water content were monitored in controlled water-scarce conditions. After an extended air-drying period, the monolith was used as an undisturbed peat lysimeter and a complete cycle of wetting and drainage was performed. Supplementary measurements of matric potential ψ and water content θ were collected by testing peat subsamples on a suction table apparatus. A set of water retention curves was determined in a range of matric potentials broader (ψ down to −7 m) than the current natural conditions in the field (minimum ψ = −1 m). While water content at saturation showed values similar to those in the original natural conditions (θ ≅ 0.8), a remarkable loss of water holding capacity (even for low potentials) has been highlighted, especially in deep layers that are now permanently below the water table. The retention curves changed shape and values, with a more pronounced hysteresis visible in an increasing distance between wetting and drying data. Hydraulic non-equilibrium between the water content and water potential could be a possible cause and it is worth modelling in future studies. The parameters of the van Genuchten retention curves were obtained for the wetting and the drying phases.  相似文献   

Moisture dynamics and hydrophysical properties of a transplanted acrotelm on a cutover peatland     

J. P. Cagampan  J. M. Waddington 《水文研究》2008,22(12):1776-1787

The natural carbon storage function of peatland ecosystems can be severely affected by the abandonment of peat extraction, influencing peatland drainage, leading to large and persistent sources of atmospheric CO₂. Moreover, these cutover peatlands have a low and variable water table position and high tension at the surface, creating harsh ecohydrological conditions for vegetation re‐establishment, particularly peat forming Sphagnum moss. Standard restoration techniques aim to restore the peatland to a carbon accumulating system through various water management techniques to improve hydrological conditions and by reintroducing Sphagnum at the surface. However, restoring the hydrology of peatlands can be expensive due to the cost of implementing the various restoration techniques. This study examines a peat extraction‐restoration technique where the acrotelm is preserved and replaced directly on the cutover peat surface. An experimental peatland adopting this acrotelm transplant technique had both a high water table and peat moisture conditions providing sufficient water at the surface for Sphagnum moss. Average water table conditions were higher at the experimental site (?8·4 ± 4·2 cm) compared to an adjacent natural site (?12·7 ± 6·0 cm) suggesting adequate moisture conditions at the restored surface. However, the experimental site experienced high variability in volumetric moisture content (VMC) in the capitula zone (upper 2 cm) where large diurnal changes in VMC (～30%) were observed, suggesting possible disturbance to the peat matrix structure during the extraction‐restoration process. However, soil–water retention analysis and physical peat properties (porosity and bulk density) suggest that no significant differences existed between the natural and experimental sites. Any structural changes within the peat matrix were therefore minimal. Moreover, low soil‐water tensions were maintained well above the laboratory measured critical Sphagnum threshold of 33% (?100 mb) VMC, further indicating favourable conditions for Sphagnum moss survival and growth. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Experimental hydrological forcing to illustrate water flow processes of a subarctic ladder fen peatland          下载免费PDF全文

Colin P. R. McCarter  Jonathan S. Price 《水文研究》2017,31(8):1578-1589

Large peatland complexes dominate the landscape of the James Bay Lowland in subarctic Ontario, Canada. However, there is not a thorough understanding of the hydrological processes occurring in these important systems, particularly how ladder fens connect large domed bogs to the aquatic ecosystems that drain the peatland complex. Ladder fens consist of a pool‐rib topography where flow downgradient is controlled by the peat ribs. Within the ribs, low‐lying preferential flow paths typically enhance the transmission of water, whereas the elevated ridge microforms impede water flow to downgradient aquatic ecosystems. To assess the hydrological connectivity, we study the role of the water table, peat transmissivity, and microtopography of a small ladder fen for 3 summers (2013–2015) in the James Bay Lowland. The system was manipulated with a sustained hydrological forcing (water addition) to the upslope boundary of the fen during 2014 (38 m³/day) and 2015 (30 m³/day). There was an exponential increase in transmissivity towards the peat surface due to extremely high‐hydraulic conductivities within the upper few centimeters of the peat deposit. At the maximum water table, the saturated hydraulic conductivity of the 0.1 m layer of peat below the water table varied depending on peat microtopography (preferential flow paths = 42–598 m/day and ridges = 16–52 m/day), resulting in high‐hydrological connectivity periods. Furthermore, during 2015, there was an abnormally large amount of precipitation (300 mm vs. long‐term average ~ 100 mm) that resulted in complete surface water connectivity of the site. This caused rapid movement of water from the head of system to the outlet (~15 hr) and runoff ratios >1, compared to low‐water table periods (runoff ratio ~ 0.05). This study highlights the profound importance of the transmissivity–water table feedback mechanism in ladder fens, on controlling the water retention and drainage of large peatland complexes.  相似文献   

Effects of managed burning in comparison with vegetation cutting on dissolved organic carbon concentrations in peat soils     

Fred Worrall  Simon Dixon 《水文研究》2013,27(26):3994-4003

Given the continuing concern about rising concentrations of dissolved organic carbon (DOC) in stream water leaving peat‐covered catchments, this study has considered the impact of managed burning or cutting of Calluna vulgaris, a dominant vegetation cover in many UK peatlands. Pristine mature Calluna stands were compared with those that had been subject to cutting and or managed burning up to 5 years after intervention. The study measured the DOC concentration of both soil and surface runoff water over a period of 12 months in comparison with water table depth, conductivity, and pH. The results show the following:

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Hydrological behaviour of a drained agricultural peat catchment in the tropics. 1: Rainfall,runoff and water table relationships     

Ayob Katimon  Shamsuddin Shahid  Ahmad Khairi Abd Wahab  Md. Hazrat Ali 《水文科学杂志》2013,58(6):1297-1309

Abstract

Hydrological data of a drained tropical peat catchment have been analysed through conventional quantitative hydrological approaches to characterize its hydrological behaviours and changes due to continuous drainage for a long period. The results show that the hydrology of the catchment is extremely dynamic and the catchment is flashy in nature. A decreasing trend in peak flow amount and an increasing trend in baseflow amount was observed in the catchment, indicating that continuous drainage has reduced the risk of both flooding and water scarcity in the catchment. Correlation analysis among rainfall, runoff and groundwater table reveals that saturation excess-near surface flow is the dominant mechanism responsible for rapid runoff generation in the catchment. Therefore, any physical alterations or disturbances to the upper part of the peat profile would definitely affect the overall hydrological behaviour of the peat catchment.

Editor Z.W. Kundzewicz; Associate editor D. Hughes

Citation Katimon, A., Shahid, S., Abd Wahab, A.K., and Ali, M.H., 2013. Hydrological behaviour of a drained agricultural peat catchment in the tropics. Part 1: Rainfall, runoff and water table relationships. Hydrological Sciences Journal, 58 (6), 1297–1309.  相似文献   

Tide-induced variability in beach surface moisture: Observations and modelling     

Laura B. Brakenhoff  Yvonne Smit  Jasper J. A. Donker  Gerben Ruessink 《地球表面变化过程与地形》2019,44(1):317-330

The moisture content w_s of a beach surface strongly controls the availability of sand for aeolian transport. Our predictive capability of the spatiotemporal variability in w_s, which depends to a large extent on water table depth, is, however, limited. Here we show that water table fluctuations and surface moisture content observed during a 10-day period on a medium-grained (365μm) planar (1:30) beach can be predicted well with the nonlinear Boussinesq equation extended to include run-up infiltration and a soil–water retention curve under the assumption of hydrostatic equilibrium. On the intertidal part of the beach the water table is observed and predicted to continuously fall from the moment the beach surface emerges from the falling tide to just before it is submerged by the incoming tide. We find that on the lower 30% of the intertidal beach the water table remains within 0.1–0.2 m from the surface and that the sand is always saturated (w_s≈20%, by mass). Higher up on the intertidal beach, the surface can dry to about 5% when the water table has fallen to 0.4–0.5 m beneath the surface. Above the high-tide level the water table is always too deep (>0.5 m) to affect surface moisture and, without precipitation, the sand is dry (w_s < 5 − 8%). Because the water table depth on the emerged part of the intertidal beach increases with time irrespective of whether the (ocean) tide falls or rises, we find no need to include hysteresis (wetting and drying) effects in the surface-moisture modelling. Model simulations suggest that at the present planar beach only the part well above mean sea level can dry sufficiently (w_s < 10%) for sand to become available for aeolian transport. ©2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

FIELD ESTIMATION OF MACROPORE FUNCTIONING AND SURFACE HYDRAULIC CONDUCTIVITY IN A FEN PEAT     

ANDREW J. BAIRD 《水文研究》1997,11(3):287-295

A limitation of existing models of water and solute movement in fen peats is that they fail to represent processes in the unsaturated zone. This limitation is largely due to a lack of data on the hydraulic properties of unsaturated peat, in particular the relationship between hydraulic conductivity (K) and pressure head (ψ). A tension infiltrometer was used to measure K(ψ) of a fen peat in Somerset, England. It was found that macropores could be important in water and solute movement in this soil type. It was also found that (i) variability of K in this peat was less than that reported for other peats and mineral soils, and (ii) the K data were better described by a log-normal distribution than a normal distribution in accord with findings from other peat and mineral soils. Recommendations on improving the understanding of water and solute movement in the unsaturated zone of this soil type are made. © 1997 by John Wiley & Sons, Ltd.  相似文献   

Scale‐dependency of hydraulic conductivity: an example from Thorne Moor,a raised mire in South Yorkshire,UK     

J. Bromley  M. Robinson  J. A. Barker 《水文研究》2004,18(5):973-985

The hydraulic conductivity (K) of many low permeability materials is strongly scale‐dependent. In raised mires and other types of peat deposit the effects of features such as abandoned infilled ditches, root holes and localized woody material, cause K to be heterogeneous and scale‐dependent. Despite this, field measurements are routinely made using auger hole (slug) tests at the scale of only a few tens of centimetres. Such measurements are locally valid, but where the regional subsurface movement of water through peat bogs is simulated using groundwater models, typically at the scale of hundreds of metres, they give rise to a systematic underestimate of flows and an overprediction of water table elevations. Until now, techniques to obtain values at a scale sufficiently large to include the effects of localized features of higher permeability have not been applied routinely. Research at Thorne Moor, a large raised mire, demonstrates that the K of peat varies over several orders of magnitude when measured at different scales, using a variety of techniques. Laboratory and auger hole tests cannot be relied upon to provide results that represent the hydraulic conductivity of large expanses of peatland. This has significant implications for the management and long‐term restoration of peatlands where both regional and local control of water levels is crucial. For groundwater models to be used successfully to plan such schemes, it is essential to apply the K values relevant to the scale of the simulation. This paper describes and tests novel techniques, using ditches, for the derivation of K at large scales which overcome many of the problems that have been identified with conventional techniques and are capable of producing estimates that are appropriate to the application of physically based regional flow models. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Evidence that piezometers vent gas from peat soils and implications for pore‐water pressure and hydraulic conductivity measurements     

J. M. Waddington  S. J. Ketcheson  E. Kellner  M. Strack  A. J. Baird 《水文研究》2009,23(8):1249-1254

Entrapped gas bubbles in peat can alter the buoyancy, storativity, void ratio and expansion/contraction properties of the peat. Moreover, when gas bubbles block water‐conducting pores they can significantly reduce saturated hydraulic conductivity and create zones of over‐pressuring, perhaps leading to an alteration in the magnitude and direction of groundwater flow and solute transport. Some previous researches have demonstrated that these zones of over‐pressuring are not observed by the measurements of pore‐water pressures using open‐pipe piezometers in peat; rather, they are only observed with pressure transducers sealed in the peat. In has been hypothesized that open‐pipe piezometers vent entrapped CH₄ to the atmosphere and thereby do not permit the natural development of zones of entrapped gas. Here we present findings of the study to investigate whether piezometers vent subsurface CH₄ to the atmosphere and whether the presence of piezometers alters the subsurface concentration of dissolved CH₄. We measured the flux of methane venting from the piezometers and also determined changes in pore‐water CH₄ concentration at a rich fen in southern Ontario and a poor fen in southern Quebec, in the summer of 2004. Seasonally averaged CH₄ flux from piezometers was 1450 and 37·8‐mg CH₄ m^?2 d^?1 at the southern Ontario site and Quebec site, respectively. The flux at the Ontario site was two orders of magnitude greater than the diffusive flux at the site. CH₄ pore‐water concentrations were significantly lower in open piezometers than in water taken from sealed samplers at both the Ontario and Quebec sites. The flux of CH₄ from piezometers decreased throughout the season suggesting that CH₄ venting through the piezometer exceeded the rate of methanogenesis in the peat. Consequently we conclude that piezometers may alter the gas dynamics of some peatlands. We suggest that less‐invasive techniques (e.g. buried pressure transducers, tracer experiments) are needed for the accurate measurement of pore‐water pressures and hydraulic conductivity in peatlands with a large entrapped gas component. Furthermore, we argue that caution must be made in interpreting results from previous peatland hydrology studies that use these traditional methods. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Effect of anisotropy on solute transport in degraded fen peat soils     

Miaorun Wang  Haojie Liu  Dominik Zak  Bernd Lennartz 《水文研究》2020,34(9):2128-2138

Peat soils are heterogeneous, anisotropic porous media. Compared to mineral soils, there is still limited understanding of physical and solute transport properties of fen peat soils. In this study, we aimed to explore the effect of soil anisotropy on solute transport in degraded fen peat. Undisturbed soil cores, taken in vertical and horizontal direction, were collected from one drained and one restored fen peatland both in a comparable state of soil degradation. Saturated hydraulic conductivity (K _s) and chemical properties of peat were determined for all soil cores. Miscible displacement experiments were conducted under saturated steady state conditions using potassium bromide as a conservative tracer. The results showed that (1) the K _s in vertical direction (K _sv) was significantly higher than that in horizontal direction (K_sh), indicating that K _s of degraded fen peat behaves anisotropically; (2) pronounced preferential flow occurred in vertical direction with a higher immobile water fraction and a higher pore water velocity; (3) the 5% arrival time (a proxy for the strength of preferential flow) was affected by soil anisotropy as well as study site. A strong correlation was found between 5% arrival time and dispersivity, K _s and mobile water fraction; (4) phosphate release was observed from drained peat only. The impact of soil heterogeneity on phosphate leaching was more pronounced than soil anisotropy. The soil core with the strongest preferential flow released the highest amount of phosphate. We conclude that soil anisotropy is crucial in peatland hydrology but additional research is required to fully understand anisotropy effects on solute transport.  相似文献   

Quantification of peatland water storage capacity using the water table fluctuation method          下载免费PDF全文

Marc‐André Bourgault  Marie Larocque  Michelle Garneau 《水文研究》2017,31(5):1184-1195

Peat specific yield (S_Y) is an important parameter involved in many peatland hydrological functions such as flood attenuation, baseflow contribution to rivers, and maintaining groundwater levels in surficial aquifers. However, general knowledge on peatland water storage capacity is still very limited, due in part to the technical difficulties related to in situ measurements. The objectives of this study were to quantify vertical S_Y variations of water tables in peatlands using the water table fluctuation (WTF) method and to better understand the factors controlling peatland water storage capacity. The method was tested in five ombrotrophic peatlands located in the St. Lawrence Lowlands (southern Québec, Canada). In each peatland, water table wells were installed at three locations (up‐gradient, mid‐gradient, and down‐gradient). Near each well, a 1‐m long peat core (8 cm × 8 cm) was sampled, and subsamples were used to determine S_Y with standard gravitational drainage method. A larger peat sample (25 cm × 60 cm × 40 cm) was also collected in one peatland to estimate S_Y using a laboratory drainage method. In all sites, the mean water table depth ranged from 9 to 49 cm below the peat surface, with annual fluctuations varying between 15 and 29 cm for all locations. The WTF method produced similar results to the gravitational drainage experiments, with values ranging between 0.13 and 0.99 for the WTF method and between 0.01 and 0.95 for the gravitational drainage experiments. S_Y was found to rapidly decrease with depth within 20 cm, independently of the within‐site location and the mean annual water table depth. Dominant factors explaining S_Y variations were identified using analysis of variance. The most important factor was peatland site, followed by peat depth and seasonality. Variations in storage capacity considering site and seasonality followed regional effective growing degree days and evapotranspiration patterns. This work provides new data on spatial variations of peatland water storage capacity using an easily implemented method that requires only water table measurements and precipitation data.  相似文献   

Bog surface oscillation (mire breathing): A useful measure in raised bog restoration          下载免费PDF全文

Sarah A. Howie  Richard J. Hebda 《水文研究》2018,32(11):1518-1530

Oscillation of the peat surface is an important mechanism for hydrological self‐regulation in bogs. As the water table rises in the wet season, the peat body expands, raising the bog surface and increasing water storage. With seasonal drying, the water table declines, the peat loses volume, and the bog surface drops, thereby keeping Sphagnum mosses in close contact with the water table. The oscillation of surface elevation in a Pacific coastal temperate raised bog was monitored at multiple sites for 4–12 years in 8 different plant communities of both peat‐harvested and unharvested sites to determine how bog surface oscillation relates to site conditions. The multiyear averages of bog surface oscillation for the different sites ranged from 2 to 34 cm (mean: 10.8 cm). In harvested sites, surface oscillation was linked to a larger water level amplitude and a shallower water table. In unharvested sites, a shallow water table was also a strong predictor of surface oscillation, but water level amplitude was negatively correlated to surface oscillation. This discrepancy was attributed to rewetting and regeneration of harvested sites, as well as historic drainage in many of the unharvested sites that reduced the elasticity of the peat. Surface oscillation differed significantly between some of the plant communities, generally between drier and wetter sites. In disturbed bogs, regeneration of a more elastic surface peat can increase the magnitude of peat volume change and bring about the return of self‐regulating mechanisms. Bog surface oscillation may be an important metric for assessing the restoration success or storage capacity of raised bogs in similar climatic settings.  相似文献   

A partial explanation of the dependency of hydraulic conductivity on positive pore water pressure in peat soils     

Andrew J. Baird  Simon W. Gaffney 《地球表面变化过程与地形》1995,20(6):561-566

Part of the relationship between positive pore water pressures and hydraulic conductivity in peat soils may be explained by accumulations of methane bubbles. We show how compression and expansion of gas bubbles with changes in pore water pressure could cause changes in hydraulic conductivity and thus help to explain some observations of dependency of hydraulic conductivity in peats on pore water pressure. Consideration is also given to the effect on hydraulic conductivities of methane gas going into solution with increase in pore water pressure.  相似文献   

Hydro-ecology of groundwater-dependent ecosystems: applying basic science to groundwater management     

Allison R. Aldous  Leslie B. Bach 《水文科学杂志》2014,59(3-4):530-544

Abstract

Effective policies to protect groundwater-dependent ecosystems require robust methods to determine the environmental flows and levels required to support species and processes. Frameworks to support groundwater management must incorporate the relationships between hydrology and species and ecological processes. These hydro-ecological relationships can be used to develop quantitative, measurable thresholds that are sensitive to changes in groundwater quantity. Here we provide a case study from a group of fens in central Oregon, USA, that are used for cattle watering, but also support numerous sensitive species. We developed quantitative relationships between the position of the water table and wetland indicator plant species and the process of peat development, to propose groundwater withdrawal thresholds. A maximum depth to water table of –0.9 to –34.8 cm for fen plants and –16.6 to –32.2 cm for peat accretion can be tolerated in these wetlands. Defining hydro-ecological relationships as thresholds can support management decisions.