Peat Hydraulic Properties Research Articles

Controls on Saturated Hydraulic Conductivity in a Degrading Permafrost Peatland Complex

AbstractPermafrost peatlands are vulnerable to rapid structural changes under climatic warming, including vertical collapse. Peatland water budgets, and therefore peat hydraulic properties, are important determinants of vegetation and carbon fluxes. Measurements of hydraulic properties exist for only a limited number of permafrost peatland locations, primarily concentrated in North America. The impacts of thaw‐induced collapse upon properties such as horizontal saturated hydraulic conductivity (Kh), and thus lateral drainage, remain poorly understood. We made laboratory determinations of Kh from 82 peat samples from a degrading Swedish palsa mire. We fitted a linear mixed‐effects model (LMM) to establish the controls on Kh, which declined strongly with increasing depth, humification and dry bulk density. Depth exerted the strongest control on Kh in our LMM, which demonstrated strong predictive performance (r2 = 0.605). Humification and dry bulk density were influential predictors, but the high collinearity of these two variables meant only one could be included reliably in our LMM. Surprisingly, peat Kh did not differ significantly between desiccating and collapsed palsas. We compared our site‐specific LMM to an existing, multi‐site model, fitted primarily to boreal and temperate peatlands. The multi‐site model made less skillful predictions (r2 = 0.528) than our site‐specific model, possibly due to latitudinal differences in peat compaction, floristic composition and climate. Nonetheless, low bias means the multi‐site model may still be useful for estimating peat Kh at high latitudes. Permafrost peatlands remain underrepresented in multi‐site models of peat hydraulic properties, and measurements such as ours could be used to improve future iterations.

A process-based model for quantifying the effects of canal blocking on water table and CO2emissions in tropical peatlands

Abstract. Drainage in tropical peatlands increases CO2 emissions, the rate of subsidence, and the risk of forest fires. To a certain extent, these effects can be mitigated by raising the water table depth (WTD) using canal or ditch blocks. The performance of canal blocks in raising WTD is, however, poorly understood because the WTD monitoring data are limited and spatially concentrated around canals and canal blocks. This raises the following question: how effective are canal blocks in raising the WTD over large areas? In this work, we composed a process-based hydrological model to assess the peatland restoration performance of 168 canal blocks in a 22 000 ha peatland area in Sumatra, Indonesia. We simulated daily WTD over 1 year using an existing canal block setup and compared it to the situation without blocks. The study was performed across two contrasting weather scenarios representing dry (1997) and wet (2013) years. Our simulations revealed that, while canal blocks had a net positive impact on WTD rise, they lowered WTD in some areas, and the extent of their effect over 1 year was limited to a distance of about 600 m around the canals. We also show that canal blocks are most effective in peatlands with high hydraulic conductivity. Averaging over all modeled scenarios, blocks raised the annual mean WTD by only 1.5 cm. This value was similar in the dry (1.44 cm) and wet (1.57 cm) years, and there was a 2.13 fold difference between the scenarios with large and small hydraulic conductivities (2.05 cm versus 0.96 cm). Using a linear relationship between WTD and CO2 emissions, we estimated that, averaging over peat hydraulic properties, canal blocks prevented the emission of 1.07 Mg ha−1 CO2 in the dry year and 1.17 Mg ha−1 CO2 in the wet year. We believe that the modeling tools developed in this work could be adopted by local stakeholders aiming at a more effective and evidence-based approach to canal-block-based peatland restoration.

Fen ecohydrologic trajectories in response to groundwater drawdown with an edaphic feedback.

Fens are high conservation value ecosystems that depend on consistent discharge of groundwater that saturates the near surface for most of the growing season. Reduced groundwater inputs can result in losses of native diversity, decreases in rare-species abundance and increased invasion by non-native species. As such, fen ecosystems are known to be particularly susceptible to changes in groundwater conditions including reduction in water levels due to nearby groundwater pumping. However, research is lacking on whether floristic degradation is influenced by feedbacks between hydrology and soil properties. We present a model of an archetype hillslope fen that couples a hydrological niche model with a variably saturated groundwater flow model to predict changes in vegetation composition in response to different groundwater drawdown scenarios. The model explores a potential edaphic feedback through the use of an observed relationship between fen floristic quality and soil/peat water retention characteristics that is attenuated with separate edaphic and floristic memory terms representing lags in biophysical responses to dewatering. Model parameters were determined based on data collected from six fens in Wisconsin under various states of degradation. We observed different water retention characteristics between sites that were minimally impacted versus degraded that are likely due to peat decomposition, oxidation and compaction at the degraded sites. These characteristics were also correlated with floristic quality. The results reveal a complex response to drawdown where changes in peat hydraulic properties following dewatering lead to even drier conditions and further shifts away from typical fen species.

Canal blocking optimization in restoration of drained peatlands

Abstract. Drained peatlands are one of the main sources of carbon dioxide (CO2) emissions globally. Emission reduction and, more generally, ecosystem restoration can be enhanced by raising the water table using canal or drain blocks. When restoring large areas, the number of blocks becomes limited by the available resources, which raises the following question: in which exact positions should a given number of blocks be placed in order to maximize the water table rise throughout the area? There is neither a simple nor an analytic answer. The water table response is a complex phenomenon that depends on several factors, such as the topology of the canal network, site topography, peat hydraulic properties, vegetation characteristics and meteorological conditions. We developed a new method to position the canal blocks based on the combination of a hydrological model and heuristic optimization algorithms. We simulated 3 d dry downs from a water saturated initial state for different block positions using the Boussinesq equation, and the block configurations maximizing water table rise were searched for by means of genetic algorithm and simulated annealing. We applied this approach to a large drained peatland area (931 km2) in Sumatra, Indonesia. Our solution consistently outperformed traditional block locating methods, indicating that drained peatland restoration can be made more effective at the same cost by selecting the positions of the blocks using the presented scheme.

Hydraulic properties of peat soils along a bulk density gradient—A meta study

AbstractOur understanding of hydraulic properties of peat soils is limited compared with that of mineral substrates. In this study, we aimed to deduce possible alterations of hydraulic properties of peat soils following degradation resulting from peat drainage and aeration. A data set of peat hydraulic properties (188 soil water retention curves [SWRCs], 71 unsaturated hydraulic conductivity curves [UHCs], and 256 saturated hydraulic conductivity [Ks] values) was assembled from the literature; the obtained data originated from peat samples with an organic matter (OM) content ranging from 23 to 97 wt% (weight percent; and according variation in bulk density) representing various degrees of peat degradation. The Mualem‐van Genuchten model was employed to describe the SWRCs and UHCs. The results show that the hydraulic parameters of peat soils vary over a wide range confirming the pronounced diversity of peat. Peat decomposition significantly modifies all hydraulic parameters. A bulk density of approximately 0.2 g cm−3 was identified as a critical threshold point; above and below this value, macroporosity and hydraulic parameters follow different functions with bulk density. Pedotransfer functions based on physical peat properties (e.g., bulk density and soil depth) separately computed for bog and fen peat have significantly lower mean square errors than functions obtained from the complete data set, which indicates that not only the status of peat decomposition but also the peat‐forming plants have a large effect on hydraulic properties. The SWRCs of samples with a bulk density of less than 0.2 g cm−3 could be grouped into two to five classes for each peat type (botanical composition). The remaining SWRCs originating from samples with a bulk density of larger than 0.2 g cm−3 could be classified into one group. The Mualem‐van Genuchten parameter values of α can be used to estimate Ks if no Ks data are available. In conclusion, the derived pedotransfer functions provide a solid instrument to derive hydraulic parameter values from easily measurable quantities; however, additional research is required to reduce uncertainty.

Effects of different irrigation strategies on the evolution of peat hydraulic properties during cultivation

The combined effects of root development and irrigation strategies on the physical and hydraulic properties of peat substrates were studied within a culture of Rosa × hybrida ‘Radrazz’ during five months when watering was managed in three different ways: (1) water potential always maintained at container capacity, (2) irrigation triggered when water potential reached -10 kPa or (3) -30 kPa. Root volume, total volume, air and water retention properties, saturated hydraulic conductivity, relative gas diffusivity, and wettability were measured at the beginning and then at the end of the experiment. The study showed important changes over time, mainly due to the hydric history, and the irrigation strategies leading to large modifications of pore size distribution and connectivity. Furthermore, the intensity of drying amplified the degradation of peat wettability, and altered the physical qualities of peat. However, positive effects of the root system were shown by limiting the decrease in wettability.

Moss and peat hydraulic properties are optimized to maximize peatland water use efficiency

AbstractPeatland ecosystems are globally important carbon and terrestrial surface water stores that have formed over millennia. These ecosystems have likely optimized their ecohydrological function over the long‐term development of their soil hydraulic properties. The optimization of peat hydraulic properties is examined to determine which of the following conditions peatland ecosystems target during this development: (i) maximize carbon accumulation, (ii) maximize water storage, or (iii) balance carbon profit across hydrological disturbances. To identify this control, the short‐term hydrological response of a 0.5‐m‐deep peat profile was simulated during a 50‐day rain‐free period. A total of 5000 Monte‐Carlo model realizations were conducted, with peat hydraulic properties differing between each realization (values derived from known probability distributions). Saturated hydraulic conductivity (Ks) and empirical van Genuchten water retention parameter α were shown to provide a first order control on simulated water tensions. For hypothetical combinations of Ks and α, the probability that water tension exceeds the ecologically important threshold of 100 mb within 24 h showed a bimodal distribution. A peak at high probabilities was associated with profiles of high Ks and low α. Such a profile is optimized for water storage. A peak at low probabilities is associated with low Ks, high α, and is optimized for carbon accumulation. Actual hydraulic properties from five northern peatlands fall between this binominal distribution, balancing the competing demands of carbon accumulation and water storage. We argue that peat hydraulic properties are thus optimized to maximize water use efficiency and that this optimization occurs over a centennial to millennial timescale. Copyright © 2015 John Wiley & Sons, Ltd.

Modelling Sphagnum moisture stress in response to projected 21st‐century climate change

AbstractSphagnum moss is an important genus of plants responsible for large stores of soil organic carbon associated with wet habitats, such as northern peatlands. Northern peatlands, which contain globally significant quantities of legacy carbon, may be vulnerable to enhanced summer moisture deficits due to climate change. We adapted HYDRUS‐1D and a semi‐arid soil‐moisture model to investigate the role of microtopographic position and depth dependence of peat hydraulic properties on Sphagnum moisture‐stress response to current and projected climate conditions in an idealized northern Michigan peatland. Water table (WT) level was shown to have a strong control on pore‐water pressure (ψ) and thus on Sphagnum moisture stress. As a result of the close correspondence between surface peat hydraulic properties for hummocks and hollows used to parameterize our model, the microtopographic position was shown to have a greater impact on Sphagnum moisture stress. Model behaviour suggests that, while ψ maintains equilibrium‐profile values relative to the WT level for relatively shallow values, surface ψ becomes nonlinearly related to the WT level below a value of approximately −0.4 m, thus, greatly increasing the likelihood of desiccation under future climate scenarios, where growing‐season soil‐moisture deficits are projected to increase. The simulated range of instantaneous and cumulative moisture stress for hollows under future climate conditions closely corresponds to the contemporary range exhibited by hummocks. Therefore, in order to assess the competitive advantage of various Sphagnum species to future climate conditions, we argue that more data are needed to better inform a physiological ψ‐based moisture‐stress threshold, the evolution of the stress response to increasing levels of desiccation and its subsequent recovery dynamics. Copyright © 2015 John Wiley & Sons, Ltd.

IMPORTANCE OF PEAT PARTICLE SIZE EFFECTS ON SPATIAL ROOT DISTRIBUTION AND CHANGES ON AERATION PROPERTIES

The evolution of peat hydraulic properties in different parts of containers was studied during root growth. For that, a 4-month long experiment was carried out in a greenhouse in 1L-containers at constant water regime (-1 kPa water potential), with Rosa ‘Knock Out’® and two different particle-size sphagnum peats, a fine one (0-10 mm) and a coarse one (20-40 mm). Every month, aerial biomass was quantified, whereas root biomass and distribution, water retention, relative gas diffusivity was studied in relation with the depth and the proximity of the container border. The study showed that root growth mainly influenced aeration properties (air filled porosity, relative gas diffusivity), with large differences regarding spatial distribution of roots. But the evolution of aeration properties depended on the peat particle size.

The role of hydrological transience in peatland pattern formation

Abstract. The sloping flanks of peatlands are commonly patterned with non-random, contour-parallel stripes of distinct micro-habitats such as hummocks, lawns and hollows. Patterning seems to be governed by feedbacks among peatland hydrological processes, plant micro-succession, plant litter production and peat decomposition. An improved understanding of peatland patterning may provide important insights into broader aspects of the long-term development of peatlands and their likely response to future climate change. We recreated a cellular simulation model from the literature, as well as three subtle variants of the model, to explore the controls on peatland patterning. Our models each consist of three submodels, which simulate: peatland water tables in a gridded landscape, micro-habitat dynamics in response to water-table depths, and changes in peat hydraulic properties. We found that the strength and nature of simulated patterning was highly dependent on the degree to which water tables had reached a steady state in response to hydrological inputs. Contrary to previous studies, we found that under a true steady state the models predict largely unpatterned landscapes that cycle rapidly between contrasting dry and wet states, dominated by hummocks and hollows, respectively. Realistic patterning only developed when simulated water tables were still transient. Literal interpretation of the degree of hydrological transience required for patterning suggests that the model should be discarded; however, the transient water tables appear to have inadvertently replicated an ecological memory effect that may be important to peatland patterning. Recently buried peat layers may remain hydrologically active despite no longer reflecting current vegetation patterns, thereby highlighting the potential importance of three-dimensional structural complexity in peatlands to understanding the two-dimensional surface-patterning phenomenon. The models were highly sensitive to the assumed values of peat hydraulic properties, which we take to indicate that the models are missing an important negative feedback between peat decomposition and changes in peat hydraulic properties. Understanding peatland patterning likely requires the unification of cellular landscape models such as ours with cohort-based models of long-term peatland development.

Peat particle size effects on spatial root distribution, and changes on hydraulic and aeration properties

During ornamental plant growth, the spatial heterogeneity of root distribution in containers raises the question of air and water availability within growing media, and of their evolution during plant growth. The aim of the present study was to characterise the evolution of peat hydraulic properties in different parts of containers during root growth. A 4-month long experiment was carried out in a greenhouse in 1L-containers at constant water regime (−1kPa water potential). We studied Rosa “Knock Out”® growth in two different particle-size sphagnum peats, a fine one (0–10mm) and a coarse one (20–40mm). Every month, aerial biomass and root biomass were quantified. Root distribution was studied relative to the depth and the proximity of the container border. Water retention, hydraulic conductivity and relative gas diffusivity of the growing media were measured. The study showed that root growth increased water retention. The higher root density at the bottom of the container highlighted a potential risk of anoxia, particularly in fine peat. In coarser peat, whose porosity is more important, both hydraulic conductivity and relative gas diffusivity were improved during root growth.

Hyporheic Flows Along a Channelled Peatland: Influence of Beaver Dams

Investigation into the effects of beaver dams on hyporheic fluxes in channelled peatlands is needed to better understand how biological processes drive stream-riparian area connections and thus nutrient export, and to improve our overall conceptual model of water storage and flow through peatlands. The objective of this study was to determine the influence of beaver dams on vertical and lateral hyporheic exchange. Hydrometric methods were used to determine subsurface flow pathways and estimate hyporheic water fluxes for a third-order stream draining a Canadian Rocky Mountain peatland in 2006 and 2007. Three sites were studied two contained small, in-channel beaver dams and the third was a control. Vertical hyporheic fluxes equaled or exceeded lateral hyporheic fluxes despite the fact that hydraulic conductivity of the stream bed tended to be lower than the banks, suggesting peat hydraulic properties were not the dominant factor in the development of hyporheic exchange in stream systems draining peatlands as has been reported for streams underlain by mineral substrates. Instead, vertical fluxes were partially influenced by the presence of a mineral lens 0.65 m below the ground surface. As well, high riparian water tables in relation to stream stage were key to limiting lateral fluxes. Steep hydraulic gradients above the two dams created looping flow pathways beneath and around them. However, little water actually flowed along these pathways. Measures of larger fluxes of water to the riparian area above the dams than those returning to the stream below the dams suggests either that hyporheic flow paths are longer than those measured in other studies or that the beaver dams generated recharge to the groundwater flow system.