Jonathan S. Price

Hydrological processes in abandoned and restored peatlands : an overview of management approaches

Mined peatlands do not readily recover their hydrological function, mainly because the dominant peat-forming plant genus, Sphagnum, cannot easily reestablish on the degraded surface peat found on cutover sites. Drainage and removal of the acrotelm can result in surface subsidence of up to3.7 cm y-1 m-1 of peat shortly after drainage (compression), and long-term rates up to 0.3 cm y-1 m-1 (compression and oxidation). This can decrease the hydraulic conductivity by over 75%, and decrease thewater retention capacity and specific yield. In old abandoned systems, drainage ditches may continue to facilitate a significant seasonal water loss.Colonization of abandoned sites by trees may increase the evapotranspirative losses by as much as 25%, and interception losses can be as high as 32% of rainfall. Without natural or planned occlusion of ditches, some peatlands become drier over time. Blocking ditches may largely restore water balance components, although the hydrological regime requires years to stabilise sufficiently for Sphagnum recolonization, especially where residualpeat is well decomposed, having inadequate water storage capacity. Consequently,winter precipitation (Europe) and spring snowmelt (North America) are criticalrecharge periods. Over the long term, consolidation of the peat due to drainageand methane production (where drainage systems are blocked and soils reflooded)decreases hydraulic conductivity, thereby reducing lateral seepage losses. This may actually assist in Sphagnum recolonization. A regenerated cover ofSphagnum increases soil wetness and reduces water tension (increases pore-water pressure) in the substrate, thus ameliorating its own environment. However, natural recolonization and recovery of many hydrological and ecological processes may not occur, or may require many decades. Water management and selective plant reintroduction can accelerate this. Water management options such as blocking ditches, constructing bunds, reconfiguring the surface and managing microclimate have met with varying degrees of success. No standard management prescription can be made because each site presents unique challenges.

Importance of shrinkage and compression in determining water storage changes in peat: the case of a mined peatland

This study examines changes in peat volume in a mined peatland near Lac St Jean, Quebec, during the spring and summer of 1995 and 1996, and the implication for water storage changes. Lowering of the water table caused drainage above the water table, but the specific yield (S y ) of the peat was relatively small (0.48), and did not adequately describe the water storage change. Lowering of the water table also caused surface subsidence, which was shown to be partly due to shrinkage above the water table (<3.6 cm), and partly due to compression of the saturated peat (about 6 cm). Measured total surface subsidence ranged from 6.5 to 10 cm. The change in peat volume occurred over the entire depth of the peat deposit (h), and so the storativity due to peat compression (bS s ), estimated to be 0.13, was more important than specific yield in determining water storage changes. Total storativity (S tot ) was best estimated as the sum S y + bS s . Changes in peat volume in the 0-3 cm layer were evident in the temporally variable bulk density (83-101 kg m -3 ). Its relationship with volumetric moisture content was highly hysteretic, reflecting the complexity of the process in the unsaturated zone. However, there was a linear relationship with water tension, suggesting a more direct causal relationship. Changes in peat volume were also recorded below the water table, as the volumetric water content of the saturated peat decreased by 3.5% over the season. Since this applied to a relatively thick layer of peat, its total effect was greater than shrinkage in the zone above the water table. It was concluded that most peatland water balances should make account of storage changes associated with peat volume changes, and that peat volume changes may increase the water limitations to plants when the water table drops below the surface.

Soil moisture, water tension, and water table relationships in a managed cutover bog

Abstract This study evaluates the hydrological conditions in a harvested bog where various water management schemes have been implemented to ameliorate conditions limiting Sphagnum regeneration. The study sites included a natural bog (natural), a recently drained and harvested bog (drained), which provided the hydrological extremes. Also included are a drained harvested bog with ditches blocked with (1) no other management (blocked), (2) peat bounded by open water at 5-m intervals (5-m), and (3) with straw mulch on the surface (mulch). The study period from May to September 1995 was drier than normal. The water table in the drained site descended to −107 cm by late August, compared with −72.5 cm at the blocked site. Both the 5-m and mulch sites (ditches also blocked) had water table recessions similar to the natural site (minimum approx. −62 cm). In the drained and blocked sites, little variation in water table depth occurred after early July, suggesting water exchanges with the atmosphere occurred to and from the unsaturated zone only. Soil moisture in the upper 3 cm layer on the drained and blocked site were similar, in spite of greatly different water table depths, dropping below 20% by volume, compared with a minimum of 30% when mulch was used. Soil water tension profiles suggest most of this storage change occurred in the upper 30 cm. The water table depth, therefore, was not a good indicator of water availability at the surface. Pressure head in the unsaturated zone (1 cm below the surface) in the drained and blocked sites was maintained between 0 and −100 cm (mb) 18 and 34% of the time, respectively, and with water management with open water (5-m) and mulch, increased to 55 and 97% of the time, respectively. The greatest tension, however, was observed on the blocked site (−355 cm), rather than the drained site (−247 cm), suggesting lower suitability for Sphagnum at the former. This was attributed to the higher bulk density (hence smaller pore structure) at the blocked site (ρb=92.1 kg compared with 55.7 kg m−3). Higher bulk density at the blocked site was ascribed to its longer time since disturbance. This implies restoration should begin as soon as possible after harvesting is finished.

Soil water flow dynamics in a managed cutover peat field, Quebec: Field and laboratory investigations

In this paper concerned with soil water dynamics in a managed cutover peat field, the microscale hydrological processes and parameters governing water flow and storage through variably saturated peat are investigated. An open water ditch-reservoir enhanced wetting of adjacent cutover peat, maintaining the water table depth above 43 cm during the summer, surface soil moisture above 45%, and water tension in the surface layer above 245 mbar. Desaturation of pores was noted in the 22 and 210 cm depths, but at 230 and 250 cm a decrease in moisture content of several percent was associated with compression of the peat as the water table dropped. Air entry occurred only at pressures below 215 mbar. Seasonal subsidence resulted in cumulative vertical displacement in excess of 10 cm during the study period. Typical settlements in the peat ranged between 11 and 23% of the lowering of the water table. Considerable hysteresis was observed, and vertical displacement was 5 times greater in response to water loss, compared to rewetting. The specific storage (S s) in the 180 cm thick deposit averaged 9.4 3 10 24 cm 21 during drying periods but averaged only 2.6 3 10 24 cm 21 on rewetting. S s was more important than specific yield (Sy) in the overall aquifer storativity. Transient hydraulic properties resulted from the shifting soil structure. The increase in peat bulk density caused by drying increased the water retention capacity and decreased hydraulic conductivity. Mean saturated hydraulic conductivity was 15 cm d 21 and decreased 2 orders of magnitude as the degree of saturation dropped from 1 to 0.4. The horizontal/vertical anisotropy ratio was 4. The changing surface elevation in response to seasonal subsidence had a profound influence on the nature of the storage changes and hydraulic parameters of the peat soil.

Energy and moisture considerations on cutover peatlands: surface microtopography, mulch cover and Sphagnum regeneration

This study examined (i) the effect of artificially created microtopography and straw mulch on the soil moisture and (ii) energy balance and the establishment of a Sphagnum cover on a cutover peatland. Straw mulch caused rainfall interception approaching 2 mm per event. Although interception represented 44% of the total rainfall over the measurement period, water that evaporated from the mulch used energy that would otherwise have been used to evaporate soil water. Thus, the net effect of interception by mulch was negligible. The soil heat flux below the mulch was only 13% of the bare soil value and was decoupled from the daily net radiation. Net radiation over the bare soil was 15% greater than over the mulch. However, because of the greater heat flux into the bare peat, the energy available for sensible and latent heat fluxes was similar between the mulch covered and bare peat. Average evaporation from mulch and bare soil was estimated to be 2.6 and 3.1 mm d 1 , respectively. Soil water tension 1 cm below the surface remained above 100 cm (mb) all season (100% of the time) when a mulch was used, compared to only 30% of the time in the bare soil. Correspondingly, the water table was sustained above the 40 cm depth, 60% of time in the mulch covered site, compared to only 40% of the time in the bare peat site. Negative relief

The effects of matrix diffusion on solute transport and retardation in undisturbed peat in laboratory columns

Experiments were performed to assess the nature of solute transport in peat by using step-inputs of a NaCl solution in laboratory columns of undisturbed peat. Peat has a dual-porosity matrix with inter-connected pores that actively transmit water, and dead-end and closed pores formed by the remains of plant cells. The proportion of dead-end and closed pores increased at depth, where the state of decomposition of organic material is more advanced. These dead-end and closed pores act as a sink for solute. Breakthrough at CCo = 0.5 occurred much later than the total active pore volume in the column, indicating that solute retardation occurred. This retardation was attributed to diffusion of the flowing solute into the closed and dead-end pores (matrix diffusion). Greater retardation occurred at depth, increasing from 2.7 at 0.20 m to 7.3 at 0.62 m, corresponding to the greater volume of closed and dead-end pores there. Retardation was also velocity dependent, with higher velocity resulting in less retardation of solute since there was less time available for solute to be abstracted from the flowing water into closed pores. Matrix diffusion was shown to enhance dispersion at lower flow velocities, and dispersion increased with depth. Peat effectively attenuated the conservative solute through matrix diffusion, and heterogeneity in peat properties influenced the effectiveness of this retardation.

HYDROLOGY AND MICROCLIMATE OF A PARTLY RESTORED CUTOVER BOG, QUÉBEC

Peatlands do not readily return to functional wetland ecosystems after harvesting (cutting), because the harsh hydrological and microclimatic conditions are unsuitable for Sphagnum regeneration. In this study, drainage ditches blocked after harvesting restored the water balance to a condition similar to a nearby natural bog. Evaporation averaged 2.9 and 2.7 mm day -1 on the cutover and natural bog, respectively. Evaporation consumed most of the rainfall input (86 and 80%, respectively), whereas runoff was minor at both sites (6 and 4%, respectively). However, the water table position was markedly different at these sites. Median water table depth was 0.05m below the surface in the natural bog, compared with 0.44m in the cutover bog (ditches blocked). Changes to the peak soil matrix owing to drainage and cutting reduced the specific yield (Sy) of the peat to 0.04-0.06 from 0.35-0.55, causing exaggerated water table changes in the cutover site. Nevertheless, volumetric soil moisture in the cutover site (0.67 ±.08) had low variability, and was maintained above moisture contents found in Sphagnum hummocks in the natural bog (0.48 ±.10), although less than on Sphagnum lawn (0.84 ±.11). Poor Sphagnum regeneration on cutover surfaces can therefore be attributed to its inability to extract water from the underlying peat, which retains water at matric suction greater than the non-vascular Sphagnum can generate. The corrupted iron pan under main ditches has permitted partial recharge of the underlying aquifer, reducing local hydraulic gradients, thereby decreasing vertical seepage loss.

DEVELOPING HYDROLOGIC THRESHOLDS FOR SPHAGNUM RECOLONIZATION ON AN ABANDONED CUTOVER BOG

After 30 years of abandonment, a block-cut peatland near Cacouna, Quebec, Canada, has naturally regenerated Sphagnum mosses on <10% of its area, typical for these disturbed systems. Distinct hydrologic conditions were observed where Sphagnum has successfully recolonized, providing a basis for establishing thresholds that can be targeted by peatland restoration managers. Sites where Sphagnum mosses recolonized were characterized by high water table (mean−24.9±14.3 cm), soil moisture (ϕ) > 50%, and soil-water pressure (Ψ)≥ 100 mb. These hydrologic indicators were spatially organized according to the morphology of block- cut trenches, which typically include raised baulks, shallow ditches, and the convex skag (unused turf) deposits along the central axis of the trench. Topographically low areas like the shallow ditches (D) and lower parts of the skag (LS) adjacent to the ditches maintained ϕ and Ψ > 50% and − 100 mb, respectively, for 100% of the summer period. About 83% of all Sphagnum recolonization that occurred in the study trench did so in these areas. In more raised areas like the mid- and center portion of the skag, ϕ and Ψ eventually fell below these thresholds, and these areas generally did not support Sphagnum, except in a few localized microtopographic depressions in the lower (downslope) end of the trench. While this lower end of the trench had all the Sphagnum species that were present in the trench, even there it was only 38% of the total area. It seems that even short periods of low Ψ may restrict Sphagnum reestablishment in an otherwise favorably wet site.

EFFECT OF PEATLAND DRAINAGE, HARVESTING, AND RESTORATION ON ATMOSPHERIC WATER AND CARBON EXCHANGE

There is a limited understanding of the hydrological and microclimatic processes from drained, harvested, or restored peatlands, and by extension the nature of the carbon balance is uncertain. However, understanding the symbiotic processes governing water and gas exchange is essential to the development of appropriate management plans for peatland restoration. In this paper, we highlight and contrast the suite of processes governing the atmospheric exchange of water and carbon on natural, harvested, and restored peatlands. Evapotranspiration from harvested sites is important throughout the spring and summer, and is not greatly different from the adjacent natural bog. On cutover peatlands, strong capillary water movement compensates for the lack of vascular plants, which on the natural bog become increasingly important as the capillary water movement within drying Sphagnum decreases. Methane emissions from cutover bogs are an order of magnitude lower than in natural sites, however, carbon dioxide (CO2) emissions are approximately three times greater. Peatland restoration enhances CO2 sequestration, although restoration (at least in the short term) does not restore the net carbon sink function to that in natural bogs. A conceptual model highlighting these changes in the atmospheric exchange of water and carbon from a natural bog through drainage, harvesting, abandonment, and restoration is presented. [Key words: peatlands, restoration, hydrology, climatology, evaporation, carbon dioxide, methane, harvested peatlands.]

Dynamics of biogenic gas bubbles in peat: Potential effects on water storage and peat deformation

[1] Dynamics of biogenic bubbles in peat soils were studied at a field site in southern Quebec, Canada. The maximum gas content measured in this study varied spatially with a maximum seasonal increase in volumetric gas content of 0.15. The size of changes in total gas content of a 1 m deep profile was comparable to the seasonal water storage change. Changes in bubble volume in the saturated zone alter the water table level and, consequently, the water content in the unsaturated zone and the apparent water budget. In highly compressible soils (and floating root mats), buoyancy forces from bubbles also cause relations between the surface and the water table to change. These effects cannot be omitted in modeling the hydrology of peatlands. Our results indicate a great spatial variability of trapped bubbles. Using pressure transducers sealed to the surface, we found pressure deviations indicating small areas closed off by bubbles clogging the pores. The hydrological influence of these areas may be considerable as they may restrict or deflect water flows. Open pipe piezometers did not show these pressure deviations, possibly because the closed zones were too small to influence the head in pipes or because of less amount of gas close to the pipe screen.