Bridging the gap between models and measurements of peat hydraulic conductivity

Abstract

Peat saturated hydraulic conductivity, K sat , declines strongly with increasing degree of decomposition, providing a potentially important negative ecohydrological feedback that may buffer peatlands from climate-induced drying. However, the quantitative nature of this relationship is poorly understood. We measured downcore changes in K sat and carbon-to-nitrogen concentration quotients (C/N) in 14 shallow ( 0.5 m deep, 0.1 m diameter) peat cores from a Swedish raised bog. We used the C/N measurements to approximate the fraction of original peat mass remaining. A linear mixed effects (LME) model predicts log 10 (K sat ) from (i) our C/N-derived estimate of fractional remaining mass; (ii) depth; (iii) microhabitat (hummock, hollow); and (iv) location (treeless bog center, treed bog margin). The LME model indicated no significant random effects or interactions between predictors, so we derived a nonlinear multiple regression (NLMR) model to predict K sat on its original scale. Both LME and NLMR models predict that K sat decreases exponentially with depth and that K sat is lower beneath hollows than beneath hummocks for equivalent depths below the surface. Fractional remaining mass was an important predictor in the LME model, but not in the NLMR model. The distinction between central and marginal areas of the bog was not an important predictor. We demonstrate for the first time that the relationship between fractional remaining mass and K sat is log-linear, and suggest revisions that should be made to peatland development models. In particular, depth-usually ignored in modeling studies-exerted a strong control over K sat independently of decomposition and should be included explicitly in model algorithms.

ORCHIDEE-PEAT (revision 4596), a model for northern peatland CO<sub>2</sub>, water, and energy fluxes on daily to annual scales

“…Peat hydraulic parameters values used in this study were applied after Largeron et al (2017), based on Letts et al (2000) and Dawson (2006) ( Table 1). The peat-saturated hydraulic conductivity value of 2.45 × 10 −5 m s −1 is comparable to the harmonic mean value (6 × 10 −5 m s −1 ) of Morris et al (2015). The values of the other Van Genuchten parameters for peat (Table 1) are similar to those employed in other peatland models (Wania et al, 2009a;Wu et al, 2016).…”

“…This is partly related to the degree of decomposition and compression of organic matter (Gnatowski et al, 2010). Morris et al (2015) reported near-surface saturated hydraulic conductivities (K) of 2.69 × 10 −2 to 7.16 × 10 −6 m s −1 in bogs. Gnatowski et al (2010) measured values of 5 × 10 −6 m s −1 in a mosscovered peat, which was 2 orders of magnitude larger than for a woody peat (5.56 × 10 −8 m s −1 ).…”

A pore-size classification for peat bogs derived from unsaturated hydraulic properties

“…An increase in overburden leads to vertical shrinkage due to compression and a resulting increase in dry bulk density with depth (Clymo, 1978;Johnson et al, 1990;Price, 2003). The resulting strong decrease in saturated hydraulic conductivity, K s (L T −1 ) (Boelter, 1969;Ingram, 1978;Quinton et al, 2008), is positively correlated with the state of decomposition as shown by Ingram (1978), Clymo (1970Clymo ( , 1984, Clymo and Hayward (1982), Hayward and Clymo (1982), and Morris et al (2015). Exceptions to this rule have been observed in cases where pipe flow (Holden, 2005), fire disturbances (Sherwood et al, 2013), and rapid climate change resulting in changes in vegetation and subsequent peat deposition history (Rydin and Jeglum;Hedwall et al, 2017) occur.…”

“…Quinton et al, 2008;Price et al, 2008;Price, 2012, 2014;Morris et al, 2015;Weber et al, 2017a). As Sphagnum decomposes, the fibrous material is biochemically broken up into smaller solid fragments (Rezanezhad et al, 2016).…”

ORCHIDEE-MICT (v8.4.1), a land surface model for the high latitudes: model description and validation

“…SOM lowers thermal conductivity and increases heat capacity (e.g., , and increases soil porosity, which in turn increase saturated hydraulic conductivity and available water capacity (e.g., Hudson, 1994;Morris et al, 2015). As a consequence, the presence of SOM modulates heat transfer from the surface through the soil column, typically leading to cooler soil temperature during summer.…”