Abstract
Abstract. Infiltration into frozen and unfrozen soils is critical in hydrology, controlling active layer soil water dynamics and influencing runoff. Few Land Surface Models (LSMs) and Hydrological Models (HMs) have been developed, adapted or tested for frozen conditions and permafrost soils. Considering the vast geographical area influenced by freeze/thaw processes and permafrost, and the rapid environmental change observed worldwide in these regions, a need exists to improve models to better represent their hydrology. In this study, various infiltration algorithms and parameterisation methods, which are commonly employed in current LSMs and HMs were tested against detailed measurements at three sites in Canada's discontinuous permafrost region with organic soil depths ranging from 0.02 to 3 m. Field data from two consecutive years were used to calibrate and evaluate the infiltration algorithms and parameterisations. Important conclusions include: (1) the single most important factor that controls the infiltration at permafrost sites is ground thaw depth, (2) differences among the simulated infiltration by different algorithms and parameterisations were only found when the ground was frozen or during the initial fast thawing stages, but not after ground thaw reaches a critical depth of 15 to 30 cm, (3) despite similarities in simulated total infiltration after ground thaw reaches the critical depth, the choice of algorithm influenced the distribution of water among the soil layers, and (4) the ice impedance factor for hydraulic conductivity, which is commonly used in LSMs and HMs, may not be necessary once the water potential driven frozen soil parameterisation is employed. Results from this work provide guidelines that can be directly implemented in LSMs and HMs to improve their application in organic covered permafrost soils.
Highlights
Absolute values of θT were estimated from the changes with respect to a reference measurement the previous fall or subsequent summer when the active layer is thawed. θT was not regularly measured at SC P site, values for θT prior to snow melt were estimated from two θT profile measurements, one by TDR in the fall of 2002 immediately before the freeze-up and another by two 0.7 m deep frozen peat cores sampled near the study pit on 6 April 2003 (Hayashi et al, 2007), using the procedure described in Wright et al (2008)
Many Land Surface Models (LSMs) and Hydrological Models (HMs) assume a minimum value for liquid water content (e.g. CLASS, Verseghy, 1991; Simultaneous Heat and Water (SHAW), Flerchinger and Saxton, 1989)
Based on various field measurements at three discontinuous permafrost sites, this study evaluated five infiltration algorithms, three soil hydraulic property parameterisations, various ice impedance schemes on frozen soil hydraulic conductivity, and their influences and sensitivity on water infiltration into organic covered permafrost soils
Summary
Infiltration of snowmelt or rain into frozen ground or the unfrozen active layer is a critical hydrological process in permafrost regions (Woo, 1986) and its simulation is a key component in almost all process-based Land Surface Models (LSMs) (e.g. Bonan, 1991; Verseghy, 1991; Desborough and Pitman, 1998; Gusev and Nasonova, 2003; Dai et al, 2003) and Hydrological Models (HMs) (e.g. Beven and Kirkby, 1979; Liang et al, 1994; Yang and Niu, 2003; Pomeroy et al, 2007; Peckham, 2008). Mathematically quantifying infiltration has always been a challenge (Smith et al, 2002), due mainly to the heterogeneity of most natural soils and highly dynamic changes of soil water status and hydraulic properties during infiltration Those difficulties become extreme in permafrost environments due to ground thawing/freezing processes and a surface organic layer that frequently mantles permafrost terrain (Kane and Stein, 1983; Kane and Chacho, 1990; Slater et al, 1998). Flerchinger and Saxton, 1989; Zhao and Gray, 1997; Zhang et al, 2000; Pomeroy et al, 2007), infiltration schemes for frozen soil have been explicitly designed Their algorithms vary widely from first order empirical estimation The overall objective is to provide guidelines for the implementation of appropriate infiltration algorithms/parameterisations in LSMs and HMs to improve their performance in permafrost regions
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