What conditions favor the influence of seasonally frozen ground on hydrological partitioning? A systematic review
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TOPICAL REVIEW • OPEN ACCESS
What conditions favor the influence of seasonally frozen ground on
hydrological partitioning? A systematic review
To cite this article: P Ala-Aho et al 2021 Environ. Res. Lett. 16 043008
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Environ. Res. Lett. 16 (2021) 043008 https://doi.org/10.1088/1748-9326/abe82c
TOPICAL REVIEW
What conditions favor the influence of seasonally frozen ground
OPEN ACCESS
on hydrological partitioning? A systematic review
RECEIVED
18 September 2020 P Ala-Aho1, A Autio1, J Bhattacharjee1, E Isokangas1, K Kujala1, H Marttila1, M Menberu1,2,
REVISED L-J Meriö1, H Postila1, A Rauhala3, A-K Ronkanen1, P M Rossi1, M Saari1, A Torabi Haghighi1
18 February 2021
and B Kløve1
ACCEPTED FOR PUBLICATION
1
19 February 2021 Water, Energy and Environmental Engineering Research Unit, University of Oulu, PO Box 4300, 90014 Oulu, Finland
2
PUBLISHED
Freshwater Centre, Finnish Environment Institute (SYKE), Paavo Havaksen tie 3, Oulu FI-90570, Finland
3
30 March 2021 Structures and Construction Technology Research Unit, University of Oulu, PO Box 4300, 90014 Oulu, Finland
E-mail: pertti.ala-aho@oulu.fi
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this work may be used Keywords: seasonally frozen ground, hydrology, infiltration, frozen soils, snowmelt runoff, soil water
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Abstract
the author(s) and the title The influence of seasonally frozen ground (SFG) on water, energy, and solute fluxes is important in
of the work, journal
citation and DOI. cold climate regions. The hydrological role of permafrost is now being actively researched, but the
influence of SFG has received less attention. Intuitively, SFG restricts (snowmelt) infiltration,
thereby enhancing surface runoff and decreasing soil water replenishment and groundwater
recharge. However, the reported hydrological effects of SFG remain contradictory and appear to be
highly site- and event-specific. There is a clear knowledge gap concerning under what
physiographical and climate conditions SFG is more likely to influence hydrological fluxes. We
addressed this knowledge gap by systematically reviewing published work examining the role of
SFG in hydrological partitioning. We collected data on environmental variables influencing the
SFG regime across different climates, land covers, and measurement scales, along with the main
conclusion about the SFG influence on the studied hydrological flux. The compiled dataset allowed
us to draw conclusions that extended beyond individual site investigations. Our key findings were:
(a) an obvious hydrological influence of SFG at small-scale, but a more variable hydrological
response with increasing scale of measurement, and (b) indication that cold climate with deep
snow and forest land cover may be related to reduced importance of SFG in hydrological
partitioning. It is thus increasingly important to understand the hydrological repercussions of SFG
in a warming climate, where permafrost is transitioning to seasonally frozen conditions.
1. Introduction seasonal freezing is wide, with around 25% of the
Northern Hemisphere’s land surface currently being
Seasonally frozen ground (SFG) has a major influ- subject to seasonal freezing of the ground outside
ence on land surface energy and water balance, and the permafrost zone, and 25% within the permafrost
thereby on all ecological, hydrological, pedological, zone (Zhang et al 2003). This has major ramifications
and biological activities at the Earth’s regions with for nutrient and carbon fluxes (Goulden et al 1998,
seasonal below-freezing ground surface temperat- Wagner-Riddle et al 2017), soil erosion (Edwards and
ures. Soils and sediments in cold regions can freeze Burney 1989, Sharratt et al 2000), vegetation dynam-
and thaw seasonally, or stay frozen perennially for two ics (Hayashi 2013, Bjerke et al 2015), heat exchange
or more consecutive years, a condition defined as per- between atmosphere and ground surface (Hagemann
mafrost (Dobinski 2011). Seasonal ground freeze and et al 2016), and land-use practices (Christensen et al
thaw occurs both in permafrost zones and perma- 2013, Yanai et al 2017).
frost free zones (Lemke et al 2007). Ground overly- Climate change will alter the frozen ground
ing the permafrost layer that freezes and thaws sea- regime and extent (Chadburn et al 2017, Biskaborn
sonally is called the active layer. The incidence of et al 2019, Wang et al 2019). The predicted warming
© 2021 The Author(s). Published by IOP Publishing Ltd
Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
of cold regions is likely to increase the frequency of occur because of limited storage capacity in the top-
freeze-thaw events (Venäläinen et al 2001). This could soil (Hinzman et al 1991, Kane et al 1991). In con-
significantly affect the biogeochemistry of rivers and trast, the influence of SFG extends to relatively shal-
alter ecosystem functioning, increasing the transport low depth (typically less than a meter), and insulating
of organic matter, inorganic nutrients, and major effect of the snowpack may reduce or even fully pre-
ions to oceans, e.g. the Arctic Ocean (Arctic Climate vent the development of ground frost even in below-
Impact Assessment 2004, Frey and McClelland 2009, freezing air temperatures (Hardy et al 2001, Lind-
Ala-Aho et al 2018, Box et al 2019). Furthermore, ström et al 2002, Iwata et al 2018). However, if the
a rise in temperature could cause the carbon-rich ground is frozen during the onset of snowmelt, it
frozen ground to release carbon and other green- is commonly assumed that SFG affects partitioning
house gases into the atmosphere, as positive feed- of snowmelt water through increasing surface run-
back to climate change (Schaefer et al 2014, Koven off and decreasing groundwater recharge (Dunne and
et al 2015, Serikova et al 2018). The hydrological Black 1971, Okkonen and Kløve 2011, Ireson et al
repercussions of thawing permafrost and changes 2013). This means that in winter and spring, SFG
in the active layer have been actively researched in seasonally promotes shallow water flow paths and
the past 10 years, whereas the hydrological influence reduces deep flow paths. In other words, SFG is con-
of permafrost free SFG has received less attention sidered to decrease the hydrological connectivity from
(but with a recent acceleration in research interest, ground surface to subsurface water reservoirs, but
see supplementary material figure S9 (available to increase the connectivity between hillslopes and
online at stacks.iop.org/ERL/16/043008/mmedia)). channels (Covino 2017).
We focused our review on permafrost free but SFG, Rain and snowmelt on frozen ground can cause
and consequently from hereon use the term SFG to major flooding events and erosion. Examples of
refer to SFG in permafrost free conditions. ground frost-induced floods were documented in the
Long-term observations and records of seasonal interior Pacific Northwest USA in 1973 (Johnson and
frost penetration depths are important for detect- McArthur 1973) and 1996 (Halpert and Bell 1997)
ing spatiotemporal variability and trends in SFG, and and around the Rhine River in Germany 1993 and
therefore instrumental in understanding the past and 1995 (Barredo 2007). In addition to floods, there are
future hydrological regime of cold regions. These data examples of more subtle hydrological influences of
suggest that the SFG regime is already under consid- SFG. Lack of ground frost intensifies groundwater
erable change, with decreasing maximum frost depth recharge, especially during winter months, which can
penetration and duration in the late part of the 19th result in increased base flow to rivers and streams
century (Frauenfeld et al 2004, Zhao et al 2004, Sinha (Peterson et al 2002, Ploum et al 2019).
et al 2010). Simulations based on climate change There are several existing review papers with mer-
scenarios suggest that the trend is likely to continue its in synthesizing the hydrology of SFG. Ireson et al
(Venäläinen et al 2001, Wang et al 2019). However, it (2013) prepared an overview of the physical processes
is unclear how the already recorded, and further anti- taking place in frozen ground using numerous field
cipated, changes in SFG are reflected in the hydrolo- studies as examples. Based on this, they developed a
gical regime, an issue that was the scope of this review. conceptual model of surface and subsurface hydrolo-
The fundamental reason why ground freezing is gical processes in semi-arid seasonally frozen regions.
important for hydrological processes is that when Hayashi (2013) reviewed the hydrological processes in
the ground freezes, ice blocks a part of the previ- frozen ground with special emphasis on the implic-
ously water-filled soil pores and prevents water flow ations of SFG on ecological functions and nutrient
through these pores. It is typically perceived that cycling. Kurylyk and Watanabe (2013) performed a
frozen ground, whether seasonal or permafrost, lim- comprehensive review of mathematical representa-
its the degree by which different parts of the landscape tions of ground freezing and thawing processes and
interact through the exchange of water, i.e. are hydro- suggested ways of advancing hydrological modeling
logically connected. However, regions with perma- in frozen ground. Lundberg et al (2016a) reviewed
frost differ in their hydrological response to regions the spatiotemporal interactions between snow cover
where only the top ground layer freezes seasonally and SFG, and their hydrological repercussions, and
(for which we use the term SFG). Permafrost penet- concluded that groundwater models consider snow
rates deep into subsurface layers (meters to hundreds and frozen ground processes in an overly simplistic
of meters) and thereby affects the hydraulic prop- manner. Mohammed et al (2018) focused on the pro-
erties of the ground over significant depths. Many cess and hydrological importance of macroporosity
studies indicate that permafrost impedes or fully pre- in frozen ground and developed a strong argument
vents deep groundwater flow (Beilman et al 2001, for taking soil macroporosity better into account in
Woo et al 2008, Walvoord et al 2012). During snow- both conceptual understanding and numerical mod-
melt when the active layer is not yet thawed, shallow eling of water flow in frozen ground. Despite the
subsurface flow and rapid surface runoff generation documented evidence of the influence of SFG on
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Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
hydrology, the universal hydrological responses to conclusions reached, together with information on
SFG remain controversial and contradictory. A col- the physiographical and environmental conditions
lective finding from previous work is that the hydro- of a given study, allowed us to explore another
logical influence of SFG is site- and event-specific question: ‘Under what conditions does SFG influence
(Laudon et al 2007, Appels et al 2018). Laborat- hydrological fluxes?’. Our systematic review adds to
ory studies typically show a distinct effect of ground the existing literature by not only reviewing current
frost on the infiltration capacity at small scale (Burt knowledge, but also establishing a new dataset to
and Williams 1976, Iwata et al 2010a, Watanabe explore the importance of SFG on hydrological par-
and Kugisaki 2017), whereas catchment-scale studies titioning. By compiling a dataset with global cov-
show considerable variability, with sometimes little or erage, we can draw conclusions that extend beyond
no evidence of SFG influencing hydrology (Granger individual site investigations. Our findings are of
et al 1984, Cherkauer and Lettenmaier 2003, Stähli wide interest to earth science researchers striving to
2017). Individual investigations are always defined by: understand the environmental repercussions of cli-
(a) a region’s prevailing ‘static’ physiographical con- mate change for water resources in seasonally frozen
ditions, such as soil type and surface topography; and regions.
(b) temporally varying environmental conditions, The remainder of this paper is organized as fol-
such as air temperature, snow conditions, and veget- lows: In section 2, we review the observational field
ation cover. For this reason, each individual study methods and numerical modeling techniques used to
encompasses only a small subset of the environmental study the hydrology of SFG. In section 3, we describe
conditions in seasonally frozen regions, and the con- our systematic reviewing methodology, which pro-
clusions reached on SFG influence are necessarily tied duced a dataset we used to analyze why some studies
to the context and conditions of the study site, obscur- report SFG to be more hydrologically relevant than
ing the generalization of the results. others. In section 4, we review key factors reported
To further complicate matters, the hydrological to determine the hydrological response in SFG, e.g.
influence of SFG cannot be measured and reported climate conditions such as air temperature and snow
with a well-defined universal metric. Instead, inter- amount, land use, soil characteristics, and scale of
pretation relies on observing changes in a selected measurement in observing the hydrological response.
hydrological flux variable brought about by the frozen After describing each factor, we discuss its influence
state of the ground. Therefore, the conclusion on on SFG hydrology in light of the findings in our sys-
whether a given study site is influenced by SFG is tematic review of published data. In section 5, we
based on the interpretation of each dataset, with a discuss expected changes in the hydrology of SFG.
degree of subjectivity by the investigators. Further- In section 6, we highlight critical future research
more, past research on ground freezing and thawing needed to better understand the role of SFG in
was fragmented over time and performed within dif- cold region hydrology, and finish with conclusion in
ferent scientific fields, including forestry, civil engin- section 7.
eering, soil science, climate sciences, hydrology, and
hydrogeology. This led to a variety of research meth- 2. Determining the hydrological influence
ods being used in both field-based observations and of SFG: observational techniques and
numerical modeling, resulting in varied practices in numerical modeling
acknowledging and reporting hydrological fluxes.
The problems of variable site environmental con- 2.1. Observational techniques to study the
ditions and non-comparability of data make it diffi- hydrological influence of SFG
cult to synthesize the information on the hydrological In studying the hydrological influence of SFG, two
influence of SFG that has so far remained ‘frozen’ main questions must be considered: (a) what is the
in the literature. Thus, there has been no compre- extent of frozen ground; and (b) how does the frozen
hensive analysis of the reasons why SFG influences state of the ground alter the flow of water? Exist-
hydrology at one site but not at another, or in 1 year ing measurement techniques to study SFG processes
but not in another. Inconsistent conclusions based on primarily answer different versions of the question
site investigations are typical of hydrological studies, (a), i.e. What is the freeze/thaw status of the ground?
so a wider perspective needs to be gained through How deep is the frost penetration? and, less com-
a review of the literature, summarizing the findings monly, What is the volumetric ice content in the
at many individual sites (Evaristo and McDonnell ground? A wide set of tools, such as frost tubes filled
2017). There is a clear knowledge gap concerning with methylene blue solution, temperature sensors,
under what physiographical and climate conditions soil moisture sensors, and geophysical techniques like
SFG is more likely to influence hydrological fluxes. ground penetrating radar, have been used to determ-
Therefore, we conducted a systematic review ine ground frost penetration depth (Steelman and
of scientific publications providing data-based Endres 2009, Steelman et al 2010, Butnor et al 2014,
answers to the question ‘To what degree does Ma et al 2015). Frost tubes and temperature sensors
SFG influence hydrological fluxes?’. Extracting the use temperature as a proxy to estimate whether water
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Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
Figure 1. Key hydrological fluxes and water quality parameters (in italic) where the influence of SFG on the hydrological cycle can
be seen. Our review focused on studies observing SFG-induced changes in the fluxes of infiltration, percolation,
evapotranspiration, groundwater recharge, runoff (surface or near surface), hydrograph (stream response), and water chemistry
using tracer techniques .
in ground is frozen. On comparing the results with snow cover manipulations. Measurement and com-
direct observations Iwata et al (2012) found satis- parison of infiltration or runoff fluxes in different
factory agreement, with root mean square error of frost regimes, while other factors remain identical,
about 3 cm for both frost tubes and temperature allows a direct conclusion on the influence of SFG
sensors. Determination of the temporal changes in on hydrological partitioning (Iwata et al 2011). The
soil moisture or liquid water content (LWC) has been hydrological influence of SFG has been studied by
used to measure ground freezing with time domain observing changes SFG brings about in the following
reflectometry (Stähli and Stadler 1997). This tech- hydrological fluxes or variables: infiltration, percola-
nique is based on the dielectric change in the ground tion, evapotranspiration, groundwater recharge, run-
as the LWC decreases with freezing. A comprehens- off (surface or near surface), stream response (hydro-
ive review of ground-based techniques for measuring graph), and water chemistry using tracer techniques
and monitoring ground frost depth was performed by (figure 1). In this systematic review, we identified and
Lundberg et al (2016b). analyzed previous experimental data on the hydrolo-
However, measuring the presence or properties of gical influence of SFG on these variables.
ground frost is not sufficient to evaluate the hydro- The influence of SFG on infiltration can be
logical influence of SFG, i.e. answer the question (b) determined as the difference between frozen and
above. To address this, other techniques are needed unfrozen infiltration rates at the ground surface. The
to observe changes in the hydrological system caused research methods primarily used to date to quantify
by SFG. However, there is no commonly recognized the difference have been small-scale experimental set-
and standardized method for measuring and determ- ups in field conditions, e.g. infiltration rings (Kane
ining the influence of SFG on hydrology. Ideally, the and Stein 1983b, Hayashi et al 2003). Percolation, i.e.
hydrological influence of frozen ground should be water movement through the soil matrix, has typic-
observed by measuring a specific hydrological flux ally been measured in the context of determining soil
when the ground is in frozen and unfrozen state, and hydraulic conductivity. Hydraulic conductivity meas-
comparing the results. For example, hydraulic con- urements have usually been conducted in a laboratory
ductivity is typically determined by measuring fluid environment, in small-scale experiments using soil
flux through the soil matrix under a given hydraulic cores or monoliths in which hydraulic conductivity
gradient in laboratory conditions. If this flux is smal- has been determined with and without the presence
ler for frozen than unfrozen soil samples, while other of frost (Wiggert et al 1997, Watanabe and Kugisaki
factors remain unchanged, this leads to an indir- 2017). The influence of SFG on groundwater recharge
ect conclusion that frozen water in the soil matrix has been studied using field-based techniques meas-
has hydrological repercussions (Burt and Williams uring groundwater level or groundwater–surface
1976). As a field study example, the ground frost pen- water interactions and numerical modeling of the
etration depth and ice content can be changed by groundwater system (Thorne et al 1998, Daniel and
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Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
Staricka 2000, Okkonen and Kløve 2011). Observed The first physically-based thermo-hydrological mod-
or simulated changes in groundwater recharge flux els emerged in the early 1970s, but their applicab-
have provided indirect evidence of the hydrological ility was not in line with the needs of hydrologists
influence of SFG. In the present review, we considered and hydrogeologists of the era. Computational tech-
runoff flux to be measured (or simulated) near- niques and resources were not sufficient to solve the
horizontal water flow on the ground surface or in sur- complex differential equations for spatially extensive
ficial soil layers. Runoff studies have typically been study areas of interest and management, i.e. catch-
performed in hillslope-scale plots with constructed ments and aquifers. The need to simulate streamflow
runoff water collection systems to evaluate surface from snowmelt for operational applications has led
and/or near-surface runoff generation under different to the development of conceptual approaches to deal
ground frost conditions and in different soil horizons with the hydrological influence of SFG.
(Willis et al 1961, Dunne and Black 1971, Bayard et al The first implementations were carried out by
2005). Hydrograph responses to rainfall or snowmelt Gray et al (1985) and Anderson and Neuman (1984).
have been used to study whether stream discharge dif- Gray et al (1985) modeled SFG influence on a small
fers in frozen and unfrozen catchments. Hydrographs catchment in Saskatchewan, Canada. Their approach
integrate the response of the whole hydrological sys- was based on the concept that infiltration into frozen
tem, and the scale of measurement is the catchment ground can be categorized into three distinct classes:
above the stream gauging point. A typical assump- restricted, unlimited, and limited. They defined the
tion is that stream response is more pronounced when limited (winter) category of infiltration as a simple
the ground is frozen, which has been reported e.g. as function of pre-winter soil water content and snow
runoff coefficient (Granger et al 1984, Stähli 2017) or water equivalent (SWE), and integrated it into their
changes in hydrograph recession (Ploum et al 2019). bucket-type rainfall-runoff model. This modification
To further analyze the streamflow response, model- significantly improved model performance, demon-
ing approaches have been used to evaluate the influ- strating that conceptual ground frost models can
ence of SFG on catchment hydrology and stream be successfully applied for operational purposes in
runoff generation (Prévost et al 1990, Sterte et al hydrograph simulations. Similar improvements in
2018). A typical modeling study introduces a math- simulations were obtained by Anderson and Neuman
ematical representation of frozen ground into the (1984) for larger river basins in Minnesota, USA.
model and tests whether the new model demonstrates They developed an empirical ‘Frost Index’ based on
better performance in simulating the hydrological meteorological variables and snow and soil moisture
flux of interest, most typically a stream hydrograph. conditions. The calculated frost index was used to reg-
More unconventional study variables to evaluate SFG ulate water percolation rates in a hydrological model.
influence on water fluxes have also been tested. SFG Instead of fully conceptual SFG formulations,
may reduce water availability for evapotranspiration, mixed approaches combining hydrological bucket-
which has been quantified through measurements or type models with a simplified version of the heat
simulations (Mellander et al 2004, Wu et al 2016, transport equation, pioneered by Koren (1980), have
Miao et al 2017). Water chemistry (Fuss et al 2016) been developed (Pomeroy et al 2007, Mohammed
and environmental tracers such as stable water iso- et al 2013, Koren et al 2014, Gao et al 2018). Such
topes (δ2 H and δ18 O), have been used to separate mixed approaches allow more data on soil proper-
hydrological flow paths in different ground frost con- ties and state variables, such as soil water content or
ditions (Laudon et al 2007, Smith et al 2019), allowing ground temperature, to be used in the model cal-
a conclusion on SFG influence on hydrology. ibration and validation, potentially reducing model
uncertainty. Another mixed approach type, which
2.2. Representation of SFG in numerical combines a conceptual frozen ground representation
hydrogeological and land surface modeling with physically-based models, has been proposed and
We categorized the existing SFG modeling applied for SFG sites (Mahmood et al 2017, Sterte et al
approaches into: (a) hydrological conceptual models; 2018).
(b) thermo-hydrogeological models; and (c) land-
surface models (table 1). Although these categor-
ies are not well-defined and many models could fall 2.2.2. Thermo-hydrogeological modeling
into several categories, we considered this categoriz- The first efforts to create physically-based models by
ation useful in providing context for the history and coupling thermal energy transport and water flow
present state of SFG model applications in hydrolo- were made in the early 1970s (Harlan 1973, Guymon
gical research. and Luthin 1974, Motovilov 1977). These models
were developed to assist with cold region infrastruc-
2.2.1. Hydrological conceptual modeling ture and were based on the analogy between Darcian
Various methodologies have been used to simulate fluid flow in unsaturated unfrozen porous media
ground freeze/thaw processes and the movement of and flow in saturated partially frozen ground. At
water in or at the surface of a frozen soil matrix. the time, computational technology restricted model
5
Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
Table 1. Overview of available models for simulating the hydrological influence of SFG categorized to their scope and applications.
Model category Hydrological Land surface Thermo-hydrogeological
Typical frozen ground Conceptual Mixed conceptual and Physical
formulation to model physical
equations
Main variable of interest Stream runoff Soil moisture status Subsurface water fluxes
affected by frozen ground
Primary simulation cases Hydrological field studies Large-scale domain with Theoretical, idealized
point field verification examples
Typical scale of application Catchment Continental Hillslope or soil profile
Typical ground freezing Permafrost or SFG (not both Permafrost and SFG (both in Permafrost (increasingly also
regime simulated in the same scheme) the same scheme) SFG)
Key advantages Low data requirements Able to utilize data at Model equations and
different scales parameters physically based
Key disadvantages Empirical model Limited model validation High data requirements
parameterization using hydrological fluxes
development models to one dimension and compar- are differences in the hydrological responses of per-
isons to soil column laboratory experiments (Jame ennially and SFG, the physical principles produ-
1977, Taylor and Luthin 1978), although the prin- cing the responses are the same. For this reason,
ciples described in the literature of the time have the physically-based models developed to simu-
served as the base for current thermo-hydrogeological late thawing of permafrost are suitable for applic-
models. These models couple the Richards equation ation to SFG regions. Most of the currently avail-
(Richards 1931) of groundwater flow with the energy able 3D thermo-hydrogeological models, e.g. SUTRA
balance equation of heat transport (equations (1) and (Voss and Provost 2002), HGS (Aquanty 2015), and
(2) in supplementary material S2), which can still be FEFLOW (Diersch 2013), have been developed from
considered the state-of-the-art representation of the a groundwater modeling perspective, as an exten-
physics of ground freeze-thaw. sion of the current hydrogeological and fully integ-
Building on this pioneering work, 1D models rated groundwater–surface water simulators. A wide
like SHAW (Flerchinger and Saxton 1989), COUP range of mathematical representations is currently
(Jansson 2004), and HYDRUS-1D (Hansson et al used in thermo-hydrogeological codes. These mod-
2004) have been used to couple thermal energy and els differ in their numerical, coupling, and dis-
water flow in hydrological simulations of SFG. The cretization schemes, and also in the form of the
key attraction of existing 1D models is their ability Clapeyron equation applied (equations (3) in sup-
to couple above-ground energy balance to subsur- plementary material S2), soil freezing curve deriv-
face process in their model domain without a great ations, and frozen hydraulic conductivity functions
increase in computational burden. This has allowed (Kurylyk and Watanabe 2013). Including state-of-
ecohydrological simulations of plant influence on the-art, physically-based freezing-thawing processes
water and energy partitioning (Ala-aho et al 2015a, for surface and subsurface domains involves highly
Metzger et al 2016), and very importantly for SFG non-linear relationships and is numerically challen-
hydrology, of good simulations of snow accumula- ging, requiring the application of advanced solution
tion and melt (Stähli et al 2001, Assefa and Woodbury schemes and leading to long running times and para-
2013, Okkonen et al 2017). Capturing snow cover meterization issues. It should be also noted that, des-
dynamics is essential both in estimating ground sur- pite their physical basis, many 3D physically-based
face temperature over winter as frost layer builds, and models employ a simplified representation of surface
in estimating water input on SFG during spring snow- hydrology and/or snow accumulation and melt.
melt, where SFG influence on hydrology is greatest. Development and refinement of thermo-
1D models have also been used to provide boundary hydrogeological models has continued and more 3D
conditions for groundwater models in SFG regions computer codes integrating surface and subsurface
(Okkonen and Kløve 2011, Ala-aho et al 2015b) thermal hydrology have emerged, e.g. GeoTOP 2.0
and 3D thermo-hydrogeological models in perma- (Endrizzi et al 2013) and ATS (Painter et al 2016).
frost regions (Langford et al 2019), because of more Simplified, physically-based modeling approaches
complete above-ground process description and bet- based on analytical solutions of energy balance
ter numerical efficiency than with 3D models. equations have also been proposed or used for large-
In the past decade, there has been growing interest scale modeling (Hayashi et al 2007, Semenova et al
in hydrological research on ground freezing and 2014, Kurylyk and Hayashi 2016). One such approach
thawing, and development of 3D physically-based is Stefan’s formula (Stefan 1891), a simple algorithm
models to simulate these processes. Although there utilizing the analytical solution of the conduction
6Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
equation for calculating frost/thaw depth penetra- has been a mix of physically-based and empirical
tion (equation (4) in supplementary material S2). approaches. Ground freeze-thaw status has typ-
The analytical model requires data on land surface ically been simulated using physically-based 1D
temperatures, which are often not available, leading heat transport equations (Gouttevin et al 2012), or
to the development of empirical and quasi-empirical simplified to the Stefan equation only accounting
factors relating to meteorological variables and land for latent heat requirements to freeze or thaw the
surface temperature (Kurylyk and Hayashi 2016). ground (Hagemann et al 2016). Storage and flow
In terms of field applicability, the physically-based of water in the ground has typically been solved in
thermo-hydrogeological models are in their infancy. the vertical dimension, using the Richards equation
Test cases are at the stage of intercomparisons of (Clark et al 2015). In principle, this approach has
idealized test cases for permafrost thawing, e.g. the been similar to that in thermo-hydrogeological mod-
Interfrost project (Grenier et al 2018). Most exist- els, but the dimensionality has been reduced to ver-
ing catchment-scale applications deal with perma- tical and the model equations have been solved in a
frost regions, typically with an outlook on hydro- more simplified, computationally inexpensive man-
logical change due to permafrost thaw (Bense et al ner. A typical approach to account for SFG influ-
2009, McKenzie and Voss 2013, Krogh et al 2017, ence on hydrology has been to reduce the bulk
Rawlins et al 2019). At the moment, catchment- hydraulic conductivity of frozen ground, with a
scale applications of state-of-the-art, physically-based physically- or empirically-based approach, and incor-
thermo-hydrogeological models that focus on perma- porate the temporarily modified parameter value in
frost free SFG regions are basically non-existent. Two- a 1D Richards equation to solve for water flow (e.g.
dimensional applications for hillslope-scale stud- Cherkauer and Lettenmaier 1999, Koren et al 1999).
ies are rare and almost exclusively performed with Slater et al (1998) explicitly solved ice content in
SUTRA code (McKenzie et al 2007, Kurylyk et al the ground as part of Richards equation formula-
2014, Evans and Ge 2017, Evans et al 2018), with tion, which decreased the soil hydraulic conductiv-
the exception of recent work by Schilling et al (2019) ity in their LSM. Luo et al (2003) found that includ-
who integrated SFG processes into a fully integrated ing ground freeze/thaw improved the simulation of
groundwater–surface water model, HydroGeoSphere. ground temperature and its variability at seasonal
Overall, the lack of field-scale applications of thermo- and interannual scales in multiple land-surface para-
hydrogeological models has been attributed to poor meterization schemes. Niu and Yang (2006) made
data availability and challenges in monitoring ground several advances in SFG representation in LSMs by
freeze-thaw processes (Hayashi 2013, Ireson et al bringing in a physical process understanding, where
2013). the ground was assumed to retain some permeabil-
ity when frozen, to LSM SFG parameterization. They:
2.2.3. Land-surface modeling (a) allowed unfrozen water to coexist with ice in the
Land surface models (LSMs), also referred to as ground over a wide range of temperatures below 0 ◦ C
Earth system models, couple water and energy fluxes by using a modified form of the Clapeyron equation
with biogeochemical and ecological processes at the (Flerchinger and Saxton 1989); (b) computed vertical
interface of the Earth surface and the lower atmo- water fluxes by introducing the concept of a frac-
sphere (Overgaard et al 2006, Best et al 2011). Good tional permeable area, which partitioned the model
simulation of soil moisture conditions is crucial in grid into an impermeable part (no vertical water flow)
LSMs, but for a long time, the influence of freez- and a permeable part; and (c) used the total soil
ing on soil moisture reservoirs received little atten- moisture (liquid water and ice) to calculate soil mat-
tion in the LSM modeling community (Slater et al ric potential and hydraulic conductivity. Comparis-
1998, Maxwell and Miller 2005). This is surpris- ons between the original and modified frozen ground
ing, given the spatial coverage and importance of schemes, with data from a small catchment and from
permafrost and SFG in the Northern Hemisphere. the six largest cold region river basins, showed that
Inclusion of ground freeze/thaw routines has recently the modified scheme produced better estimates of
been deemed particularly important in permafrost monthly runoff and terrestrial water storage change
regions, because of positive feedback between the cli- (Niu and Yang 2006).
mate system and organic soil carbon sources exposed Gouttevin et al (2012) specifically simulated the
by permafrost thaw (Schuur et al 2015). In recog- hydrology of SFG with their LSM, and reported
nition of this, recent state-of-the-art LSMs account improved runoff simulations at both catchment and
for the thermo-hydrological processes of ground continental river basin scale. Ekici et al (2014) used
freeze/thaw both in permafrost and seasonally frozen a physically-based formulation that coupled the heat
condition (Lawrence et al 2012, Chadburn et al 2015, transfer and Richards equation in one dimension,
Guimberteau et al 2018, Yang et al 2018, Melton and incorporated the co-existence of ice and unfrozen
et al 2019). water according to Niu and Yang (2006). Hagemann
The mathematical approach employed to date to et al (2016) used the same model formulation and
represent the hydrology of frozen ground in LSMs reported large improvements in simulated snowmelt
7Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
peak runoff due to frozen ground in spring in the and other screening steps are given in supplementary
SFG-influenced Baltic Sea basin. material S1.
Recent thermo-hydrological schemes in LSMs
largely remain adopted from Koren et al (1999), Niu 3.1.2. Step (b): screening by scope
and Yang (2006), Gouttevin et al (2012), except for From the set of identified papers, we further screened
improvements to water phase change equations and those included in the analysis by applying two criteria.
parameterization suggested by Yang et al (2018) and For a study to be included, it had to have:
hydraulic conductivity and matric potential formu-
lations by Ganji et al (2017). Other recent advances (a) An explicit conclusion, result, or data-based
that are not strictly related to conceptualizing energy speculation that SFG has (or does not have) an
transport and water flow, but still have hydrolo- influence on hydrological fluxes, i.e. the partition-
gical importance, are done on improving land sur- ing of water to infiltration, runoff, percolation,
face parameterization and soil profile representation. groundwater recharge, or evapotranspiration.
Deeper soil profiles (tens of meters instead of 3–5 m) (b) Conclusion/s based on original measurements
have been found important in correctly simulating or hydrological simulations calibrated/validated
active layer thaw in the permafrost region due to heat with original measurements presented in the
sink in deep cold permafrost (Chadburn et al 2015, paper.
Melton et al 2019), but not tested or proven important
in SFG regions because this deep heat sink is missing. The first criterion made our scope unique, as
Another advance in improving thermo-hydrological much of the SFG-related literature focuses on frost
simulations has been including a layer of moss and/or penetration depth or ground temperature change.
organic soil that increases insulation properties of the Our interest was in determining whether the frozen
top soil (Guimberteau et al 2018, Melton et al 2019). state of ground has hydrological consequences. The
Again, LSMs have demonstrated the importance of second criterion, to account only for studies that
mosses only in simulating the active layer in the per- founded their conclusions on original measurements,
mafrost. From a physical perspective mosses, lichens, excluded studies that developed hydrological mod-
or organic soil layers are equally important in SFG eling algorithms but did not validate the models
regions by causing more insulating (less freezing) and against measured data (Kulik 1977, e.g. Flerchinger
water interception before entering the mineral soil and Saxton 1989, Tao and Gray 1993). Even though
(lower water and ice content) (Ala-aho et al 2015a). our title word search (see supplementary material
S1) attempted to exclude work done in permafrost
regions, at this point we did a second screening to
3. Systematic review of hydrological
ensure the original research was done on SFG by
influence on SFG
reviewing the study site description.
Screening in step (b) revealed that 143 of the 379
3.1. Systematic review methodology
publications identified in step (a) contained data-
Our systematic review process consisted of three main
based conclusions (or discussions) on whether ‘the
steps: (a) study identification (screening by title);
frozen state of ground influences hydrological fluxes’.
(b) screening the selected studies for applicability in
However, some of these 143 studies produced mul-
the analysis (screening by scope); and (c) double-
tiple entries in our analysis (i.e. multiple study sites
blind review (by two people) of the studies to extract
or methods yielded multiple individual conclusions
data for further analysis. The workflow of the liter-
from one study), which resulted in a total of 162
ature identification, screening, and review process is
analyzed entries in our dataset. Although this data-
presented in supplementary material (figure S1).
set is extensive in coverage, we cannot claim to have
included all studies conducted to date, because of
3.1.1. Step (a): screening by title limited availability, language barriers, or failure to
We identified a total of 379 studies in which frozen identify all relevant work in the literature search, par-
ground was an essential part of the study set-up. ticularly in the case of early research in the Soviet
Studies applying a range of research methods, from Union.
laboratory and field experiments to numerical sim-
ulations, were included in the review. Papers relev- 3.1.3. Step (c): double-blind review
ant to our research topic were identified through The most important piece of information extracted
three pathways: (a) a title word search in Scopus and from each study was the nature of the conclusion/s
Web of Science, (b) reference crawling: incorporat- reached about the influence of SFG on hydrological
ing relevant work cited in other reviewed papers, and partitioning, in terms of the observed or simulated
(c) miscellaneous sources, including primarily papers flux of water infiltration, surface runoff, percolation
known/suspected to be relevant from past experi- through soil matrix, groundwater recharge, or tran-
ence of the writing team, or recent work through spiration. In our analysis, we categorized the influ-
publication alerts. Details on the title word search ence of SFG into four classes:
8Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
(a) EVIDENT: the data show indisputable evidence the location of the study site (for full description, see
of SFG influence in all hydrological events/years table S1). The global datasets we used were Köppen–
studied. Geiger climate classification based on data for the
(b) OCCASIONAL: the data show clear evidence of period 1976–2000 (Kottek et al 2006), mean and
SFG influence in some, but not all, hydrological annual maximum SWE for the period 1980–2014
events/years studied. (Luojus et al 2013), and mean air temperature of the
(c) UNCERTAIN: the data suggest that SFG coldest quarter in the years 1970–2000 (Fick and Hij-
can explain some aspects of the hydrological mans 2017). The variables were selected to encom-
response, but the data/conclusions are not defin- pass physiographical and environmental conditions
itive. that previous studies have found to be important
(d) ABSENT: the data show no evidence of SFG for explaining the hydrological response in frozen
influence on hydrological fluxes. ground.
Quotation or summary of the decisive argu-
ment(s) in each paper, on which our conclusion cat- 3.2. Hydrological influence of SFG is common, but
egory is based, is included in our dataset. Below we not universal
give examples for studies and conclusions in each cat- Most of the SFG influences reported in the studies
egory. EVIDENT: Kane (1980) evaluated how ground fell into the EVIDENT or OCCASIONAL categories
ice and moisture content was related to infiltration (75.9%), implying that the frozen state of ground has
rates using infiltration test in the field. They found hydrological repercussions in most experimental or
that: ‘the greater the moisture content, the greater simulation settings (table 2). However, it is import-
the ice present in the frozen soil; thus the infilt- ant to point out that the majority of studies reviewed
ration rate and the saturated hydraulic conductiv- suggested that even if SFG influences water move-
ity were reduced’. OCCASIONAL: Stähli et al (2004) ment, it rarely completely blocks it. The remaining
used a dye tracer in an excavated vertical soil pro- 24.1% of studies reviewed reported that SFG has only
file to examine the infiltration pathways in alpine vague or non-existent impacts on hydrological fluxes
soils. They found that ‘Soil frost conditions var- (UNCERTAIN or ABSENT categories), confirming
ied between winters…During the first winter the the assumption underlying our initial hypotheses that
water infiltration showed a pronounced preferen- the influence of SFG on hydrological fluxes is not
tial behavior…During the second winter the imped- universal. The differences in reported SFG influence
ing impact of soil frost was clearly seen.’ UNCER- allowed us to explore the experimental settings or
TAIN: Komiskey et al (2011) studied nutrient and environmental conditions in which SFG is hydrolo-
sediment loading runoff from frozen agricultural gically less important.
fields by monitoring runoff volumes, meteorological The field studies reviewed spanned the North-
variables, and ground frost regime. They concluded ern Hemisphere and mostly fell between the seasonal
that ‘…it appeared that runoff amounts were more frost boundary in the south and the continuous per-
related to the timing and type (snow/ sleet/rain) mafrost boundary in the north (figure 2), demon-
of precipitation, intensity of precipitation (rainfall), strating a successful screening to exclude hydrolo-
air temperatures, and snowpack properties such as gical studies in permafrost settings. A majority (58%)
depth, water equivalent, ice layers, and temperat- of the field studies were conducted in Canada and
ure, rather than soil temperatures or frost depth the USA (figures 2 and 3). Other regions with high
alone’. ABSENT: Nyberg et al (2001) studied the influ- numbers of studies were Scandinavia, Alpine Central
ence of ground frost on water flow paths along a Europe, Japan, and western parts of the Tibetan Plat-
hillslope by monitoring ground moisture and frost eau. Interestingly, studies from Europe reported the
regime and runoff. They stated: ‘…there was no clear smallest percentage of EVIDENT hydrological influ-
frost effect on runoff during the three investigated ence of SFG and, conversely, the highest percentage
winters’. of ABSENT influence. The ABSENT reports of SFG
In addition to the reported conclusion on SFG influence were clustered in Fennoscandia (figure 2).
hydrological influence, we extracted data that could The earliest reviewed studies dated from the 1950s,
explain why some site investigations or laborator- since when the number of studies on the topic has
y/modeling studies show a more evident hydrolo- been steadily increasing (figure 3). This increase may
gical response to frozen ground than others. The be related to the higher numbers of scientific papers
data included spatial and temporal scale of measure- published overall and to our better access to more
ment, soil physical properties, climate characteristics, recent literature, rather than to growing interest in the
land cover, frost penetration depth, and other relev- research topic. However, it is worth noting that relat-
ant attributes either reported directly in the studies ively fewer studies reported an EVIDENT influence
reviewed, or extracted from global datasets based on after 1990 than before.
9Table 2. The reviewed literature grouped to study method (columns) and the conclusion about the influence of soil frost on hydrological partitioning made in the study (rows). For full details of the conclusions and data extracted
from each study see Ala-aho et al (2020).
SFG influence Laboratory Field Simulation
Evident (Benoit and Bornstein 1970, Burt and Williams 1976, (Diebold 1938, Garstka 1944, Stoeckeler and Weitzman (Molnau and Bissell 1983, Anderson and Neuman 1984,
Rudra et al 1986, Engelmark 1988, Edwards and Bur- 1960, Kuznik and Bezmenov 1963, Larin 1963, Haupt Gray et al 1985, Sand and Kane 1986, Barry et al 1990,
ney 1989, Andersland et al 1996, Seyfried and Murdock 1967, Dunne and Black 1971, Murray and Gillies 1971, Molnau et al 1990, Prévost et al 1990, Johnsson and
1997, Wiggert et al 1997, Stadler et al 2000, McCauley Harris 1972, Alexeev et al 1973, Romanov et al 1974, Lundin 1991, Koren et al 1995, 2014, Cherkauer and
et al 2002, Weigert and Schmidt 2005, Fouli et al 2013, Kane 1980, 1981, Zuzel et al 1982, Kane and Stein 1983a, Lettenmaier 1999, 2003, Li and Simonovic 2002, Zhao
Watanabe et al 2013, Zhao et al 2013b, Campbell et al 1983b, 1987, Granger et al 1984, Price and Woo 1988, et al 2002, Koren 2006, Niu and Yang 2006, Okkonen and
Environ. Res. Lett. 16 (2021) 043008
2014, Sutinen et al 2014, Ban et al 2017, Watanabe and Byrne 1989, Thunholm et al 1989, Blackburn et al 1990, Kløve 2011, Hagemann et al 2016, Qin et al 2017, Sterte
Kugisaki 2017, Watanabe and Osada 2017, Appels et al Seyfried et al 1990, Pikul et al 1992, Tao and Gray 1994, et al 2018)
2018, Wu et al 2018) Auckenthaler 1995, Seyfried and Wilcox 1995, Stadler
et al 1996, Derby and Kinighton 1997, Pikul and Aase
1998, Radke and Berry 1998, Thorne et al 1998, Pit-
man et al 1999, Daniel and Staricka 2000, Sharratt et al
10
2000, Jones and Pomeroy 2001, Xiuqing and Flerchinger
2001, Xiuqing et al 2001, Hayashi et al 2003, Laudon et al
2004, Hejduk and Kasprzak 2010, Iwata et al 2010a, 2013,
Redding and Devito 2011, Zhang et al 2013, Zhao et al
2013a, Anis and Rode 2015, Coles et al 2016, Eskelinen
et al 2016, Wu et al 2016, Coles and McDonnell 2018)
Occasional (Hess 2017) (Baker and Mace 1976, Blackburn and Wood 1990, Baker (Mohammed et al 2013, Mahmood et al 2017)
and Spaans 1997, Pomeroy et al 1997, Stähli et al 1999,
2004, Bayard et al 2005, Orradottir et al 2008, Sutinen
et al 2009, Iwata et al 2011, Christensen et al 2013, He
et al 2015, Miao et al 2017, Pan et al 2017, Starkloff et al
2017)
Uncertain (Hale 1951, Willis et al 1961, Mace 1968, Kapotov 1990, (Emerson 1994, Stadler et al 1997)
Bengtsson et al 1992, Woo and Rowsell 1993, Stein et al
1994, Shanley and Chalmers 1999, Hardy et al 2001,
Shanley et al 2002, van der Kamp et al 2003, Fritz 2004,
Iwata et al 2010b, Komiskey et al 2011, Ketcheson et al
2012, Fuss et al 2016)
Absent (Juusela 1941, Zavodchikov 1962, Karvonen et al 1986, (Lindström et al 2002, Luo et al 2003)
Munter 1986, Vehvilainen and Motovilov 1989, Nyberg
et al 2001, Leenders and Woo 2002, Mellander et al 2004,
Laudon et al 2007, Sutinen et al 2008, Stähli 2017)
P Ala-Aho et alEnviron. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
Figure 2. The geographical location of field studies on the hydrological relevance of SFG included in this review. Colors indicate
the nature of the conclusion on SFG influence (EVIDENT/OCCASIONAL/UNCERTAIN/ABSENT), with sites with multiple
conclusions show by split colors. For overlapping study site locations, points are displaced horizontally for clarity. Region of SFG
was estimated as the 0 ◦ C contour of mean air temperature of the three coldest months (Fick and Hijmans 2017), as in Zhang et al
(2003), with permafrost regions as in Brown et al (2002).
Figure 3. (a) The major geographical regions of published field studies and the nature of the responses reported in each and (b)
change in the number of publications reporting a hydrological influence of SFG over time.
4. Key findings of ground freezing regime general, formation of ground frost starts when the
influence on hydrological fluxes ground temperature is below 0 ◦ C and frost melt-
ing commences when the air temperature above the
4.1. Climate conditions associated with ground rises above 0 ◦ C. In the context of SFG, frost
hydrological relevance of SFG penetration can be assumed to be deeper in a colder
Air temperature and precipitation (both rain and climate and, with deeper frost penetration, one might
snowfall) affect the formation of ground frost. In expect a more intensive hydrological response. Our
11Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
Figure 4. (a) Maximum frost depth penetration reported in each study reviewed, as a function of average air temperature of the
coldest quarter of the year, and (b) maximum frost depth penetration, grouped to reported hydrological response to SFG.
review data confirmed the first assumption, and (Takala et al 2011). Therefore snow conditions spe-
showed a negative association between average air cific to the study year s−1 were not accounted for,
temperature in the coldest quarter of a year and even though year-to-year variability in snow depth
maximum observed frost depth (figure 4(a)). EVID- can be significant. In addition to snow depth, other
ENT hydrological responses were reported across factors such as snow stratigraphy, snow density, and
the range of frost depths from few centimeters to interaction with micrometeorological conditions can
two meters (figure 4(b)). Studies with frost depths greatly influence the ground thermal regime and frost
more than 1 m predominantly reported EVIDENT penetration (Zhang 2005, Lundberg et al 2016a). In
responses. While there was a slight decrease towards agreement with our results, a similar weak relation-
lower frost depths (median value) on going from the ship between ground temperature (not frost penetra-
EVIDENT → OCCASIONAL → UNCERTAIN cat- tion explicitly) and snow depth has been reported in
egory, the differences were minor and the trend was other large-scale studies (Karjalainen et al 2019).
reversed for the ABSENT category. In addition to the relationship between snow and
The onset and total depth of ground frost penetra- frost penetration, we were able to explore how snow
tion are highly dependent on snow conditions. Snow and climate are reflected in the hydrological response
has low thermal conductivity and acts as thermal in SFG regions. In our review, studies reporting
insulation, and was thus found to have a major influ- ABSENT hydrological influence of SFG were asso-
ence on ground energy balance (Zhang 2005). At ciated with higher snow cover, measured as SWE
the plot scale, snowpack height was often found to (figures 5(a) and (b)). Similar relationship was not
be negatively correlated with ground frost penetra- found in other key climate variables of mean annual
tion depth (Stähli et al 2004, Iwata et al 2011). As a precipitation (figure S4) or average air temperature
result, a thick snowpack in early winter may greatly of coldest quarter (figure S6). However, the SWE
reduce or even fully prevent the formation of ground was positively correlated with mean annual precipit-
frost, even though air temperatures would be below ation and negatively with cold season air temperature
freezing (Hardy et al 2001, Iwata et al 2018). The and elevation (figure S8). This highlights that disen-
data in the studies reviewed did not support the tangling the influence of one individual climate para-
assumption of shallower frost penetration with more meter is difficult, and integrative measures of climate
snow (figure S2). The snow data were long-term aver- conditions such as climate zones may be more appro-
age maximum values from a remote sensing product priate.
12Environ. Res. Lett. 16 (2021) 043008 P Ala-Aho et al
Figure 5. (a) Average annual maximum SWE calculated from Luojus et al (2013) with the study site locations in the papers
reviewed here, and (b) SWE grouped to reported hydrological response of SFG.
Studies at sites located in the Köppen–Geiger cli- ground water content before freezing. Soil moisture
mate zone snow—fully humid—cool summer (Dfc) conditions in autumn, at the onset of freezing, have
(figure 6) had the greatest annual snowpack (median been suggested to greatly influence the hydrological
SWE of sites 140.4 mm) and the highest percentage of response of SFG (Willis et al 1961, Bayard et al 2005,
ABSENT response (31.8%) (figure 6(b)). In contrast, Mahmood et al 2017, Appels et al 2018). The studies
in studies at sites located in the arid and temperate reviewed here rarely reported any explicit measure of
warm Köppen–Geiger climate zone (the group ‘Arid soil ice content, making our dataset insufficient for
and temperate’ in figure 6), the median SWE was detailed analysis of this relationship. However, as a
lowest (6.9 mm) and the majority (77.3%) of stud- proxy we noted studies that flagged soil water content
ies reported an EVIDENT snow frost influence on at the onset of freezing, or soil ice saturation, as an
hydrology (figure 6(b)). Most of the studies analyzed important factor influencing the hydrological parti-
(46.6%, n = 61) were located in the Köppen–Geiger tioning in SFG. Soil water or ice content was shown or
climate zone snow—fully humid—warm summer speculated to be important in 53% of all entries ana-
(Dfb), with intermediate SWE values (median of lyzed. This reinforces the intuitive idea that high ice
sites 76.7 mm). saturation, either from soil moisture conditions prior
A plausible explanation for more frequent to freezing or from midwinter snowmelt events, is a
ABSENT hydrological responses in regions with key factor in the hydrological response of SFG.
deeper snow, in our dataset namely northern Europe,
is that these regions are more likely to have persist- 4.2. Soil type characteristics not clearly linked with
ent snow cover throughout winter, with fewer mid- the hydrological response of SFG
winter snowmelt events or rain-on-snow events. In Hydrological and thermal regimes in SFG are closely
contrast, regions with less snow are more likely to coupled through intertwined interactions. Ground
have water entering the ground and refreezing over thermal properties and the associated likelihood of
the snow cover season, which would lead to more ground frost formation are affected by many factors,
ice in the topsoil matrix and less permeable ground. including soil water content and soil composition.
Ice blocks and lenses have been found to form in Soil thermal conductivity is typically lower in organic
soils with high water content, thus preventing infilt- soils (such as peat) than in mineral soils (Brovka
ration (He et al 2015, Pan et al 2017, Mohammed and Rovdan 1999). In general, thermal conductivity
et al 2019). The complex interplay between snow and increases with increasing water and ice content. On
ground energy balance, due to temporally evolving one hand this enhances heat conduction and speeds
snow thermal properties, and latent heat sources and up ground frost penetration. On the other hand
sinks due to water phase (in snow and ground) leaves increasing water content increases the heat capacity
this discussion speculative and in need for more of the soil, and phase change associated with freezing
research. soil in high water content releases more energy due to
In any case, mid-winter snowmelt events alone high latent heat of freezing, which slows down ground
do not determine the water and ice content in frost penetration. Soil with a low water content has
the SFG, since autumn rainfall largely dictates the high air-filled porosity and low heat capacity (Stähli
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