Geoscience Reference
In-Depth Information
In Sect.
3.1
, relevant hydrological processes are described that occur in permafrost areas
and that should preferably represented in models simulating interactions of permafrost
hydrology with vegetation, climate and the carbon cycle. The current state of the repre-
sentation of permafrost processes in ESMs is tackled in Sect.
3.2
, while Sect.
3.3
deals with
the specific topic of wetlands.
3.1 Basic Hydrological Processes in Permafrost Areas
Apart from climatic cold conditions, the occurrence of permafrost is largely controlled by
physiographic features such as aspect, slope and elevation. Other factors such as soil types,
soil moisture, vegetation cover and disturbances (e.g., wildfire) can also influence the
distribution of permafrost (Haugen et al.
1982
; Yoshikawa et al.
2002
). The most basic
process in permafrost areas is the seasonal melting and freezing of soil water in the
presence of continuously frozen ground below a certain depth. The depth to which the soil
is thawed is called the active layer. Regions that are affected by permafrost or extensive
seasonal ground freezing show a specific behaviour of important hydrological variables:
(1) Soil moisture is often rather high in near-surface layers, despite low precipitation rates
in many regions; (2) river discharge observations display very low wintertime values; and
(3) surface runoff shows a steep spring peak after snowmelt that can deliver a substantial
part of the annual total runoff (Swenson et al.
2012
). Several reasons are responsible for
these features, which will be described below.
Firstly, the phase change exerts a drop in liquid water content, and the freezing front can
be seen as a water sink within the soil. This leads to the development of a gradient in liquid
water content and thus induces water movement towards the freezing front. This process is
called cryosuction. It leads to unique characteristics of soil moisture in regions with
permafrost and extensive seasonal ground freezing, namely to the increase in total soil
moisture in the upper layers as well as to the development of large ice bodies in the ground
(see also description of ice wedges below).
Secondly, the permeability for liquid water flow is reduced in frozen soils. This might
be the main and most obvious effect frozen soil exerts on hydrology (Niu and Yang
2006
).
According to Staehli et al. (
1999
), there are two possible pathways for the flow of liquid
water when soil temperature is below 0 C. Transport channels for slow water flows are
provided by thin films of adsorptive and capillary water, which are still existing in liquid
phase and whose amount depends mainly on soil texture type. Alternatively, fast water
movement is possible through air-filled macropores. Soils contain such pores through
structural variations like cracks, holes and channels, e.g., from dead roots and soil
inhabitants like worms.
Nearly impermeable soil layers can develop due to the freezing of the soil during winter
and spring seasons (Koren et al.
1999
) as ice bodies in the ground impede liquid water
movement through blocking of the pore space (Swenson et al.
2012
). Moreover, a strongly
frozen soil will contain only very limited amounts of unfrozen water so that the ability of
the
subsurface
material
to
conduct
liquid
water,
i.e.
the
hydraulic
conductivity,
is
decreased, yet not totally set to zero.
Frozen ground and snow cover also influence rainfall-runoff partitioning, the timing of
spring runoff and the amount of soil moisture that subsequently is available for evapo-
transpiration in spring and summer (Koren et al.
1999
). For the infiltration of surface water
into the soil, the above-explained principles lead to the same general behaviour as for the
hydraulic conductivity, as the infiltration process is lastly determined by the soil's ability to
conduct water away from the surface. Nevertheless, infiltration can vary even more than
