Energy states of soil water – a thermodynamic perspective on soil water dynamics and storage-controlled streamflow generation in different landscapes
2019
Abstract. The present study confirms
that a thermodynamic perspective on soil water is well suited to distinguishing the
typical interplay of gravity and capillarity controls on soil water dynamics
in different landscapes. To this end, we express the driving matric and
gravity potentials by their energetic counterparts and characterize soil
water by its free energy state. The latter is the key to defining a new
system characteristic determining the possible range of energy states of soil
water, reflecting the joint influences of soil physical properties and height
over nearest drainage (HAND) in a stratified manner. As this characteristic
defines the possible range of energy states of soil water in the root zone,
it also allows an instructive comparison of top soil water dynamics observed
in two distinctly different landscapes. This is because the local
thermodynamic equilibrium at a given HAND and the related equilibrium storage
allow a subdivision of the possible free energy states into two different
regimes. Wetting of the soil in local equilibrium implies that free energy of
soil water becomes positive, which in turn implies that the soil is in a
state of storage excess, while further drying of the soil leads to a negative
free energy and a state of storage deficit. We show that during 1
hydrological year the energy states of soil water visit distinctly different
parts of their respective energy state spaces. The two study areas compared here exhibit
furthermore a threshold-like relation between the observed free energy of
soil water in the riparian zone and observed streamflow, while the tipping
points coincide with the local equilibrium state of zero free energy. We
found that the emergence of a potential energy excess/storage excess in the
riparian zone coincides with the onset of storage-controlled direct
streamflow generation. While such threshold behaviour is not unusual, it is
remarkable that the tipping point is consistent with the underlying theoretical
basis.
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