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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 thresholdlike
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.