Energy states of soil water - A thermodynamic perspective on soil water dynamics and storage-controlled streamflow generation in different landscapes

Energy states of soil water - A thermodynamic perspective on soil water dynamics and storage-controlled streamflow generation in different landscapes

Zehe, Erwin (Karlsruhe Institut für Technologie)
Loritz, Ralf (Karlsruhe Institut für Technologie)
Jackisch, Conrad (Karlsruhe Institut für Technologie)
Westhoff, Martijn (Vrije Universiteit Amsterdam)
Kleidon, Axel (Max-Planck Institute of Iron and Steel Research)
Blume, Theresa (German Research Centre for Geosciences)
Hassler, Sibylle K. (Karlsruhe Institut für Technologie)
Savenije, Hubert (TU Delft Water Resources)

2019

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.

http://resolver.tudelft.nl/uuid:3e3fa7a4-3b4d-4e77-83c5-13bf6a2d4505

https://doi.org/10.5194/hess-23-971-2019

1027-5606

Hydrology and Earth System Sciences, 23 (2), 971-987

Institutional Repository

journal article

© 2019 Erwin Zehe, Ralf Loritz, Conrad Jackisch, Martijn Westhoff, Axel Kleidon, Theresa Blume, Sibylle K. Hassler, Hubert Savenije