Energy states of soil water n a thermodynamic perspective on storage dynamics and the underlying controls

Abstract. The present study corroborates that the free energy state of soil water offers a new perspective on storage dynamics and similarity of hydrological systems that cannot be inferred from the usual comparison of soil moisture observations or groundwater levels. We show that the unsaturated zone of any hydrological system is characterized by a system- specific balance of storage and release. This storage equilibrium, which is jointly controlled by the soil physical and topographical system characteristics, reflects the thermodynamic equilibrium state of minimum free energy the system approaches when relaxing from external disturbances. Rainfall or radiation frequently forces parts of the system out of this storage equilibrium, storage dynamics can hence be visualized as sequences of deviations from and relaxations back to equilibrium. This perspective reveals that storage dynamics operates in two distinctly different energetic regimes, where either capillarity dominates over gravity or vice versa. As these regimes are associated either with a storage deficit or a storage excess, relaxation requires either recharge or release. This implies that the terms wet and dry should be used with respect to the equilibrium storage as meaningful reference point. We show furthermore that the free energy state of the soil water stock, the storage equilibrium which separates the two dynamic regimes, as well as the degree of non-linearity within those regimes depend on the joint controls of catchment topography and the physical properties of the soils. We express these joint controls in form of a new characteristic function of the unsaturated zone we call the energy state function . By comparing the energy state functions of different systems we demonstrate their distinct sensitivity to topography and soil water characteristics and their usefulness for inter-comparing storage dynamics among those systems. This ultimately reveals that storage dynamics at the system level may operate by far more linearly than suggested by the retention function of the soils.