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Linking landscape structure and rainfall runoff behaviour in a thermodynamic optimality context

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Term from 2014 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 182331427
 
Final Report Year 2021

Final Report Abstract

This project explored, within the frame of the CAOS research unit FOR 1598, a new thermodynamic perspective on hydrological dynamics. To this end, Zehe et al. (2019) characterized the soil water content by its Gibbs free energy, which jointly reflects gravity and capillary controls. From this, they derived a new system characteristic determining the possible range of energy states of soil water, which is well suited to distinguish the typical interplay of gravity and capillarity controls on soil water dynamics in different landscapes. Moreover, the energy state functions consist of two different regimes associated either with a storage excess or with a storage deficit. Zehe et al. (2019) showed that storage dynamics into different landscaped is straightforwardly visualized as distinctly pseudo oscillations of the corresponding free energy state around the local equilibrium. The free energy state of soil water in the riparian zone of both study catchments provides furthermore a theoretically sound explain of the threshold like onset in streamflow generation. Complementary to that Loritz et al. (2019) proposed a novel thermodynamic index to explain topographic controls on runoff generation named reduced dissipation per unit length (rDUNE). rDUNE jointly accounts for the energetic driver and the dissipative loss along the flow path and a provided stronger discrimination of catchments into groups of similar runoff generation than HAND or the topographic wetness index (TWI). Moreover, we corroborated the central CAOS hypothesis postulating the existence of functional units of similar hydrological behaviour within two related model studies. The first study corroborated that the water balance of two different mesoscale catchments, the Colpach and the Wollefsbach, can successfully be simulated by a single 2d representative hillslope. In both catchments, the representative models yielded simulations of streamflow, optionally sap flow and distributed soil moisture dynamics in good accordance with observations. This success is explained by the fact that both models preserve the relevant information about the driving gradients and resistance terms that control runoff generation and hydrological dynamics. Furthermore, Loritz et al. (2018) showed by means of the Shannon entropy that simulations by means of a fully distributed setup of the same Colpach catchment using 105 different hillslopes yielded strongly redundant contributions of streamflow. They further showed that the fully distributed model, consisting of 105 hillslopes, could be compressed to a model using six hillslopes with distinctly different runoff responses, without a loss in simulation performance. In both catchments, the representative models yielded simulations of streamflow, optionally sap flow and distributed soil moisture dynamics in good accordance with observations. Furthermore, we successfully advanced research activities started within the first funding Phase within Project I ‘From subsurface structures to functions and texture linking virtual realities and experiments at the plot and hillslope scales’. This includes firstly new ways for in-situ imaging of rapid subsurface flow by combining time-lapse ground-penetrating radar with TDR profiling. Secondly, we started to develop Lagrangian models for simulating soil water dynamics and solute transport in structure heterogeneous soils. These studies underpin that Lagrangian models provide many assets to simulate flow and transport in heterogeneous soils compared to the traditional Richards and Advection-Dispersion equations. Overall these findings corroborate that a thermodynamic perspective on hydrological systems offer holistic information for judging and inter-comparing soil water storage and runoff generation as well as new avenues in modeling and upscaling, which cannot be inferred from the traditional water balance thinking alone.

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