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Recent research explored an alternative energy-centred perspective on hydrological processes, extending beyond the classical analysis of the catchment's water balance. Particularly, streamflow and the structure of river networks have been analysed in an energy-centred framework, which allows for the incorporation of two additional physical laws: (1) energy is conserved and (2) entropy of an isolated system cannot decrease (first and second law of thermodynamics). This is helpful for understanding the self-organized geometry of river networks and open-catchment systems in general. Here we expand this perspective, by exploring how hillslope topography and the presence of rill networks control the free-energy balance of surface runoff at the hillslope scale. Special emphasis is on the transitions between laminar-, mixed- and turbulent-flow conditions of surface runoff, as they are associated with kinetic energy dissipation as well as with energy transfer to eroded sediments. Starting with a general thermodynamic framework, in a first step we analyse how typical topographic shapes of hillslopes, representing different morphological stages, control the spatial patterns of potential and kinetic energy of surface runoff and energy dissipation along the flow path during steady states. Interestingly, we find that a distinct maximum in potential energy of surface runoff emerges along the flow path, which separates upslope areas of downslope potential energy growth from downslope areas where potential energy declines. A comparison with associated erosion processes indicates that the location of this maximum depends on the relative influence of diffusive and advective flow and erosion processes. In a next step, we use this framework to analyse the energy balance of surface runoff observed during hillslope-scale rainfall simulation experiments, which provide separate measurements of flow velocities for rill and for sheet flow. To this end, we calibrate the physically based hydrological model Catflow, which distributes total surface runoff between a rill and a sheet flow domain, to these experiments and analyse the spatial patterns of potential energy, kinetic energy and dissipation. This reveals again the existence of a maximum of potential energy in surface runoff as well as a connection to the relative contribution of advective and diffusive processes. In the case of a strong rill flow component, the potential energy maximum is located close to the transition zone, where turbulence or at least mixed flow may emerge. Furthermore, the simulations indicate an almost equal partitioning of kinetic energy into the sheet and the rill flow component. When drawing the analogy to an electric circuit, this distribution of power and erosive forces to erode and transport sediment corresponds to a maximum power configuration. [ABSTRACT FROM AUTHOR]

Recent developments in hydrology have led to a new perspective on runoff processes, extending beyond the classical mass dynamics of water in a catchment. For instance, stream flow has been analysed in a thermodynamic framework, which allows the incorporation of two additional physical laws and enhances our understanding of catchments as open environmental systems. Related investigations suggested that energetic extremal principles might constrain hydrological processes, because the latter are associated with conversions and dissipation of free energy. Here we expand this thermodynamic perspective by exploring how hillslope structures at the macro- and microscale control the free energy balance of Hortonian overland flow. We put special emphasis on the transitions of surface runoff processes at the hillslope scale, as hillslopes energetically behave distinctly different in comparison to fluvial systems. To this end, we develop a general theory of surface runoff and of the related conversion of geopotential energy gradients into other forms of energy, particularly kinetic energy as the driver of erosion and sediment transport. We then use this framework at a macroscopic scale to analyse how combinations of typical hillslopes profiles and width distributions control the spatial patterns of steady-state stream power and energy dissipation along the flow path. At the microscale, we analyse flow concentration in rills and its influence on the distribution of energy and dissipation in space. Therefore, we develop a new numerical method for the Catflow model, which allows a dynamical separation of Hortonian surface runoff between a rill- and a sheet flow domain. We calibrated the new Catflow-Rill model to rainfall simulation experiments and observed overland flow in the Weiherbach catchment and found evidence that flow accumulation in rills serves as a means to redistribute energy gradients in space, therefore minimizing energy expenditure along the flow path, while also maximizing overall power of the system. Our results indicate that laminar sheet flow and turbulent rill flow on hillslopes develop to a dynamic equilibrium that corresponds to a maximum power state, and that the transition of flow from one domain into the other is marked by an energy maximum in space. [ABSTRACT FROM AUTHOR]

The Budyko curve is a widely used framework for predicting the steady-state water balance --solely based on the hydro-climatic setting of river basins. While this framework has been tested and verified across a wide range of climates and settings around the globe, numerous catchments have been reported to considerably deviate from the predicted behavior. Here, we hypothesize that storage capacity and field capacity of the root zone are important controls of the water limitation of evapotranspiration and thus deviations of the mean annual water balance from the Budyko curve. For testing our hypothesis, we selected 16 catchments of different climatic settings and varied the corresponding parameters of a simple water balance model that was previously calibrated against long-term data and investigated the corresponding variations of the simulated water balance in the Budyko space. We found that total soil storage capacity --by controlling water availability and limitation of evapotranspiration-- explains deviations of the evaporation ratio (EVR) from the Budyko curve. Similarly, however to a lesser extent, the evaporation ratio showed sensitivity to alterations of the field capacity. In most cases, the parameter variations generated evaporation ratios enveloping the Budyko curve. The distinct soil storage volumes that matched the Budyko curve clustered at a normalized storage capacity equivalent to 5-15 % of mean annual precipitation. The second, capillarity-related soil parameter clustered at around 0.6-0.8, which is in line with its hydropedological interpretation. A simultaneous variation of both parameters provided additional insights into the interrelation of both parameters and their joint control on offsets from the Budyko curve. Here we found three different sensitivity patterns and we conclude the study with a reflection relating these offsets to the concept of catchment coevolution. The results of this study could also be useful to facilitate evaluation of the water balance in data-scarce regions, as they help constrain parameterizations for hydrological models a priori using the Budyko curve as a predictor. [ABSTRACT FROM AUTHOR]

Recent developments in hydrology have led to a new perspective on runoff processes, extending beyond the classical mass dynamics of water in a catchment. For instance, stream flow has been analyzed in a thermodynamic framework, which allows the incorporation of two additional physical laws and enhances our understanding of catchments as open environmental systems. Related investigations suggested that energetic extremal principles might constrain hydrological processes, because the latter are associated with conversions and dissipation of free energy. Here we expand this thermodynamic perspective by exploring how macro and micro hillslope structures control the free energy balance of Hortonian overland flow. This may ultimately help understanding why these structures have evolved to their present shape. To this end, we develop a general theory of surface runoff and of the related conversion of geopotential energy gradients into other forms of energy, particularly kinetic energy as driver of erosion and sediment transport. We then use this framework to analyze how combinations of typical hillslopes profiles and width distributions control the spatial patterns of steady state stream power and energy dissipation along the flow path. Additionally, we provide a first order estimate whether and when rills reduce the overall energy dissipation compared to sheet flow. Finally, we relate accumulated stream power of linear hillslopes to slope angles, closing the loop to Horton's original formulation of erosion force. The analytical analysis of stream power reveals that the common formulation, a function of the depth-discharge product is a reduced version of the more general equations if we neglect changes in velocity and discharge in space. The full equations of stream power result in maximum energy fluxes in space for sinusoidal and exponential hillslope profiles, while linear and negative exponential forms unlimitedly increase these fluxes in the downstream direction. Depending on geometry, rill flow increases or decreases kinetic energy fluxes downslope, effectively counteracting or increasing the dissipation of potential energy. For accumulated power in space for steady state runoff, we find that on linear hillslopes a slope angle of 45° maximizes the conversion of potential energy into dissipation and an angle of 35° maximizes the conversion of potential energy into kinetic energy. [ABSTRACT FROM AUTHOR]

Diagnostics of hydrological models are pivotal for a better understanding of catchment functioning, and the analysis of dominating model parameters plays a key role for region-specific calibration or parameter transfer. A major challenge in the analysis of parameter sensitivity is the assessment of both temporal and spatial differences of parameter influences on simulated streamflow response. We present a methodological approach for global sensitivity analysis of hydrological models. The multilevel approach is geared towards complementary forms of streamflow response targets, and combines sensitivity analysis directed to hydrological fingerprints, i.e. temporally independent and temporally aggregated characteristics of streamflow (INDPAS), with the conventional analysis of the temporal dynamics of parameter sensitivity (TEDPAS). The approach was tested in 14 mesoscale headwater catchments of the Ruhr River in western Germany using simulations with the spatially distributed hydrological model mHM. The multilevel analysis with diverse response characteristics allowed us to pinpoint parameter sensitivity patterns much more clearly as compared to using TEDPAS alone. It was not only possible to identify two dominating parameters, for soil moisture dynamics and evapotranspiration, but we could also disentangle the role of these and other parameters with reference to different streamflow characteristics. The combination of TEDPAS and INDPAS further allowed us to detect regional differences in parameter sensitivity and in simulated hydrological functioning, despite the rather small differences in the hydroclimatic and topographic setting of the Ruhr headwaters. [ABSTRACT FROM AUTHOR]

Diagnostics of hydrological models is pivotal for a better understanding of catchment functioning. The analysis of dominating parameters for the simulation of streamflow plays a key role for region specific model diagnostics, model calibration or parameter transfer. A major challenge in this analysis of parameter sensitivity is the assessment of both temporal and spatial differences of parameter influences on simulated streamflow response. A methodical approach is presented, wherein a two-tiered global sensitivity analysis on a spatially distributed hydrological model is applied to 14 mesoscale headwater catchments of the river Ruhr in western Germany. The analysis of parameter sensitivity is geared towards two complementary forms of streamflow response targets. The analysis of the temporal dynamics of parameter sensitivity (TEDPAS) is contrasted with sensitivity analysis directed to hydrological fingerprints, i.e. temporally independent and temporally aggregated characteristics of streamflow (INDPAS). The two-tiered approach allows to discern a clarified sensitivity pattern pinpointed to diverse response characteristics, to detect regional differences and to reveal the regional relevance of the response target. Small local differences in the hydroclimatic and topographic setting of the headwaters lead to slight differences in the hydrological functioning, which was revealed by gradual differences in TEDPAS and INDPAS. [ABSTRACT FROM AUTHOR]

Despite the numerous hydrological models existing in hydrology we are limited to a few forms of conceptualization when abstracting hydrological systems into different model frameworks. Speaking in black and white terms, in most cases hydrological systems are either represented spatially lumped with conceptual models or spatially explicit with physically-based models. Physically-based models are often parameter-rich, making the parametrization challenging, while conceptual models are parsimonious, with only a few parameters needing to be identified. But this simplistic mathematical expression is often also their drawback since their model states and parameters are difficult to translate to the physical properties of a catchment. It is interesting to note that both hydrological modeling approaches often start with the drawing of a perceptual model. This follows the hydrologist's philosophy to separate dominant patterns and processes from idiosyncratic system details. Due to the importance of hillslopes as key landscape elements perceptual models are often displayed as 2D cross-sections. In this study we examine whether we can step beyond the qualitative character of perceptual models by using them as blueprint for setting up representative hillslope models. Thereby we test the hypothesis if a single hillslope can represent the functioning of an entire lower mesoscale catchment in a spatially aggregated way. We do this by setting up and testing two hillslope models in catchments located in two different geological settings, Schist and Marl, using a two-dimensional physically-based model. Both models are parametrized based on intensive field data and literature values without automatic calibration. Remarkably we are able to not only match the water balance of both catchments but further have some success in simulating runoff generation as well as soil moisture and sap flow dynamics. Particularly, our findings demonstrate that both models performed well during the winter season and clearly worse during the summer period. Virtual experiments revealed that this was most likely either due to a poor representation of the onset of vegetation in the Schist catchment or due to emergence of soil cracks in the Marl area. Both findings underpin that a static parameterization of hydrological models might be problematical in case of emergent behavior. Additional virtual experiments indicate that the storage of water in the bedrock and not so much the topographic gradient is a first order control on the hydrological functioning of the Schist catchment. We conclude that the representative hillslope concept is a feasible approach in data rich regions and that this form of abstraction provides an added value to the established conceptualization frameworks in hydrology. [ABSTRACT FROM AUTHOR]

Slow-moving landslides are a wide-spread type of active mass movement, can cause severe damages to infrastructure, and may be a precursor of sudden catastrophic slope failures. Pore-water pressure is commonly regarded as the most important among a number of possible factors controlling landslide velocity. We used high-resolution monitoring data to explore the relations of landslide mobility and hydrologic processes at the Heumöser landslide in Austria, which is characterized by continuous slow movement along a shear zone. Movement rates showed a seasonality that was associated with elevated pore-water pressures. Pore pressure monitoring revealed a system of confined and separated aquifers with differing dynamics. Analysis of a simple infinite slope mobility model showed that small variations in parameters, along with measured pore pressure dynamics, provided a perfect match to our observations. Modeling showed a stabilizing effect of snow cover due to the additional load. This finding was supported by a multiple regression model, which further suggested that effective pore pressures at the slip surface were partially differing from the borehole observations and were related to preferential infiltration and subsurface flow in adjacent areas. It appears that in a setting like the Heumöser landslide, hydrologic processes delicately influence slope mobility through their control on pore pressure dynamics and the weight of the landslide body, which challenges observation and modeling. Moreover, it appears that their simplicity, and especially their high sensitivity to parameter variations, limits the conclusions that can be drawn from infinite slope models. [ABSTRACT FROM AUTHOR]

• Four years of continuous denitrification bed observation led to a process model. • Temperature and hydraulic retention time influence nitrate removal the most. • Hydraulic retention time impacts the flow path through a denitrification bed. • Bioinformatic analysis reveals a division of labor and a low amount of methanogens. Environmental contamination with fertilizers is threatening biodiversity in many ecosystems due to nitrate-based eutrophication. One opportunity for a cost-efficient nitrate elimination are denitrification beds in which a microbial community thrives under anoxic conditions with polymeric plant material as a carbon and an electron source. Incoming nitrate is used as electron acceptor and reduced to molecular nitrogen. Projects realizing denitrification beds in field scale are sparse and robust data on their efficiency throughout the year mostly not available. This study analyzed the nitrate elimination efficiency and microbiology of a 216 m3 denitrification bed over the time course of more than three years. Phylogenetic as well as transcriptomic analysis revealed that the reactor contained a biofilm community growing on the surface of the wood chips and a planktonic community. Both differed in composition but their variance was affected only to a minor extend by seasonal temperature changes. Cellulose degradation was mainly conducted by the biofilm population while denitrification was mostly conducted by the planktonic community. Methanogens were detectable only to a very minor extend. Using online data from the nitrate concentration of in- and outflowing water as well as a hydrological model to predict the water inflow, it was possible to establish a process model that sufficiently describes the denitrification process. This model clearly indicates that the denitrification efficiency is mostly impacted by temperature and hydraulic retention time. It also suggests that the simple design of the denitrification bed most likely leads to different flow paths through the reactor depending on the volumetric flow rate. This study allows for the first time a robust estimation of the necessary reactor size for nitrate removal in a moderate continental climate setting. It also suggests how future denitrification beds could be improved for better performance. Image, graphical abstract [ABSTRACT FROM AUTHOR]