Julian Klaus

University of Bonn | Uni Bonn ·  Department of Geography

Recent studies on the relationships between catchment storage and water ages using Travel Time Distributions (TTDs), Residence Time Distributions (RTDs), and StorAge Selection (SAS) functions have led to the hypothesis that streamflow preferentially mobilizes younger water when catchment storage is high. This so-called “Inverse Storage Effect” (ISE...

Hillslopes exert critical controls on the quality and quantity of downstream waters. To understand and model dominant headwater catchment processes, we need to estimate the relative importance of different runoff generation processes. In this work we analyze published data from studies of 17 hillslopes from a range of landscapes to better understan...

In this study, we explored the spatio-temporal variability of surface saturation within a forested headwater catchment using a combined simulation–observation approach. We simulated the occurrence of surface saturation in the Weierbach catchment (Luxembourg) with the physically based model HydroGeoSphere. We confronted the simulation with thermal i...

Roots exhibit plasticity in morphology and physiology when exposed to fluctuating nutrient and water availability. However, the dynamics of daily time-scale adjustments to changes in water availability are unclear and experimental evidence of the rates of such adjustments is needed. In this study we investigated how the root system responds within...

Interflow, throughflow and subsurface stormflow are interchangeable terms that refer to the lateral subsurface flow above a restricting layer of lower hydraulic conductivity that occurs during and following storm events. Interflow (used here) is a more dominant process in steeper catchments with high infiltration capacity soils overlying a more imp...

Understanding the interrelation between topography and vegetation across different environments is important to assess how hydrological and climatic conditions affect tree physiological activity. This becomes especially important given the expected reduction in water availability and the increase in water demand driven by climate change. These extr...

Currently, the global water cycle is experiencing radical shifts and the associated global water crisis requires rapid action by stakeholders to mitigate adverse impacts on both human populations and ecosystems. This urgency in action is driven by the combined effect of Climate Change and Land-use land cover change (LULCC) and the associated challe...

Soil water content (SWC) is a fundamental variable involved in several hydrological processes governing catchment functioning. Comparative analysis of hydrological processes in different catchments based on SWC data is therefore beneficial to infer driving factors of catchment response. Here, we explored the use of high‐temporal resolution SWC data...

Understanding the interaction between topography and vegetation across different environments is important to assess how hydrological and climatic conditions affect tree physiological activity. This becomes especially important given the expected reduction in water availability and increase in water demand driven by climate change. These extremes c...

ParFlow is an open-source high performance parallel hydrological model designed for the integrated modeling of the hydrological cycle from the hillslope to continental scales. Due to its complexity, ParFlow requires intricate data preprocessing steps that might hinder its application by the wider modeling community. Here, we present the PFGIS-Tool...

Contradictory interpretations of transient storage modeling (TSM) results of past studies hamper the understanding of how hydrologic conditions control solute transport in streams. To address this issue, we conduct 30 instantaneous tracer experiments in the Weierbach stream, Luxembourg. Using an iterative modeling approach, we calibrate TSM paramet...

Catchment-scale transpiration is commonly determined by the use of sap-flow sensors, and its quantification, which is critical for water and forest management, relies crucially on the total catchment’s sapwood area (As). Species-specific allometric relationships between the trees As and the trees diameter at breast height (DBH) are widely used for...

Identifying the vegetation and topographic variables influencing the isotopic variability of xylem water of forest vegetation remains crucial to interpret and predict ecohydrological processes in landscapes. In this study, we used temporally and spatially distributed xylem stable water isotopes measurements from two growing seasons to examine the t...

Climate change and other human activities are modifying river water temperature globally. A more holistic understanding of river temperature dynamics in an integrated climate–land–hydrology–human framework is urgently needed for sustainable river management and adaptation strategies.

The transport of solutes in river networks is controlled by the interplay of processes such as in-stream solute transport and the exchange of water between the stream channel and dead zones, in-stream sediments, and the hyporheic zone. Transient storage models (TSMs) are a powerful tool for testing hypotheses related to solute transport in streams....

Water transit time is now a standard measure in catchment hydrological and ecohydrological research. The last comprehensive review of transit time modeling approaches was published 15+ years ago. But since then the field has largely expanded with new data, theory and applications. Here, we review these new developments with focus on water‐age‐balan...

In most tree species, xylem consists of two different functional parts: sapwood and heartwood. While sapwood, as the flowpath for sap, has received more attention in isotope studies assessing water sources accessed by trees (e.g., soil water from different depths, groundwater, stream water or a mixture of these), much remains unknown about heartwoo...

Catchment-scale transpiration is commonly determined by the use of sap-flow sensors, and its quantification, which is critical for water and forest management, relies crucially on the total catchment’s sapwood area (A s ). Species-specific allometric relationships between the trees A s and the trees diameter at breast height (DBH) are widely used f...

Emerging drought stress on vegetation over western Eurasia is linked to varying teleconnection patterns. The North-Sea Caspian pattern (NCP) is a relatively less studied Eurasian teleconnection pattern which has a role on drought conditions and the consequence of changing conditions on vegetation. Between 1981 and 2015, we found that the Standardiz...

Precipitation extremes are commonly linked with land use types. The UKCP18 Convection-Permitting Model (CPM) Projections at 5km high resolution simulation provide opportunities to investigate probable relationships between precipitation extremes and land use types. Changes in the duration and severity of extreme precipitation events can be linked t...

In river systems, headwater networks contain the vast majority of the stream length. Thus, climate and land‐use change in headwaters have disproportionate impacts on downstream ecosystems and societies that rely on them. Despite decades of hydrological research, difficulties in observing hydrological properties across scales means that scientific k...

Knowledge of the sources of surface water in riparian zones and floodplains is critical to understanding its role in runoff generation and impact on biogeochemical and ecological processes. In this study, we demonstrate the potential of integrated surface‐subsurface hydrologic modeling (HydroGeoSphere) in combination with a hydraulic mixing‐cell ap...

The role of landscape topography in mediating subsurface water availability and ultimately tree transpiration is still poorly understood. To assess how hillslope position affects tree water use, we coupled sap velocity with xylem isotope measurements in a temperate beech‐oak forest along a hillslope transect in Luxembourg. We generally observed gre...

The substantial tree-to-tree variability of transpiration poses a major challenge to a reliable stand-scale quantification of transpiration. The diameter at breast height (DBH) and landscape characteristics have been identified as drivers of tree-to-tree variability, but it remains unclear if their control on sap velocity varies between species-spe...

Ocean-atmosphere modes of climate variability in the Pacific and Indian oceans, as well as monsoons, regulate the regional wet and dry episodes in tropical regions. However, how those modes of climate variability, and their interactions, lead to spatial differences in drought patterns over tropical Asia at seasonal- to interannual time scales remai...

Understanding near-stream groundwater dynamics and flow directions is important for predicting hillslope-stream connectivity, streamflow generation, and hydrologic controls of streamwater quality. To determine the drivers of groundwater flow in the stream corridor (i.e., the stream channel and the adjacent groundwater in footslopes and riparian are...

West Africa exhibits decadal patterns in the behaviour of droughts and floods, creating challenges for effective water resources management. Proposed drivers of prolonged shifts in hydrological extremes include the impacts of land-cover change and climate variability in the region. However, while future land-degradation or land-use are highly unpre...

Widespread observations of ecohydrological separation are interpreted by suggesting that water flowing through highly conductive soil pores resists mixing with matrix storage over periods of days to months (i.e., two ‘water worlds’ exist). These interpretations imply that heterogeneous flow can produce ecohydrological separation in soils, yet littl...

Drought conditions of Southeast China are associated with the sea surface temperature warm pool in the tropical Western Pacific, which is related to low-frequency hydroclimatic patterns and their teleconnections. Empirically, the moisture influx to the region is linked to the interannual and decadal teleconnections, including the Pacific Decadal Os...

We present a method to simulate fluid flow with reactive solute transport in structured, partially saturated soils using a Lagrangian perspective. In this context, we extend the scope of the Lagrangian Soil Water and Solute Transport Model (LAST) (Sternagel et al., 2019) by implementing vertically variable, non-linear sorption and first-order degra...

The Weierbach experimental catchment (0.45 km²) is the most instrumented and studied sub‐catchment in the Alzette River basin in Luxembourg. Within the last decade, it has matured towards an interdisciplinary critical zone observatory focusing on a better understanding of hydrological and hydro‐geochemical processes. The Weierbach catchment is embe...

Catchment travel time distributions (TTDs) are an efficient concept for summarizing the time-varying 3D transport of water and solutes towards an outlet in a single function of a water age and for estimating catchment storage by leveraging information contained in tracer data (e.g., deuterium 2H and tritium 3H). It is argued that the preferential u...

Submitted for publication in Hydrological Processes: Widespread observations of ecohydrological separation are interpreted by suggesting that water flowing through highly conductive soil pores resists mixing with matrix storage over periods of days to months (i.e., two water worlds). We quantified the separation between mobile water moving through...

We present an approach to simulate reactive solute transport within the Lagrangian Soil Water and Solute Transport Model framework (LAST). The LAST-Model is based on a Lagrangian perspective describing the (1-D) movement of discrete water particles, which travel at different velocities and carry solutes through a heterogeneous, partially saturated...

Ecohydrological separation has been observed across climates and biomes, and at a fundamental level suggests that water in mobile versus immobile domains may resist mixing over varying periods of time; however little mechanistic evidence exists to explain this separation at a process scale. Non-equilibrium flow in the vadose zone may partially acco...

Travel time distributions (TTDs) are concise descriptions of transport processes in catchments based on water ages, and they are particularly efficient as lumped hydrological models to simulate tracers in outflows. Past studies have approximated catchment TTDs with unimodal probability distribution functions (pdf) and have successfully simulated tr...

The occurrence of preferential flow in the subsurface has often been shown in field experiments. However, preferential flow is rarely included in models simulating the hydrological response at the catchment scale. If it is considered, preferential flow parameters are typically determined at the plot scale and then transferred to larger‐scale simula...

Surface saturated areas are key features in generating runoff. A detailed characterisation of the expansion and contraction of surface saturation in riparian zones and its connectivity to the stream is fundamental to improve our understanding of the spatial and temporal variability of streamflow generation processes. In this first contribution of a...

The occurrence of preferential flow in the subsurface has often been shown in field experiments. However, preferential flow is rarely included in models simulating the hydrological response at the catchment scale. If it is considered, preferential flow parameters are typically determined at the plot scale and then transferred to larger‐scale simula...

Catchment travel times integrate the multitude of hydrological flow processes and provide insights into catchment functioning. StorAge Selection (SAS) functions describe how residence times of water in storage are related to travel times of water in catchment outflows. As such, SAS functions are useful to summarize transport processes in catchments...

Catchment travel time distributions (TTDs) are an efficient concept to summarize the time-varying 3-dimensional transport of water and solutes to an outlet in a single function of water age and to estimate catchment storage by leveraging information contained in tracer data (e.g. ²H and ³H). It is argued that the increasing use of the stable isotop...

Investigating the spatio‐temporal variability of streamflow generation is fundamental to interpret the hydrological and bio‐chemical functioning of catchments. In humid temperate environments, streamflow generation is often linked to the occurrence of near stream surface saturated areas, which mediate hydrological connectivity between hillslope and...

Core Ideas We tested a range of dual‐permeability parameterizations at plot and catchment scale. Well‐performing parameters at plot scale did not clearly improve catchment simulation. Vertical preferential flow was important for simulating plot‐scale observations. At catchment scale, it appeared more important to consider fast lateral subsurface fl...

The inundation of flood-prone areas varies in space and time and can have crucial impacts on runoff generation and water quality when the surface saturated areas become connected to the stream. In this study, we aimed to investigate and explain the variability of surface saturation patterns and dynamics within a forested headwater catchment. On the...

Catchment storage controls most eco-hydrological processes by modulating, among other things, the hydrological connectivity between landscape units. However, little is known about the internal dynamics of storage and its feedback with hydraulic connectivity. Here, we evaluate the capability of a combined lumped approach (i.e. combining catchment wa...

Monsoons affect millions of lives in West Africa. Regional and decadal droughts and floods are proposed to be related to the anomalies of the monsoon strength, timing and duration. In multiple scales, causes and effects of interannual monsoon variability are valuable in predicting extreme regional hydroclimatic conditions. Driven by climate indices...

Surface saturation can have a critical impact on runoff generation and water quality. Saturation patterns are dynamic, thus their potential control on discharge and water quality is also variable in time. In this study, we assess the practicability of applying thermal infrared (TIR) imagery for mapping surface-saturation dynamics. The advantages of...

In this commentary, we summarize and build upon discussions that emerged during the workshop “Isotope-based studies of water partitioning and plant–soil interactions in forested and agricultural environments” held in San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zo...

In this study we assess the practicability of applying thermal infrared (TIR) imagery for mapping surface saturation dynamics. The advantage of TIR imagery compared to other surface saturation mapping methods is its large spatial and temporal flexibility combined with a non-invasive and intuitive character. Based on an 18-month field campaign, we r...

In this commentary, we build on discussions that emerged during the workshop "Isotope-based studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to...

Diel fluctuations of stream water DOC concentrations are generally explained by a complex interplay of different instream processes. We measured the light absorption spectrum of water and DOC concentrations in situ and with high frequency by means of a UV–Vis spectrometer during 18 months at the outlet of a forested headwater catchment in Luxembour...

Recent advancements in analytical solutions to quantify water and solute travel time distributions (TTDs) and the related StorAge Selection (SAS) functions synthesize catchment complexity into a simplified, lumped representation. While these analytical approaches are efficient in application, they require rarely available long-term and high-frequen...

The cover image, by Laurent Pfister et al, is based on Advanced Review Terrestrial diatoms as tracers in catchment hydrology: a review, DOI: 10.1002/wat2.1241.

Diatoms are remarkable organisms. They are present in almost all habitats containing water (e.g., lakes, streams, soils, bark) and rank among the most common algal groups in both freshwaters and marine ecosystems. The ubiquitous character of aquatic diatoms has triggered countless applications as environmental tracers for studies in water quality,...

We monitored dissolved organic carbon (DOC) and nitrate concentrations and fluxes in situ with a UV-Vis spectrometer for two years at a high temporal resolution of 15 minutes in the forested Weierbach headwater catchment. The catchment exhibits a characteristic double peak runoff response to incident rainfall during periods with wet initial conditi...

Quantifying the travel times, pathways, and dispersion of solutes moving through stream environments is critical for understanding the biogeochemical cycling processes that control ecosystem functioning. Validation of stream solute transport and exchange process models requires data obtained from in-stream measurement of chemical concentration chan...