Browsing by Author "Klaus, Julian"
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- Importance of tree diameter and species for explaining the temporal and spatial variations of xylem water delta O-18 and delta H-2 in a multi-species forestFresne, Maelle; Chun, Kwok P.; Hrachowitz, Markus; McGuire, Kevin J.; Schoppach, Remy; Klaus, Julian (Wiley, 2023-05)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 temporal and spatial variations of xylem stable water isotopes and their relationships with vegetation and topographic variables in a Luxembourgish temperate mixed forest. Species-specific temporal variations of xylem stable water isotopes were observed during both growing seasons with a higher variability for beeches than oaks. Principal component regressions revealed that tree diameter at breast height explains up to 55% of the spatial variability of xylem stable water isotopes, while tree species explains up to 24% of the variability. Topographic variables had a marginal role in explaining the spatial variability of xylem stable water isotopes (up to 6% for elevation). During the drier growing season (2020), we detected a higher influence of vegetation variables on xylem stable water isotopes and a lower temporal variability of the xylem water isotopic signatures than during the wetter growing season (2019). Our results reveal the dominant influence of vegetation on xylem stable water isotopes across a forested area and suggest that their spatial patterns arise mainly from size- and species-specific as well as water availability-dependent water use strategies rather than from topographic heterogeneity. The identification of the key role of vegetation on xylem stable water isotopes has critical implications for the representativity of isotopes-based ecohydrological and catchments studies.
- Interflow, subsurface stormflow and throughflow: A synthesis of field work and modellingMcGuire, Kevin J.; Klaus, Julian; Jackson, C. Rhett (Wiley, 2024-09-03)Interflow, throughflow and subsurface stormflow are interchangeable terms that refer to the lateral subsurface flow above a restricting layer of lower hydraulic con- ductivity 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 impermeable soil or geologic layer. Interflow as a runoff process was first recognised in the early 1900s, yet hydrologists still struggle to predict its occurrence, persistence, importance, interaction with other streamflow generation processes, and potential to connect to valleys and streams during and following storms. We review the history of interflow research and address some of the chal- lenges in understanding its role in runoff production. We argue that characterising the controls on interflow initiation and occurrence relies on detailed field observa- tions of subsurface properties, which exist only in limited experimental settings. This data shortcoming contributes to our inability to predict interflow or determine its contribution to streamflow more broadly. There remain many opportunities to advance our understanding of interflow that include both modelling and experimental or observational approaches in hydrology.
- Temporal dynamics of catchment transit times from stable isotope dataKlaus, Julian; Chun, K. P.; McGuire, Kevin J.; McDonnell, J. J. (American Geophysical Union, 2015-06-01)Time variant catchment transit time distributions are fundamental descriptors of catchment function but yet not fully understood, characterized, and modeled. Here we present a new approach for use with standard runoff and tracer data sets that is based on tracking of tracer and age information and time variant catchment mixing. Our new approach is able to deal with nonstationarity of flow paths and catchment mixing, and an irregular shape of the transit time distribution. The approach extracts information on catchment mixing from the stable isotope time series instead of prior assumptions of mixing or the shape of transit time distribution. We first demonstrate proof of concept of the approach with artificial data; the Nash-Sutcliffe efficiencies in tracer and instantaneous transit times were >0.9. The model provides very accurate estimates of time variant transit times when the boundary conditions and fluxes are fully known. We then tested the model with real rainfall-runoff flow and isotope tracer time series from the H.J. Andrews Watershed 10 (WS10) in Oregon. Model efficiencies were 0.37 for the 18O modeling for a 2 year time series; the efficiencies increased to 0.86 for the second year underlying the need of long time tracer time series with a long overlap of tracer input and output. The approach was able to determine time variant transit time of WS10 with field data and showed how it follows the storage dynamics and related changes in flow paths where wet periods with high flows resulted in clearly shorter transit times compared to dry low flow periods.
- Testing the 'two water worlds' hypothesis under variable preferential flow conditionsRadolinski, Jesse; Pangle, Luke; Klaus, Julian; Stewart, Ryan D. (2021-06)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 little mechanistic evidence exists to explain this phenomenon. We quantified the separation between mobile water moving through preferential flow paths versus less mobile water remaining in the soil matrix after free-drainage to identify the amount of preferential flow necessary to maintain a two water world's scenario. Soil columns of varying macropore structure were subjected to simulated rainfall of increasing rainfall intensity (26 mm h(-1), 60 mm h(-1), and 110 mm h(-1)) whose stable isotope signatures oscillated around known baseline values. Prior to rainfall, soil matrix water delta H-2 nearly matched the known value used to initially wet the pore space whereas soil delta O-18 deviated from this value by up to 3.4 parts per thousand, suggesting that soils may strongly fractionate O-18. All treatments had up to 100% mixing between rain and matrix water under the lowest (26 mm h(-1)) and medium (60 mm h(-1)) rainfall intensities. The highest rainfall intensity (110 mm h(-1)), however, reduced mixing of rain and matrix water for all treatments and produced significantly different preferential flow estimates between columns with intact soil structure compared to columns with reduced soil structure. Further, artificially limiting exchange between preferential flow paths and matrix water reduced bypass flow under the most intense rainfall. We show that (1) precipitation offset metrics such as lc-excess and d-excess may yield questionable interpretations when used to identify ecohydrological separation, (2) distinct domain separation may require extreme rainfall intensities and (3) domain exchange is an important component of macropore flow.
- Time-Varying Storage-Water Age Relationships in a Catchment With a Mediterranean ClimateRodriguez, Nicolas B.; McGuire, Kevin J.; Klaus, Julian (American Geophysical Union, 2018-06-01)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) needs further evaluation in more catchments with diverse climates and physiographical features. In this work, we assessed the validity of the ISE in WS10 (H. J. Andrews forest, Oregon, USA), a forested headwater catchment in a Mediterranean climate. A conceptual model of the catchment, developed based on experimental observations of water flow paths in WS10, was calibrated to streamflow and δ18O in streamflow. Based on the calibrated model results, we determined RTDs, and streamflow TTDs and SAS functions by assuming that the soil reservoir and the groundwater reservoir act as well-mixed systems. The streamflow SAS functions and travel time dynamics showed that the ISE generally applies in WS10. Yet, during transitions from dry summer periods to wet winter periods and vice versa, the marked seasonal climate caused rapid and strong storage variations in the catchment, which led to deviations from the ISE. The seasonality of streamflow travel times in WS10 is the result of the seasonal contributions of younger water from the hillslopes added to the rather constant groundwater contributions of older water. The streamflow SAS functions were able to capture the relative importance of contrasting flow paths in the soils and in the bedrock highlighted by previous studies in WS10.
- Transit Time Estimation in Catchments: Recent Developments and Future DirectionsBenettin, Paolo; Rodriguez, Nicolas B.; Sprenger, Matthias; Kim, Minseok; Klaus, Julian; Harman, Ciaran J.; Velde, Ype; Hrachowitz, Markus; Botter, Gianluca; McGuire, Kevin J.; Kirchner, James W.; Rinaldo, Andrea; McDonnell, Jeffrey J. (American Geophysical Union, 2022-11-14)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-balance approaches and data-based approaches. We discuss and compare methods including StorAge-Selection functions, well/partially mixed compartments, water age tracking through spatially distributed models, direct transit time estimates from controlled experiments, young water fractions, and ensemble hydrograph separation. We unify some of the heterogeneity in the literature that has crept in with these many new approaches, in an attempt to clarify the key differences and similarities among them. Finally, we point to open questions in transit time research, including what we still need from theory, models, field work, and community practice.