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  • Stable Isotopes
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  • 1
    In: New Phytologist, May 2016, Vol.210(3), pp.839-849
    Description: Plants rely primarily on rainfall infiltrating their root zones – a supply that is inherently variable, and fluctuations are predicted to increase on most of the Earth's surface. Yet, interrelationships between water availability and plant use on short timescales are difficult to quantify and remain poorly understood. To overcome previous methodological limitations, we coupled high‐resolution in situ observations of stable isotopes in soil and transpiration water. We applied the approach along with Bayesian mixing modeling to track the fate of 2H‐labeled rain pulses following drought through soil and plants of deciduous tree ecosystems. We resolve how rainwater infiltrates the root zones in a nonequilibrium process and show that tree species differ in their ability to quickly acquire the newly available source. Sessile oak (Quercus petraea) adjusted root uptake to vertical water availability patterns under drought, but readjustment toward the rewetted topsoil was delayed. By contrast, European beech (Fagus sylvatica) readily utilized water from all soil depths independent of water depletion, enabling faster uptake of rainwater. Our results demonstrate that species‐specific plasticity and responses to water supply fluctuations on short timescales can now be identified and must be considered to predict vegetation functional dynamics and water cycling under current and future climatic conditions. See also the Commentary on this article by
    Keywords: Climate Change ; Deciduous Trees ; Ecohydrology ; Laser Spectroscopy ; Plant–Water Relations ; Root Uptake ; Soil Water ; Stable Isotopes
    ISSN: 0028-646X
    E-ISSN: 1469-8137
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  • 2
    Language: English
    In: Journal of Hydrology, 27 November 2014, Vol.519, pp.340-352
    Description: Assessing temporal variations in soil water flow is important, especially at the hillslope scale, to identify mechanisms of runoff and flood generation and pathways for nutrients and pollutants in soils. While surface processes are well considered and parameterized, the assessment of subsurface processes at the hillslope scale is still challenging since measurement of hydrological pathways is connected to high efforts in time, money and personnel work. The latter might not even be possible in alpine environments with harsh winter processes. Soil water stable isotope profiles may offer a time-integrating fingerprint of subsurface water pathways. In this study, we investigated the suitability of soil water stable isotope (δ O) depth profiles to identify water flow paths along two transects of steep subalpine hillslopes in the Swiss Alps. We applied a one-dimensional advection–dispersion model using δ O values of precipitation (ranging from −24.7 to −2.9‰) as input data to simulate the δ O profiles of soil water. The variability of δ O values with depth within each soil profile and a comparison of the simulated and measured δ O profiles were used to infer information about subsurface hydrological pathways. The temporal pattern of δ O in precipitation was found in several profiles, ranging from −14.5 to −4.0‰. This suggests that vertical percolation plays an important role even at slope angles of up to 46°. Lateral subsurface flow and/or mixing of soil water at lower slope angles might occur in deeper soil layers and at sites near a small stream. The difference between several observed and simulated δ O profiles revealed spatially highly variable infiltration patterns during the snowmelt periods: The δ O value of snow (−17.7 ± 1.9‰) was absent in several measured δ O profiles but present in the respective simulated δ O profiles. This indicated overland flow and/or preferential flow through the soil profile during the melt period. The applied methods proved to be a fast and promising tool to obtain time-integrated information on soil water flow paths at the hillslope scale in steep subalpine slopes.
    Keywords: Stable Isotopes ; Soil Water ; Steep Hillslopes ; Modeling ; Water Pathways ; Snowmelt ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Environmental Earth Sciences, 2012, Vol.65(8), pp.2377-2389
    Description: For water management purposes, information about an entire aquifer system is generally more important than information about a specific spring. Since a karstic aquifer system might drain to several outlets, conclusions derived from a single spring can be misleading for characterization and modeling. In this study we apply a conceptual model to an Alpine dolomite karst system in Austria. The particular challenge was that several small springs with strongly varying hydrological behavior and diffuse flow into surrounding streams drain this system. Instead of applying the model to a single spring, it was calibrated simultaneously to several observations within the system aiming to identify the karst system’s intrinsic hydrodynamic parameters. Parameter identification is supported by modeling the transport of water isotopes (δ 18 O). The parameters were transferred to the whole system with a simple upscaling procedure and a sensitivity analysis was performed to unfold influence of isotopic information on parameter sensitivity and simulation uncertainty. The results show that it is possible to identify system intrinsic parameters. But the sensitivity analysis revealed that some are hardly identifiable. Only by considering uncertainty reasonable predictions can be provided for the whole system. Including isotopic information increases the sensitivity of some intrinsic parameters, but it goes along with a sensitivity decrease for others. However, a possible reduction of prediction uncertainty by isotopic information is compensated by deficiencies in the transport modeling routines.
    Keywords: Karst aquifer ; Karst modeling ; Water isotopes ; Solute transport modeling ; Upscaling ; Rainfall-runoff modeling
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 4
    In: Water Resources Research, March 2012, Vol.48(3), pp.n/a-n/a
    Description: We developed a method to measure in situ the isotopic composition of liquid water with minimal supervision and, most important, with a temporal resolution of less than a minute. For this purpose a microporous hydrophobic membrane contactor (Membrana) was combined with an isotope laser spectrometer (Picarro). The contactor, originally designed for degassing liquids, was used with N as a carrier gas in order to transform a small fraction of liquid water to water vapor. The generated water vapor was then analyzed continuously by the Picarro analyzer. To prove the membrane's applicability, we determined the specific isotope fractionation factor for the phase change through the contactor's membrane across an extended temperature range (8°C–21°C) and with different waters of known isotopic compositions. This fractionation factor is needed to subsequently derive the liquid water isotope ratio from the measured water vapor isotope ratios. The system was tested with a soil column experiment, where the isotope values derived with the new method corresponded well (R = 0.998 for δO and R = 0.997 for δH) with those of liquid water samples taken simultaneously and analyzed with a conventional method (cavity ring‐down spectroscopy). The new method supersedes taking liquid samples and employs only relatively cheap and readily available components. This makes it a relatively inexpensive, fast, user‐friendly, and easily reproducible method. It can be applied in both the field and laboratory wherever a water vapor isotope analyzer can be run and whenever real‐time isotope data of liquid water are required at high temporal resolution. No more trade‐off between limited temporal resolution and extensive lab work No more significant time lags between sampling and data acquisition New method is field‐deployable and utilizes readily available components only
    Keywords: Crds ; Continuous Analysis ; Equilibrium Fractionation ; Hydrophobic Membrane ; In Situ Monitoring ; Stable Water Isotopes
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 5
    In: Hydrological Processes, 15 December 2015, Vol.29(25), pp.5174-5192
    Description: The vadose zone plays a crucial role in the water cycle for storing water, providing water to vegetation and transporting solutes or degrading contaminants. Earth scientists have long acknowledged the importance of the vadose zone, and numerous methods have been developed to better understand and predict hydrological processes within this ‘critical zone’. For several decades, stable isotopes (O and H) of pore water have been used as environmental tracers to gain insights into vadose zone water movement and other processes. To determine the pore water stable isotopic composition, various sampling procedures have been developed. We present the procedure and the accompanied advantages and drawbacks of each method. We further discuss possible opportunities and limitations regarding the scale of interest and the pore space that is sampled. The methodological review reveals that the choice of sampling method is crucial for the interpretation of pore water stable isotopes in the vadose zone, but a thorough comparison between the different methods is yet missing. Spiking experiments, where water of known isotopic composition is added to oven‐dried soil, have been shown to be questionable, as the extracted water is usually depleted compared with the standard water. A comparative study analysing soil samples with the recently developed direct water vapour equilibration method and the widely used cryogenic extraction shows deviations, which can only be partly explained, but discloses the need for a more thorough experimental comparative study. Especially promising are developments of continuous isotope measurements based on laser‐based spectrometry that will open up new opportunities for analysing pore water isotopes with higher temporal and spatial resolutions, revealing new insights into hydrological processes across various temporal and spatial scales. Copyright © 2015 John Wiley & Sons, Ltd.
    Keywords: Vadose Zone ; Water Stable Isotopes ; Soil Hydrology ; Ecohydrology
    ISSN: 0885-6087
    E-ISSN: 1099-1085
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  • 6
    In: Water Resources Research, August 2016, Vol.52(8), pp.5727-5754
    Description: Water travel times reflect hydrological processes, yet we know little about how travel times in the unsaturated zone vary with time. Using the soil physical model HYDRUS‐1D, we derived time variable travel time distributions for 35 study sites within the Attert catchment in Luxembourg. While all sites experience similar climatic forcing, they differ with regard to soil types (16 Cambisols, 12 Arenosols, and 7 Stagnosols) and the vegetation cover (29 forest and 6 grassland). We estimated site specific water flow and transport parameters by fitting the model simulations to observed soil moisture time series and depth profiles of pore water stable isotopes. With the calibrated model, we tracked the water parcels introduced with each rainfall event over a period of several years. Our results show that the median travel time of water from the soil surface to depths down to 200 cm is mainly driven by the subsequent rainfall amounts. The median time until precipitation is taken up by roots is governed by the seasonality of evapotranspiration rates. The ratio between the amount of water that leaves the soil profile by on the one hand and evaporation and transpiration on the other hand also shows an annual cycle. This time variable response due to climatic forcing is furthermore visible in the multimodal nature of the site specific master transit time distribution representing the flow‐averaged probability density for rainwater to become recharge. The spatial variability of travel times is mainly driven by soil texture and structure, with significant longer travel times for the clayey Stagnosols than for the loamy to sandy Cambisols and Arenosols. Seasonal effects produce multimodal patterns in travel time distributions Rainfall patterns control temporal and soil type spatial variation of travel times Unsaturated zone travel times are long compared to those observed at the catchment outlet
    Keywords: Vadose Zone ; Modeling ; Soils ; Catchment ; Stable Isotope Geochemistry
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 7
    In: New Phytologist, September 2018, Vol.219(4), pp.1300-1313
    Description: We assessed how the seasonal variability of precipitation δ2H and δ18O is propagated into soil and xylem waters of temperate trees, applied a hydrological model to estimate the residence time distribution of precipitation in the soil, and identified the temporal origin of water taken up by Picea abies and Fagus sylvatica over 4 yr. Residence times of precipitation in the soil varied between a few days and several months and increased with soil depth. On average, 50% of water consumed by trees throughout a year had precipitated during the growing season, while 40% had precipitated in the preceding winter or even earlier. Importantly, we detected subtle differences with respect to the temporal origin of water used by the two species. We conclude that both current precipitation and winter precipitation are important for the water supply of temperate trees and that winter precipitation could buffer negative impacts of spring or summer droughts. Our study additionally provides the means to obtain realistic estimates of source water δ2H and δ18O values for trees from precipitation isotope data, which is essential for improving model‐based interpretations of δ18O and δ2H values in plants.
    Keywords: Fagus Sylvatica ; Global Network Of Isotopes In Precipitation Gnip ; Picea Abies ; Residence Time ; Soil Water ; Stable Isotopes ; Temporal Origin ; Xylem Water
    ISSN: 0028-646X
    E-ISSN: 1469-8137
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  • 8
    In: Water Resources Research, June 2012, Vol.48(6), pp.n/a-n/a
    Description: The transit time of water is an important indicator of catchment functioning and affects many biological and geochemical processes. Water entering a catchment at one point in time is composed of water molecules that will spend different amounts of time in the catchment before exiting. The next water input pulse can exhibit a totally different distribution of transit times. The distribution of water transit times is thus best characterized by a time‐variable probability density function. It is often assumed, however, that the variability of the transit time distribution is negligible and that catchments can be characterized with a unique transit time distribution. In many cases this assumption is not valid because of variations in precipitation, evapotranspiration, and catchment water storage and associated (de)activation of dominant flow paths. This paper presents a general method to estimate the time‐variable transit time distribution of catchment waters. Application of the method using several years of rainfall‐runoff and stable water isotope data yields an ensemble of transit time distributions with different moments. The combined probability density function represents the master transit time distribution and characterizes the intra‐annual and interannual variability of catchment storage and flow paths. Comparing the derived master transit time distributions of two research catchments (one humid and one semiarid) reveals differences in dominant hydrologic processes and dynamic water storage behavior, with the semiarid catchment generally reacting slower to precipitation events and containing a lower fraction of preevent water in the immediate hydrologic response. Water transit time distributions are highly irregular and variable in time Water transit time distributions differ from hydrologic response functions Differences between the two functions yield information on storage dynamics
    Keywords: Catchment Response Classification ; Event‐Preevent Water ; Hydrologic Response Functions ; Storage Dynamics ; Time Variant ; Transit Time Distributions
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 9
    In: Hydrological Processes, 30 May 2017, Vol.31(11), pp.2000-2015
    Description: One of the most important functions of catchments is the storage of water. Catchment storage buffers meteorological extremes and interannual streamflow variability, controls the partitioning between evaporation and runoff, and influences transit times of water. Hydrogeological data to estimate storage are usually scarce and seldom available for a larger set of catchments. This study focused on storage in prealpine and alpine catchments, using a set of 21 Swiss catchments comprising different elevation ranges. Catchment storage comparisons depend on storage definitions. This study defines different types of storage including definitions of dynamic and mobile catchment storage. We then estimated dynamic storage using four methods, water balance analysis, streamflow recession analysis, calibration of a bucket‐type hydrological model Hydrologiska Byråns Vattenbalansavdelning model (HBV), and calibration of a transfer function hydrograph separation model using stable isotope observations. The HBV model allowed quantifying the contributions of snow, soil and groundwater storages compared to the dynamic catchment storage. With the transfer function hydrograph separation model both dynamic and mobile storage was estimated. Dynamic storage of one catchment estimated by the four methods differed up to one order of magnitude. Nevertheless, the storage estimates ranked similarly among the 21 catchments. The largest dynamic and mobile storage estimates were found in high‐elevation catchments. Besides snow, groundwater contributed considerably to this larger storage. Generally, we found that with increasing elevation the relative contribution to the dynamic catchment storage increased for snow, decreased for soil, but remained similar for groundwater storage.
    Keywords: Elevation Gradient ; Hbv ; Storage Estimation ; Swiss Alps ; Tracer Hydrology ; Transep ; Water Availability
    ISSN: 0885-6087
    E-ISSN: 1099-1085
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  • 10
    Language: English
    In: Hydrology and Earth System Sciences, July 21, 2017, Vol.21(7), p.3727
    Description: The phrase form and function was established in architecture and biology and refers to the idea that form and functionality are closely correlated, influence each other, and co-evolve. We suggest transferring this idea to hydrological systems to separate and analyze their two main characteristics: their form, which is equivalent to the spatial structure and static properties, and their function, equivalent to internal responses and hydrological behavior. While this approach is not particularly new to hydrological field research, we want to employ this concept to explicitly pursue the question of what information is most advantageous to understand a hydrological system. We applied this concept to subsurface flow within a hillslope, with a methodological focus on function: we conducted observations during a natural storm event and followed this with a hillslope-scale irrigation experiment. The results are used to infer hydrological processes of the monitored system. Based on these findings, the explanatory power and conclusiveness of the data are discussed. The measurements included basic hydrological monitoring methods, like piezometers, soil moisture, and discharge measurements. These were accompanied by isotope sampling and a novel application of 2-D time-lapse GPR (ground-penetrating radar). The main finding regarding the processes in the hillslope was that preferential flow paths were established quickly, despite unsaturated conditions. These flow paths also caused a detectable signal in the catchment response following a natural rainfall event, showing that these processes are relevant also at the catchment scale. Thus, we conclude that response observations (dynamics and patterns, i.e., indicators of function) were well suited to describing processes at the observational scale. Especially the use of 2-D time-lapse GPR measurements, providing detailed subsurface response patterns, as well as the combination of stream-centered and hillslope-centered approaches, allowed us to link processes and put them in a larger context. Transfer to other scales beyond observational scale and generalizations, however, rely on the knowledge of structures (form) and remain speculative. The complementary approach with a methodological focus on form (i.e., structure exploration) is presented and discussed in the companion paper by Jackisch et#xC2;#xA0;al.(2017).
    Keywords: Fluid Dynamics – Models ; Hydrology – Models
    ISSN: 1027-5606
    E-ISSN: 16077938
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