It is important to understand how precipitation is stored in catchments, released via evapotranspiration (ET), or recharges aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (2H and 18O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years, comprising >550 rainfall and >980 stream water samples. They were complemented by fortnightly plant-water-isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or discharged as streamflow, using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach, fitting the model solely to stream water isotope data, as well as a multi-objective calibration approach, in which the model was simultaneously fitted to tree-xylem-water isotope data.
Our results showed that the conventional calibration approach was not able to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed streamwater and xylem water isotope ratios, if those xylem water isotope observations were used in calibration. This multi-objective-calibration approach led to a more constrained parameter space, facilitating parameter value identification. The model was tested on a segment of data reserved for validation, showing a Kling-Gupta Efficiency of 0.72, compared to the 0.83 observed during in the calibration period.
The water age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective-calibration model. The latter suggested that the water supplying evapotranspiration is much older (median age 150-300 days) than what was suggested by the former (median age 50-200 days). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates, and that ET was largely sourced by winter precipitation, that must have resided in the subsurface across seasons.
}, author={Sprenger, Matthias and Llorens, Pilar and Gallart, Francesc and Benettin, Paolo and Allen, Scott and Latron, Jérôme}, year={2022}, month={Sep} } @article{carroll_deems_sprenger_maxwell_brown_newman_beutler_williams_2022, title={Modeling Snow Dynamics and Stable Water Isotopes Across Mountain Landscapes}, volume={3}, url={https://doi.org/10.1002/essoar.10510911.1}, DOI={10.1002/essoar.10510911.1}, abstractNote={Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Geophysical Research Letters. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Modeling Snow Dynamics and Stable Water Isotopes Across Mountain LandscapesAuthorsRosemary W.H.CarrolliDJeffrey SDeemsiDMatthiasSprengeriDReed M.MaxwellWendy SBrownAlexanderNewmaniDCurtis ABeutlerKenneth HurstWilliamsiDSee all authors Rosemary W.H. CarrolliDCorresponding Author• Submitting AuthorDesert Research InstituteiDhttps://orcid.org/0000-0002-9302-8074view email addressThe email was not providedcopy email addressJeffrey S DeemsiDUniversity of Colorado BoulderiDhttps://orcid.org/0000-0002-3265-8670view email addressThe email was not providedcopy email addressMatthias SprengeriDLawrence Berkeley National LaboratoryiDhttps://orcid.org/0000-0003-1221-2767view email addressThe email was not providedcopy email addressReed M. MaxwellPrinceton Universityview email addressThe email was not providedcopy email addressWendy S BrownRocky Mountain Biological Laboratoryview email addressThe email was not providedcopy email addressAlexander NewmaniDRocky Mountain Biological LaboratoryiDhttps://orcid.org/0000-0002-1574-8754view email addressThe email was not providedcopy email addressCurtis A BeutlerRocky Mountain Biological Laboratoryview email addressThe email was not providedcopy email addressKenneth Hurst WilliamsiDLawrence Berkeley National Laboratory (DOE)iDhttps://orcid.org/0000-0002-3568-1155view email addressThe email was not providedcopy email address}, publisher={Wiley}, author={Carroll, Rosemary W.H. and Deems, Jeffrey S and Sprenger, Matthias and Maxwell, Reed M. and Brown, Wendy S and Newman, Alexander and Beutler, Curtis A and Williams, Kenneth Hurst}, year={2022}, month={Mar} } @article{carroll_deems_sprenger_maxwell_brown_newman_beutler_williams_2022, title={Modeling Snow Dynamics and Stable Water Isotopes Across Mountain Landscapes}, url={https://doi.org/10.1029/2022GL098780}, DOI={10.1029/2022GL098780}, abstractNote={AbstractA coupled hydrologic and snowpack stable water isotope model assesses controls on isotopic inputs across a mountainous basin. Annually, the most depleted isotope conditions occur in the upper subalpine where snow accumulation is high, and rainfall is low. Snowmelt isotopic evolution over time indicates fractionation processes account for <25% snowmelt enrichment. Meltwater isotopic inputs are largely determined by controls on the amount, phase and isotopic mass of precipitation coincident with the ablation period. Effect of vapor loss from the snowpack on d‐excess in snowmelt is a balance between energy and snow‐availability. It is highest above treeline, and in the grass and aspen‐dominated portions of the upper montane where vegetation shading is low. Deep snowpack in conifer forests limit the influence of vapor loss in the subalpine. Wet years reduce the effects of vapor loss on snowmelt across the basin, except in the lower montane where added snowfall bolsters snow‐limited conditions.}, journal={Geophysical Research Letters}, author={Carroll, Rosemary W. H. and Deems, Jeffrey and Sprenger, Matthias and Maxwell, Reed and Brown, Wendy and Newman, Alexander and Beutler, Curtis and Williams, Kenneth H.}, year={2022}, month={Oct} } @article{sprenger_llorens_gallart_benettin_allen_latron_2022, title={Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data}, url={https://doi.org/10.5194/hess-2022-93}, DOI={10.5194/hess-2022-93}, abstractNote={Abstract. To predict hydrologic responses to inputs and perturbations, it is important to understand how precipitation is stored in catchments, released back to the atmosphere via evapotranspiration (ET), or transported to aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (2H and 18O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years of measurements, comprising > 550 rainfall and > 980 stream water samples, capturing intra-event variations. They were complemented by fortnightly plant-water-isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or discharged as streamflow, using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach, fitting the model solely to stream water isotope data, as well as a multi-objective calibration approach, in which the model was simultaneously fitted to tree xylem-water isotope data. Our results showed that the conventional model-fitting approach was not able to constrain the model parameters that represented the age of water supplying ET. Consequently, the ET isotope ratios simulated by the conventionally calibrated model failed to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed stream water and xylem water isotope ratios, if those xylem water isotope observations were used in calibration (i.e., the multi-objective approach). The multi-objective-calibration approach led to a more constrained parameter space, facilitating parameter value identification. The model was tested on a segment of data reserved for validation, showing a Kling-Gupta Efficiency of 0.72, compared to the 0.83 observed during in the calibration period. The water-age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective-calibration model. The latter suggested that the median ages of water supplying evapotranspiration is much older (150–300 days) than what was suggested by the former (50–200 days). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates, and that ET was largely sourced by winter precipitation, that must have resided in the subsurface across seasons. }, author={Sprenger, Matthias and Llorens, Pilar and Gallart, Francesc and Benettin, Paolo and Allen, Scott T. and Latron, Jérôme}, year={2022}, month={Mar} } @article{sprenger_llorens_gallart_benettin_allen_latron_2022, title={Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data}, volume={26}, ISSN={["1607-7938"]}, url={https://doi.org/10.5194/hess-26-4093-2022}, DOI={10.5194/hess-26-4093-2022}, abstractNote={Abstract. To predict hydrologic responses to inputs and perturbations, it is important to understand how precipitation is stored in catchments, released back to the atmosphere via evapotranspiration (ET), or transported to aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (18O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years of measurements, comprising >550 rainfall and >980 stream water samples, capturing intra-event variations. They were complemented by fortnightly plant water isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or to be discharged as streamflow using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach fitting the model solely to stream water isotope data and a multi-objective calibration approach in which the model was simultaneously fitted to tree xylem water isotope data. Our results showed that the conventional model-fitting approach was not able to constrain the model parameters that represented the age of water supplying ET. Consequently, the ET isotope ratios simulated by the conventionally calibrated model failed to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed stream water and xylem water isotope ratios, if those xylem water isotope observations were used in calibration (i.e., the multi-objective approach). The multi-objective calibration approach led to a more constrained parameter space facilitating parameter value identification. The model was tested on a segment of data reserved for validation showing a Kling–Gupta Efficiency of 0.72 compared to the 0.83 observed during in the calibration period. The water-age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective calibration model. The latter suggested that the median ages of water supplying evapotranspiration is much older (150–300 d) than what was suggested by the former (50–200 d). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates and that ET was largely sourced by winter precipitation that must have resided in the subsurface across seasons. }, number={15}, journal={HYDROLOGY AND EARTH SYSTEM SCIENCES}, author={Sprenger, Matthias and Llorens, Pilar and Gallart, Francesc and Benettin, Paolo and Allen, Scott T. and Latron, Jerome}, year={2022}, month={Aug}, pages={4093–4107} } @article{sprenger_llorens_gallart_benettin_allen_latron_2022, title={Supplementary material to "Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data"}, url={https://doi.org/10.5194/hess-2022-93-supplement}, DOI={10.5194/hess-2022-93-supplement}, abstractNote={Fig. 1 Median water age in stream runoff (Q, blue), the catchment's storage (S, grey), and the evapotranspiration flux (ET, orange).Age estimates shown for the best simulation (dark color) and the top 100 simulations (light color) for the conventional calibration approach, KGEQ.Compared to the results of the multi-objective calibration (Figure 5), the median ages of Q, S and ET are more uncertain and tend to overlap.}, author={Sprenger, Matthias and Llorens, Pilar and Gallart, Francesc and Benettin, Paolo and Allen, Scott T. and Latron, Jérôme}, year={2022}, month={Mar} } @article{benettin_rodriguez_sprenger_kim_klaus_harman_velde_hrachowitz_botter_mcguire_et al._2022, title={Transit Time Estimation in Catchments: Recent Developments and Future Directions}, url={https://doi.org/10.1029/2022WR033096}, DOI={10.1029/2022WR033096}, abstractNote={AbstractWater 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.}, journal={Water Resources Research}, author={Benettin, Paolo and Rodriguez, Nicolas B. and Sprenger, Matthias and Kim, Minseok and Klaus, Julian and Harman, Ciaran J. and Velde, Ype and Hrachowitz, Markus and Botter, Gianluca and McGuire, Kevin J. and et al.}, year={2022}, month={Nov} } @article{carroll_deems_maxwell_sprenger_brown_newman_beutler_bill_hubbard_williams_2022, title={Variability in observed stable water isotopes in snowpack across a mountainous watershed in Colorado}, volume={36}, url={https://doi.org/10.1002/hyp.14653}, DOI={10.1002/hyp.14653}, abstractNote={AbstractIsotopic information from 81 snowpits was collected over a 5‐year period in a large, Colorado watershed. Data spans gradients in elevation, aspect, vegetation, and seasonal climate. They are combined with overlapping campaigns for water isotopes in precipitation and snowmelt, and a land‐surface model for detailed estimates of snowfall and climate at sample locations. Snowfall isotopic inputs, describe the majority of δ18O snowpack variability. Aspect is a secondary control, with slightly more enriched conditions on east and north facing slopes. This is attributed to preservation of seasonally enriched snowfall and vapour loss in the early winter. Sublimation, expressed by decreases in snowpack d‐excess in comparison to snowfall contributions, increases at low elevation and when seasonal temperature and solar radiation are high. At peak snow accumulation, post‐depositional fractionation appears to occur in the top 25 ± 14% of the snowpack due to melt‐freeze redistribution of lighter isotopes deeper into the snowpack and vapour loss to the atmosphere during intermittent periods of low relative humidity and high windspeed. Relative depth of fractionation increases when winter daytime temperatures are high and winter precipitation is low. Once isothermal, snowpack isotopic homogenization and enrichment was observed with initial snowmelt isotopically depleted in comparison to snowpack and enriching over time. The rate of δ18O increase (d‐excess decrease) in snowmelt was 0.02‰ per day per 100‐m elevation loss. Isotopic data suggests elevation dictates snowpack and snowmelt evolution by controlling early snow persistence (or absence), isotopic lapse rates in precipitation and the ratio of energy to snow availability. Hydrologic tracer studies using stable water isotopes in basins of large topographic relief will require adjustment for these elevational controls to properly constrain stream water sourcing from snowmelt.}, number={8}, journal={Hydrological Processes}, publisher={Wiley}, author={Carroll, Rosemary W. H. and Deems, Jeffery and Maxwell, Reed and Sprenger, Matthias and Brown, Wendy and Newman, Alexander and Beutler, Curtis and Bill, Markus and Hubbard, Susan S. and Williams, Kenneth H.}, year={2022}, month={Aug} } @article{sprenger_carroll_dennedy‐frank_siirila‐woodburn_newcomer_brown_newman_beutler_bill_hubbard_et al._2022, title={Variability of Snow and Rainfall Partitioning Into Evapotranspiration and Summer Runoff Across Nine Mountainous Catchments}, volume={49}, url={https://doi.org/10.1029/2022GL099324}, DOI={10.1029/2022GL099324}, abstractNote={AbstractUnderstanding the partitioning of snow and rain contributing to either catchment streamflow or evapotranspiration (ET) is of critical relevance for water management in response to climate change. To investigate this partitioning, we use endmember splitting and mixing analyses based on stable isotope (18O) data from nine headwater catchments in the East River, Colorado. Our results show that one third of the snow partitions to ET and 13% of the snowmelt sustains summer streamflow. Only 8% of the rainfall contributes to the summer streamflow, because most of the rain (67%) partitions to ET. The spatial variability of precipitation partitioning is mainly driven by aspect and tree cover across the sub‐catchments. Catchments with higher tree cover have a higher share of snow becoming ET, resulting in less snow in summer streamflow. Summer streamflow did not contain more rain with higher rainfall sums, but more rain was taken up in ET.}, number={13}, journal={Geophysical Research Letters}, publisher={American Geophysical Union (AGU)}, author={Sprenger, Matthias and Carroll, Rosemary W. H. and Dennedy‐Frank, James and Siirila‐Woodburn, Erica R. and Newcomer, Michelle E. and Brown, Wendy and Newman, Alexander and Beutler, Curtis and Bill, Markus and Hubbard, Susan S. and et al.}, year={2022}, month={Jul} } @article{sprenger_carroll_dennedy-frank_siirila-woodburn_newcomer_brown_newman_beutler_bill_hubbard_et al._2022, title={Variability of snow and rainfall partitioning into evapotranspiration and summer runoff across nine mountainous catchments}, url={https://doi.org/10.1002/essoar.10511257.1}, DOI={10.1002/essoar.10511257.1}, abstractNote={Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Geophysical Research Letters. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Variability of snow and rainfall partitioning into evapotranspiration and summer runoff across nine mountainous catchmentsAuthorsMatthiasSprengeriDRosemary W.H.CarrolliDP. James JDennedy-FrankErica R.Siirila-WoodburniDMichelle E.NewcomeriDWendy SBrownAlexanderNewmaniDCurtis ABeutlerMarkusBilliDSusan S.HubbardKenneth H.WillamsiDSee all authors Matthias SprengeriDCorresponding Author• Submitting AuthorLawrence Berkeley National LaboratoryiDhttps://orcid.org/0000-0003-1221-2767view email addressThe email was not providedcopy email addressRosemary W.H. CarrolliDDesert Research InstituteiDhttps://orcid.org/0000-0002-9302-8074view email addressThe email was not providedcopy email addressP. James J Dennedy-FrankStanford Universityview email addressThe email was not providedcopy email addressErica R. Siirila-WoodburniDLawrence Berkeley National Laboratory (DOE)iDhttps://orcid.org/0000-0001-9406-124Xview email addressThe email was not providedcopy email addressMichelle E. NewcomeriDLawrence Berkeley National Laboratory (DOE)iDhttps://orcid.org/0000-0001-5138-9026view email addressThe email was not providedcopy email addressWendy S BrownRocky Mountain Biological Laboratoryview email addressThe email was not providedcopy email addressAlexander NewmaniDRocky Mountain Biological LaboratoryiDhttps://orcid.org/0000-0002-1574-8754view email addressThe email was not providedcopy email addressCurtis A BeutlerRocky Mountain Biological Laboratoryview email addressThe email was not providedcopy email addressMarkus BilliDLawrence Berkeley National LaboratoryiDhttps://orcid.org/0000-0001-7002-2174view email addressThe email was not providedcopy email addressSusan S. HubbardLawrence Berkeley National Laboratory (DOE)view email addressThe email was not providedcopy email addressKenneth H. WillamsiDLawrence Berkeley National LabiDhttps://orcid.org/0000-0002-3568-1155view email addressThe email was not providedcopy email address}, author={Sprenger, Matthias and Carroll, Rosemary W.H. and Dennedy-Frank, P. James J and Siirila-Woodburn, Erica R. and Newcomer, Michelle E. and Brown, Wendy S and Newman, Alexander and Beutler, Curtis A and Bill, Markus and Hubbard, Susan S. and et al.}, year={2022}, month={May} } @article{zhao_sprenger_barzegar_tang_adamowski_2022, title={Withdrawn: Similar Isotopic Biases of Plant Stem Bulk Water From Different Water Sources by Cryogenic Vacuum Distillation Demonstrated Through Rehydration Experiments}, volume={49}, url={https://doi.org/10.1029/2021GL096474}, DOI={10.1029/2021GL096474}, abstractNote={AbstractZhao, P., Sprenger, M., Barzegar, R., Tang, X., & Adamowski, J. (2022). Similar isotopic biases of plant stem bulk water from different water sources by cryogenic vacuum distillation demonstrated through rehydration experiments. Geophysical Research Letters, 49, e2021GL096474. The above article from Geophysical Research Letters, published online on 30 March 2022 in Wiley Online Library (http://wileyonlinelibrary.com), has been withdrawn by agreement among the authors, the Editor‐in‐Chief Harihar Rajaram, the American Geophysical Union, and Wiley Periodicals, LLC. The withdrawal has been agreed because the authors discovered, after acceptance of the article but before final publication, some accidental transcription/typo errors in the data (for some δ2H and δ18O values) which would affect some of the results and cannot be sufficiently corrected to the authors' satisfaction.}, number={13}, journal={Geophysical Research Letters}, publisher={American Geophysical Union (AGU)}, author={Zhao, Pei and Sprenger, Matthias and Barzegar, Rahim and Tang, Xiangyu and Adamowski, Jan}, year={2022}, month={Jul} } @article{carroll_deems_maxwell_sprenger_brown_newman_beutler_bill_hubbard_williams_2021, title={Stable Water Isotope Inputs Across Mountain Landscapes}, url={https://doi.org/10.1002/essoar.10507500.1}, DOI={10.1002/essoar.10507500.1}, abstractNote={Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Water Resources Research. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Stable Water Isotope Inputs Across Mountain LandscapesAuthorsRosemary W.H.CarrolliDJeffrey SDeemsiDReed M.MaxwellMatthiasSprengerWendy SBrownAlexanderNewmaniDCurtisBeutlerMarkusBilliDSusan S.HubbardKenneth HurstWilliamsiDSee all authors Rosemary W.H. CarrolliDCorresponding Author• Submitting AuthorDesert Research InstituteiDhttps://orcid.org/0000-0002-9302-8074view email addressThe email was not providedcopy email addressJeffrey S DeemsiDUniversity of Colorado BoulderiDhttps://orcid.org/0000-0002-3265-8670view email addressThe email was not providedcopy email addressReed M. MaxwellPrinceton Universityview email addressThe email was not providedcopy email addressMatthias SprengerNCSUview email addressThe email was not providedcopy email addressWendy S BrownRocky Mountain Biological Laboratoryview email addressThe email was not providedcopy email addressAlexander NewmaniDRocky Mountain Biological LaboratoryiDhttps://orcid.org/0000-0002-1574-8754view email addressThe email was not providedcopy email addressCurtis BeutlerRocky Mountain Biological Laboratoryview email addressThe email was not providedcopy email addressMarkus BilliDLawrence Berkeley National LaboratoryiDhttps://orcid.org/0000-0001-7002-2174view email addressThe email was not providedcopy email addressSusan S. HubbardLawrence Berkeley National Laboratory (DOE)view email addressThe email was not providedcopy email addressKenneth Hurst WilliamsiDLawrence Berkeley National Laboratory (DOE)iDhttps://orcid.org/0000-0002-3568-1155view email addressThe email was not providedcopy email address}, author={Carroll, Rosemary W.H. and Deems, Jeffrey S and Maxwell, Reed M. and Sprenger, Matthias and Brown, Wendy S and Newman, Alexander and Beutler, Curtis and Bill, Markus and Hubbard, Susan S. and Williams, Kenneth Hurst}, year={2021}, month={Jul} } @article{gallart_valiente_llorens_cayuela_sprenger_latron_2020, title={Investigating young water fractions in a small Mediterranean mountain catchment: Both precipitation forcing and sampling frequency matter}, volume={6}, url={https://doi.org/10.1002/hyp.13806}, DOI={10.1002/hyp.13806}, abstractNote={AbstractThe proportion of water younger than 2–3 months (young water fraction, Fyw) has become increasingly investigated in catchment hydrology. Fyw is typically estimated by comparing seasonal tracer cycles in precipitation and streamflow, through water sampling. However, some open research questions remain, such as: (i) whether part of the summer precipitation should be discarded because the high evapotranspiration demand, (ii) how well Fyw serves as a metric to compare catchments, and (iii) how sampling frequency affects Fyw estimates. To address these questions, we investigated Fyw in soil‐, ground‐ and stream waters for the small Mediterranean Can Vila catchment. Rainfall was sampled at 5‐mm intervals. Mobile soil water and groundwater were sampled fortnightly. Stream water was sampled depending on flow at variable time intervals (30 min to 1 week). Over 58 months, this sampling provided 1,529 δ18O determinations. Isotopic analyses results led us to include summer precipitation in the input signal. We found the highest Fyw in mobile soil waters (34%), while this was almost zero for groundwater except during wet periods. For stream waters, Fyw depended on the discharge variations, so that the flow‐weighted young water fraction () was 22.6%, whereas the time‐weighted Fyw was just 6.2%. Both and its discharge sensitivity (Sd) varied when different 12‐month sampling periods were investigated. The young water fraction that would be obtained from a virtual thorough sampling () was estimated from the Sd and the observed stream flow. This showed an underestimation of by 25% for the frequent dynamic sampling and 66% for weekly sampling, due to missing high flows. Our results confirm that Fyw and its discharge sensitivity are metrics very sensitive to meteorological forcing during the analysed period. Thus, comparisons between catchments need long‐term mean annual values and their variability. Our findings also support the dependence of Fyw estimates on the sampling rate and show the advantages of flow‐weighted analysis. Finally, catchment water turnover investigations should be accompanied by the analysis of flow duration curves.}, journal={Hydrological Processes}, publisher={Wiley}, author={Gallart, Francesc and Valiente, María and Llorens, Pilar and Cayuela, Carles and Sprenger, Matthias and Latron, Jérôme}, year={2020}, month={Aug} } @article{knighton_kuppel_smith_soulsby_sprenger_tetzlaff_2020, title={Using isotopes to incorporate tree water storage and mixing dynamics into a distributed ecohydrologic modelling framework}, volume={2}, url={https://doi.org/10.1002/eco.2201}, DOI={10.1002/eco.2201}, abstractNote={AbstractRoot water uptake (RWU) by vegetation influences the partitioning of water between transpiration, evaporation, percolation, and surface runoff. Measurements of stable isotopes in water have facilitated estimates of the depth distribution of RWU for various tree species through methodologies based on end member mixing analysis (EMMA). EMMA often assumes that the isotopic composition of tree‐stored xylem water (δXYLEM) is representative of the isotopic composition of RWU (δRWU). We tested this assumption within the framework of EcH2O‐iso, a process‐based distributed tracer‐aided ecohydrologic model, applied to a small temperate catchment with a vegetation cover of coniferous eastern hemlock (Tsuga canadensis) and deciduous American beech (Fagus grandifolia). We simulated three scenarios for tree water storage and mixing: (a) zero storage (ZS), (b) storage with a well‐mixed reservoir (WM), and (c) storage with piston flow (PF). Simulating tree storage (WM and PF) improved the fit to δXYLEMobservations over ZS in the summer and fall seasons and substantially altered calibrated RWU depths and stomatal conductance. Our results suggest that there are likely to be advantages to considering tree storage and internal mixing when attempting to interpret δXYLEMin the estimation of RWU depths and critical zone water residence times, particularly during periods of low transpiration. Improved representations of tree water dynamics could yield more accurate ecohydrologic and earth system model representations of the critical zone.}, journal={Ecohydrology}, publisher={Wiley}, author={Knighton, James and Kuppel, Sylvain and Smith, Aaron and Soulsby, Chris and Sprenger, Matthias and Tetzlaff, Doerthe}, year={2020}, month={Apr} } @article{sprenger_allen_2020, title={What Ecohydrologic Separation Is and Where We Can Go With It}, volume={56}, url={https://doi.org/10.1029/2020WR027238}, DOI={10.1029/2020WR027238}, abstractNote={AbstractThe “ecohydrologic separation” hypothesis challenged assumptions of translatory flow through the rooting zone. However, studies aiming to test ecohydrologic separation have largely done so with insufficient discussion of infiltration and rooting zone recharge processes and instead have mostly focused on either isotopic differences between stream water and plant water or the presence of fractionated isotope ratios in plant water. Based on extensively observed heterogeneities in soils and watersheds, we posit that differences in isotopic compositions of water in plants, streams, and other subsurface pools are expected in most scenarios. Interpretation of those plant‐and‐stream water isotopic differences is important, but diagnosing the role of any specific process is typically confounded by the diversity of potential mechanisms contributing to those isotopic differences. Thus, we should progress from simply describing their occurrences and refocus the discussion of ecohydrologic separation on how heterogeneous infiltration and root uptake processes lead to such differences. Consequently, we outline areas where plant and soil‐water stable isotope data may be useful for advancing our understanding and representation of soil‐water transport and plant‐water recharge.}, number={7}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Sprenger, Matthias and Allen, Scott T.}, year={2020}, month={Jul} } @article{zhu_wang_kong_zheng_feng_zhang_yuan_song_sprenger_2019, title={Interaction of Surface Water and Groundwater Influenced by Groundwater Over-Extraction, Waste Water Discharge and Water Transfer in Xiong'an New Area, China}, volume={11}, ISSN={["2073-4441"]}, url={https://doi.org/10.3390/w11030539}, DOI={10.3390/w11030539}, abstractNote={Understanding the interaction of surface water and groundwater affected by anthropogenic activities is of great importance for water resource and water quality management. The Xiong’an New Area, located in the North China Plain, has been designated a new building area by China’s government. Groundwater has been over pumped and artificial water was transferred to meet the water supply in this region. Therefore, the natural interaction of surface water and groundwater has been greatly changed and there has been a complex impact of the groundwater from anthropogenic activities. In this study, we used water chemical ions and stable isotopes of δ2H and δ18O to assess the interaction of surface water and groundwater in the Xiong’an New Area. We carried out field surveys and water sampling of the Fu River (domestic waste water discharge), Lake Baiyangdian (artificial water transfer), and the underlying groundwater along the water bodies. Results show that the artificial surface water (discharged and transferred) became the major recharge source for the local groundwater due to the decline of groundwater table. We used groundwater table observations, end-member mixing analysis of the stable isotopic composition and chloride tracers to estimate the contributions of different recharge sources to the local groundwater. Due to the over pumping of groundwater, the lateral groundwater recharge was dominant with a contribution ratio ranging from 12% to 78% in the upper reach of the river (Sections 1–3). However, the contribution of lateral groundwater recharge was estimated to be negligible with respect to the artificial water recharge from Lake Baiyangdian. Seepage from the Fu River contributed a significant amount of water to the connecting aquifer, with a contribution ranging from 14% to 75% along the river. The extent of the river influence into the aquifer ranges as far as 1400 m to the south and 400 m to the north of the Fu River. Estimations based on isotopic fractionation shows that about 25% of Lake Baiyangdian water was lost by evaporation. By using the stable isotopes of oxygen and hydrogen in the lake water, an influencing range of 16 km west of the lake was determined. The interaction of the surface water and groundwater is completely changed by anthropogenic activities, such as groundwater over pumping, waste water discharge and water transfer. The switched interaction of surface water and groundwater has a significant implication on water resources management.}, number={3}, journal={WATER}, author={Zhu, Meijia and Wang, Shiqin and Kong, Xiaole and Zheng, Wenbo and Feng, Wenzhao and Zhang, Xianfu and Yuan, Ruiqiang and Song, Xianfang and Sprenger, Matthias}, year={2019}, month={Mar} } @article{sprenger_llorens_cayuela_gallart_latron_2019, title={Mechanisms of consistently disjunct soil water pools over (pore) space and time}, volume={23}, ISSN={["1607-7938"]}, url={https://doi.org/10.5194/hess-23-2751-2019}, DOI={10.5194/hess-23-2751-2019}, abstractNote={Abstract. The storage and release of water in soils is critical for sustaining plant transpiration and groundwater recharge. However, how much subsurface mixing of water occurs, and how much of the water is available for plants or otherwise percolates to streams and the groundwater is not yet understood. Based on stable isotope (2H and 18O) data, some studies have found that water infiltrating into soils can bypass older pore water. However, the mechanisms leading to the separation of water routed to the streams and water held tightly in smaller pores are still unclear. Here, we address the current limitations of the understanding of subsurface mixing and their consequences regarding the application of stable isotopes in ecohydrological studies. We present an extensive data set, for which we sampled the isotopic composition of mobile and bulk soil water in parallel with groundwater at a fortnightly temporal resolution and stream water and rainfall at a much higher resolution in a Mediterranean long-term research catchment, in Vallcebre, Spain. The data reveal that the mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix well; however, they constitute two disjunct subsurface water pools with little exchange, despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentalization results from the rewetting of small soil pores by isotopically depleted winter/spring rain. Thus, stable isotopes, and, in turn, water residence times, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing the water uptake by plants or the process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone. }, number={6}, journal={HYDROLOGY AND EARTH SYSTEM SCIENCES}, publisher={Copernicus GmbH}, author={Sprenger, Matthias and Llorens, Pilar and Cayuela, Carles and Gallart, Francesc and Latron, Jerome}, year={2019}, month={Jun}, pages={2751–2762} } @article{zheng_wang_sprenger_liu_cao_2019, title={Response of soil water movement and groundwater recharge to extreme precipitation in a headwater catchment in the North China Plain}, volume={576}, ISSN={["1879-2707"]}, url={https://doi.org/10.1016/j.jhydrol.2019.06.071}, DOI={10.1016/j.jhydrol.2019.06.071}, abstractNote={Soil water storage and movement are highly heterogeneous across landscapes and their response to spatiotemporal variations in meteorological forcing is complex. While different pools of soil water (including bound and mobile water) are observed, the mechanisms of soil water movement in semi-arid and sub-humid regions are not well understood due to high variation in soil water storage conditions. The Taihang Mountain is a headwater region that recharges both groundwater and surface water systems of the North China Plain, where groundwater levels have been declining and water storage loss is serious. Increasing land cultivation in the Taihang Mountain areas has increased evapotranspiration and reduced both surface runoff and groundwater recharge. Although extreme precipitation is critical for groundwater recharge in the headwater regions, the response mechanism of soil water movement and groundwater recharge remains unclear. In this study, soil water movement and groundwater recharge mechanisms in a cultivated farmland (FL) and land under natural vegetation (NV) were determined for a normal and an extreme precipitation year through the combined use of soil water content and stable isotopes of water (18O and 2H). Soil water got enriched in δ18O and δ2H (δ18O changed from −11.2 to −7.0‰ at NV and from −11.1 to −4.4‰ at FL; δ2H changed from −71 to −49‰ at NV and from −73 to −30% at FL) with increasing soil depth during the growing season suggesting that winter precipitation was generally transported via advection dispersion flow mechanism. However, this process was accompanied by the mixing of previously enriched soil water after large rain events (20–50 mm/day) during the rainy season in a normal precipitation year. Water movement changed from translatory flow to preferential flow after extreme precipitation in a wet precipitation year. Cultivation intensified water evaporation in the top soil layer (upper 10–20 cm), and induced preferential flow down to 50 cm soil depth under FL relative to land under NV. Thus, cropping significantly reduced groundwater recharge. Excessive storm during a wet year produced bypass flow after the first rainstorm, which rapidly recharged deep soil layers (50–100 cm depth). Bypass flow induced by excessive precipitation and contributed the most to groundwater in FL. The observed rapid response of soil water and groundwater to extreme precipitation events is critical for soil and water management to mitigate problems such as nitrate leaching and groundwater contamination in headwater regions of semi-arid and sub-humid areas.}, journal={JOURNAL OF HYDROLOGY}, publisher={Elsevier BV}, author={Zheng, Wenbo and Wang, Shiqin and Sprenger, Matthias and Liu, Bingxia and Cao, Jiansheng}, year={2019}, month={Sep}, pages={466–477} } @misc{sprenger_stumpp_weiler_aeschbach_allen_benettin_dubbert_hartmann_hrachowitz_kirchner_et al._2019, title={The Demographics of Water: A Review of Water Ages in the Critical Zone}, volume={57}, ISSN={["1944-9208"]}, url={https://doi.org/10.1029/2018RG000633}, DOI={10.1029/2018RG000633}, abstractNote={AbstractThe time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil‐atmosphere or soil‐groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone.}, number={3}, journal={REVIEWS OF GEOPHYSICS}, publisher={American Geophysical Union (AGU)}, author={Sprenger, Matthias and Stumpp, Christine and Weiler, Markus and Aeschbach, Werner and Allen, Scott T. and Benettin, Paolo and Dubbert, Maren and Hartmann, Andreas and Hrachowitz, Markus and Kirchner, James W. and et al.}, year={2019}, month={Sep}, pages={800–834} } @article{sprenger_tetzlaff_buttle_laudon_leistert_mitchell_snelgrove_weiler_soulsby_2018, title={Measuring and Modeling Stable Isotopes of Mobile and Bulk Soil Water}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045392401&partnerID=MN8TOARS}, DOI={10.2136/vzj2017.08.0149}, abstractNote={Core Ideas Bulk soil water isotopes have an evaporation signal, but mobile water isotopes do not. These differences are time variant and linked to the volume and age of the mobile water. Two pore domains (fast and slow) improve simulations of soil water isotope dynamics. A new model accounts for isotopic exchange via water vapor between two pore domains. This exchange is relevant for proper simulation of the evaporation signal in bulk soil water. Recent findings from stable isotope studies have opened up new questions about differences in the isotopic composition (δ2H and δ18O) of mobile (MW) and bulk water (BW) in soils. We sampled the isotopic compositions of MW using suction lysimeters and BW with the direct‐equilibration method. The study was conducted at two landscape units in each of three catchments: the Bruntland Burn (Scotland), Dorset (Canada), and Krycklan (Sweden). We further used the numerical one‐dimensional flow model SWIS (Soil Water Isotope Simulator) to simulate the hydrometric and isotopic dynamics. The model included evaporation fractionation, allowed differentiation between a fast and a slow flow domain, and included isotopic exchange via water vapor. Our measurements showed that MW plots along the local meteoric water lines, whereas BW plots below, which is indicative of evaporation fractionation. We suggest that the relative volume of MW to BW is relevant for explaining these isotopic differences because MW volumes are usually relatively low during periods of high evaporation. Under this condition, differences between MW and plant water isotopes are not paradoxical but rather related to the water that cannot be sampled with suction lysimeters but is still available for plant water uptake. The simulations accounting for fast and slow flow supported the conceptualization of the two soil pore domains with isotopic exchange via vapor exchange because this model setup resulted in the best model performance. Overall, these findings are of high relevance for current understanding related to the source and isotopic composition of water taken up by plants.}, number={1}, journal={Vadose Zone Journal}, publisher={Soil Science Society of America}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Buttle, Jim and Laudon, Hjalmar and Leistert, Hannes and Mitchell, Carl P.J. and Snelgrove, Jenna and Weiler, Markus and Soulsby, Chris}, year={2018}, pages={0} } @article{sprenger_tetzlaff_buttle_carey_mcnamara_laudon_shatilla_soulsby_2018, title={Storage, mixing, and fluxes of water in the critical zone across northern environments inferred by stable isotopes of soil water}, volume={6}, url={https://doi.org/10.1002/hyp.13135}, DOI={10.1002/hyp.13135}, abstractNote={AbstractQuantifying soil water storage, mixing, and release via recharge, transpiration, and evaporation is essential for a better understanding of critical zone processes. Here, we integrate stable isotope (2H and 18O of soil water, precipitation, and groundwater) and hydrometric (soil moisture) data from 5 long‐term experimental catchments along a hydroclimatic gradient across northern latitudes: Dry Creek (USA), Bruntland Burn (Scotland), Dorset (Canada), Krycklan (Sweden), and Wolf Creek (Canada). Within each catchment, 6 to 11 isotope sampling campaigns occurred at 2 to 4 sampling locations over at least 1 year. Analysis for 2H and 18O in the bulk pore water was done for >2,500 soil samples either by cryogenic extraction (Dry Creek) or by direct equilibration (other sites). The results showed a similar general pattern that soil water isotope variability reflected the seasonality of the precipitation input signal. However, pronounced differences among sampling locations occurred regarding the isotopic fractionation due to evaporation. We found that antecedent precipitation volumes mainly governed the fractionation signal, temperature and evaporation rates were of secondary importance, and soil moisture played only a minor role in the variability of soil water evaporation fractionation across the hydroclimatic gradient. We further observed that soil waters beneath conifer trees were more fractionated than beneath heather shrubs or red oak trees, indicating higher soil evaporation rates in coniferous forests. Sampling locations closer to streams were more damped and depleted in their stable isotopic composition than hillslope sites, revealing increased subsurface mixing towards the saturated zone and a preferential recharge of winter precipitation. Bulk soil waters generally comprised a high share of waters older than 14 days, which indicates that the water in soil pores are usually not fully replaced by recent infiltration events. The presented stable isotope data of soil water were, thus, a useful tool to track the spatial variability of water fluxes within and from the critical zone. Such data provide invaluable information to improve the representation of critical zone processes in spatially distributed hydrological models.}, number={12}, journal={Hydrological Processes}, publisher={Wiley}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Buttle, Jim and Carey, Sean K. and McNamara, James P. and Laudon, Hjalmar and Shatilla, Nadine J. and Soulsby, Chris}, year={2018}, month={Jun}, pages={1720–1737} } @article{sprenger_tetzlaff_buttle_laudon_soulsby_2018, title={Water ages in the critical zone of long-term experimental sites in northern latitudes}, volume={22}, url={https://doi.org/10.5194/hess-22-3965-2018}, DOI={10.5194/hess-22-3965-2018}, abstractNote={Abstract. As northern environments undergo intense changes due to a warming climate and altered land use practices, there is an urgent need for improved understanding of the impact of atmospheric forcing and vegetation on water storage and flux dynamics in the critical zone. We therefore assess the age dynamics of water stored in the upper 50 cm of soil, and in evaporation, transpiration, or recharge fluxes at four soil–vegetation units of podzolic soils in the northern latitudes with either heather or tree vegetation (Bruntland Burn in Scotland, Dorset in Canada, and Krycklan in Sweden). We derived the age dynamics with the physically based SWIS (Soil Water Isotope Simulator) model, which has been successfully used to simulate the hydrometric and isotopic dynamics in the upper 50 cm of soils at the study sites. The modelled subsurface was divided into interacting fast and slow flow domains. We tracked each day's infiltrated water through the critical zone and derived forward median travel times (which show how long the water takes to leave the soil via evaporation, transpiration, or recharge), and median water ages (to estimate the median age of water in soil storage or the evaporation, transpiration, and recharge fluxes). Resulting median travel times were time-variant, mainly governed by major recharge events during snowmelt in Dorset and Krycklan or during the wetter winter conditions in Bruntland Burn. Transpiration travel times were driven by the vegetation growth period with the longest travel times (200 days) for waters infiltrated in early dormancy and the shortest travel times during the vegetation period. However, long tails of the travel time distributions in evaporation and transpiration revealed that these fluxes comprised waters older than 100 days. At each study site, water ages of soil storage, evaporation, transpiration, and recharge were all inversely related to the storage volume of the critical zone: water ages generally decreased exponentially with increasing soil water storage. During wet periods, young soil waters were more likely to be evapotranspired and recharged than during drier periods. While the water in the slow flow domain showed long-term seasonal dynamics and generally old water ages, the water ages of the fast flow domain were generally younger and much flashier. Our results provide new insights into the mixing and transport processes of soil water in the upper layer of the critical zone, which is relevant for hydrological modelling at the plot to catchment scales as the common assumption of a well-mixed system in the subsurface holds for neither the evaporation, transpiration, or recharge. }, number={7}, journal={Hydrology and Earth System Sciences}, publisher={Copernicus GmbH}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Buttle, Jim and Laudon, Hjalmar and Soulsby, Chris}, year={2018}, month={Jul}, pages={3965–3981} } @article{sprenger_tetzlaff_buttle_laudon_soulsby_2018, title={Water ages in the critical zone of long-term experimental sites in northern latitudes}, volume={3}, url={https://doi.org/10.5194/hess-2018-144}, DOI={10.5194/hess-2018-144}, abstractNote={Abstract. As northern environments undergo intense respond due to a warming climate and altered land use practices, there is an urgent need for improved understanding of the impact of atmospheric forcing and vegetation on water storage and flux dynamics in the critical zone. We therefore assess the age dynamics of water stored in the upper 50 cm of soil, and in evaporation, transpiration or recharge fluxes at four soil-vegetation units of podzolic soils in the northern latitudes with either heather or tree vegetation (Bruntland Burn in Scotland, Dorset in Canada, and Krycklan in Sweden). We derived the age dynamics with the physically based SWIS (Soil Water Isotope Simulator) model, which has been successfully used to simulate the hydrometric and isotopic dynamics in the upper 50 cm of soils at the study sites. The modelled subsurface was divided into interacting fast and slow flow domains. We tracked each day's infiltrated water through the critical zone and derived forward median travel times (which show how long the water takes to leave the soil via evaporation, transpiration or recharge), and median water ages (to estimate the median age of water in soil storage or the evaporation, transpiration and recharge fluxes). Resulting median travel times were time-variant, mainly governed by major recharge events during snow melt in Dorset and Krycklan or during the wetter winter conditions in Bruntland Burn. Transpiration travel times were driven by the vegetation growth period with longest travel times (200 days) for waters infiltrated in early dormancy and shortest travel times during the vegetation period. However, long tails of the travel time distributions in evaporation and transpiration revealed that these fluxes comprised waters older than 100 days. At each study site, water ages of soil storage, evaporation, transpiration and recharge were all inversely related to the storage volume of the critical zone: water ages generally decreased exponentially with increasing soil water storage. During wet periods, young soil waters were more likely to be evapotranspired and recharged than during drier periods. While the water in the slow flow domain showed long-term seasonal dynamics and generally old water ages, the water ages of the fast flow domain were generally younger and much flashier. Our results provide new insights into the mixing and transport processes of soil water in the upper layer of the critical zone, which is relevant for hydrological modelling at the plot to catchment scales as the common assumption of a well-mixed system in the subsurface neither holds for the evaporation, transpiration nor recharge. }, publisher={Copernicus GmbH}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Buttle, Jim and Laudon, Hjalmar and Soulsby, Chris}, year={2018}, month={Mar} } @article{sprenger_tetzlaff_tunaley_dick_soulsby_2017, title={Evaporation fractionation in a peatland drainage network affects stream water isotope composition}, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85011063373&partnerID=MN8TOARS}, DOI={10.1002/2016WR019258}, abstractNote={AbstractThere is increasing interest in improving understanding of evaporation within a catchment for an enhanced representation of dominant processes in hydrological models. We used a dual‐isotope approach within a nested experimental design in a boreal catchment in the Scottish Highlands (Bruntland Burn) to quantify the spatiotemporal dynamics of evaporation fractionation in a peatland drainage network and its effect on stream water isotopes. We conducted spatially distributed water sampling within the saturated peatland under different wetness conditions. We used the lc‐excess—which describes the offset of a water sample from the local meteoric water line in the dual‐isotope space—to understand the development of kinetic fractionation during runoff in a peatland network. The evaporation fractionation signal correlated positively with the potential evapotranspiration and negatively with the discharge. The variability of the isotopic enrichment within the peatland drainage network was higher with higher potential evapotranspiration and lower with higher discharge. We found an increased evaporation fractionation toward the center of the peatland, while groundwater seepage from minerogenic soils influenced the isotopic signal at the edge of the peatland. The evaporation signal was imprinted on the stream water, as the discharge from a peatland dominated subcatchment showed a more intense deviation from the local meteoric water line than the discharge from the Bruntland Burn. The findings underline that evaporation fractionation within peatland drainage networks affects the isotopic signal of headwater catchments, which questions the common assumption in hydrological modeling that the isotopic composition of stream waters did not undergo fractionation processes.}, number={1}, journal={Water Resources Research}, publisher={Wiley-Blackwell}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Tunaley, Claire and Dick, Jonathan and Soulsby, Chris}, year={2017}, month={Jan}, pages={851–866} } @article{angermann_jackisch_allroggen_sprenger_zehe_tronicke_weiler_blume_2017, title={Form and function in hillslope hydrology: characterization of subsurface flow based on response observations}, volume={21}, url={https://doi.org/10.5194/hess-21-3727-2017}, DOI={10.5194/hess-21-3727-2017}, abstractNote={Abstract. 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 al.(2017). }, number={7}, journal={Hydrology and Earth System Sciences}, publisher={Copernicus GmbH}, author={Angermann, Lisa and Jackisch, Conrad and Allroggen, Niklas and Sprenger, Matthias and Zehe, Erwin and Tronicke, Jens and Weiler, Markus and Blume, Theresa}, year={2017}, month={Jul}, pages={3727–3748} } @article{jackisch_angermann_allroggen_sprenger_blume_tronicke_zehe_2017, title={Form and function in hillslope hydrology: in situ imaging and characterization of flow-relevant structures}, volume={21}, url={https://doi.org/10.5194/hess-21-3749-2017}, DOI={10.5194/hess-21-3749-2017}, abstractNote={Abstract. The study deals with the identification and characterization of rapid subsurface flow structures through pedo- and geo-physical measurements and irrigation experiments at the point, plot and hillslope scale. Our investigation of flow-relevant structures and hydrological responses refers to the general interplay of form and function, respectively. To obtain a holistic picture of the subsurface, a large set of different laboratory, exploratory and experimental methods was used at the different scales. For exploration these methods included drilled soil core profiles, in situ measurements of infiltration capacity and saturated hydraulic conductivity, and laboratory analyses of soil water retention and saturated hydraulic conductivity. The irrigation experiments at the plot scale were monitored through a combination of dye tracer, salt tracer, soil moisture dynamics, and 3-D time-lapse ground penetrating radar (GPR) methods. At the hillslope scale the subsurface was explored by a 3-D GPR survey. A natural storm event and an irrigation experiment were monitored by a dense network of soil moisture observations and a cascade of 2-D time-lapse GPR trenches. We show that the shift between activated and non-activated state of the flow paths is needed to distinguish structures from overall heterogeneity. Pedo-physical analyses of point-scale samples are the basis for sub-scale structure inference. At the plot and hillslope scale 3-D and 2-D time-lapse GPR applications are successfully employed as non-invasive means to image subsurface response patterns and to identify flow-relevant paths. Tracer recovery and soil water responses from irrigation experiments deliver a consistent estimate of response velocities. The combined observation of form and function under active conditions provides the means to localize and characterize the structures (this study) and the hydrological processes (companion study Angermann et al., 2017, this issue). }, number={7}, journal={Hydrology and Earth System Sciences}, publisher={Copernicus GmbH}, author={Jackisch, Conrad and Angermann, Lisa and Allroggen, Niklas and Sprenger, Matthias and Blume, Theresa and Tronicke, Jens and Zehe, Erwin}, year={2017}, month={Jul}, pages={3749–3775} } @article{soulsby_braun_sprenger_weiler_tetzlaff_2017, title={Influence of forest and shrub canopies on precipitation partitioning and isotopic signatures}, volume={31}, url={https://doi.org/10.1002/hyp.11351}, DOI={10.1002/hyp.11351}, abstractNote={AbstractOver a 4‐month summer period, we monitored how forest (Pinus sylvestris) and heather moorland (Calluna spp. and Erica spp.) vegetation canopies altered the volume and isotopic composition of net precipitation (NP) in a southern boreal landscape in northern Scotland. During that summer period, interception losses were relatively high and higher under forests compared to moorland (46% of gross rainfall [GR] compared with 35%, respectively). Throughfall (TF) volumes exhibited marked spatial variability in forests, depending upon local canopy density, but were more evenly distributed under heather moorland. In the forest stands, stemflow was a relatively small canopy flow path accounting for only 0.9–1.6% of NP and only substantial in larger events. Overall, the isotopic composition of NP was not markedly affected by canopy interactions; temporal variation of stable water isotopes in TF closely corresponded to that of GR with differences of TF‐GR being −0.52‰ for δ2H and −0.14‰ for δ18O for forests and 0.29‰ for δ2H and −0.04‰ for δ18O for heather moorland. These differences were close to, or within, analytical precision of isotope determination, though the greater differences under forest were statistically significant. Evidence for evaporative fractionation was generally restricted to low rainfall volumes in low intensity events, though at times, subtle effects of liquid–vapour moisture exchange and/or selective transmission though canopies were evident. Fractionation and other effects were more evident in stemflow but only marked in smaller events. The study confirmed earlier work that increased forest cover in the Scottish Highlands will likely cause an increase in interception and green water fluxes at the expenses of blue water fluxes to streams. However, the low‐energy, humid environment means that isotopic changes during such interactions will only have a minor overall effect on the isotopic composition of NP.}, number={24}, journal={Hydrological Processes}, publisher={Wiley-Blackwell}, author={Soulsby, Chris and Braun, Hannah and Sprenger, Matthias and Weiler, Markus and Tetzlaff, Doerthe}, year={2017}, month={Nov}, pages={4282–4296} } @article{sprenger_tetzlaff_soulsby_2017, title={No influence of CO2 on stable isotope analyses of soil waters with off-axis integrated cavity output spectroscopy (OA-ICOS)}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85011665212&partnerID=MN8TOARS}, DOI={10.1002/rcm.7815}, abstractNote={RationaleIt was recently shown that the presence of CO2 affects the stable isotope (δ2H and δ18O values) analysis of water vapor via Wavelength‐Scanned Cavity Ring‐Down Spectroscopy. Here, we test how much CO2 is emitted from soil samples and if the CO2 in the headspace influences the isotope analysis with the direct equilibration method by Off‐Axis Integrated Cavity Output Spectroscopy (OA‐ICOS).MethodsThe headspace above different amounts of sparkling water was sampled, and its stable isotopic composition (δ2H and δ18O values) and CO2 concentration were measured by direct equilibration and by gas chromatography, respectively. In addition, the headspace above soil samples was analyzed in the same way. Furthermore, the gravimetric water content and the loss on ignition were measured for the soil samples.ResultsThe experiment with the sparkling water showed that CO2 does not influence the stable isotope analysis by OA‐ICOS. CO2 was emitted from the soil samples and correlated with the isotopic fractionation signal, but no causal relationship between the two was determined. Instead, the fractionation signal in pore water isotopes can be explained by soil evaporation and the CO2 can be related to soil moisture and organic matter which both enhance microbial activity.ConclusionsWe found, despite the high CO2 emissions from soil samples, no need for a post‐correction of the pore water stable isotope analysis results, since there is no relation between CO2 concentrations and the stable isotope results of vapor samples obtained with OA‐ICOS. © 2016 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.}, number={5}, journal={Rapid Communications in Mass Spectrometry}, publisher={Wiley-Blackwell}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Soulsby, Chris}, year={2017}, month={Mar}, pages={430–436} } @article{sprenger_tetzlaff_soulsby_2017, title={Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone}, volume={21}, url={https://doi.org/10.5194/hess-21-3839-2017}, DOI={10.5194/hess-21-3839-2017}, abstractNote={Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn long-term experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ2H and δ18O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15–20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate tracer-aided hydrological models either at the plot to the catchment scale. }, number={7}, journal={Hydrology and Earth System Sciences}, publisher={Copernicus GmbH}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Soulsby, Chris}, year={2017}, month={Jul}, pages={3839–3858} } @article{sprenger_tetzlaff_soulsby_2017, title={Stable isotopes reveal evaporation dynamics at the soil-plant-atmosphere interface of the critical zone}, volume={2}, url={https://doi.org/10.5194/hess-2017-87}, DOI={10.5194/hess-2017-87}, abstractNote={Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn long-term experimental catchment in the Scottish Highlands; a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analysed for their isotopic composition (δ2H and δ18H) with the direct equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15–20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate tracer-aided hydrological models either at the plot to the catchment scale. }, publisher={Copernicus GmbH}, author={Sprenger, Matthias and Tetzlaff, Doerthe and Soulsby, Chris}, year={2017}, month={Feb} } @article{sprenger_weiler_2017, title={Tracing Water Through the Critical Zone}, volume={98}, DOI={10.1029/2018eo074313}, abstractNote={The authors of a recent paper in Reviews of Geophysics describe how isotope hydrology offers new insights into interactions at the interface between soil, vegetation, and the atmosphere.}, journal={Eos}, publisher={American Geophysical Union (AGU)}, author={Sprenger, Matthias and Weiler, Markus}, year={2017}, month={Jun} } @article{sprenger_erhardt_riedel_weiler_2016, title={Historical tracking of nitrate in contrasting vineyards using water isotopes and nitrate depth profiles}, volume={222}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84958984676&partnerID=MN8TOARS}, DOI={10.1016/j.agee.2016.02.014}, abstractNote={The European Water Framework Directive (EWFD) aims to achieve a good chemical status for the groundwater bodies in Europe by the year 2015. Despite the effort to reduce the nitrate pollution from agriculture within the last two decades, there are still many groundwater aquifers that exceed nitrate concentrations above the EWFD threshold of 50 mg L⿿1. Viticulture is seen as a major contributor of nitrate leaching and sowing of a green cover was shown to have a positive effect on lowering the nitrate loads in the upper 90 cm of the soil. However, the consequences for nitrate leaching into the subsoil were not yet tested. We analyzed the nitrate concentrations and pore water stable isotope composition (δ 2H) to a depth of 380 cm in soil profiles under an old vineyard and a young vineyard with either soil tillage or permanent green cover in between the grapevines. The pore water δ 2H data was used to calibrate a soil physical model, which was then used to infer the age of the soil water at different depths. This way, we could relate elevated nitrate concentrations below an old vineyard to tillage processes that took place during the winter two years before the sampling. We further showed that the elevated nitrate concentration in the subsoil of a young vineyard can be related to the soil tillage prior to the planting of the new vineyard. If the soil was kept bare due to tillage, a nitrate concentration of 200 kg NO3⿿-N ha⿿1 was found in 290⿿380 cm depth 2.5 years after the set-up of the vineyard. The amount of nitrate leaching was considerably reduced due to a seeded green cover between the grapevines that took up a high share of the mineralized nitrate reducing a potential contamination of the groundwater.}, journal={Agriculture, Ecosystems & Environment}, publisher={Elsevier BV}, author={Sprenger, Matthias and Erhardt, Martin and Riedel, Monika and Weiler, Markus}, year={2016}, month={Apr}, pages={185–192} } @article{sprenger_leistert_gimbel_weiler_2016, title={Illuminating hydrological processes at the soil-vegetation-atmosphere interface with water stable isotopes}, volume={54}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988028235&partnerID=MN8TOARS}, DOI={10.1002/2015rg000515}, abstractNote={AbstractWater stable isotopes (18O and 2H) are widely used as ideal tracers to track water through the soil and to separate evaporation from transpiration. Due to the technical developments in the last two decades, soil water stable isotope data have become easier to collect. Thus, the application of isotope methods in soils is growing rapidly. Studies that make use of soil water stable isotopes often have a multidisciplinary character since an interplay of processes that take place in the vadose zone has to be considered. In this review, we provide an overview of the hydrological processes that alter the soil water stable isotopic composition and present studies utilizing pore water stable isotopes. The processes that are discussed include the water input as precipitation or throughfall, the output as evaporation, transpiration, or recharge, and specific flow and transport processes. Based on the review and supported by additional data and modeling results, we pose a different view on the recently proposed two water world hypothesis. As an alternative to two distinct pools of soil water, where one pool is enriched in heavy isotopes and used by the vegetation and the other pool does not undergo isotopic fractionation and becomes recharge, the water gets successively mixed with newly introduced rainwater during the percolation process. This way, water initially isotopically enriched in the topsoil loses the fractionation signal with increasing infiltration depth, leading to unfractionated isotopic signals in the groundwater.}, number={3}, journal={Rev. Geophys.}, publisher={Wiley-Blackwell}, author={Sprenger, Matthias and Leistert, Hannes and Gimbel, Katharina and Weiler, Markus}, year={2016}, pages={674–704} } @article{jackisch_angermann_allroggen_sprenger_blume_weiler_tronicke_zehe_2016, title={In situ investigation of rapid subsurface flow: Identification of relevant spatial structures beyond heterogeneity}, volume={5}, url={https://doi.org/10.5194/hess-2016-190}, DOI={10.5194/hess-2016-190}, abstractNote={Abstract. Rapid subsurface flow in structured soils facilitates fast vertical and lateral redistribution of event water. Despite their significance and omnipresence the related processes are challenging hydrological exploration, monitoring, modeling and theory. One reason for this is that flow processes at high velocities are difficult to observe in the subsurface. Another reason is that advective flow is channeled in distinct connected structures several orders of magnitude smaller than commonly resolved observation volumes. This is the second part of a companion paper with a focus on \\textit{in situ} experimental exploration of rapid subsurface flow. Complementary to the temporal dynamics, this study looks into the identification of spatially organized structures. We present a bottom-up approach with point-scale measurements, plot-scale multi-tracer experiments and a hillslope-scale irrigation experiment. Special emphasis is given to the employed 2D and 3D time-lapse ground penetrating radar monitoring under field conditions on forested, young soils on periglacial slope deposits. The study highlights the difficulty to draw conclusions beyond overall heterogeneity from point observations in a basically unknown and structured domain. We also spotlight the challenge to identify relevant structures based on a single quasi-static exploration. A coherent combination of different hydrological and geophysical methods to monitor the system under driven conditions was key to reduce ambiguity in the identification of hydrologically relevant structures and the overall process understanding. }, publisher={Copernicus GmbH}, author={Jackisch, Conrad and Angermann, Lisa and Allroggen, Niklas and Sprenger, Matthias and Blume, Theresa and Weiler, Markus and Tronicke, Jens and Zehe, Erwin}, year={2016}, month={May} } @article{angermann_jackisch_allroggen_sprenger_zehe_tronicke_weiler_blume_2016, title={In situ investigation of rapid subsurface flow: Temporal dynamics and catchment-scale implication}, volume={5}, url={https://doi.org/10.5194/hess-2016-189}, DOI={10.5194/hess-2016-189}, abstractNote={Abstract. Preferential flow is omnipresent in natural systems. It links multiple scales from single pores to entire hillslopes and potentially influences the discharge dynamics of a catchment. However, there is still a lack of appropriate monitoring techniques and thus, process understanding. In this study, a promising combination of 2D time-lapse ground-penetrating radar (GPR) and soil moisture monitoring was used to observe preferential flow processes in highly structured soils during a hillslope-scale irrigation experiment. The 2D time-lapse GPR data were interpreted using structural similarity attributes, highlighting changes between individual time-lapse measurements. These changes are related to soil moisture variations in the subsurface. In combination with direct measurements of soil moisture, the spatial and temporal characteristics of the resulting patterns can give evidence about subsurface flow processes. The response dynamics at the hillslope were compared to the runoff response behavior of the headwater catchment. The experiment revealed a fast establishment of hillslope-scale connectivity despite unsaturated conditions, with high response velocities of up to 10−3 m s−1 or faster, and a high portion of mobile water. These processes substantially impact the overall catchment response behavior. While the presented approach is a good way to observe the temporal dynamics and general patterns, the spatial characteristics of small-scale preferential flow path could not be fully resolved. }, publisher={Copernicus GmbH}, author={Angermann, Lisa and Jackisch, Conrad and Allroggen, Niklas and Sprenger, Matthias and Zehe, Erwin and Tronicke, Jens and Weiler, Markus and Blume, Theresa}, year={2016}, month={May} } @article{sprenger_seeger_blume_weiler_2016, title={Travel times in the vadose zone: Variability in space and time}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84980337504&partnerID=MN8TOARS}, DOI={10.1002/2015wr018077}, abstractNote={AbstractWater 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.}, number={8}, journal={Water Resour. Res.}, publisher={Wiley-Blackwell}, author={Sprenger, Matthias and Seeger, Stefan and Blume, Theresa and Weiler, Markus}, year={2016}, month={Jun}, pages={5727–5754} } @article{sprenger_herbstritt_weiler_2015, title={Established methods and new opportunities for pore water stable isotope analysis}, volume={29}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84956594756&partnerID=MN8TOARS}, DOI={10.1002/hyp.10643}, abstractNote={AbstractThe 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 (18O and 2H) 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.}, number={25}, journal={Hydrol. Process.}, publisher={Wiley-Blackwell}, author={Sprenger, Matthias and Herbstritt, Barbara and Weiler, Markus}, year={2015}, month={Sep}, pages={5174–5192} } @article{sprenger_volkmann_blume_weiler_2015, title={Estimating flow and transport parameters in the unsaturated zone with pore water stable isotopes}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84930960378&partnerID=MN8TOARS}, DOI={10.5194/hess-19-2617-2015}, abstractNote={Abstract. Determining the soil hydraulic properties is a prerequisite to physically model transient water flow and solute transport in the vadose zone. Estimating these properties by inverse modelling techniques has become more common within the last 2 decades. While these inverse approaches usually fit simulations to hydrometric data, we expanded the methodology by using independent information about the stable isotope composition of the soil pore water depth profile as a single or additional optimization target. To demonstrate the potential and limits of this approach, we compared the results of three inverse modelling strategies where the fitting targets were (a) pore water isotope concentrations, (b) a combination of pore water isotope concentrations and soil moisture time series, and (c) a two-step approach using first soil moisture data to determine water flow parameters and then the pore water stable isotope concentrations to estimate the solute transport parameters. The analyses were conducted at three study sites with different soil properties and vegetation. The transient unsaturated water flow was simulated by solving the Richards equation numerically with the finite-element code of HYDRUS-1D. The transport of deuterium was simulated with the advection-dispersion equation, and a modified version of HYDRUS was used, allowing deuterium loss during evaporation. The Mualem–van Genuchten and the longitudinal dispersivity parameters were determined for two major soil horizons at each site. The results show that approach (a), using only the pore water isotope content, cannot substitute hydrometric information to derive parameter sets that reflect the observed soil moisture dynamics but gives comparable results when the parameter space is constrained by pedotransfer functions. Approaches (b) and (c), using both the isotope profiles and the soil moisture time series, resulted in good simulation results with regard to the Kling–Gupta efficiency and good parameter identifiability. However, approach (b) has the advantage that it considers the isotope data not only for the solute transport parameters but also for water flow and root water uptake, and thus increases parameter realism. Approaches (b) and (c) both outcompeted simulations run with parameters derived from pedotransfer functions, which did not result in an acceptable representation of the soil moisture dynamics and pore water stable isotope composition. Overall, parameters based on this new approach that includes isotope data lead to similar model performances regarding the water balance and soil moisture dynamics and better parameter identifiability than the conventional inverse model approaches limited to hydrometric fitting targets. If only data from isotope profiles in combination with textural information is available, the results are still satisfactory. This method has the additional advantage that it will not only allow us to estimate water balance and response times but also site-specific time variant transit times or solute breakthrough within the soil profile. }, number={6}, journal={Hydrol. Earth Syst. Sci.}, publisher={Copernicus GmbH}, author={Sprenger, M. and Volkmann, T. H. M. and Blume, T. and Weiler, M.}, year={2015}, pages={2617–2635} } @article{sprenger_oelmann_weihermüller_wolf_wilcke_potvin_2013, title={Tree species and diversity effects on soil water seepage in a tropical plantation}, volume={309}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84884816813&partnerID=MN8TOARS}, DOI={10.1016/j.foreco.2013.03.022}, abstractNote={Plant diversity has been shown to influence the water cycle of forest ecosystems by differences in water consumption and the associated effects on groundwater recharge. However, the effects of biodiversity on soil water fluxes remain poorly understood for native tree species plantations in the tropics. Therefore, we estimated soil water fluxes and assessed the effects of tree species and diversity on these fluxes in an experimental native tree species plantation in Sardinilla (Panama). The study was conducted during the wet season 2008 on plots of monocultures and mixtures of three or six tree species. Rainfall and soil water content were measured and evapotranspiration was estimated with the Penman-Monteith equation. Soil water fluxes were estimated using a simple soil water budget model considering water input, output, and soil water and groundwater storage changes and in addition, were simulated using the physically based one-dimensional water flow model Hydrus-1D. In general, the Hydrus simulation did not reflect the observed pressure heads, in that modeled pressure heads were higher compared to measured ones. On the other hand, the results of the water balance equation (WBE) reproduced observed water use patterns well. In monocultures, the downward fluxes through the 200 cm-depth plane were highest below Hura crepitans (6.13 mm day−1) and lowest below Luehea seemannii (5.18 mm day−1). The average seepage rate in monocultures (±SE) was 5.66 ± 0.18 mm day−1, and therefore, significantly higher than below six-species mixtures (5.49 ± 0.04 mm day−1) according to overyielding analyses. The three-species mixtures had an average seepage rate of 5.63 ± 0.12 mm day−1 and their values did not differ significantly from the average values of the corresponding species in monocultures. Seepage rates were driven by the transpiration of the varying biomass among the plots (r = 0.61, p = 0.017). Thus, a mixture of trees with different growth rates resulted in moderate seepage rates compared to monocultures of either fast growing or slow growing tree species. Our results demonstrate that tree-species specific biomass production and tree diversity are important controls of seepage rates in the Sardinilla plantation during the wet season.}, journal={Forest Ecology and Management}, publisher={Elsevier BV}, author={Sprenger, Matthias and Oelmann, Yvonne and Weihermüller, Lutz and Wolf, Sebastian and Wilcke, Wolfgang and Potvin, Catherine}, year={2013}, pages={76–86} }