Groundwater levels in steep headwater catchments typically respond quickly to rainfall, but the timing of the response may vary spatially across the catchment. In this study, we investigated the ...topographic controls and the effects of rainfall and antecedent conditions on the groundwater response timing for 51 groundwater monitoring sites in a 20‐ha pre‐alpine catchment with low permeability soils. The median time to rise and median duration of recession for the 133 rainfall events were highly correlated to the topographic characteristics of the site and its upslope contributing area. The median time to rise depended more on the topographic characteristics than on the rainfall characteristics or antecedent soil wetness conditions. The median time to rise decreased with Topographic Wetness Index (TWI) for sites with TWI < 6 and was almost constant for sites with a higher TWI. The slope of this relation was a function of rainfall intensity. The rainfall threshold for groundwater initiation was also a function of TWI and allowed extrapolation of point measurements to the catchment scale. The median lag time between the rainfall centroid and the groundwater peak was 75 min. The groundwater level peaked before peak streamflow at the catchment outlet for half of the groundwater monitoring sites, but only by 15 to 25 min. The stronger correlations between topographic indices and groundwater response timing in this study compared to previous studies suggest that surface topography affects the groundwater response timing in catchments with low permeability soils more than in catchments with more transmissive soils.
Tree water uptake processes and ecohydrological travel times have gained more attention in recent ecohydrological studies. In situ measurement techniques for stable water isotopes offer great ...potential to investigate these processes but have not been applied much to tree xylem and soils so far. Here, we used in situ probes for stable water isotope measurements to monitor the isotopic signatures of soil and tree xylem water before and after two deuterium-labeled irrigation experiments. To show the potential of the method, we tested our measurement approach with 20-year-old trees of three different species (Pinus pinea, Alnus incana and Quercus suber). They were planted in large pots with homogeneous soil in order to have semi-controlled experimental conditions. Additional destructive sampling of soil and plant material allowed for a comparison between destructive (cryogenic vacuum extraction and direct water vapor equilibration) and in situ isotope measurements. Furthermore, isotope-tracer-based ecohydrological travel times were compared to travel times derived from sap flow measurements. The time to first arrival of the isotope tracer signals at 15 cm stem hight were ca. 17 h for all tree species and matched well with sap-flow-based travel times. However, at 150 cm stem height tracer-based travel times differed between tree species and ranged between 2.4 and 3.3 d. Sap-flow-based travel times at 150 cm stem hight were ca. 1.3 d longer than tracer-based travel times. The isotope signature of destructive and in situ isotope measurements differed notably, which suggests that the two types of techniques sampled water from different pools. In situ measurements of soil and xylem water were much more consistent between the three tree pots (on average standard deviations were smaller by 8.4 ‰ for δ2H and by 1.6 ‰ for δ18O for the in situ measurements) and also among the measurements from the same tree pot in comparison to the destructive methods (on average standard deviations were smaller by 7.8 ‰ and 1.6 ‰ for δ2H and δ18O, respectively). Our study demonstrates the potential of semi-controlled large-scale pot experiments and very frequent in situ isotope measurements for monitoring tree water uptake and ecohydrological travel times. It also shows that differences in sampling techniques or sensor types need to be considered when comparing results of different studies and within one study using different methods.
While the concept of connectivity has gained popularity in fields like hydrology and ecology, little agreement exists on its definition, which hinders its use in both scientific and legal contexts. ...In contrast, neuroscientists have developed not only strong conceptualizations of connectivity but also tools to quantify it: a clear distinction is made between structural connectivity, which is determined from brain anatomy; functional connectivity, which is estimated based on statistical dependencies between neuronal electric timeseries; and effective connectivity, which infers causal relations from the same timeseries based on the assumption that “true” interactions occur with a certain time delay. The motivation of this review arose from the hypothesis that connectivity-related statistical techniques, which are applied to timeseries of electrical currents measured by placing electrodes on the scalp of the human brain, could also apply to high-frequency hydrological timeseries acquired to characterize catchment response to precipitation. Here we bring together existing conceptualizations of structural, functional and effective connectivity in hydrology and ecology and compare them with those used in brain neuroscience. We then summarize the most important brain connectivity measures and their associated mathematical frameworks before evaluating the potential of those measures to help advance our understanding of hydrologic connectivity properties – in terms of the frequency, magnitude, timing, duration and rate of water movement linking two disparate locations. Lastly, we present a short case study where a selection of brain connectivity measures is applied to 35 groundwater and streamflow timeseries from a Swiss catchment to infer subsurface flow-driven hydrologic connectivity. Our literature review combined with our short case study suggest that an ensemble of functional and effective connectivity measures should be used and constrained not only by structural connectivity measures but also by interpretation thresholds in order to make results parsimonious. We highlight challenges associated with transferring brain connectivity measures to hydrology, especially those related to choosing the appropriate length and sampling frequency of input timeseries when assessing perennial versus ephemeral connectivity, appropriately detecting and differentiating noisy from indirect connections, and interpreting unbounded connectivity measures. We then offer recommendations for future research and propose that hydrologists use a common classification system encompassing all potential connectivity assessment approaches and measures in order to facilitate scientific communication.
Groundwater levels are typically measured at only a limited number of points in a catchment. Thus, upscaling these point measurements to the catchment scale is necessary to determine subsurface flow ...paths and runoff source areas. Here we present a data‐driven approach composed of time series clustering and topography‐based upscaling of shallow, perched groundwater dynamics using groundwater data from 51 monitoring sites in a 20‐ha prealpine headwater catchment in Switzerland. The agreement between the upscaled (modeled) and measured groundwater dynamics was strong for most of the 19‐month study period for the upslope and footslope locations but weaker at the beginning of events and for the midslope locations. However, these differences between measured and modeled groundwater levels did not significantly affect modeled groundwater activation, that is, the time when groundwater levels were within the more transmissive soil layers near the soil surface. The resulting groundwater activation maps represent the groundwater response across the catchment and highlight the dynamic expansion and contraction of the subsurface runoff source areas, particularly along the channel network. This is in agreement with the variable source area concept. However, there were also isolated active zones that did not get connected to the stream during rainfall events, highlighting the need to distinguish between variable active and variable stream‐connected runoff source areas. Our data‐driven approach to upscale point measurements of shallow groundwater levels appears useful for studying catchment‐scale variations in groundwater storage and connectivity and thus may help to better understand runoff generation in mountain catchments.
Plain Language Summary
For a better understanding of how runoff in streams is generated, we need to know how groundwater levels respond across a catchment. However, groundwater can usually only be measured at a few selected points, and interpolation between these points does often not result in realistic groundwater response patterns. Here we present a data‐driven approach based on groundwater level data from 51 sites in a catchment in Switzerland for a 19‐month study period. We grouped the monitoring sites into six clusters with similar groundwater level dynamics. We then determined the topographic characteristics of the sites in each cluster and assigned the average relative groundwater level for the monitoring sites in a cluster to all other sites in the catchment with similar topographic characteristics. By doing so, we created sequences of maps of groundwater levels across the entire study catchments. These maps show an expansion and contraction of the areas where the groundwater level is close to the surface and which of these areas are connected to the stream channels. These maps are useful to identify from which parts of the catchment streamwater may come during a rain event, which helps to improve our understanding of runoff generation processes.
Key Points
We present a data‐driven approach composed of time series clustering and topography‐based upscaling of groundwater data to the catchment
The groundwater activation maps highlight the dynamic expansion and contraction of subsurface runoff source areas along the channel network
Results agree with the variable source area concept, but isolated groundwater response areas were not hydrologically connected to the stream
The 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.
Plain language Summary
Investigating how long it takes for a drop of rainwater until it is either evaporated back to the atmosphere, taken up by plants, or infiltrated into groundwater or discharged in streams provides new understanding of how water flows through the water cycle. Knowledge about the time water travels further helps assessing groundwater recharge, transport of contaminants, and weathering rates. Such water age studies typically focus either on water in individual compartments of the water cycle such as soils, groundwater, or stream runoff. But we argue that the interfaces between these compartments can have an influence on the water age. Here, we present methods how water ages can be estimated using tracers and hydrological models. We further discuss the “demographics of water” (water age distribution) in the critical zone that spans from the tree canopy to the bottom of the groundwater. Our review highlights how water flows and mixes between plants, soils, groundwater, and streams and how this interaction affects the water ages. This way, our work contributes toward a better understanding of vital resource water sustaining the life in the Earth's living skin.
Key Points
New tracer techniques now allow tracking water at high spatiotemporal resolution across the vastly varying water ages in the water cycle
Exchanges of water between hydrological compartments at key interfaces influence the water age distribution more than previously assumed
Variation from complete to nearly absent mixing of water at the interfaces in the critical zone affects the water ages in compartments
Methodological advancements have been made in in situ observations of water stable isotopes that have provided valuable insights into ecohydrological processes. The continuous measurement ...capabilities of laser-based analyzers allow for high temporal resolutions and non-destructive minimally invasive study designs of such in situ approaches. However, isotope analyzers are expensive, heavy, and require shelter and access to electrical power, which impedes many in situ assays. Therefore, we developed a new inexpensive technique to collect discrete water vapor samples in the field via diffusion-tight inflatable bags that can later be analyzed in the lab. In a series of structured experiments, we tested different procedural settings, bag materials, and closure types for diffusion tightness during storage as well as for practical handling during filling and extraction. To facilitate reuse of sampling bags, we present a conditioning procedure using ambient air as primer. In order to validate our method, direct measurements through hydrophobic in situ probes were compared to repeated measurements of vapor sampled with our bags from the same source. All steps are summarized in a detailed standard operating procedure (SOP). This procedure represents the preparation and measurement of calibration and validation vapor standards necessary for processing of unknown field-collected vapor samples in the foreseen application. By performing pertinent calibration procedures, accuracy was better than 0.4 ‰ for δ18O and 1.9 ‰ for δ2H after 1 d of storage. Our technique is particularly suitable when used in combination with minimally invasive water vapor sampling in situ probes that have already been employed for soils and tree xylem. It is an important step towards minimally invasive monitoring of stable isotope distributions and also time series in virtually undisturbed soils and trees without the need to have an analyzer in the field. It is therefore a promising tool for many applications in ecohydrology and meteorology.
Abstract
For a better understanding of plant nutrition processes, it is important to study the flux of nutrients within plants. However, existing xylem sap sampling methods are typically destructive ...and do not allow for repeated, highly frequent measurements of nutrient concentration. In this paper, we present a novel use of microdialysis (MD) for characterizing xylem sap phosphate (PO43−) concentration as a possible alternative to destructive sampling. First, MD probes were tested under laboratory conditions in vitro, in a stirred solution test, and in vivo, using beech tree stem segments. Exponential decline in the relative recovery (RR) with an increasing MD pumping rate allows for determining an optimal sampling interval (i.e., the maximum amount of sample volume with the minimum required concentration). The RR changed only minimally, with a change in the simulated sap flow velocity during the in vivo stem segment test. This suggests that MD can be applied over a range of naturally occurring sap flow velocities. Differences in the ionic strength between the xylem sap and the perfusate pumped through the MD did not influence the RR. Then, MD was successfully applied in a 24 h field campaign in two beech trees of different ages and allowed for in situ assessments of the diurnal variation of PO43− concentration and (together with xylem flow measurements) flux variability in living trees. Both beech trees exhibited the same diurnal pattern in PO43− concentrations with higher concentrations in the younger tree. The xylem PO43− concentration measured with MD was in the same order of magnitude as that received through destructive sampling in the younger tree. The MD probes did not show a decline in RR after the field application. We showed that MD can be applied to capture the PO43− concentration dynamics in the xylem sap with bihourly resolution under field conditions.
Information about catchment‐scale groundwater dynamics is necessary to understand how catchments store and release water and why water quantity and quality varies in streams. However, groundwater ...level monitoring is often restricted to a limited number of sites. Knowledge of the factors that determine similarity between monitoring sites can be used to predict catchment‐scale groundwater storage and connectivity of different runoff source areas. We used distance‐based and correlation‐based similarity measures to quantify the spatial and temporal differences in shallow groundwater similarity for 51 monitoring sites in a Swiss prealpine catchment. The 41 months long time series were preprocessed using Dynamic Time‐Warping and a Flow‐corrected Time Transformation to account for small timing differences and bias toward low‐flow periods. The mean distance‐based groundwater similarity was correlated to topographic indices, such as upslope contributing area, topographic wetness index, and local slope. Correlation‐based similarity was less related to landscape position but instead revealed differences between seasons. Analysis of variance and partial Mantel tests showed that landscape position, represented by the topographic wetness index, explained 52% of the variability in mean distance‐based groundwater similarity, while spatial distance, represented by the Euclidean distance, explained only 5%. The variability in distance‐based similarity and correlation‐based similarity between groundwater and streamflow time series was significantly larger for midslope locations than for other landscape positions. This suggests that groundwater dynamics at these midslope sites, which are important to understand runoff source areas and hydrological connectivity at the catchment scale, are most difficult to predict.
Key Points
Distance‐based similarity captured spatial differences, correlation‐based similarity temporal differences in groundwater dynamics
Landscape position explained a larger portion of the variability in distance‐based groundwater similarity than spatial distance
Topographic indices were good predictors of groundwater similarity and allow upscaling groundwater dynamics to the catchment scale
pCatchment response to precipitation is often investigated using two-component isotope-based hydrograph separation, which quantifies the contribution of precipitation (i.e., event water Q.sub.e) or ...water from storage (i.e., pre-event water Q.sub.pe) to total discharge (Q) during storm events. In order to better understand streamflow-generating mechanisms, two-component hydrograph separation studies often seek to relate the event-water fraction Q.sub.e /Q to storm characteristics or antecedent wetness conditions. However, these relationships may be obscured because the same factors that influence Q.sub.e also necessarily influence total discharge Q as well. Here we propose that the fractions of event water and pre-event water relative to total precipitation (Q.sub.e /P and Q.sub.pe /P), instead of total discharge, provide useful alternative tools for studying catchment storm responses. These two quantities separate the well-known runoff coefficient (Q/P, i.e., the ratio between total discharge and precipitation volumes over the event timescale) into its contributions from event water and pre-event water. Whereas the runoff coefficient Q/P quantifies how strongly precipitation inputs affect streamflow, the fractions Q.sub.e /P and Q.sub.pe /P track the sources of this streamflow response.
Understanding groundwater recharge processes is important for sustainable water resource management. Experimental approaches to study recharge in
karst areas often focus on analysing the aquifer ...response using a disintegration of its outlet signals, but only a few approaches directly investigate the
recharge processes that occur at the surface of the system. Soil moisture measurements have a high potential to investigate water infiltration to
deeper soil depth or epikarst with an easy and not too intrusive installation. They can yield long-term measurements with high temporal
resolution. Using these advantages, we developed and tested a method to estimate recharge based on soil moisture measurements. The method consists
of the extraction of linked events in rainfall, soil moisture, and discharge time series, as well as a subsequent fitting of the parameters of a simple
drainage model to calculate karst recharge from soil moisture metrics of individual events. The fitted parameters could be interpreted in physically
meaningful terms and were related to the properties of the karstic system. The model was tested and validated in a karst catchment located in
southwest Germany with hourly precipitation, soil moisture, and discharge data of 8 years duration. The soil moisture measurements were
distributed among grassland (n = 8) and woodland areas (n = 7) at 20 cm depth. A threshold of about 35 % (± 8 %) of
volumetric water content was necessary to initiate effective infiltration. Soil moisture averaged during the wetting period of each event was the
best metric for the prediction of recharge. The model performed reasonably well, estimating recharge during single rainfall events. It was also
capable of simulating 88 % of the average annual recharge volume despite considerable differences in the performance between years. The
event-based approach is potentially applicable to other karstic systems where soil moisture and precipitation measurements are available to predict
karst groundwater recharge.
PDF
