Isotopic hydrograph separation in two small mountain catchments during multiple events

Environmental isotopes, such as oxygen-18 and deuterium, have been used increasingly to separate stormflow into its event and pre-event components in order to elucidate the sources, pathways and residence times of water in drainage basins. The majority of isotopic hydrograph separations indicate that pre-event water supplies at least 50% of streamflow at peak discharge in small- and medium-sized basins; however, there is no consensus as to the means by which pre-event water is rapidly exported from drainage basins. The hydrological processes that have been invoked to explain the observed isotopic response of streamflow to rainfall and snowmelt inputs in various environments are reviewed. These processes include groundwater ridging, translatory flow, macropore flow, saturation overland flow, kinematic waves and release of water from surface storage. Tests of the ability of the hypothesized mechanisms to explain the isotopic signature of stormflow from drainage basins will require a more complete integration of hydrometric methods with the use of environmental isotopes than has been achieved previously. Along with various methodological issues associated with the isotopic hydrograph separation technique, the overall relevance of these hydrograph separations to the understanding and prediction of stream hydrochemistry must be evaluated critically.

Scientific limnological research in the Tatra Mountains were initiated by Stanislaw Staszic in the early XIX century. After the World War II, the evolution of Tatra lakes was investigated by Kondracki, Klimaszewski, Baumgart-Kotarba and. Extensive paleolimnological investigations in the Tatra Mountains were started by the group of scientists led by K. Starmach in the beginning of the second half of the 20 th century. There has been not much research concerned to the regional distribution of lakes and their properties in the Tatra Mountains (Pociask-Karteczka 2013). Very early division of lakes presented A. Gadomski (1922), which distinguished four types of lakes: a) tarns (cirque lake or corrie loch), b) bedrock-dammed lakes, c) moraine lakes. This division was concerned in subsequent publications (Choinski 2007). M. Luknis (1973, 1985) recognized additional types: kettles and landslide-dammed lakes and M. Klimaszewski (1988) – inter-sheepback lakes. J. Pacl and K. Wit-Joźwik in Klima Tatier (Pacl, Wit-Joźwik 1974) were focused on the temperature of water in lakes in Polish and Slovak parts and M. Borowiak (2000a,b) provided a comprehensive analysis of types, dimensions, temperature and chemical composition of water in lakes in the Tatra Mountains. According to present day state of knowledge, one may distinguish following genetic types of lakes: I) glacial, II) not-glacial. There are four types of the glacial origin lakes in the Tatra Mountains (Fig. 1): a) tarns (cirque lakes or corrie loch), b) bedrock-moraine dammed lakes, c) inter-sheepback lakes, d) moraine lakes, e) kettles. Most of lakes in the Tatra Mountains are tarns and bedrock-moraine dammed lakes, and they are located at the elevation over 1400 m a.s.l. in the Western Tatra Mountains, and over 1600 m a.s.l. in the High Tatra Mountains. Some of them are paternoster lakes – a series of stair-stepped lakes formed in individual rock basins aligned down the course of a glaciated valley. Lakes in the Five Polish Lakes Valley is an example of such type. The inter-sheepback lakes occur at higher elevations (e.g. Wyznie Mnichowe Stawki, Zamrznute oka). There is a little number of moraine lakes in the Tatra Mountains (e.g. Smreczynski, Toporowy Nizni) and kettles (e.g. Strbske Pleso, Kotlinowy Stawek). Some lakes are located among rock debris or rock debris and moraine material (Dwoisty Staw Gąsienicowy, Anitino ocko) – they are of polygenetic origin.

Regional flow duration curve estimation in small ungauged catchments using instantaneous flow measurements and a censored data approach

Understanding how streamflow is generated is essential for managing water quantity, quality, aquatic ecosystems and drinking water resources. Stable water isotopes are widely used to separate streamflow into event and pre-event water, offering valuable insights into catchment storage dynamics, water ages and transit times of water. Typically, precipitation or stream water for isotope analysis are sampled using autosamplers to reduce effort and ensure an adequate temporal resolution.However, hydrograph separation using stable isotopes is often limited by reliance on single-point rainfall sampling, which therefore assumes that precipitation inputs are spatially uniform across the catchment. This can introduce substantial errors, as spatial variability in the isotopic composition of precipitation—even within small catchments—may lead to misestimations of event water endmember contributions. Also, the various transit time models may experience biases due to an incorrect precipitation input time series of stable isotopes. Furthermore, typical autosamplers are susceptible to evaporative losses from the stored water samples, resulting in isotopic fractionation and compromised data integrity.To solve these difficulties, we have developed and deployed the low-cost, evaporation-proof Portable Autosampler for Liquids (PAUL). Nine PAUL units were distributed across the 1.5 km² Krummenbach sub-catchment of the Brugga watershed, a mountainous headwater catchment located in the Black Forest, Germany. Eight units measured precipitation and one sampled streamflow, with biweekly collection over the course of one month.Our findings show that spatially distributed, evaporation-secure sampling significantly improves the characterization of event water inputs and reduces uncertainty in hydrograph separation. The PAUL system provides a robust and accessible solution for high-resolution, catchment-scale isotope monitoring, providing spatial and temporal coverage that was previously unfeasible with standard autosamplers. This approach advances process-based hydrology by increasing the accuracy and reliability of isotope-based precipitation and streamflow analyses.

Exploring the contributions of new and old water to runoff during precipitation events in agricultural catchments is essential for understanding runoff generation, solute transport, and soil erosion. The aim of this study was to compare two isotope hydrograph separation methods in the Hydrological Open Air Laboratory (HOAL) in Austria, a 66-ha large experimental catchment dominated by agricultural land use. The classical two-component (IHS) and the ensemble isotope hydrograph separation (EIHS) methods were applied to multiple events in May-October of 2013-2018 using δ18O and δ2H. The new water contributions obtained by the IHS during peak flow were compared with the average new water fraction from the EIHS. The results showed that EIHS provided average new water fractions during peak flows (0.46 for δ18O and 0.47 for δ2H) that were close to the averages obtained by IHS (0.48 for δ18O and 0.50 for δ2H). While the EIHS may be a more robust approach compared to IHS, as it relaxes some of the assumptions of IHS and it gives a reliable average of the new water contribution, the IHS can provide useful information on the new water contribution variability for individual events.

Runoff components of the Zastler catchment (18\4 km2, southern Black Forest, Germany) were analysed with hydrograph separations using stable oxygen isotopes and dissolved silica. It was shown that event water and components with low silica contributed only small amounts to total runoff. In addition, comparison of the two-component hydrograph separations showed that the low-silica components are generated by both event water and pre-event water fractions, depending on the state of the system. A modified three-component hydrograph separation method was introduced using dissolved silica and 18O. During storm events an interaction of three runoff components having distinct silica concentrations could be shown. Based on the geological and geomorphological genesis of the study site, it was appropriate to assign (i) the low silica component to the riparian zones and impermeable areas, (ii) the medium silica component to the periglacial debris cover and (iii) the high silica component to the crystalline detritus and crystalline hard rock. Exact quantification of the runoff components remained difficult. However, runoff components with medium silica concentrations reacted very sensitively and intensely. The contribution of this component to total runoff is comparatively large. This shows the important role of the periglacial debris to runoff generation of the study site and emphasizes the importance of runoff generation processes occurring in this reservoir. Copyright © 2000 John Wiley & Sons, Ltd.

This study investigates the temporal variation of stable isotopic composition in precipitation, soil water, and streamflow water during the plum rainfall events in an upland headwater watershed which is mainly covered with bamboo. The results show that the isotopic composition of various water sources exhibit significant temporal variation. The local meteoric water line is established by using the relationship between the stable isotope of oxygen-18 and deuterium, which is slightly different from the meteoric water line of China. The isotopic temporal variation of precipitation is closely related to exchange effect between raindrops and environmental vapor, evaporation fractionation and rainfall intensity. The isotopic variation of shallow soil water is mainly determined by canopy interception, ground evaporation and the mixing with pre-event water; as for the isotopic variation of deep soil water, it is virtually influenced by pre-event water. The most enriched isotopic composition of streamflow and deuterium excess (d-excess) differences between streamflow and rainfall both indicate that streamflow is recharged not only by event water but also by pre-event water. Hence, a better understanding of precipitation formation and the hydrological response under the plum rainfall system may be instructive for the management of water resources in humid watersheds in southern China.

The “Dynamic SlowDown” concept (noted hereafter “DSD") promotes flood mitigation by slowing down, using transverse obstacles or temporary storage works distributed throughout the catchment. In accordance with Sustainable Development, it aims at reducing the use of the flood defence structures that damage riverine ecosystems. DSD hydraulic projects must be planned at catchment scale and must take all the problems into account, including ecological issues. Cemagref and Cracow Institute of Water Engineering carried out a DSD feasibility study in a small mountainous catchment in Poland, the Isepnica catchment. To deal with flood and erosion problems, they proposed small storage works on hillslopes and a dry reservoir. To assess their impact on design floods, they developed two simple distributed hydrologic models, and linked them with a hydraulic model built for the main torrent. They concluded that the proposed strategy was efficient. In a further stage, it is envisaged to build and monitor pilot structures, in order to check their behaviour, better calibrate the models, and issue maintenance requirements.

Storm flow in forested basins on the Canadian Shield is largely supplied by subsurface water; however, mechanisms by which this water reaches the stream remain unclear. Side slope contributions to storm flow were studied using throughflow trenches on slopes in a headwater basin near Dorset, Ontario. Discharge, soil water content, and chemical and isotopic signatures of subsurface water were monitored at each site. Four hypotheses were tested: (1) most flow occurs at the soil‐bedrock interface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathways and laterally over the bedrock surface; (3) relative preevent water contribution to subsurface flow on shield slopes is a function of soil thickness; and (4) a significant portion of event water flux in storm flow from forested basins with shallow soil cover is supplied from side slopes via subsurface flow along the soil‐bedrock interface. Hypothesis 1 was confirmed from hydrometric observations during spring and fall rainstorms. Hypotheses 2 and 3 were supported by temporal trends in dissolved organic carbon and 18O in flow at the soil‐bedrock interface and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux from the basin in excess of that attributable to direct precipitation onto near‐channel saturated areas could be supplied by flow along the bedrock surface (hypothesis 4). Flow at the soil‐bedrock interface on side slopes also contributed ∼25% of preevent water flux from the basin. Much of the event water component of basin storm flow may travel considerable distances via subsurface routes and is not necessarily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes little interaction with the soil during its passage downslope may be unwarranted here.

This study demonstrates the effectiveness of a multi-objective validation approach for a distributed hydrological model in a high mountain river basin. Focusing on the Baksan River Basin in the Central Caucasus, where snow and glacier melt play a crucial role in runoff formation, we applied the ECOMAG model, which has proven its reliability in high-altitude hydrology. To enhance the validation accuracy, we integrated diverse data sources, including observed river discharge, MODISderived snow cover, stable isotope hydrograph separation, glacier mass balance observations, and glacial runoff simulations from the A-Melt model. The results confirm the high performance of the model across multiple hydrological components. The simulated and observed discharge values showed strong agreement, with the Nash-Sutcliffe efficiency exceeding 0.8 for both the calibration and validation periods. The model successfully captured seasonal snow cover variations, achieving an R² of 0.85 when compared with the MODIS data. Isotopic hydrograph separation further validated the accuracy of the simulated meltwater and rainfall contributions to runoff. Although glacier ablation simulations showed some deviations, particularly for the Djankuat Glacier, these findings highlight opportunities for refining glacial process representation. Overall, this study confirms the robustness and applicability of multi-objective validation for hydrological modeling in complex mountainous regions. The integration of multiple observational datasets significantly enhances the reliability of modeling results, providing valuable insights into water resource management, climate impact assessments, and sustainable development in glacier-fed river basins.

Climate change is affecting the discharge of headstreams from mountainous areas on the Qinghai–Tibet Plateau. To constrain future changes in discharge, it is important to understand the present-day formation mechanism and components of runoff in the basin. Here we explore the sources of runoff and spatial variations in discharge through measurements of δ2H and δ18O in the Naqu River, at the source of the Nu River, on the Qinghai–Tibet plateau, during the month of August from 2016 to 2018. We established thirteen sampling sites on the main stream and tributaries, and collected 39 samples from the river. We examined all the water samples and analyzed them for isotopes. We find a significant spatial variation trend based on one-way analysis of variance (ANOVA) (p < 0.05) between Main stream-2 and tributaries. The local meteoric water-line (LMWL) can be described as: δ2H = 7.9δ18O + 6.29. Isotopic evaporative fractionation in water and mixing of different water sources are responsible for the spatial difference in isotopic values between Main stream-2 and tributaries. Based on isotopic hydrograph separation, the proportion of snowmelt in runoff components ranges from 15% to 47%, and the proportion of rainwater ranges from 3% to 35%. Thus, the main components of runoff in the Naqu River are snowmelt and groundwater.

Virtual experiments: a new approach for improving process conceptualization in hillslope hydrology

<strong class="journal-contentHeaderColor">Abstract.</strong> Hydrological modelling of small mountainous catchments is particularly challenging because of the high spatio-temporal resolution required for the meteorological forcings. In-situ measurements of precipitation are typically scarce in these remote areas, particularly at high elevations. Precipitation reanalyses propose different alternative forcings for the simulation of streamflow using hydrological models. In this paper, we evaluate the performances of two hydrological models representing some of the key processes for small mountainous catchments, using different meteorological products with a fine spatial and temporal resolution. The evaluation is performed on 55 small catchments of the Northern French Alps. While the simulated streamflows are adequately reproduced for most of the configurations, these evaluations emphasize the added value of radar measurements, in particular for the reproduction of flood events. However, these better performances are only obtained because the hydrological models correct the underestimations of accumulated amounts (e.g. annual) from the radar data in high-elevation areas.

Inverse modeling of mass transfer characterizes the dynamic processes affecting the function of karst systems and can be used to identify karst properties. An inverse model is proposed to calculate unit hydrographs as well as impulse response of fluxes from rainfall‐runoff or rainfall‐flux data, the purpose of which is hydrograph separation. Contrary to what hydrologists have been doing for years, hydrograph separation is carried out by using transfer functions in their entirety, which enables accurate separation of fluxes, as was explained in the companion paper [Pinault et al., this issue]. The unit hydrograph as well as impulse response of fluxes is decomposed into a quick and a slow component, and, consequently, the effective rainfall is decomposed into two parts, one contributing to the quick flow (or flux) and the other contributing to the slow flow generation. This approach is applied to seven French karstic aquifers located on the Larzac plateau in the Grands Causses area (in the south of France). Both hydrodynamical and hydrogeochemical data have been recorded from these springs over several hydrological cycles. For modeling purposes, karst properties can be represented by the impulse responses of flow and flux of dissolved species. The heterogeneity of aquifers is translated to time‐modulated flow and transport at the outlet. Monitoring these fluxes enables the evaluation of slow and quick components in the hydrograph. The quick component refers to the “flush flow” effect and results from fast infiltration in the karst conduit network when connection is established between the infiltration and phreatic zones, inducing an increase in water head. This component reflects flood events where flow behavior is nonlinear and is described by a very short transfer function, which increases and decreases according to water head. The slow component consists of slow and fast infiltration, underground runoff, storage in annex‐to‐drain systems, and discharge from the saturated zone. These components can be further subdivided by measuring chemical responses at the karst outlet. Using such natural tracers enables the slow component of the unit hydrograph to be separated into preevent water, i.e., water of the reservoir and event water, i.e., water whose origin can be related to a particular rainfall event. These measurements can be used to determine the rate of water renewal. Since the preevent water hydrograph is produced by stored water when pushed by a rainfall event and the event water hydrograph reflects rainwater transfer, separating the two components can yield insights into the characteristics of karst aquifers, the modes of infiltration, and the mechanisms involved in karstification, as well as the degree of organization of the aquifer.

Abstract. In this study, precipitation isotopic variations at Barrow, AK, USA, are linked to conditions at the moisture source region, along the transport path, and at the precipitation site. Seventy precipitation events between January 2009 and March 2013 were analyzed for δ2H and deuterium excess. For each precipitation event, vapor source regions were identified with the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) air parcel tracking program in back-cast mode. The results show that the vapor source region migrated annually, with the most distal (proximal) and southerly (northerly) vapor source regions occurring during the winter (summer). This may be related to equatorial expansion and poleward contraction of the polar circulation cell and the extent of Arctic sea ice cover. Annual cycles of vapor source region latitude and δ2H in precipitation were in phase; depleted (enriched) δ2H values were associated with winter (summer) and distal (proximal) vapor source regions. Precipitation δ2H responded to variation in vapor source region as reflected by significant correlations between δ2H with the following three parameters: (1) total cooling between lifted condensation level (LCL) and precipitating cloud at Barrow, ΔTcool, (2) meteorological conditions at the evaporation site quantified by 2 m dew point, Td, and (3) whether the vapor transport path crossed the Brooks and/or Alaskan ranges, expressed as a Boolean variable, mtn. These three variables explained 54 % of the variance (p&lt;0. 001) in precipitation δ2H with a sensitivity of −3.51 ± 0.55 ‰ °C−1 (p&lt;0. 001) to ΔTcool, 3.23 ± 0.83 ‰ °C−1 (p&lt;0. 001) to Td, and −32.11 ± 11.04 ‰ (p = 0. 0049) depletion when mtn is true. The magnitude of each effect on isotopic composition also varied with vapor source region proximity. For storms with proximal vapor source regions (where ΔTcool &lt;7 °C), ΔTcool explained 3 % of the variance in δ2H, Td alone accounted for 43 %, while mtn explained 2 %. For storms with distal vapor sources (ΔTcool &gt; 7°C), ΔTcool explained 22 %, Td explained only 1 %, and mtn explained 18 %. The deuterium excess annual cycle lagged by 2–3 months during the δ2H cycle, so the direct correlation between the two variables is weak. Vapor source region relative humidity with respect to the sea surface temperature, hss, explained 34 % of variance in deuterium excess, (−0.395 ± 0.067 ‰ %−1, p&lt;0. 001). The patterns in our data suggest that on an annual scale, isotopic ratios of precipitation at Barrow may respond to changes in the southerly extent of the polar circulation cell, a relationship that may be applicable to interpretation of long-term climate change records like ice cores.