A comparison of catchment travel times and storage deduced from deuterium and tritium tracers using StorAge Selection functions

Catchment travel time distributions (TTDs) are an efficient concept for summarizing the time-varying 3D transport of water and solutes towards an outlet in a single function of a water age and for estimating catchment storage by leveraging information contained in tracer data (e.g., deuterium $^{2}$H and tritium $^{3}$H). It is argued that the preferential use of the stable isotopes of O and H as tracers, compared to tritium, has truncated our vision of streamflow TTDs, meaning that the long tails of the distribution associated with old water tend to be neglected. However, the reasons for the truncation of the TTD tails are still obscured by methodological and data limitations. In this study, we went beyond these limitations and evaluated the differences between streamflow TTDs calculated using only deuterium ($^{2}$H) or only tritium ($^{3}$H). We also compared mobile catchment storage (derived from the TTDs) associated with each tracer. For this, we additionally constrained a model that successfully simulated high-frequency stream deuterium measurements with 24 stream tritium measurements over the same period (2015–2017). We used data from the forested headwater Weierbach catchment (42 ha) in Luxembourg. Time-varying streamflow TTDs were estimated by consistently using both tracers within a framework based on StorAge Selection (SAS) functions. We found similar TTDs and similar mobile storage between the $^{2}$H- and $^{3}$H-derived estimates, despite statistically significant differences for certain measures of TTDs and storage. The streamflow mean travel time was estimated at 2.90±0.54 years, using $^{2}$H, and 3.12±0.59 years, using $^{3}$H (mean ± 1 SD – standard deviation). Both tracers consistently suggested that less than 10 % of the stream water in the Weierbach catchment is older than 5 years. The travel time differences between the tracers were small compared to previous studies in other catchments, and contrary to prior expectations, we found that these differences were more pronounced for young water than for old water. The found differences could be explained by the calculation uncertainties and by a limited sampling frequency for tritium. We conclude that stable isotopes do not seem to systematically underestimate travel times or storage compared to tritium. Using both stable and radioactive isotopes of H as tracers reduced the travel time and storage calculation uncertainties. Tritium and stable isotopes both had the ability to reveal short travel times in streamflow. Using both tracers together better exploited the more specific information about longer travel times that 3H inherently contains due to its radioactive decay. The two tracers thus had different information contents overall. Tritium was slightly more informative than stable isotopes for travel time analysis, despite a lower number of tracer samples. In the future, it would be useful to similarly test the consistency of travel time estimates and the potential differences in travel time information contents between those tracers in catchments with other characteristics, or with a considerable fraction of stream water older than 5 years, since this could emphasize the role of the radioactive decay of tritium in discriminating younger water from older water

Atmospheric conditions favouring extreme precipitation and flash floods in temperate regions of Europe

In recent years, flash floods have repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past (especially in the 1990s), floods have been characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes. Here, we explore potential links and causes between the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions. We hypothesize that a change in atmospheric conditions has led to more frequent extreme precipitation events that have subsequently triggered flash flood events in central western Europe. To test this hypothesis, we compiled data on flash floods in central western Europe and selected precipitation events above 40 mm h$^{−1}$ from radar data (the RADOLAN “Radar-Online-Adjustment” dataset from the German Weather Service). Moreover, we identified proxy parameters representative of extreme precipitation favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity (q) in the lower troposphere (q≥0.004 kg kg$^{−1}$), sufficient latent instability (convective available potential energy (CAPE) ≥ 327 J kg$^{−1}$), and weak wind speeds between 10 m a.g.l. and 500 hPa (WS$_{10 m−500 hPa}$ ≤ 6 m s$^{−1}$) proved to be characteristic of intense rainfall that can potentially trigger flash floods. We relied on linear models to analyse 40 years worth (1981–2020) of atmospheric parameters as well as related precipitation events. We found significant increases in the atmospheric moisture content and increases in atmospheric instability. Parameters representing the motion and organization of convective systems remained largely unchanged in the considered period (1981–2020); however, the number of precipitation events, their maximum 5 min intensities, and their hourly sums were characterized by large interannual variations, and no trends could be identified between 2002 and 2020. Our study shows that there is no single mechanistic path leading from atmospheric conditions to extreme precipitation and subsequently to flash floods. The interactions between the processes involved are so intricate that more analyses which consider other potentially relevant factors, such as intra-annual precipitation patterns or catchment-specific parameters, are required

Saturated areas through the lens: 2. Spatio-temporal variability of streamflow generation and its relationship with surface saturation

Investigating the spatio-temporal variability of streamflow generation is fundamental to interpret the hydrological and biochemical functioning of catchments. In humid temperate environments, streamflow generation is often linked to the occurrence of near stream surface saturated areas, which mediate hydrological connectivity between hillslopes and streams. In this second contribution of a series of two papers, we used salt dilution gauging to investigate the spatio-temporal variability of streamflow in different subcatchments and for different reaches in the Weierbach catchment (0.42 km2) and explored the topographical controls on streamflow variability. Moreover, we mapped stream network expansion and contraction dynamics. Finally, we combined the information on the spatio-temporal variability of streamflow with the characterization of riparian surface saturation dynamics of seven different areas within the catchment (mapped with thermal infrared imagery, as presented in our first manuscript). We found heterogeneities in the streamflow contribution from different portions of the catchment. Although the size of the contributing area could explain differences in subcatchments' and reaches' net discharge, no clear topographic controls could be found when considering the area-normalized discharge. This suggests that some local conditions exert control on the variability of specific discharge (e.g., local bedrock characteristics and occurrence of perennial springs). Stream network dynamics were found not to be very responsive to changes in catchment's discharge (i.e., total active stream length vs. stream outlet discharge relationship could be described through a power law function with exponent = 0.0195). On the contrary, surface saturation dynamics were found to be in agreement with the level of streamflow contribution from the correspondent reach in some of the investigated riparian areas. This study represents an example of how the combination of different techniques can be used to characterize the internal heterogeneity of the catchment and thus improve our understanding of how hydrological connectivity is established and streamflow is generated.</p

Rare Earth Elements as Hydrological Tracers of Anthropogenic and Critical Zone Contributions: A Case Study at the Alzette River Basin Scale.

AbstractEnrichments in Gd and LREE/HREE fractionation have been observed for the dissolved and colloidal fractions in waters of the Alzette River during low flow conditions. They can be related to the effluents of the waste water treatment plants (Gd/Gd*: 8-380 and LaN/YbN: 0.02-0.07). Mean daily flux balance calculations at the basin scale show that during low flow conditions Gd only comes from the anthropogenic effluents. When flood events occur, the Gd anomaly progressively disappears and gives way to a chemical water signature, which is closer to that of natural REE sources in the basin (Gd/Gd*<1.4 and 0.11<LaN/YbN<1.5)

Towards the sequential assimilation of SAR-derived water stages into hydraulic models using the Particle Filter : proof of concept

With the onset of new satellite radar constellations (e.g. Sentinel-1) and advances in computational science (e.g. grid computing) enabling the supply and processing of multimission satellite data at a temporal frequency that is compatible with real-time flood forecasting requirements, this study presents a new concept for the sequential assimilation of Synthetic Aperture Radar (SAR)-derived water stages into coupled hydrologic-hydraulic models. The proposed methodology consists of adjusting storages and fluxes simulated by a coupled hydrologic-hydraulic model using a Particle Filterbased data assimilation scheme. Synthetic observations of water levels, representing satellite measurements, are assimilated into the coupled model in order to investigate the performance of the proposed assimilation scheme as a function of both accuracy and frequency of water level observations. The use of the Particle Filter provides flexibility regarding the form of the probability densities of both model simulations and remote sensing observations. We illustrate the potential of the proposed methodology using a twin experiment over a widely studied river reach located in the Grand-Duchy of Luxembourg. The study demonstrates that the Particle Filter algorithm leads to significant uncertainty reduction of water level and discharge at the time step of assimilation. However, updating the storages of the model only improves the model forecast over a very short time horizon. A more effective way of updating thus consists in adjusting both states and inputs. The proposed methodology, which consists in updating the biased forcing of the hydraulic model using information on model errors that is inferred from satellite observations, enables persistent model improvement. The present schedule of satellite radar missions is such that it is likely that there will be continuity for SAR-based operational water management services. This research contributes to evolve reactive flood management into systematic or quasi-systematic SAR-based flood monitoring services

Pruning Neural Belief Propagation Decoders

We consider near maximum-likelihood (ML) decoding of short linear block codes based on neural belief propagation (BP) decoding recently introduced by Nachmani et al.. While this method significantly outperforms conventional BP decoding, the underlying parity-check matrix may still limit the overall performance. In this paper, we introduce a method to tailor an overcomplete parity-check matrix to (neural) BP decoding using machine learning. We consider the weights in the Tanner graph as an indication of the importance of the connected check nodes (CNs) to decoding and use them to prune unimportant CNs. As the pruning is not tied over iterations, the final decoder uses a different parity-check matrix in each iteration. For Reed-Muller and short low-density parity-check codes, we achieve performance within 0.27 dB and 1.5 dB of the ML performance while reducing the complexity of the decoder