Comment on “A comparison of catchment travel times and storage deduced from deuterium and tritium tracers using StorAge Selection functions” by Rodriguez et al. (2021)

The combined use of deuterium and tritium to determine travel time distributions (TTDs) in streams is an important development in catchment hydrology (Rodriguez et al., 2021). This comment takes issue with Rodriguez et al.'s assertion that the truncation hypothesis may not hold for catchments in general, i.e. that the use of stable isotopes alone may not lead to underestimation of travel times or storage compared to tritium. We discuss reasons why the truncation hypothesis may not appear to hold for the catchment studied by Rodriguez et al. (2021) but could still apply to the majority of catchments. We also discuss more generally future applications of tritium in Northern Hemisphere and Southern Hemisphere catchments.

​The Significance Impact Assessment of Morphological Parameters on Watershed: A Review

Watershed morphological analysis is momentous for controlling floods risk, forethought and management of the watershed area, as well as it is foremost useful to perceive catchment hydrology. Remote sensing and geographic information system are used in recent times as a tool for watershed delineation and its planning. Many types of input parameters generally use for watershed delineation such as Toposheet, ALOS, SRTM DEM, ASTER DEM and CARTOSAT DEM. Based on analysis SRTM DEM gives meticulous and clear results compared to other DEM files. Morphometric based prioritization of watershed was given in many research papers but an appropriate result of priority range was not given and this type of study confusing to evaluate the rank of priority based on its erosional behaviour. In many papers results of morphometric parameters were not indicate how to retaliate these results of morphometric parameters to a watershed. This paper deals with the implication of different values of morphometric parameters with adequate contextual information. This review paper can give useful information for the morphometric analysis of watersheds.

Application of Water Stable Isotopes for Hydrological Characterization of the Red River (Asia)

Fraction of young water (Fyw) and mean transit time (MTT, τ) calculated from water isotope profiles are valuable information for catchment hydrological assessment, especially in anthropogenically impacted region where natural conditions may not be decisive to catchment hydrology. The calculation of Fyw and MTT were performed on three subsets of δ18O_H2O data collected at the Hanoi meteo-hydrological station, Red River, in three periods; 2002–2005, 2015, and 2018–2019. The mean (min and max) values of δ18O_H2O in rainwater over the three periods are, respectively, −5.3‰ (−11.0 and −1.2‰), −5.4‰ (−10.7 and −1.4‰), and −4.5‰ (−13.9 and 1.7‰). The corresponding values in river water are −8.4‰ (−9.8 and −6.9‰), −8.5‰ (−9.1 and −7.7‰), and −8.4‰ (−9.5 and −7.2‰), respectively. The mean of Fyw calculated from the δ18O_H2O data for different periods is 22 ± 9%, 10 ± 5%, and 8 ± 3%. Mean transit time is 4.69 ± 15.57, 1.65 ± 1.53, and 2.06 ± 1.87 years. The calculated Fyw (MTT) is negatively (positively) proportional to change in reservoir volume over the three periods, which is logical, since reservoirs tend to keep more water in the catchment and slower down water flow. The strong variation of Fyw and τ, two essential variables characterizing the catchment hydrology, represents an anthropogenic impact in the Red River system.

Accuracy vs Realism: Does including reservoirs improve hydrological models?

Brazil has invested considerably in the reservoir construction during the past decades, mainly for irrigation and hydro-power generation. Despite their large impact on catchment hydrology, reservoir dynamics are often not included in hydrological models due to their complexity. In this study, we investigated the effect of including reservoir dynamics (realism) in hydrological models on the model performance (accuracy). Combined, realism and accuracy form the model fidelity. We used the HBV-EC and GR4J models to simulate hydrological processes and daily streamflow of 403 catchments across Brazil in two scenarios, with and without reservoirs. The model performances were assessed with the Kling Gupta Efficiency (KGE) and its components, and were compared between the models and scenarios. We found a significant increase in the HBV-EC model performance when the reservoirs were taken into account, although the overall performance was relatively poor. The average KGE increased from 0.21 without the reservoirs to 0.40 with the reservoirs. The GR4J model, on the other hand, showed better overall performance, but without the improvement when including the reservoirs; the average KGE slightly decreased from 0.57 to 0.56. In the catchments with the largest reservoir capacity, HBV-EC in the scenario with reservoirs outperformed GR4J in both scenarios. We note that better model performance can still be obtained with a smaller spatial scale or other methods of including reservoirs, which require more data and detailed studies. With this paper, we demonstrate that model performance can improve when including reservoir dynamics, but this depends on model structure and does not always increase model fidelity.

Diagnosing the impacts of permafrost on catchment hydrology: field measurements and model experiments in a mountainous catchment in western China

Increased attention directed at permafrost hydrology has been prompted by climate change. In spite of an increasing number of field measurements and modeling studies, the impacts of permafrost on hydrological processes at the catchment scale are still unclear. Permafrost hydrology models at the catchment scale were mostly developed based on a “bottom-up” approach, hence by aggregating prior knowledge at the spot/field scales. In this study, we followed a “top-down” approach to learn from field measurement data to understand permafrost hydrology at the catchment scale. In particular, we used a stepwise model development approach to examine the impact of permafrost on streamflow response in the Hulu catchment in western China. We started from a simple lumped model (FLEX-L), and step-wisely included additional complexity by accounting for topography (i.e. FLEX-D) and landscape heterogeneity (i.e. FLEX-Topo). The final FLEX-Topo model, was then analyzed using a dynamic identifiability analysis (DYNIA) to investigate parameters’ temporal variation. By enabling temporal dynamics on several parameters, we diagnosed the physical relationships between parameter variation and permafrost impacts. We found that in the Hulu catchment: 1) the improvement associated to the model modifications suggest that topography and landscape heterogeneity are dominant controls on catchment response; 2) baseflow recession in permafrost regions is the result of a linear reservoir, and slower than non-permafrost regions; 3) parameters variation infers seasonally non-stationary precipitation-runoff relationships in permafrost catchment; 4) permafrost impacts on streamflow response mostly at the beginning of the melting season; 5) allowing the temporal variations of frozen soil related parameters, i.e. the unsaturated storage capacity and the splitter of fast and slow streamflow, improved model performance. Our findings provide new insights on the impact of permafrost on catchment hydrology in vast mountain regions of western China. More generally, they help to understand the effect of climate change on permafrost hydrology.

Comment on “A comparison of catchment travel times and storage deduced from deuterium and tritium tracers using StorAge Selection functions” by Rodriguez et al. (2021)

The combined use of deuterium and tritium to determine travel time distributions (TTDs) in streams is an important development in catchment hydrology (Rodriguez et al, 2021). This comment takes issue with Rodriguez et al.'s general rejection of the truncation hypothesis, i.e. that the almost exclusive use of stable isotopes has truncated our vision of streamflow TTDs and caused us to miss the long tails of old water often shown by tritium. We discuss reasons why this hypothesis may not hold for the catchment described by Rodriguez et al. (2021), but could still apply to a large proportion of all catchments. We also discuss more generally future applications of tritium in northern and southern hemisphere catchments.

Global glacio-hydrological model coupling for streamflow prediction

<p>Global hydrological models (GHMs) have become an increasingly valuable tool in a range of global impact studies related to water resources. However, the parameterization of glaciers is often overly simplistic or non-existent in GHMs. The representation of glacier dynamics and evolution, including related products such as glacier runoff, can be improved by relying on dedicated global glacier models (GGMs). Coupling a GGM to a GHM could consequently lead to increased GHM predictive skills, decreased GHM uncertainty, and an increased understanding of the contribution of glaciers to catchment hydrology, particularly in light of climate change.</p><p>To test this hypothesis, the GHM PCR-GLOBWB 2 (Sutanudjaja et al., 2018) is coupled with the GGM GloGEM (Huss and Hock, 2015) using eWaterCycle (Hut et al., 2018). For the years 2001-2012, the coupled model is evaluated against the uncoupled benchmark in 25 large (>50.000 km2) glacierized basins using GRDC streamflow observations. Across all basins, the coupled model produces higher runoff throughout the melt season, which can principally be attributed to the underrepresentation of glacial melt in PCR-GLOBWB 2. In highly glaciated basins this difference is pivotal, while in lowly glaciated basins it is negligible. In the evaluation against the GRDC observations, the performance increment of the coupled model at the peak of the melt season in highly glaciated basins stands out.</p><p>This study underlines the importance of glacier representation in GHMs and demonstrates the potential of coupling a GHM with a GGM for better glacier representation and runoff predictions in glaciated basins.</p><p> </p>

A Budyko-like framework for exploring the controls of long-term flood risk in coupled human-flood systems

<p>Long term dynamics in human-flood systems differ due to differences in hydrological and societal characteristics. By contrasting and comparing different human-flood systems we increase our understanding of which characteristics lead to which dynamics, which might help to counteract unfavorable developments. We propose a framework for comparing human-flood systems analogous to the Budyko one for traditional catchment hydrology. While in the Budyko framework catchments are classified as either water limited or energy limited, in the framework proposed here the human-flood systems are classified as either hydrology limited or exposure limited. In analogy to the precipitation, potential evapotranspiration and actual evapotranspiration components of the Budyko space we formulate the components of the “flood risk space” as hydrological potential loss, manmade potential loss and actual flood loss. The framework is applied to four stylised theoretical systems, investigating how their position in the flood risk space may change under the influence of hydrological, technical and demographic changes. Results show that hydrological changes have the largest effect on a system’s position in the flood risk space: with an increasing skewness and CV systems become more hydrology limited. The framework’s value for comparing empirical case studies is demonstrated through an application to two case studies in Germany: Dresden on the Elbe and Cologne on the Rhine. The framework allows us to identify the differences in dynamics between the two case studies, as they are located in different areas of the flood risk space. The difference in dynamics between the Dresden and Cologne systems seems to be mostly caused by the hydrological parameters (i.e. the skewness) rather than the social parameters. The flood frequency distribution is more skewed in the case of the Elbe in Dresden than in the case of the Rhine in Cologne. Therefore, Dresden experiences more shocks to the system (i.e. unexpectedly large floods) than Cologne.</p>

Closing the climate-hydrology feedback loop: Variations in greenhouse gas fluxes resulting from changes in catchment hydrology due to human-induced climate change

<p>Climate change during the Anthropocene has caused many disturbances to Earth’s system, including altering patterns of precipitation and temperature. This has led to hydrological extremes with increases in both floods and droughts globally. We know and recognise this large effect on the global water cycle, but the consequent influence on biogeochemistry and greenhouse gas production has received less attention. Changes in greenhouse gas emissions due to increases in hydrological extremes may be an unrecognised climate feedback, having large implications for future climate and in turn, catchment hydrology. Here we present a synthesis from field studies and a review of the literature to investigate the effects of hydrological extremes on greenhouse gas production and emissions. We focus on variations in greenhouse gas emissions as a result of changes in both discharge and temperature, which are affected by hydrological extremes.</p>