A multi‐scale study of the dominant catchment characteristics impacting low‐flow metrics

Low flows can impact water use and instream ecology. Therefore, reliable predictions of low-flow metrics are crucial. In this study, we assess which catchment characteristics (climate, topography, geology and landcover) can explain the spatial variability of low-flow metrics at two different scales: the regional scale and the small headwater catchment scale. For the regional-scale analysis, we calculated the mean 7-day annual minimum flow (qmin), the mean of the flow that is exceeded 95% of the year (q95), and the master recession constant (C) for 280 independent gauging stations across the Swiss Plateau and the Swiss Alps for the 2000–2018 period. We assessed the relation between 44 catchment characteristics and the three low-flow metrics based on correlation analysis and a random forest model. Low-flow magnitudes across the Swiss Plateau were positively correlated with the fraction of the area covered by sandstone bedrock or alluvium, and with the area that has a slope between 10° and 30°. Across the Swiss Alps, low-flow magnitudes were positively correlated with the fraction of area with slopes between 30° and 60°, and the area with glacial deposits and debris cover. There was good agreement between observations and predictions by the random forest regression model with the top 11 catchment characteristics for both regions: for 80% of the Swiss Plateau catchments and 60% of the Swiss Alpine catchments, we could predict the three low-flow metrics within an error of 30%. The residuals of the regression model, however, varied across short distances, suggesting that local catchment characteristics affect the variability of low-flow metrics. For the local-scale headwater catchments, we conducted 1-day snapshot field campaigns in 16 catchments during low-flow periods in 2015 and 2016. The measurements in these sub-catchments also showed that areas with sandstone bedrock and a good storage-to-river connectivity had above average low-flow magnitudes. Including knowledge on local catchment characteristics may help to improve regional low-flow predictions, however, not all local catchment characteristics were useful descriptors at larger scales

Creating Community for Early-Career Geoscientists:Student involvement in geoscience unions: A case study from hydrology

The American Geophysical Union (AGU) and the European Geosciences Union (EGU) play central roles in nurturing the next generation of geoscientists. Students and young scientists make up about one-quarter of the unions’ active memberships [American Geophysical Union, 2013; European Geosciences Union, 2014], creating a major opportunity to include a new generation of geoscientists as more active contributors to the organizations’ activities, rather than merely as consumers

Determination of the Effective Viscosity of Non-newtonian Fluids Flowing Through Porous Media

When non-Newtonian fluids flow through porous media, the topology of the pore space leads to a broad range of flow velocities and shear rates. Consequently, the local viscosity of the fluid also varies in space with a non-linear dependence on the Darcy velocity. Therefore, an effective viscosity μeff is usually used to describe the flow at the Darcy scale. For most non-Newtonian flows the rheology of the fluid can be described by a (non linear) function of the shear rate. Current approaches estimate the effective viscosity by first calculating an effective shear rate mainly by adopting a power-law model for the rheology and including an empirical correction factor. In a second step this averaged shear rate is used together with the real rheology of the fluid to calculate μeff. In this work, we derive a semi-analytical expression for the local viscosity profile using a Carreau type fluid, which is a more broadly applicable model than the power-law model. By solving the flow equations in a circular cross section of a capillary we are able to calculate the average viscous resistance 〈μ〉 directly as a spatial average of the local viscosity. This approach circumvents the use of classical capillary bundle models and allows to upscale the viscosity distribution in a pore with a mean pore size to the Darcy scale. Different from commonly used capillary bundle models, the presented approach does neither require tortuosity nor permeability as input parameters. Consequently, our model only uses the characteristic length scale of the porous media and does not require empirical coefficients. The comparison of the proposed model with flow cell experiments conducted in a packed bed of monodisperse spherical beads shows, that our approach performs well by only using the physical rheology of the fluid, the porosity and the estimated mean pore size, without the need to determine an effective shear rate. The good agreement of our model with flow experiments and existing models suggests that the mean viscosity 〈μ〉 is a good estimate for the effective Darcy viscosity μeff providing physical insight into upscaling of non-Newtonian flows in porous media

CAMELS-CH: hydro-meteorological time series and landscape attributes for 331 catchments in hydrologic Switzerland

We present CAMELS-CH (Catchment Attributes and MEteorology for Large-sample Studies – Switzerland), a large-sample hydro-meteorological data set for hydrologic Switzerland in central Europe. This domain covers 331 basins within Switzerland and neighboring countries. About one-third of the catchments are located in Austria, France, Germany and Italy. As an Alpine country, Switzerland covers a vast diversity of landscapes, including mountainous environments, karstic regions, and several strongly cultivated regions, along with a wide range of hydrological regimes, i.e., catchments that are glacier-, snow- or rain dominated. Similar to existing data sets, CAMELS-CH comprises dynamic hydro-meteorological variables and static catchment attributes. CAMELS-CH (Höge et al., 2023; available at https://doi.org/10.5281/zenodo.7784632) encompasses 40 years of data between 1 January 1981 and 31 December 2020, including daily time series of stream flow and water levels, and of meteorological data such as precipitation and air temperature. It also includes daily snow water equivalent data for each catchment starting from 2 September 1998. Additionally, we provide annual time series of land cover change and glacier evolution per catchment. The static catchment attributes cover location and topography, climate, hydrology, soil, hydrogeology, geology, land use, human impact and glaciers. This Swiss data set complements comparable publicly accessible data sets, providing data from the “water tower of Europe”

Evaluating capacity and drainage behavior of alpine ground water storages

Die Abschätzung der Wasserverfügbarkeit in alpinen Einzugsgebieten bei Niederwasser ist wesentlich für viele wirtschaftliche und ökologische Fragestellungen. Besseres Verständnis über die Wasserspeicherung in Böden und quartären Sedimenten könnte die Abschätzung von Niederwasserverhalten und Hochwasserretention erheblich verbessern. Selbst steile alpine Gebiete reagieren auf Grund der vorhandenen Speicher gedämpft auf Niederschlagsereignisse oder weisen auch in Niederwasserperioden einen hohen Abluss auf. Nicht nur das Volumen dieser Speicher, sondern auch die Zeit der Entwässerung ist wichtig um die Dynamik der Abflussabnahme zu verstehen. In den Wintermonaten können Rücklaufprozesse von Abflüssen in alpinen Einzugsgebieten gut untersucht werden, da die Speicher kaum durch Regen oder Schneeschmelze aufgefüllt werden. Um zu untersuchen, wie das Verhalten eines Einzugsgebiets mit der räumlichen Verteilung von Speichern zusammen hängt, wurden während der Wintersaison 2013/14 in 7 Messkampagnen in verschiedenen Teileinzugsgebieten des Oberlaufs des Poschiavino, räumlich hoch aufgelöst, Abflussmessungen durchgeführt. Das Einzugsgebiet in der Südost Schweiz ist etwa 14 km² gross und weist sehr unterschiedliche Teileinzugsgebiete auf. Ausserdem wurden elektrische Leitfähigkeiten, sowie Ionenzusammensetzung gemessen, um verschiedene Speichertypen und die Herkunft des Wassers zu identifizieren und klassifizieren. Um die Auswirkungen von Speichern auf das Verhalten bei Niederwasser zu untersuchen, wurden die unterschiedlichen Ablagerungen kartiert und nach ihrer Mächtigkeit und Typen klassifiziert. Ausserdem wurde versucht das Speicherpotential der Ablagerungen abzuschätzen. Räumliche Variationen in Drainagezeit und abgeflossenem Volumen konnten in den einzelnen Teileinzugsgebieten identifiziert werden (zwischen 54mm und 200mm in vier Monaten). Die Untersuchungen sind auf Teileinzugsgebietsebene limitiert, da unterirdischer Abfluss und punktuelle Zuflüsse die Beobachtungen auf kleinerer Skala verfälschen. Rücklaufkurven, kombiniert mit zeitlicher Veränderung in der Ionen-Zusammensetzung, wurden verwendet, um Drainage-Zeiten und Speichervolumen zu klassifizieren. Die unterschiedlichen Volumina und die zeitliche Variabilität der Abflüsse konnten auf kartierte Speichereigenschaften zurückgeführt werden. Zusammengefasst konnte gezeigt werden, dass das Verständnis über Speicher und Drainageprozesse in alpinen Einzugsgebieten helfen könnte, Herausforderungen bei der Vorhersage von Niederwassermenge, aber auch bei der Hochwasserabschätzung zu bewältigen

Isotopic offsets in throughfall and stemflow may have small effects on estimates of winter precipitation fractions

Forest canopies alter the amount and isotopic composition of precipitation reaching the forest floor. Thus retention, evaporation and transport processes in forest canopies, and their effects on water isotopes, are key to understanding forest water cycling. Using a two-year isotope dataset from a mixed beech/spruce forest in Zurich, Switzerland, we assessed the isotopic offsets between precipitation, throughfall and stemflow. We also analysed how these offsets affect estimates of the fraction of soil water that is derived from winter precipitation. Throughfall was typically enriched in heavy isotopes compared to precipitation, but isotopically lighter than stemflow, with average δ²H of -64.3‰, -59.9‰ and -56.3‰ in precipitation, throughfall and stemflow, respectively. The differences between beech and spruce were rather small compared to the seasonal differences in precipitation isotopes. Isotopic offsets between precipitation and throughfall/stemflow were smaller during the spring and summer months (March through August) than during fall and winter (September through February). Bulk and mobile soil waters at 10 and 40 cm showed smaller seasonal variations than those in precipitation, throughfall and stemflow, and were isotopically lighter than recent precipitation, with the largest offsets occurring during the summer months (June through August) for bulk soil waters. Thus, bulk soil waters at both depths contain a mixture of precipitation from previous events and seasons, with over-representation of isotopically lighter winter precipitation. Mobile soil waters were more similar to recent precipitation than bulk soil waters were. Throughfall isotopes were slightly heavier than precipitation isotopes, resulting in different sinusoidal fits for seasonal isotopic cycles in precipitation and throughfall. These differences lead to small underestimates in the fraction of soil water originating from winter precipitation, when open-field precipitation rather than throughfall is used as the input data. Together our results highlight the importance of isotope measurements in throughfall and stemflow for the assessment of precipitation seasonality and water cycling across forested landscapes.ISSN:0885-6087ISSN:1099-108

Estimating endmember chemistry of contributing sources to low flows in Alpine headwater catchments

ISSN:1029-7006ISSN:1607-796

Enrichment Processes in Throughfall and Stemflow in a Mixed Temperate Forest

Forest canopies redistribute precipitation by processes of interception and stemflow which also change the chemical signature of incoming precipitation. Understanding what controls these transformations and how they evolve across seasons is key to assess forest water cycling and nutrient transport. At the Waldlabor Zurich ecohydrological observatory (Switzerland) we measure the amount and chemical signature of precipitation since April 2020. Our measurement setup focusses on spruce (Picea abies) and beech (Fagus silvatica) trees, as they are the two most common species across Switzerland. In addition to the ion and isotope concentrations in precipitation, throughfall, stemflow as well as in the soil at different depths (10, 20, 40 and 80 cm), we also assess the canopy density in weekly resolution, groundwater depth and streamflow amount at the outlet of our forested catchment, as well as their chemical and isotopic composition We assessed the seasonal variability of throughfall and stemflow and their relation to canopy density measurements for beech, spruce and young spruce trees. Canopy density had little to no effect on interception and stemflow fractions. We found almost half of the total annual precipitation is intercepted in the canopies of spruce and beech trees, this is because most precipitation events were quite small, resulting in almost no throughfall at all. However, in general, the fraction of interception decreased with increasing event size, on the other hand events below 4 mm did not produce significant amounts of throughfall and stemflow. Water chemistry is showing that major enrichment processes took place in the canopy, subsequently the ion signature was different in throughfall and stemflow compared to open field precipitation. Ion concentrations of sodium, chloride, nitrate, ammonium and potassium were 2 - 10 times higher in throughfall and up to 14 times higher in stemflow compared to concentrations measured in open field precipitation. We hypothesize this is the result of accumulation of wind deposits, especially of anthropogenic contaminants on the tree stem, branches and leaves, as we found concentrations where generally higher during events succeeding long periods without precipitation. In accordance with the much rougher surface of spruce needles and stems compared to beech leaves and stems, we found much higher concentrations in throughfall and stemflow below spruce trees and elevated ion concentrations in the soil waters up to 40 cm depth. Overall, our results highlight the importance of forests, not only in redistributing precipitation, but also in changing the chemical signal of precipitation and thus the forest water and nutrient cycle