HS10.10

The Dead Sea is a terminal hypersaline lake at a unique location at ~430 m below sea level. Over the last several decades the Dead Sea has been drying up due to climate change: its water level has dropped at the rate of ~1 m year^-1. In this study we investigated the diurnal cycle of spatial heterogeneity in Dead Sea surface temperature (SST) using METEOSAT geostationary satellite data (2005-2015). METEOSAT data showed that, in the summer months, SST peaked at the same time, 13 LT (local time), as land surface temperature (LST) over surrounding land areas. In the presence of water mixing, the maximum of SST should be observed several hours later than that of LST due to thermal inertia of bulk water. The fact that SST and LST peaked at the same time, 13 LT, is evidence that there was no noticeable vertical water mixing. We consider that, in the absence of noticeable water mixing and under uniform solar radiation in the summer months, inhomogeneity in evaporation was the main causal factor of the observed spatial heterogeneity in Dead Sea SST. METEOSAT showed that spatial heterogeneity in SST was pronounced throughout the daytime. In summer, SST peaked at 13 LT, when SST reached 38.1 ^oC, 34.1 ^oC, and 35.4 ^oC being averaged over the east, middle and west parts of the lake, respectively. The above mentioned spatial heterogeneity in daytime SST caused a pronounced asymmetry in land surface temperature between land areas adjacent to the east and west sides of the lake. Maximal evaporation (causing maximal surface water cooling) took place at the middle part of the Dead Sea, while minimum evaporation took place at the east side of the lake. In the nighttime, METEOSAT data showed that SST values were minimal and SST spatial distribution was much more uniform compared to the daytime. We found that, in winter, when maximal solar radiation reached ~500 W/m² compared to ~900 W/m² in summer, daytime SST non-uniformity was less pronounced than that in summer. As the characteristic feature of the diurnal cycle, SST daily temperature range was equal to 7.2 °C, 2.5 °C, and 3.8 °C over the east, middle and west parts of the Dead Sea, respectively, in summer, compared to 5.3 °C, 1.2 °C, and 2.3 °C in winter.

Evaporation causes significant drying up of the Dead Sea, especially in the summer months, as the main contributor to maximal water level drop in the lake. However, no measurements of spatial distribution of Dead Sea evaporation have ever been conducted, either in situ or from space. Our findings allowed us to visualize spatial inhomogeneity in evaporation using the obtained heterogeneity in Dead Sea SST.

Reference:   Kishcha P. and Starobinets B. (2021). Spatial heterogeneity in Dead Sea surface temperature associated with inhomogeneity in evaporation. Remote Sensing  (Special Issue: Remote Sensing of Lake Properties and Dynamics), 13(1), 93; https://doi.org/10.3390/rs13010093.

How to cite: Kishcha, P., Starobinets, B., and Alpert, P.: Spatial heterogeneity in Dead Sea surface temperature caused by evaporation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-398, https://doi.org/10.5194/egusphere-egu21-398, 2021.

When a sediment laden river flows into a stratified water body, the water mass can either intrude as an overflow, interflow, or underflow depending upon the density contrast between the river and the lake. If the river is sufficiently warm or fresh to compensate for the additional mass of sediment, an overflow results, below which convective sedimentation occurs. If the sediment load is sufficiently high, then an underflow initially occurs, from which the warm/fresh interstitial material can subsequently loft as sedimentation reduces the initial density. Such convection can even potentially overturn the water column stratification if there is a very fresh, but very high sediment load turbidity current. For intermediate cases, an interflow can occur. Here it is possible for both lofting and sediment driven convection to occur above and below the pycnocline. All these different regimes can be described in terms of two dimensionless parameters: R_S and R_A, which are ratios that compare the density contrast due to sediment between the river and the upper layer with the density contrast between the upper and lower layers and the density contrast between the river and upper layer, respectively. We used laboratory experiments to describe the vigour of convection in terms of these dimensionless parameters, which then allows the behaviour in various rivers inflows into lakes to be predicted. We also apply our observations to predict how a turbidity current could lead to lofting and possible overturn of the stratification of meromictic Lake Kivu.

How to cite: Lu, G., Wells, M., Van Stygeren, I., and Hecky, R.: Intrusions of sediment laden fluids into density stratified water columns can be an unrecognized source of mixing in many lakes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1407, https://doi.org/10.5194/egusphere-egu21-1407, 2021.

The thermal structure in reservoirs affects the development of aquatic ecosystems and is substantially influenced by changing climate conditions. At the same time, reservoir management strategies can also affect the thermal structure of the water body and may enable adaptation strategies in a warmer world. We applied a two-dimensional hydrodynamicmodel to explore the response of the thermal structure in Germany's largest drinking water reservoir, Rappbode Reservoir, to future climate projections and different water withdrawal strategies. We used projections for representative concentration pathways (RCP) 2.6, 6.0 and 8.5 from an ensemble of 4 different global climate models taken from the ISIMIP project. Simulation results showed that epilimnetic water temperatures in the reservoir strongly increased under all three climate scenarios while the magnitude of warming directly corresponds to the increase in air temperatures. Hypolimnetic temperatures remained rather constant under RCP 2.6 and RCP 6.0 but increased markedly under RCP 8.5. Under the intense warming in RCP 8.5, hypolimnion temperatures were projected to rise from 5 °C to 8 °C by the end of the century. Moreover, the results suggested that surface withdrawal can be an effective adaptation strategy under strong climate warming (RCP 8.5) to reduce surface warming and even avoid hypolimnetic warming. This study documents how global scale climate projections can be translated into site-specific climate impacts to derive adaptation strategies for reservoir operation. Moreover, our results illustrate that the most intense warming scenario, i.e. RCP 8.5, demands far-reaching climate adaptation while the mitigation scenario (RCP 2.6) does not require adaptation of reservoir management before 2100.

How to cite: Rinke, K., Shatwell, T., Ma, J., Xu, Y., Su, F., and Mi, C.: Ensemble climate projections on stratification dynamics in Germany's largest drinking water reservoir and potential adaptation strategies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1426, https://doi.org/10.5194/egusphere-egu21-1426, 2021.

Lake Tarfala is an up to 50 m deep glacier-proximal Arctic lake in the Kebnekaise Mountains, northern Sweden (~67°55' N, ~18°35' E, 1162 m asl) in direct vicinity to the Tarfala Research Station run by Stockholm University, and to the glacier Storglaciären for which the world’s longest glacier mass balance record is kept since 1946. The neighboring Kebnepakte Glacier drains directly into Lake Tarfala. The site provides a unique an easily accessible natural observatory to study the impacts of climate and environmental change in an Arctic lake linked to a melting glacier.

As other Arctic lakes, Lake Tarfala is exposed to accelerated atmospheric warming in recent decades leading to increasingly shorter periods of lake freeze-over. Recent warming has also led to a widespread mass loss from glaciers with so for unclear implications for glacier-fed lakes which may receive larger amounts of meltwater and sediments from shrinking glaciers.

General atmospheric warming on the one hand and in response an increased influx of cold glacial meltwater to glacier-fed lakes on the other hand thus cause two competing processes determining the thermal state of a lake. Understanding (changing) lake thermal states and associated lake mixing dynamics is important because it has ramifications for a multitude of lake ecological, biological, and geochemical processes.

Here, we present the first continuous 3-year water temperature record from the deepest part of Lake Tarfala, acquired between 2016 and 2019. The record shows that Lake Tarfala is dimictic with overturning during spring and fall with substantial interannual variability concerning the timing, duration and intensity of mixing processes, as well as of summer and winter stratification. Particularly cold lake winter states appear to be related to elevated influx of cold glacial meltwater.

The projected high mass loss of Scandinavian glaciers with up to more than 80% of their volume under RCP8.5 until 2100 AD relative to 2015 renders Lake Tarfala a natural observatory where changes in processes, inherent timescales and impacts in response to competing drivers can be studied before they occur at other glacial lake sites where glaciers melt at a slower place.

How to cite: Kirchner, N., Schenk, F., Kuttenkeuler, J., Rosqvist, G., Weckström, J., Weckström, K., Korhola, A., Hancke, M., Granebeck, A., and Eriksson, P.: Lake Tarfala, N-Sweden – first results from a natural observatory mimicking future changes in glacier-fed Arctic lakes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1576, https://doi.org/10.5194/egusphere-egu21-1576, 2021.

Antarctica keeps great volume of water on Earth. Its cold, dry and windy climate leads peculiar balance of water in various phases (solid, liquid and gas), and it is sensitive to warming. Increase of near surface temperature enhances water transition from solid to liquid phase (melting) as well as to gas (evaporation). The melted water is accumulated in a population of glacial lakes. These water bodies are located inside glaciers (subglacial type), over their surface (supraglacial type) or contacted glaciers (proglacial or epiglacial type). The glacial lakes are connected by a network of ephemeral streams. This hydrological network is typical in a lowest zone of Antarctic ice sheet, where the melting is substantial in the continental mass balance.

Water cycle in the glacial lakes differs with their type, and various processes drive water transport in the glacial lakes. In this study, the water balance equation method was applied to evaluate the volume of water accumulated in the glacial lakes. The water balance equation was written separately for the lakes of the epiglacial and supraglacial types. We used the observations by the long-term monitoring network, the data collected by the remote sensing, and the in-situ measurements gathered in field campaigns in the evaluations of the volume of lakes, the evaporation over a lake surface, and the inflow/outflow runoff. The components were evaluated for the epiglacial lakes located in the the Shirmacher, the Larsemann Hills and the Thala Hills oases (East Antarctica). 

The lake volume was evaluated from the lake surface area and depth measured withing last 10 years. The results show that since late 1980s, the lake volume has increased on many epiglacial lakes located not only in the coastal oases but also in the continental interior. The results suggest that the evaporation in among a key components of the water balance of the glacial lakes located in the Antarctica. In the polar region, the role of the evaporation is traditionally underestimated due to lack of the observations with precise measuring techniques. The results of the study contribute with the QAntarctica with the dataset on the actual physiography of the glacial lakes in Antarctica. This study is supported by the Academy of Finland (contract number 304345) with the logistic support of the national programs on the Antarctic research. 

How to cite: Shevnina, E.: Water balance in the Antarctic lakes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2537, https://doi.org/10.5194/egusphere-egu21-2537, 2021.

Lakes are lentic environmental with unique hydrodynamic, which depends on the morphology, in and outflows, and atmospheric variables. This last driving force has its influence represented, mostly, by radiation and wind. All the interactions in the water column are harmed when the water column is divided into layers with different densities.

This condition means no gas or nutrients exchanges, impairing the food channel, and oxygen availability across the lake. Lakes and reservoirs play a key role for the development of populations, industries, human activities that need water, and also as a landscape component, this context increases the necessity to ensure its availability during the year. In this perspective, the interest in understanding lakes’ hydrodynamics and their effects on the water quality grew, aiming for appropriate management of the reservoirs and contributing areas.

To collaborate with the knowledge in this area this research intended to improve the reservoir operator’s capacity to forecast situations that can compromise their uses. This objective was achieved by investigating the possibility of a functional relationship between the atmospheric forces and the lake thermal status changing.

Stratification can be postulated as an energy balance considering the energy incident from solar radiation and the kinetic energy transferred by the wind in terms of the surface wind-drag force. The lake's thermal conditions can be affected when an instability factor is inserted in the system. The wind's speed fluctuation produces the instability that transfers an amount of energy to the water column, provoking oscillations on the isothermals or internal waves.

A curve that represents this concept was constructed by crossing high-frequency field data from four lakes from two proxies, S* Rad-1 and W* S*^-1. The proxies describe the effectiveness of energy transfer from the atmospheric to the water column, and so, which is the ruling energy on balance at the moment. The variables included in it are Rad (total amount of the incident radiation on the last 24h (J m^-²)); W* (mean of the wind’s speed variance in a time window (m s^-1), multiplied by the air density (kg m^-3) and the lake's depth (m)); and the S* (Schmidt Number mean of the last 24h (J m^-²)).

The determined curve represents the thermal condition of the lake as a balanced result of the external variables and potential energy contained in the water column. This tool was able to represent the lakes’ thermal status rapidly and well, with little data information. Its performance was tested against most known lakes’ indices (Lake Number and Wedderburn Number) presenting more accurately with fewer data. Those outcomes allow an improvement to the reservoirs’ management tools and operations.

How to cite: Amorim, L., Martins, J. R., Vinçon-Leite, B., Nogueira, F., Silva, F., Duarte, B., and Magalhães, A.: Development of thermal stability curve to forecast water column stratification, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2763, https://doi.org/10.5194/egusphere-egu21-2763, 2021.

The spring bloom phenomenon in large regions of the world oceans have been studied for decades. However, the conditions necessary to trigger spring blooms remains uncertain till date. During the past decades several hypothesis appeared, the first being critical depth hypothesis - a conventional framework put forwarded by a Norwegian researcher H.U Sverdrup in the North Atlantic. His theory predicts that phytoplankton blooms occur when the mixing depth of the water column is less than a critical threshold value. This hypothesis proposed by Sverdrup (1953) to explain the occurrence of spring bloom of phytoplankton is known as critical depth (z_cr) in oceanography. Thus, the z_cr corresponds to the depth at which integral net photosynthesis is balanced by respiratory losses.

For the computation of the growth term to explain spring bloom of phytoplankton several alternative models have been proposed which are based on grazing and mixing processes, mathematical modelling and simulations, controlled and field experiments. Mathematical expressions have been extensively investigated by means of integro-differential equation models (Platt et al., 1991; Huisman and Weissing, 1994; Weissing and Huisman, 1994). Simplifying assumption such as use of linear P-I curve by Sverdrup, series solution based on a light saturation exponential model by Platt et al. (1991) and rectangular hyperbola model by Huisman (1999) are removed. Here, we focus on selecting a high accuracy P-I curve for estimating z_cr.

The most accurate photosynthesis-intensity relationship (P-I equation), a right-angle hyperbolic function, is proposed for critical depth evaluation. An exact analytical solution is presented by performing definite depth integrations of the right-angle hyperbolic equation and examining a method to obtain the equation by considering the mathematical characteristics. The series expansion equation including Bernoulli's number was used because the right-angle hyperbolic equation does not provide analytical solutions in definite integration. Moreover, since the integration range of this series equation is mathematically limited to π/2 or less, a new series of right-angle hyperbolic P-I equation is proposed by using polynomial approximation in the depth range up to the maximum photosynthetic rate (P_m). We, therefore present concise ideas for the estimation of z_cr limited to saturation type P-I curve by comparing the obtained equation with the critical water depths evaluated in previous studies. Furthermore, we suggest that future models of bloom formation should include shape factor for water column to make realistic projections for engineering applications in inland water bodies.

References:

Huisman J (1999): Population dynamics of light-limited phytoplankton: Microcosm experiments, Ecology, 80(1), 202–210.

Huisman J & Weissing FJ (1994): Light Limited Growth and Competition for Light in Well Mixed Aquatic Environments: An Elementary Model. Ecology, 75(2), 507-520.

Platt T, Bird DF, Sathyendranath S (1991): Critical depth and marine primary production. Proc. R. Soc. Lond. B, 246(1317), 205-217.  

Sverdrup, HU (1953). On Conditions for the Vernal Blooming of Phytoplankton. Cons. int. Explor. 18(3), 287-295.

Weissing FJ & Huisman J (1994): Growth and Competition in a Light Gradient. J. theor. Biol., 168, 323 – 326.

How to cite: Bhuyan, J. K., Furusato, E., and Dutta, S.: Critical Depth Model – Primary Production by Phytoplanktons, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2957, https://doi.org/10.5194/egusphere-egu21-2957, 2021.

Some attempts to predict water temperature in lakes by means of machine learning (ML) approaches have been pursued in recent years, relying on the performances that ML showed in many different contexts. The existing literature is focused on specific applications, and does not provide a general framework. Therefore, we systematically tested the role of different forcing factors on the accuracy of the simulation of lake surface water temperature (LSWT), comparing ML results with those obtained for a synthetic case study by means of a physically-based one-dimensional model, GLM. Among the available supervised ML tools, we considered artificial neural network (ANN) with back propagation, one of the most common and successful methods.

In our modelling exercise, we found that the two most important factors influencing the ability of ML to predict LSWT in temperate climates are air temperature (AT) and the day of the year (DOY). All the other meteorological inputs provide only minor improvements if considered additionally to AT and DOY, while they cannot be used as single predictors. The analysis showed that an important role is played by lake depth because a larger volume per unit of surface area implies a larger heat capacity of the lake, which smooths the temporal evolution of LSWT.  Such a filtering behaviour of deep lakes is not reproduced by standard ML methods, and requires an ad hoc pre-processing of AT input, which needs to be averaged with a proper time window. Moreover, while shallow lakes tend to be relatively well-mixed also in summer, deeper lakes can develop a strong stratification that tends to isolate the surface layer, modifying the thermally reactive volume and thus affecting the temporal evolution of LSWT. These considerations suggest that the physical dynamics of lakes, and especially of deep lakes, needs to be carefully considered also when adopting “black-box” approaches such as ML.

How to cite: Yousefi, A. and Toffolon, M.: The influence of water depth and forcing factors on the performances of Machine Learning approaches for the simulation of lake surface water temperature, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2970, https://doi.org/10.5194/egusphere-egu21-2970, 2021.

The Aral Sea desiccation is the worst aquatic ecological disaster of the last century, important for understanding the worldwide trends to degradation of arid lakes under water use and climate change. Formerly the fourth largest lake worldwide, the Aral Sea has lost ~90% of its water since the early 1960s due to irrigation in its drainage basin. Basing on field observations and numerical simulations, we show that the former bay of the Aral Sea — Chernyshev — turned to a meromictic heliothermal water body with extreme temperature, light and chemical regimes. The heliothermal regime of Chernyshev keeps the deep monimolimnion warm (about 15-16°C) throughout cold winter. Among less than 30 heliothermal waters worldwide, Chernyshev with its area of ~80-90 km² is the largest heliothermal lake, the second one being permanently ice-covered Antarctic lake Vanda. Chernyshev is also the youngest heliothermal lake, emerged within the last half-century. Seasonal themal cycle of the basin, scenarios of its formation and possible consequences for the ecosystem are discussed.

The study is funded by the Russian Foundation for Basic Research (RFBR project № 20-55-12007) and German Research Foundation (DFG KI 853-16/1).

How to cite: Izhitskiy, A., Kirillin, G., Goncharenko, I., Kurbaniyazov, A., and Zavialov, P.: Former bay of the desiccating Aral Sea as the newly formed world’s largest heliothermal lake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3846, https://doi.org/10.5194/egusphere-egu21-3846, 2021.

It is well-known that the shallow Sea of Azov can be thought of as a large estuary receiving discharges from the Don and the Kuban rivers, therefore, the flow through the Kerch Strait towards the Black Sea usually carries a variety of pollutants. However, the flux of plastic waste through the Strait has never been quantified. In situ measurements and sampling of microplastic debris and floating plastic litter in the Kerch Strait were conducted by a team from Shirshov Institute of Oceanology on July 16-18, 2020, along with CTD and ADCP profiling in the cross-section of the strait. The microplastic debris were sampled using a 0.3 mm mesh size Manta trawl net towed behind the R/V "Peleng" cruising at 4 kts and taking material from the upper 1 m of the water column. As a result, a large set of plastic particles, fibers and films were collected. All sampled items were measured, weighted, and sorted by composition using Micro NIR 1700 spectrometer instrument. The particle sizes ranged from 0.4 to 25.0 mm and weights varied between 0.05 and 7.72 mg. With respect to the chemical composition, about 63% of the collected particles were udentified as HDPE (high-densuty polyethylene), 21% as PP (polypropylene), 5% as PET (polyethylene terephthalate), 4% as PA (polyamide), 4% as PC (polycarbonate), and 3% as all other types of plastic. The content of visually identifiable plastic litter in the Kerch Strait varied between about 10 and 200 pieces per km², with the average value close to 100 pieces/km². However, the distribution was far from homogeneous – the litter was mainly concentrated in the western part of the Strait, where the principal stream carrying the Sea of Azov water into the Black Sea is usually localized. The newly obtained data of plastic litter concentration together with the current velocity data collected in ADCP profiling enabled us to estimate for the first time the flux of plastic through the Kerch Strait from the Sea of Azov into the Black Sea. This can be done by simply multiplying the plastic concentration by the velocity and then integrating it over the cross-section of the Strait. This procedure yields an estimate of 14,700 major pieces of plastic such as bottles, bags, etc., passing through the Strait per day. Assuming the average weight of a plastic litter piece 15 g , this leads to 220 kg/day, or about 9 kg/hour. This is quite a considerable mass of plastic, although it is about 20 times smaller than the amount brought to the Black Sea by the Danube according to [Lechner et al., 2014]. It must be kept in mind, however, that our estimate for the Kerch Strait is based on instantaneous one-time measurements, and may not represent long-term average values.

The studies described in this presentation represent the Russian contribution to the PLUMPLAS Project within STI BRICS cooperative initiative, implemented through the Russian Basic Research Foundation grant 19-55-80004.

How to cite: Zavialov, P.: Plastic debris and plastic litter in the Kerch Strait of the Black Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5707, https://doi.org/10.5194/egusphere-egu21-5707, 2021.

Lake Kultuchnoe applies to lakes of lagoon type and is located in the historical center of Petropavlovsk-Kamchatsky. The area and volume of the lake in connection with human economic activity was repeatedly reduced during backfilling. In the early 90s of the last century the lake was divided into two parts. The area of the water mirror of both lakes is about 2 km². The maximum depth of the Big lake – 7 m, Small lake– 1.2 m. Compared to the conditions that took place 30 years ago, there is an improvement in the state of the aquatic ecosystem and a decrease in the level of pollution. This is due to the reconstruction of urban sewage systems, in the 2000s, the release of fecal and industrial wastewater stopped. During the open water period a direct stratification is formed in the lake, and in the deepest part of it during the summer, low water temperatures remain in the bottom horizons and there is a lack of oxygen. The stratification is due to the insufficient length of wind acceleration for mixing the lake to the bottom and creates prerequisites for the formation of oxygen-free conditions below the boundary of the mixed layer (2-3 m). The systematic discharge of drainage wastewater into Lake Kultuchnoye through sewers with three outlets in the littoral part of the Big Lake and one in the Small Lake was revealed. According to the complex of components and indicators of water quality, the water in the Big and Small Lakes has a high level of, although the concentrations of many pollutants have decreased during the last 30 years. Compared to the state of the lake in 1990s, there was a decrease in the lake water of copper and manganese, phenols, petroleum products, ammonium nitrogen and BOD. Silty bottom sediments had a uniform composition, olive color. The content of organic matter reaches 14.4-16.9%, which indicates the active mineralization of organic residues. According to the content of mineral phosphorus (more than 10 μg/l), due to the influx of polluted waters, water masses do not experience a limit for the development of biota. In the Big Kultuchnoe Lake, the content of mineral phosphorus in the bottom horizons is 74-163 µg/l. In Small Kultuchnoe Lake, the content of mineral and total phosphorus is lower – up to 60 µg/l, which may be due to a more active process of its consumption by higher algae, which the lake is almost completely overgrown. Methane emission is the highest from the surface of the Small Lake (37.4 mgC / m²h), which is due to its high content in the water and low (up to 1 m) depth. For Big Kultuchnoe lake specific flow rate not exceeding 20,7 mgC/m²h. To preserve the ecosystem of the lake, which is located in the historical part of the city near important tourist infrastructure and has great recreational value, it is proposed to create phyto-treatment facilities that would intercept drainage runoff and not violate the overall appearance of the landscape.

How to cite: Grechushnikova, M. and Chalov, S.: Hydroecological status of Kultuchnoe lake (Petropavlovsk-Kamchatskiy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6478, https://doi.org/10.5194/egusphere-egu21-6478, 2021.

Burabay National Nature Park (BNNP), which is famous for its beautiful lakes and pine forests, is an important tourist destination and biodiversity hot spot in cold, semi-arid Northern Kazakhstan, Central Asia. BNNP lake system is being influenced by increasing anthropogenic pressures and climate change impacts. Lake level declines observed from 2008 to 2013 followed by rebound from 2013 onwards raised concerns about the future of these unique lakes. Previous studies on BNNP lakes showed that its steady long-term water storage decline was mainly due to a natural water balance deficit, with evaporation (from the lakes and catchments) exceeding precipitation. Next, to obtain a deeper understanding of this complex lake system, we studied the BNNP’s catchments by applying a hydrological model. This work is the first attempt to simulate the hydrological processes in two key BNNP lakes (Ulken Shabakty and Burabay) using a semi-distributed hydrological model, Soil and Water Assessment Tool (SWAT). The available daily lake level measurements were transformed into lake volumes using the data from a recent bathymetric survey and Surface Volume tool of ArcGIS. The level of Burabay Lake is determined by its main outlet, Gromotukha river, that discharges the excess water from Burabay Lake to Ulken Shabakty. Therefore, it acts as a natural reservoir and allows to use the Reservoir function of SWAT. Calibration of the model by lake volumes was done for years 2010-2013 and the model performed well for both lakes (NSE 0.71 and 0.57; KGE 0.77 and 0.73; PBIAS -0.9 and -0.4 for Ulken Shabakty and Burabay, respectively). However, during validation for years 2014-2016 the model performance decreased considerably (NSE -23.94 and -0.35, KGE 0.12 and -0.35, PBIAS 7.6 and -0.3 for Ulken Shabakty and Burabay, respectively). SWAT substantially overestimated the lake volumes for Ulken Shabakty by 0.01 km³ on average for the validation period. This extreme overestimation highlights the specific features of both basins, which has to do with the local subsurface flows. Due to the relatively simplistic representation of groundwater in SWAT and the absence of comprehensive groundwater data, the calibrated model might not have been able to fully capture the complexity of the actual hydrogeologic system. As a result, smaller in size surface catchment boundary (in the case of Ulken Shabakty Lake) is considered in comparison to potentially larger groundwater catchment boundary. In addition, two years (2010 and 2012) used for calibration were drought years, during which the model might have compensated for the lower groundwater flows by simulating enhanced surface runoff and lateral flow. As a result, during the following years with normal and higher precipitation amounts (2013-2016) significantly higher surface runoff was generated. Further studies using coupled groundwater and surface water models are necessary to understand the interactions between groundwater and surface water.

How to cite: Ongdas, N., Yapiyev, V., Akiyanova, F., Nazhbiyev, A., Karakulov, Y., Sagintayev, Z., Samakhanov, K., and Verhoef, A.: Application of semi-distributed hydrological model to simulate the lake volumes of small closed lakes in the Northern Kazakhstan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6996, https://doi.org/10.5194/egusphere-egu21-6996, 2021.

Currently, shipborne observations using CTD-type instruments are the main method for studying the hydrological characteristics of Lake Baikal. They provide episodic information about the spatial distribution of temperature, mineralization and dissolved oxygen over the depth of the lake but do not provide detailed information about their temporal variability. As a rule, hydrochemical parameters are measured even more pointwise because they require sampling and subsequent analysis. To study spatiotemporal variability of ecosystem characteristics in more detail, it is necessary to combine shipborne observations with long-term measurements at coastal stations or develop a network of abyssal buoy stations equipped with various hydrophysical instruments.

The first step in this direction was the development and implementation of an automated hydrometeorological station at Limnological Institute SB RAS to organize online monitoring of hydrophysical, hydrochemical and meteorological parameters in the littoral zone of Lake Baikal. The developed station is based on an AAQ177 Rinko water quality profiler (JFE Advantech, Japan) and water level sensor developed at Limnological Institute SB RAS. Meteorological parameters are measured with a set of Vantage Pro 2 sensors (Davis Instruments, USA). The environmental parameters measured every 10 seconds are transmitted in real time via wireless communication channels to a remote Internet server. Functionally, this server is a data collection and data processing centre (data centre). Tasks of the data centre include receiving data from the network of monitoring stations, primary processing, storage and provision of the access through the WEB page.

The monitoring station was installed at the pier of Limnological Institute SB RAS in the Bolshiye Koty settlement in August 2020. The obtained comparatively high-frequency and quasi-continuous measurements of the indicated parameters allowed us for the first time to trace in detail their daily and monthly variations during the summer-winter transition period. A comparative analysis of the obtained data with the results of parallel chemical analyses of the daily samples revealed their good agreement. In general, it is noteworthy that the set of measured parameters of develop station is sufficient to assess water quality and track its changes over time.

The development of systems for online monitoring of water balance parameters, such as water temperature, solar irradiance, wind regime, chemical and biogenic elements, etc., can provide additional information to understand the causes of the recent ecological transformation of the littoral zone of Lake Baikal. Thus, we will be able to switch from discrete/one-time observations to quasi-continuous ones, which will significantly improve the forecasting of natural and anthropogenic phenomena that are hazardous for the residents and ecosystem of the Baikal natural territory, and will form the basis for the development of the solutions for their prevention or mitigation.         

The work was supported by the grant No. 075-15-2020-787 in the form of a subsidy for a Major scientific project from Ministry of Science and Higher Education of Russia (project "Fundamentals, methods and technologies for digital monitoring and forecasting of the environmental situation on the Baikal natural territory").

How to cite: Aslamov, I., Makarov, M., Gnatovsky, R., Chernyshov, M., and Kucher, K.: Development and testing of autonomous water quality monitoring system in the littoral zone of Lake Baikal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7004, https://doi.org/10.5194/egusphere-egu21-7004, 2021.

In temperate lakes, it is generally assumed that light rather than temperature constrains phytoplankton growth in winter. Rapid winter warming and increasing observations of winter blooms warrant more investigation of these controls. We investigated the mechanisms regulating a massive winter diatom bloom in a temperate lake. High frequency data and process-based lake modeling demonstrated that phytoplankton growth in winter was dually controlled by light and temperature, rather than by light alone. Water temperature played a further indirect role in initiating the bloom through ice-thaw, which increased light exposure. The bloom was ultimately terminated by silicon limitation and sedimentation. These mechanisms differ from those typically responsible for spring diatom blooms and contributed to the high peak biomass. Our findings show that phytoplankton growth in winter is more sensitive to temperature, and consequently to climate change, than previously assumed. This has implications for nutrient cycling and seasonal succession of lake phytoplankton communities. The present study exemplifies the strength in integrating data analysis with different temporal resolutions and lake modeling. The new lake ecological model serves as an effective tool in analyzing and predicting winter phytoplankton dynamics for temperate lakes.

How to cite: Shatwell, T., Kong, X., Seewald, M., Dadi, T., Friese, K., Mi, C., Boehrer, B., Schulze, M., and Rinke, K.: Unravelling winter diatom blooms in temperate lakes using high frequency data and ecological modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8051, https://doi.org/10.5194/egusphere-egu21-8051, 2021.

How to cite: Biemond, B., Amadori, M., Toffolon, M., Piccolroaz, S., van Haren, H., and Dijkstra, H.: Deep-mixing and deep-cooling events in Lake Garda: Simulation and Mechanisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8326, https://doi.org/10.5194/egusphere-egu21-8326, 2021.

Lake Iseo is a 256-m deep basin which underwent a dramatic deterioration of water quality since the 80ies, to the point that it now shows the most worrying environmental conditions of all the deep lake in northern Italy, with anoxia and 160 μg/l of total phosphorus (TP) below 100 m. In this lake, a permanent chemical stratification has established, preventing deep mixing and trapping the larger part of the incoming TP in the monimolimnion. The increase in air temperature foreseen for the Iseo watershed will further enhance the stability of the water column and further reduce the efficiency of the outflow in the removal of TP.  In order to rationally guide future choices of remediation strategies, a quantification of the main sources of external and internal TP load to the lake is thus essential.

At this purpose, in the period 2016-2019 the research project ISEO (Improving the lake Status from Eutrophy towards Oligotrophy) was developed, comprising field monitoring, laboratory and experimental activities. The contribution of the main watershed (covering about 80% of the whole drained area) was quantified as 111 tonns TP/year, by measuring the TP concentration at high temporal resolution in main tributary through a bank-side auto-analyser. These measurements revealed that about 50% of this load is generated by acute, storm-dependent events, in which high TP concentrations in particulate form are delivered to the lake over short periods. The contribution of the combined sewer overflows (CSO) was quantified as 7 tonns TP/year, by coupling an hydraulic model of the sewer system along the shore of the lake with the measurements of the nutrients discharged in wet periods through the sewer spillways of 3 representative CSOs. This load was foreseen to increase by 10% in a climate change scenario with amplified intense storms. With regard to the internal load, soluble reactive phosphorus (SRP) fluxes were determined across the sediment–water interface from centimetre-scale pore water SRP concentration profiles using passive pore water samplers in 3 different lake locations. The average monimolimnion-wide flux was thus established 28.7 tonns SRP/year. Interestingly, the size and speciation of the phosphorus-bearing sediment fractions at each station revealed that the available mobile TP in the sediment under the monimolimnion was only 45 tonns, so able to sustain the actual release for only ~ 1.6 years without constant renewal. These data allowed to address the current contribution of the different nutrient sources to the TP budget in lake Iseo, and to argue about their possible temporal evolution and distribution in the lake in a climate change scenario.

How to cite: Valerio, G., Pilotti, M., Hupfer, M., and Nizzoli, D.: Assessing the contributions to the phosphorus load delivered to lake Iseo. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8922, https://doi.org/10.5194/egusphere-egu21-8922, 2021.

A decrease in the ice-period on lakes against the background of climate warming improves its oxygen regime in the cold half of the year by reducing the winter anoxia. A decrease in the thickness of the snow-ice cover can contribute to an increase in under-ice irradiation, which can provoke an earlier onset of spring under-ice convection and activation of algal blooms. Do these processes affect the oxygen content in ice-covered lakes? This study examines the variability of dissolved oxygen, water temperature, currents, chlorophyll "a" and under-ice irradiation according to field measurements carried out in 2007-2020 during spring under-ice convection in a small Lake Vendyurskoe (northwestern Russia). Field data were obtained at autonomous stations with an interval of one minute. Measurements of temperature and dissolved oxygen (RBR TR- and DO-sensors) were carried out from October to May, covering the entire ice-period, while measurements of currents (ADCP), solar radiation fluxes («Star-shaped pyranometer»  «Theodor Friderich & Co, Meteorologishe Gerate und Systeme»), and chlorophyll "a" (BBE Moldaenke) were carried out for 3-12 days from late March to the third decade of April in different years. The thickness of the snow-ice cover was also measured. Analysis of the data showed that in 2007-2020 the thickness of the snow-ice cover of Lake Vendyurskoe in spring (late March – mid-April) varied significantly from 35 to 70 cm, depending on weather conditions. The under-ice solar radiation fluxes varied from close to zero to more than 150 W/m2. The duration of spring under-ice convection ranged from two to seven weeks. Chlorophyll "a" was fairly uniformly distributed within the convective layer, even below the photic zone. We assume the dual role of convective currents in the development of subglacial plankton: ascending currents facilitate the entry of algal cells and nutrients into the photic zone, activating photosynthesis, while descending currents carry them out of it, suppressing photosynthesis. With well-developed convection, oscillations of dissolved oxygen were recorded with a daily frequency, reaching 1 mgO2/L in the upper part of the convective layer. Presumably, an increase in the content of dissolved oxygen is associated with a daytime increase in photosynthesis against the background of an increase in under-ice radiation, and a decrease is associated with the destruction of organic matter. Convective currents also affect the vertical distribution of dissolved oxygen, involving the oxygen-depleted bottom waters in mixing, which leads to a certain decrease in oxygen concentrations in the convective layer. The total amount of oxygen in the water column during the period of spring under-ice convection can increase by 10% due to the photosynthesis of phytoplankton. Oxygen fluctuations from minutes to hours were identified, which can be caused by seiche activity, the convective cells, advective transport, and the dynamics of internal waves. The results obtained in this work will contribute to a better understanding of the variability of oxygen in ice-covered lakes, caused by the total impact of biological and hydrophysical processes. The study was supported by an RFBR grant 18-05-60921.

How to cite: Zdorovennova, G., Palshin, N., Zdorovennov, R., Efremova, T., Bogdanov, S., Terzhevik, A., and Fedorova, I.: Dissolved oxygen variability in a small ice-covered lake during the spring under-ice convection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9741, https://doi.org/10.5194/egusphere-egu21-9741, 2021.

Evaporation of surface water is critical to the basic functioning of lakes. It directly and, in some cases, substantially modifies the hydrologic, chemical, and energy budgets, making evaporation one of the most important physical controls on lake ecosystems. Predicting lake evaporation response to climate change is, therefore, of paramount importance. Most studies that simulate climate change impacts on lake evaporation have utilised only a single mechanistic model. Whilst such studies have merit, the advantage of applying multiple, independently developed models (i.e., an ensemble approach), is that some of the inherent uncertainties in the individual lake models due to, for example, different model structures, can be reduced thus enabling increased robustness of historic and future projections. In this study, we present results from the Inter-Sectoral Impact Model Intercomparison Project phase 2b (ISIMIP) Lake Sector, where lake evaporation responses to 20^th and 21^st century (1901-2099) climate change has been simulated with a suite of independently developed lake models under different climate change scenarios (Representative Concentration Pathways, RCP, 2.6, 6.0 and 8.5). Our study focuses on Lake Kinneret (Israel), a sub-tropical monomictic lake of socioeconomic importance. Our simulations are validated during the historic period with bulk evaporation estimates calculated from high frequency meteorological and in-lake observations. Our results demonstrate that the lake models provide an accurate representation of historical variability in lake evaporation, with promising comparisons of the magnitude, timing and seasonality of evaporative water loss. Future evaporation projections at Lake Kinneret show that evaporation anomalies will increase by the end of the century. We show that multi-model projections of lake evaporation can accurately represent the historic period and hence provide reliable future projections that will be vital for water management.

How to cite: La Fuente, S., Woolway, I., Jennings, E., Gal, G., Kirillin, G., Shatwell, T., Ladwig, R., Moore, T., Couture, R.-M., Côté, M., and Råman Vinnå, L.: Multi-model projections of evaporation in a sub-tropical lake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10186, https://doi.org/10.5194/egusphere-egu21-10186, 2021.

To test if recent climate change and pollution affected remote lake ecosystems without direct human influence, we used paleolimnological methods on lake sediments from a large, prestine, and deep lake in Yakutia, Russia. We compared diatoms and sediment-geochemistry from before and after the onset of industrialization in the mid-nineteenth century, at water depths between 12.1 and 68.3 m in Lake Bolshoe Toko. We analyzed diatom species changes and geochemical changes including mercury concentrations. Chronologies were established using ²¹⁰Pb and ¹³⁷Cs revealing sedimentation rates between 0.018 and 0.033 cm y^-1 at shallow- and deep-water sites, respectively. Increase in light planktonic diatoms (Cyclotella) and decrease in heavily silicified euplanktonic Aulacoseira through time at deep-water sites can be related to warming air temperatures and shorter periods of lake-ice cover, causing pronounced thermal stratification. Diatom beta diversity changed only significantly in shallow-water communities which can be related to the development of new habitats with macrophyte growth. Mercury concentrations increased by a factor of 1.6 as a result of atmospheric fallout. Increases in the chrysophyte Mallomonas indicates a trend towards acidification. We conclude that also remote boreal lakes are susceptible to human-induced long-distance pollution and recent climate change.

How to cite: Biskaborn, B. K., Narancic, B., Stoof-Leichsenring, K. R., Pestryakova, L. A., Appleby, P. G., Piliposian, G. T., and Diekmann, B.: Impact of climate change and industrialization on remote Lake Bolshoe Toko, Siberia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10887, https://doi.org/10.5194/egusphere-egu21-10887, 2021.

In a context of global warming an important objective is an estimation of green-house gases fluxes into the atmosphere from different sources. Methane is a crucial green-house gas in the atmosphere with specific global warming potential 28 times larger than that of carbon dioxide. Our research is focused on estimation of methane emission from artificial Mozhaysk valley reservoir, located in Russia, Moscow region. The main goal of the study is to quantify the methane emission by two approaches – measuring the fluxes in situ on the reservoir and modelling the methane emission by 1D hydrodynamic and biogeochemical model. Combination of these methods provides more reliable result compared to using them separately. We expect, that firstly tested on Mozhaysk reservoir this approach can be applied to other artificial reservoirs, which is especially important for estimating carbon footprint of hydro energetics. The measurements on Mozhaysk reservoir have been carried since 2015. The patterns of spatial-temporal variability of methane flux are demonstrated. The highest values of methane emission during open-water period are typically observed in the end of summer, at the initiation of autumnal convective mixing. In this period, methane flux into the atmosphere can reach 350 – 390 mgC-CH₄*m^-2*d^-1. High values of methane flux, up to 400 mgC-CH₄*m^-2*d^-1, were observed after the storm events. As to spatial flux distribution, the highest values of methane emission were observed in the middle part of the reservoir and in shallow areas inhabited by macrophytes plants. The 1D hydrodynamic and biogeochemical model LAKE simulated high variability of methane flux into the atmosphere in the annual cycle. According to modeling results, the main pathway of methane into the atmosphere is ebullition, constituting more than 95% in total methane evasion into the atmosphere. The highest values of CH₄ flux according to model results take place in the beginning of spring period, after the ice-off, and during the mixing events. Modelling results for methane emission demonstrate satisfactory agreement to in situ measurements – the average annual methane emission during 5 years is 430 tons of C-CH₄ per year according to observation, and 380 tons of C-CH₄ per year in model simulations.

The work is supported by Russia`s President Council of Grants for Young Scientists, grant No. MD 1850.2020.5.

How to cite: Lomov, V., Stepanenko, V., Grechushnikova, M., and Repina, I.: Quantification of the total methane emission from valley artificial reservoir – combination of measurements and process-based modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11877, https://doi.org/10.5194/egusphere-egu21-11877, 2021.

Lakes are a fundamental resource for the Insubric region (cross-border area that includes Ticino, North Lombardy and west Piedmont regions). Therefore the quality of their waters must be protected from the risks caused by the increased anthropogenic pressure and climate change. The main objective of the interreg project named SIMILE (https://interreg-italiasvizzera.eu/database_progetti/simile/) is to support decision making in the definition of management policies through an advanced information system based on data obtained from innovative monitoring systems (automatic, diversified, cost-effective and with high spatial and temporal resolution). The information system will also facilitate the identification of possible critical issues understanding the specific causes in a timely manner by using a common methodology across Switzerland and Italy: specifically for Lake Lugano, Lake Maggiore and Lake Como. The project aims at capitalizing and sharing the experiences of the project partners in the field of monitoring and management of water resources in the project area, in particular in the context of the CIPAIS programs (IT-CH international water protection commission). The information system, fully open, is designed to offer an effective, lowcost and sustainable solution that can be maintained by the project partners beyond the end of the project. From a scientific and technical point of view the project is based on the combination of advanced automatic and continuous observation systems, high resolution remote sensing data processing, citizen science and ecological and physical models. In this presentation we will discuss experiences gained from the deployment of cost-effective monitoring platform and open technologies used for data colection, archive, processing and dissemination.

How to cite: Cannata, M., Strigaro, D., Lepori, F., Capelli, C., Veronesi, M., Rogora, M., Lami, A., and Brovelli, M.: SIMILE: An integrated monitoring system to understand, protect and manage sub-alpine lakes and their ecosystem, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12404, https://doi.org/10.5194/egusphere-egu21-12404, 2021.

Distinct sub-basins and large embayments are a ubiquitous feature of many lakes. Horizontal gradients in water quality between basins can result from a number of processes. For example, different seasonal mixing regimes between basins with different maximum depths can produce biochemical gradients between their hypolimnia. Consequently, interbasin exchange can be an important process with significant ecological consequences.

Combining field observations, 3D hydrodynamic modeling, and model-based Lagrangian particle tracking, we investigated wind-driven interbasin exchange between the shallow Petit Lac (max. depth 75 m) and deep Grand Lac (max. depth 309 m) basins of Lake Geneva, Western Europe’s largest lake, during early winter. In addition to CTD casts conducted in the Petit Lac, several ADCP and thermistor chain moorings were deployed at the confluence between the two basins during the winter 2018/2019.

Following a strong northeast-bound, along-axis wind event lasting from 7 to 10 December 2018, a two-layer flow pattern established at the confluence: epilimnetic water from the Petit Lac was pushed by the wind into the Grand Lac and was compensated for by a bottom inflow of deep hypolimnetic waters from the Grand Lac into the Petit Lac. Consequently, temperatures in the lower part of the water column gradually decreased at all moorings, with the lowest temperatures corresponding to values found at 180 m depth, as indicated by full-depth temperature profiles taken in November and December 2018.

For approximately 3.5 days, deep Grand Lac water was continuously transported into the Petit Lac, with observed inflowing current velocities near the bottom exceeding 27 cm s^-1. Approximately 1.5 d after the wind subsided, the current patterns at the confluence reversed and the previously upwelled Grand Lac water was drained again from the Petit Lac in a bottom-hugging current with measured velocities reaching 19 cm s ^-1.

The current and temperature patterns at the confluence were well represented by a 3D hydrodynamic model (MITgcm). Model-based particle tracking confirmed the deep origin of the upwelled Grand Lac waters. Furthermore, it revealed that the interbasin upwelling event effectively formed a current loop, during which, over the course of more than one week, hypolimnetic water from below 150 m depth first upwelled into the Petit Lac, intruding approximately 10 km into the shallow basin, and subsequently descended back into the Grand Lac hypolimnion. Moreover, low model-based gradient Richardson numbers and temperature inversions observed in the CTD profiles indicate turbulent mixing between the deep, upwelled Grand Lac waters and the “fresher,” i.e., better quality Petit Lac waters.

Our field observations and modeling results show that enhanced wind-driven interbasin exchange and deep hypolimnetic upwelling between the shallow Petit Lac and deep Grand Lac basins of Lake Geneva frequently occur during early winter. Furthermore, our results suggest that these hypolimnetic interbasin upwelling events may present a potentially important mechanism for hypolimnetic-epilimnetic exchange and deep-water renewal in Lake Geneva and possibly in other deep multi-basin lakes under similar wind conditions; especially, when considering the expected weakening of the classical deep convective cooling during wintertime due to climate change effects.

How to cite: Reiss, R. S., Lemmin, U., and Barry, D. A.: Wind-driven interbasin exchange and hypolimnetic upwelling during wintertime in a large, deep lake (Lake Geneva), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12853, https://doi.org/10.5194/egusphere-egu21-12853, 2021.

Dissolved oxygen is a central player in water quality management of lakes and reservoirs. Low levels or absence of oxygen poses a major problem, especially in drinking water reservoirs. Usually, the focus lies on the oxygen depletion in deep water. However, in many stably stratified water bodies, significant oxygen deficits have been documented in the metalimnion, even in lakes of low trophic state. This phenomenon is known as metalimnetic oxygen minimum (MOM) and the causes of MOM have been discussed controversially. The Rappbode Dam, Germany's largest drinking water reservoir, forms a MOM every year and long-term observations indicate that the oxygen deficit may have increased in recent years. Although the data cover a long period (40 years), they are very heterogeneous in terms of temporal and spatial resolution. Our study aims at systematically analysing the available data to characterize the interannual development of the MOM with respect to existing trends and to identify relevant environmental and management factors. The results confirm increasing surfacewater temperatures and unchanged deepwater temperatures in summer (Mai to October) as well as an increasingly prolonged summer stratification in the course of global warming. In contrast to the previous working hypothesis, increasing stratification duration is not correlated with the significantly increasing (p 0.009; τ -0.26) annual maximum intensity of the MOM.

How to cite: Seewald, M., Mi, C., Donner, J., and Rinke, K.: Forty years record of the metalimnetic oxygen minimum in Germany’s largest drinking water reservoir, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12937, https://doi.org/10.5194/egusphere-egu21-12937, 2021.

Lake and reservoir water quality is impacted greatly by the input of momentum, heat, oxygen, sediment, nutrients and contaminants delivered to them by riverine inflows. When such an inflow is negatively buoyant, it will plunge upon contact with the receiving ambient water and form a gravity-driven current near the bed (density current). If such a current is sediment-laden, its bulk density can be higher than that of the surrounding ambient water, even if its carrying fluid has a density lower than that of the surrounding ambient water. After sufficient sediment particles have settled however, the buoyancy of the current can reverse and lead to the plume rising up from the bed, a process referred to as lofting. In a stratified environment, the river plume may then find its way into a layer of neutral buoyancy to form an intermediate current (interflow). A deeper understanding of the wide range of hydrodynamic processes related to the transitions from open-channel inflow to underflow (plunging) and from underflow to interflow (lofting) is crucial in predicting the fate of all components introduced into the lake or reservoir by the inflow.

Field measurements of the plunging inflow of the negatively buoyant Rhône River into Lake Geneva (Switzerland/France) are presented. A combination of a vessel-mounted ADCP and remote sensing cameras was used to capture the three-dimensional flow field of the plunging and lofting transition zones over a wide range of spatial and temporal scales.

In the plunge zone, the ADCP measurements show that the inflowing river water undergoes a lateral (perpendicular to its downstream direction) slumping movement, caused by its density surplus compared to the ambient lake water and the resulting baroclinic vorticity production. This effect is also visible in the remote sensing images in the form of a distinct plume of sediment-rich water with a triangular shape leading away from the river mouth in the downstream direction towards a sharp tip. A wide range of vortical structures, which most likely impact the amount of mixing taking place, is also visible at the surface in the plunging zone.

In the lofting zone, the ADCP measurements show that the underflow undergoes a lofting movement at its edges. This is most likely caused by a higher sedimentation rate due to the lower velocities at the underflow edges and leads to a part of the underflow peeling off and forming an interflow, while the higher velocity core of the underflow continues following the bed. Here, the baroclinic vorticity production works in the opposite direction as that in the plunge zone. Further downstream, as more particles have settled and the surrounding ambient water has become denser, the remaining underflow also undergoes a lofting motion. The remnants of these lofting processes show in the remote sensing images as intermittent ‘boils’ of sediment rich water reaching the surface and traces of surface layer leakage.

How to cite: Thorez, S., Blanckaert, K., Lemmin, U., and Barry, D. A.: From inflow to interflow, through plunging and lofting: uncovering the dominant flow processes of a sediment-rich negatively buoyant river inflow into a stratified lake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13214, https://doi.org/10.5194/egusphere-egu21-13214, 2021.

Lakes and reservoirs are standing surface water bodies that provide several environmental services and anthropic uses. As driving forces, climate conditions, sediment loads and pollutants influence the hydrodynamic behaviour of lakes, affecting the thermal stratification and mixing regime patterns that play expressive roles in the water quality condition. Therefore, the analysis of climate change scenarios allows the planning and implementation of preventive and mitigative actions. Mathematical modelling can simulate the thermal regime of lakes and reservoirs, considering different boundary conditions. Three-dimensions models are often used to better assess the changes on these environments, however, the extensive set of information required, along with its elevated processing parameters, can determine the selection of simpler models for long periods simulations, provided that the results accuracy remains appropriated. This paper intends to evaluate the differences and similarities between a one-dimension (GLM) and a three-dimensions (DELFT3D) transport models, used to assess the impacts of different climate scenarios on the thermal regime of a small lake. The case study was conducted on the Hedberg Dam, located about 90 km from Sao Paulo city, Brazil. It is a 0,2 km²-4.5m depth pond, built in the beginnings of the 19th century. Its hydrological catchment area is partially protected, with some sparse urban occupations. Both models used morphology characteristics, atmospheric variables and flow as input data. The calibration and validation were performed using water thermal profiles from high-frequency sensor data, observed from 2016 to 2018. Two climate change scenarios, optimistic and pessimistic, based on Eta Regional Climate Model, were simulated considering changes in radiation, air temperature, wind, precipitation and flow. Both results indicate changes in the thermal profiles regime, with increasing occurrence of mixing events and variations on the stratification patterns. However, differences can be noted in the water balance and in the thermal profiles results.

How to cite: Duarte, B., Amorim, L., Martins, J. R., and Bernardino, J. C.: Comparison of 1D and 3D hydrodynamic models on the assessement of climate change scenarios impact over a small tropical lake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14141, https://doi.org/10.5194/egusphere-egu21-14141, 2021.