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  • 1
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    Boundary mixing in lakes: 2. Combined effects of shear- and convectively induced turbulence on basin-scale mixing (2011)
    Wiley
    Details
    Publication Date: 2011-10-15
    Description: A detailed comparison of results from a numerical three-dimensional hydrostatic lake model with high-resolution observations of the vertical structure of the turbulent bottom boundary layer (BBL) in a medium-size lake (Lake Alpnach, Switzerland) is provided. The focus of this study is on the shear-induced generation and destruction of stratification in the BBL that may ultimately lead to unstable layers (convection). The model was shown to provide a reliable description of the internal seiching dynamics, as well as the local BBL properties, including the generation of shear-induced convection in two data sets from 2003 and 2007. Basin-scale mixing parameters, inferred from the simulations, are closely connected to the seiching motions, with the hypolimnetic mixing reacting almost immediately to the variable wind-forcing and seiching activity. During upslope flow, the BBL becomes convectively turbulent, causing low mixing efficiency on a basin-scale, whereas during downslope flow, the BBL is restratifying and shear-induced turbulence is weak but leads to a higher mixing efficiency. The overall deep-water mixing efficiency varied in the range of 5 to 10% in this system dominated by turbulent boundary processes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Comment on An additional challenge of Lake Kivu in Central Africa – upward movement of the chemoclines by Finn Hirslund (2012)
    PAGEPress
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    Publication Date: 2012-07-24
    Description: In a paper published earlier this year, Hirslund suggested to release degassed deepwater from methane harvesting to the surface layer of Lake Kivu in order to counteract an observed slow rising of the chemoclines in the lake. In this comment, we present strong evidence for the presence of subaquatic springs in the lake that maintain the chemoclines at their present levels. Furthermore, the observed changes in the stratification do not call for urgent action. We therefore strongly advise against the proposed measures, which would most probably be harmful for the ecosystem, and instead propose to further monitor the development of the stratification in the lake.
    Print ISSN: 1129-5767
    Electronic ISSN: 1723-8633
    Topics: Biology
    Published by PAGEPress
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  • 3
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    Methane sources and sinks in Lake Kivu (2011)
    Wiley
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    Publication Date: 2011-07-22
    Description: Unique worldwide, Lake Kivu stores enormous amounts of CH4 and CO2. A recent study reported that CH4 concentrations in the lake have increased by up to 15% in the last 30 years and that accumulation at this rate could lead to catastrophic outgassing by ∼2100. This study investigates the present-day CH4 formation and oxidation in Lake Kivu. Analyses of 14C and 13C in CH4 and potential carbon sources revealed that below 260 m, an unusually high ∼65% of the CH4 originates either from reduction of geogenic CO2 with mostly geogenic H2 or from direct inflows of geogenic CH4. Aerobic CH4 oxidation, performed by close relatives of type X CH4-oxidizing bacteria, is the main process preventing CH4 from escaping to the atmosphere. Anaerobic CH4 oxidation, carried out by CH4-oxidizing archaea in the SO42−-reducing zone, was also detected but is limited by the availability of sulfate. Changes in 14CCH4 and 13CCH4 since the 1970s suggest that the amount of CH4 produced from degrading organic material has increased due to higher accumulation of organic matter. This, as well as the sudden onset of carbonates in the 1960s, has previously been explained by three environmental changes: (1) introduction of nonnative fish, (2) amplified subaquatic inflows following hydrological changes, and (3) increased external inputs due to the fast growing population. The resulting enhancement of primary production and organic matter sedimentation likely caused CH4 to increase. However, given the large proportion of old CH4 carbon, we cannot exclude an increased inflow of geogenic H2 or CH4.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Sediment accumulation and carbon, nitrogen, and phosphorus deposition in the large tropical reservoir Lake Kariba (Zambia/Zimbabwe) (2011)
    Wiley
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    Publication Date: 2011-07-12
    Description: Large dams affect the aquatic continuum from land to ocean by accumulating particles and nutrients in their reservoirs. We examined sediment cores to quantify sediment, organic carbon (OC), nitrogen (N), and phosphorous (P) accumulation, and to examine historic changes and spatial variability in the sedimentation pattern in Lake Kariba, the largest hydropower reservoir in the Zambezi River Basin (ZRB). Sediment characteristics (concentrations of OC, N, P; δ13C and δ15N; wet bulk density) showed large variability both with sediment depth and between cores. While organic matter (OM) in river deltas was primarily allochthonous in origin, OM characteristics (δ13C, C:N) in lacustrine sediments suggest that autochthonous sources account for 〉45% of the OM that accumulates over large areas of the lake. At the same time, the relative contribution of allochthonous material within individual layers of lacustrine cores varied considerably with depth due to discrete flood deposits. The overall sediment accumulation rate in Lake Kariba is on the order of 4 × 106 t yr−1, and the estimated OC accumulation of 120 × 103 t C yr−1 accounts for ∼1‰ of globally buried OC in reservoirs. In addition, mass balance calculations revealed that approximately 70% and 90% of incoming total N and P, respectively, are eliminated from the water column by sedimentation (N, P) and denitrification (N). Since Lake Kariba attenuates flow from ∼50% of the ZRB, these OC, N, and P removals represent a drastic reduction in nutrient loadings to downstream riparian ecosystems and to the coastal Indian Ocean.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Particle dynamics in high-Alpine proglacial reservoirs modified by pumped-storage operation (2011)
    Wiley
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    Publication Date: 2011-09-27
    Description: Temperature and suspended particle distribution were surveyed and modeled in two high-Alpine reservoirs in Switzerland, connected by pumped-storage operations for ∼30 years. Due to different glacier coverage of the catchments, the two reservoirs exhibit different particle concentrations and temperatures. After ice-breakup, the lower reservoir with a higher glacier cover in its catchment experiences a higher particle input becoming more turbid than the upper reservoir, which in contrast becomes warmer and thermally more stratified. The pumped-storage operations, which replace the basin volumes annually at least 6 (larger lower basin) to 10 (smaller upper basin) times, modify the physical characteristics of the two reservoirs. This is especially so in winter, when they are ice-covered, without riverine input and at low water level. Our reservoir investigations between 2007 and 2009 and the subsequent particle-balance model show that the upper and lower basins have become more and less turbid, respectively. Pumped-storage operations modify the stratification and particle distribution in both reservoirs and therefore alter the particle outflow and sedimentation. However, on the basis of particle concentrations and reservoir volumes, it is evident that the annually integrated particle release to downstream (∼40% of total) and to overall sedimentation (∼60%) have hardly changed. The budget model was useful in the prediction of particle distribution and sedimentation dynamics in the pumped-storage system. It implies that this approach can be useful for further employment during planning stages of power plants in order to modify and mitigate downstream particle loads in reservoir operations.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Impact of a large tropical reservoir on riverine transport of sediment, carbon, and nutrients to downstream wetlands (2011)
    Wiley
    Details
    Publication Date: 2011-12-22
    Description: Large dams can have major ecological and biogeochemical impacts on downstream ecosystems such as wetlands and riparian habitats. We examined sediment removal and carbon (C), nitrogen (N), and phosphorus (P) cycling in Itezhi-Tezhi Reservoir (ITT; area = 364 km2, hydraulic residence time = 0.7 yr), which is located directly upstream of a high ecological value floodplain ecosystem (Kafue Flats) in the Zambezi River Basin. Field investigations (sediment cores, sediment traps, water column samples), mass balance estimates, and a numerical biogeochemical reservoir model were combined to estimate N, P, C, and sediment removal, organic C mineralization, primary production, and N fixation. Since dam completion in 1978, 330 × 103 tons (t) of sediment and 16 × 103, 1.5 × 103, 200 t of C, N, and P, respectively, have accumulated annually in ITT sediments. Approximately 50% of N inputs and 60% of P inputs are removed by the reservoir, illustrating its potential in decreasing nutrients to the downstream Kafue Flats floodplain. The biogeochemical model predicted substantial primary production in ITT (∼280 g C m−2 yr−1), and significant N-fixation (∼30% for the total primary production) was required to support primary production due to marginal inputs of inorganic N. Model simulations indicate that future hydropower development in the reservoir, involving the installation of turbines driven by hypolimnetic water, will likely result in the delivery of low-oxygen waters to downstream ecosystems and increased outputs of dissolved inorganic N and P by a factor of ∼4 and ∼2 compared to current dam management, respectively.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Atlantic hurricanes and associated insurance loss potentials in future climate scenarios: limitations of high-resolution AGCM simulations (2012)
    Co-Action Publishing
    In: Tellus A
    Details
    Publication Date: 2012-01-31
    Description: Potential future changes in tropical cyclone (TC) characteristics are among the more serious regional threats of global climate change. Therefore, a better understanding of how anthropogenic climate change may affect TCs and how these changes translate in socio-economic impacts is required. Here, we apply a TC detection and tracking method that was developed for ERA-40 data to time-slice experiments of two atmospheric general circulation models, namely the fifth version of the European Centre model of Hamburg model (MPI, Hamburg, Germany, T213) and the Japan Meteorological Agency/ Meteorological research Institute model (MRI, Tsukuba city, Japan, TL959). For each model, two climate simulations are available: a control simulation for present-day conditions to evaluate the model against observations, and a scenario simulation to assess future changes. The evaluation of the control simulations shows that the number of intense storms is underestimated due to the model resolution. To overcome this deficiency, simulated cyclone intensities are scaled to the best track data leading to a better representation of the TC intensities. Both models project an increased number of major hurricanes and modified trajectories in their scenario simulations. These changes have an effect on the projected loss potentials. However, these state-of-the-art models still yield contradicting results, and therefore they are not yet suitable to provide robust estimates of losses due to uncertainties in simulated hurricane intensity, location and frequency. Keywords: Loss potential; hurricanes; future scenarios; hurricane identification method (Published: 30 January 2012) Citation: Tellus A 2012, 64 , 15672, DOI: 10.3402/tellusa.v64i0.15672
    Print ISSN: 0280-6495
    Electronic ISSN: 1600-0870
    Topics: Geography , Geosciences , Physics
    Published by Co-Action Publishing on behalf of The International Meteorological Institute in Stockholm.
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  • 8
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    Remote Sensing, Vol. 6, Pages 9530-9551: Retrieval of Particle Scattering Coefficients and Concentrations by Genetic Algorithms in Stratified Lake Water (2014)
    MDPI Publishing
    Details
    Publication Date: 2014-10-10
    Description: We retrieved the mass-specific scattering coefficient b*sm(λ) = 0.60·(λ/650)−1.82 of the inhomogeneous and optically complex water column of eastern Lake Constance in May 2012. In-situ measured and modelled remote-sensing reflectance Rrs(λ) were matched via a parameter search procedure using genetic algorithms. The optical modelling consisted of solving the azimuthally-averaged Radiative Transfer Equation, forced with in-situ suspended matter concentration (sm) data. b*sm(λ) was univocally determined at red wavelengths. In contrast, we encountered unresolved spectral ambiguity at blue wavelengths due to the absence of organic absorption in our dataset. Despite this, a surprisingly good sm retrieval regression is achieved (R2 > 0.95 with respect to independent data) using our b*sm(λ). Acquisition of accurate inherent optical properties in future field campaigns is needed to verify the estimated b*sm(λ) and related assumptions.
    Electronic ISSN: 2072-4292
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by MDPI Publishing
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  • 9
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    Large lakes as sources and sinks of anthropogenic heat—Capacities and limits (2014)
    Wiley
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    Publication Date: 2014-08-30
    Description: The goal of reducing carbon fuel and thereby saving energy will increase the use of lake water for heating and cooling of riparian infrastructures. This raises the question of which heat use designs meet the ecological and technical requirements for lakes, particularly in regard to climate warming. Thus, this study explores heat use effects on the temperature and stratification of a large, deep, temperate lake by applying the one-dimensional k-epsilon model SimStrat to various forcing scenarios. Several design parameters, such as extraction and discharge depth, and their effects were assessed. Additionally, 21 st -century climate projections were used to evaluate the effects of climate change relative to those of heat use. Generally, the study showed only minor effects for a realistic heat demand of ±2 W m -2 quite independent of the heat extraction / discharge modes. Mean water temperature changed less than ±0.2 °C as long as there was no discharge into the deepest layers. Water extraction and discharge at the surface had the least thermal influence. To relate to climate change, heat use was scaled up to +85 W m -2 . Resultant simulations showed that such (unrealistic) anthropogenic, lake-based “ thermal pollution “ would have a comparable influence to that of climate change. Conversely, heat extraction could damp or even compensate climate-induced warming. The present study concludes that (i) there are minor effects on water temperatures, stratification and seasonal mixing due to heat use of up to ±2 W m -2 and (ii) those influences are insignificant relative to the expected climate change.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
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    Double-diffusive interfaces in Lake Kivu reproduced by direct numerical simulations (2014)
    Wiley
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    Publication Date: 2014-07-16
    Description: Double-diffusion transforms uniform background gradients of temperature and salinity into “staircases” of homogeneous mixed layers that are separated by high-gradient interfaces. Direct numerical simulations (DNS) and microstructure measurements are two independent methods of estimating double-diffusive fluxes. By performing DNS under similar conditions as found in our measurements in Lake Kivu, we are able to compare results from both methods for the first time. We find that ( i ) the DNS reproduces the measured interface thicknesses of in-situ microstructure profiles, ( ii ) molecular heat fluxes through interfaces capture the total vertical heat fluxes for density ratios larger than three and ( iii ) the commonly-used heat flux parameterization underestimates the total fluxes by a factor of 1.3 to 2.2.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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