Streamflow, biogeochemical cycling, and flux transport models rely on digital representations of river networks. At local to regional extents, such representations can be very detailed and account for individual hydrologic features such as dams and river diversions. However, at continental to global extents, these hydrologic features are often far less resolved. This lack of detail can lead to a mismatch between the resolution of hydrologic features relative to the resolution of the network itself. One solution to such mismatches is to impose a “global” standard hydrologic feature resolution. However, this approach may fail to provide critical information that is essential for accurate modeling because it removes hydrologic feature data (such as lakes) that could otherwise be passed to calibration and fitting routines. In this research, we test how variations in river network resolution may introduce such resolution mismatches. Using Viti Levu, Fiji as a case study, we show that even small “coarsening” of network resolution has a significant effect on lake representation and has carry-on effects on overall network transport. Because these effects were less pronounced for networks with larger lakes, this indicates that including lakes even in coarse models may be very informative given the extent to which available chemical and hydrologic data is skewed towards larger lakes.
Cascade hydropower group of Wujiang River in Guizhou Province, China, is an essential part of the west-east power transmission project. Hongjiadu Hydropower Station is the first reservoir in the cascade hydropower group, with a hydraulic retention time of 1.008 years, DAM height of 179.5 m, a controlled watershed area of 9,900 km2, a total storage capacity of 4.95 billion m3, an adjusted storage capacity of 3.36 billion m3, an annual average runoff of 4.89 billion m3, a total installed capacity of 0.6 million kilowatts. Since 2004, Hongjiangdu Hydropower Station has generated power. Water storage in the flood period and water power generation in the dry period led to a huge difference in water level and affected fish habitat. Firstly, the water and nitrogen mass balance model was applied. CVL Grid and Google Earth Pro were applied to build 292 orthogonal grids. Environmental fluid dynamics code was exploited to simulate the hydrodynamic, temperature, water quality, and sediment modules of Hongjiadu Reservoir in 2007, 2008 and 2017. The fieldwork and the interpolation method were used the inputted particulate refractory organic nitrogen, easily degradable organic nitrogen, dissolved organic nitrogen, Kjeldahl nitrogen, ammonia nitrogen[2,3], nitrate-nitrogen[2,3], and total nitrogen of inflow rivers. Cage farming brought large amounts of particulate nitrogen (fish feed), which were input the corresponding grids of Hongjiadu Reservoir. Fortunately, cage farming has now turned into ecological fish farming. This model was an exploration of fish habitat protection in the first stage of a cascade hydropower station. Liang, X.; et. al. 2019. Control of the hydraulic load on nitrous oxide emissions from cascade reservoirs. Environ. Sci. & Technol. 53(20): 11745-11754. Liu, X. 2010. The effects of cascade reservoirs on biogeochemical cycling of nitrogen in River-Reservoir system -take Wujiang River and its branch Maotiao River in SW China for example, Guiyang, Institute of Geochemistry, Chinese Academy of Sciences, Doctoral degre (In Chinese).
Aquatic vegetation alters the hydrodynamics of natural waters, such as rivers, lakes, and estuaries. Plants can generate turbulence that propagates throughout the entire water column, which affects gas transfer mechanisms at both air-water and water-sediment interfaces, driving changes of dissolved oxygen (DO), an important indicator of water quality. We conducted a series of laboratory experiments with rigid cylinder arrays to mimic vegetation using a staggered configuration in a recirculating race-track flume. Walnut shells were chosen as the sediment substrate, which interacts with DO in water. 2D planar Particle Image Velocimetry was used to characterize the flow field under various submergence ratios, highlighting the effect of vegetation on turbulence quantities. Gas transfer rates were determined by measuring the DO concentration during the re-aeration process based on the methodology proposed by the American Society of Civil Engineers. Our data provide new insight on Air-Water-Vegetation-Sediment interactions in streams as a function of submergence ratio, array density, and flow turbulence. A modified surface renewal model using turbulence production as an indicator of gas transfer efficiency is used to predict surface gas transfer rates. A delayed time of re-aeration between the bulk and the near-bed region was observed and varies with flow velocities and submergence ratios, which controls the oxygen flux from water to sediment. Future studies are required to investigate the cause of the delayed time to incorporate sediment oxygen demand in a substrate-to-surface transfer model.
The interior of lakes is often quiescent and most of the mixing in a lake occurs at the sloping boundaries, where wind-induced internal waves create turbulence (which leads to mixing) through interactions with the lakebed. To predict the occurrence and strength of turbulence in terms of meteorological forcing and stratification, we investigated the dependence of internal wave type, and their contribution to turbulence on the slope, on the Lake number, which compares the stabilizing tendency of stratification to the destabilizing tendency of the wind. Three thermistor chains and a meteorological station were deployed in West Okoboji Lake (length ~ 9 km, max. depth ~ 40 m) for two weeks. A wavelet analysis was conducted to determine time periods when different wave frequencies were excited, with particular focus on the first vertical mode seiche, the critical frequency with respect to the stratification and slope, and high frequency waves in the band of 1-10 times the buoyancy frequency. We measured the velocities in the bottom boundary layer (BBL) with a high resolution acoustic current profiler (2 MHz Nortek HR Aquadopp) and then computed the turbulent dissipation rate using the structure function method, which uses the spatial correlations of velocity along a beam to estimate the dissipation. This generated a two week time series of turbulent dissipation rate in the BBL which was then compared to the wavelet amplitudes. During the deployment, a strong daily wind forced near constant internal wave activity. The theoretical period of the first vertical mode seiche was ~17 hours, but the diurnal wind forcing interfered with free oscillation of this mode. Although not an obvious natural frequency of the lake, waves of the critical frequency (which had a period of ~11 hours) were activated throughout the measurement period. High-frequency waves were observed in the thermistor chain near the slope at the lowest Lake number wind events. The turbulence observed on the boundary was highest during these events, implying that the low frequency seiching was less important than higher frequency motions in driving turbulence on the slope.
A small gradient in the densities (Δρ) of two rivers was recently shown to develop coherent streamwise orientated vortices (SOVs) in the mixing interface of their confluence. We further investigate this phenomenon at the Coaticook and Massawippi confluence (Quebec, Canada) using eddy-resolved numerical modelling to examine how the magnitude and direction of Δρ; affect this secondary flow feature. Results show that a front from the denser channel always slides underneath the lighter channel independent of the direction of Δρ. When the fast tributary (Coaticook) is denser, coherent clockwise rotating density SOVs tend to form on the slow (Massawippi) side. However, when the slow Massawippi is denser by the same magnitude, anticlockwise secondary flow caused principally by shear induced interfacial instabilities develop on the fast Coaticook side. This shows the inertia of the tributary opposing the lateral propagation of the dense front shapes the secondary flow characteristics of the mixing interface. Moreover, in the absence of a density difference, anticlockwise SOVs are predicted by the model which correspond well to new aerial observations of anticlockwise SOVs at the site. A densimetric Froude number (Fd) convention accounting for the direction of Δρ is proposed to accurately convey the local inertial forces that oppose the lateral propagation of the dense front. Finally, a conceptual model of the mixing interface’s secondary flow structure over a spectrum of plausible Fd values is proposed. The Fd convention provides a flexible and consistent metric for use in future studies examining the effects of Δρ on river confluence hydrodynamics.
The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CHO in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modeled using reaction-transport equations vertical profiles of the concentration and isotopic composition (δC) of CH and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Our results reveal that methanogenesis only occurs via hydrogenotrophy in both basins. Furthermore, we calculate, from CH and DIC production rates associated with methanogenesis, that the fermenting OM has an average carbon oxidation state (COS) below −0.9. Modeling solute porewater profiles reported in the literature for four other seasonally anoxic lake basins also yields negative COS values. Collectively, the mean (±SD) COS value of −1.4 ± 0.3 for all the seasonally anoxic sites is much lower than the value of zero expected from carbohydrates fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM and that the COS should be included in the formulation of OM fermentation in models applied to lake sediments. This study highlights the need to better characterize the labile OM undergoing mineralization to interpret present-day greenhouse gases cycling and predict its alteration under environmental changes.
The interaction of a uniform cooling rate at the lake surface with sloping bathymetry efficiently drives cross-shore water exchanges between the shallow littoral and deep interior regions. The faster cooling rate of the shallows results in the formation of density-driven currents, known as thermal siphons, that flow downslope until they intrude horizontally at the base of the surface mixed layer. Existing parameterizations of the resulting buoyancy-driven cross-shore transport assume calm wind conditions which are, however, rarely observed in lakes and thereby strongly restrict their applicability. Here we examine how moderate winds (≲ 5 m s-1) affect this convective cross-shore transport. We derive simple analytical solutions that we further test against realistic three-dimensional numerical hydrodynamic simulations of an enclosed stratified basin subject to uniform and steady surface cooling rate and cross-shore winds. We show cross-shore winds modify the convective circulation, stopping or even reversing it in the upwind littoral region and enhancing the cross-shore exchange in the downwind region. The magnitude of the simulated offshore unit-width discharges in the upwind and downwind littoral regions was satisfactorily predicted by the analytical parameterization. Our scaling expands the previous formulation to a regime where both wind and buoyancy forces drive cross-shore discharges of similar magnitude. This range is defined by the non-dimensional Monin-Obukhov length scale, χMO: 0.1 ≲ χMO ≲0.5. The information needed to evaluate the scaling formula can be readily obtained from a traditional set of in-situ observations.
Lakes are traditionally classified based on their thermal regime and trophic status. While this classification adequately captures many lakes, it is not sufficient to understand seasonally ice-covered lakes, the most common lake type on Earth. Here, we propose an additional classification to differentiate under-ice stratification. When ice forms in smaller and deeper lakes, inverse stratification will form with a thin buoyant layer of cold water (near 0oC) below the ice, which remains above a deeper 4oC layer. In contrast, the entire water column can cool to ~0oC in larger and shallower lakes. We suggest these alternative conditions for dimictic lakes be termed “cryostratified” and “cryomictic.” We describe the inverse thermal stratification in 19 highly varying lakes and derive a model that predicts the temperature profile as a function of wind stress, area, and depth. The model opens up for a more precise prediction of lake responses to a warming climate.
The present study aims to quantify (estimate) the impact of human water consumption—as for irrigation, livestock, domestic, manufacturing, and thermal energy production—versus (natural) climatic variability on the water balance and storage of the Lake Urmia (LU) basin and consequently the lake desiccation during the past decades. This is indeed a curious question with high practical relevance, given the ongoing drying of the lake and scientific debates around possible causes and viable remedies. One of the strength of the study is incorporating multiple input data (both ground and remote sensing) in developing the basin’s hydrologic model. The authors have also attempted to include the groundwater data which is highly important in this basin, and has been ignored in many (not all) of the previous studies. I enjoyed reading the paper, however, as the other reviewers have already pointed out there are major shortcomings that call for a major revision. In the spirit of helping the authors to improve the manuscript, I’d like to further comment on a number of—I believe—major deficiencies and questionable assumptions of the study that undermine the reliability of their results and discussion, given my own (limited) knowledge/experience in studying the lake’s dynamics and desiccation [Khatami, 2013; Khatami and Berndtsson, 2013; Khazaei et al., in review]. I hope the authors would find my comments useful in highlighting the new insights and contribution of their study.
Environmental controls on methane (CH) emission from lakes are poorly understood at sub-daily time scales due to a lack of continuous data, especially for ebullition. We used a novel technique to partition eddy covariance CH flux observed in the littoral zone of a mid-latitude shallow lake in Japan and examined the environmental controls on diffusion and ebullitive CH flux separately at a sub-daily time scale during different seasons. Both diffusive and ebullitive flux were significantly higher in summer than winter. The contribution of ebullitive flux to total flux was 56% on average. Diffusive flux increased with increasing wind speed due to increased subsurface turbulence. For a given wind speed, diffusive flux was higher in summer than in winter due to the higher concentration of dissolved CH in the surface water during summer. The transfer of accumulated dissolved CH from the bottom layer to the surface in summer and the accumulation of dissolved CH under surface ice in winter were important for explaining the variability of diffusive flux. In summer, ebullition tended to occur following triggers such as a decrease in hydrostatic pressure or an increase in wind speed. In winter, on the other hand, the impact of triggers was not obvious, and ebullition tended to occur in the morning when the wind speed began to increase. The low CH production rate in winter slowed the replenishment of bubbles in the sediment, negating the effect of triggers on ebullition.
Water quality monitoring is an integral tool in the management of freshwater resources. It identifies trends in water quality and tracks the effects of anthropogenic influences such as shoreline development and eutrophication and the successes of restoration actions. Remote sensing presents a cost efficient complementary approach for a more comprehensive assessment of our freshwater resources. The open access to Earth observation data limits the costs to software maintenance and data processing costs and makes the integration of remote sensing into water quality monitoring programs more attractive for natural resources management agencies. The operational annual satellite retrieval of the water clarity for thousands of lakes across Wisconsin from Landsat 7 ETM+ and Landsat 8 OLI-TIRS data assists the Wisconsin DNR in trophic state assessments for the State of Wisconsin. The results are shared within the Wisconsin DNR for the Wisconsin Water Quality Reports to Congress in compliance with Section 305(b) of the Clean Water Act and with state and local water quality managers, lake organizations, and the public. Summer water clarity maps are shared through the Wisconsin Lakes & Aquatic Invasive Species Mapping Tool and GIS data portal. Current remote sensing activities at the Wisconsin DNR include the transition of image processing efforts from a desktop to cloud environment, the collection of field and satellite match-up data for waterbodies across Wisconsin to support algorithm development and validation efforts by external partners, and the development of strategies for the integration of satellite data products into our water quality monitoring program. This poster provides insights in the different components of the successful integration of satellite data analysis into our water quality monitoring program and can serve as a roadmap for natural resources management agencies.
Nutrient pollution is considered one of America’s most widespread, costly, and challenging environmental problems. Artificial Floating Islands (AFIs), a phytoremediation technology, has been proven as an efficient, environmental-friendly, and cost-effective strategy to address this issue. However, most previous studies of AFIs were done in controlled conditions at mesocosm experiments. In addition, limited information exists on the use of AFIs as a nutrient remediation/prevention strategy in Ohio. This study aims to fill these gaps. We are currently undertaking a combination of mesocosm and natural experiment to assess the nutrient-removal efficiency of AFI systems in the Milliron Research Wetlands (at the Ohio State University Mansfield campus), and establish a performance baseline for two native aquatic plant species, Carex comosa and Eleocharis palustris. In this study, 18 AFIs, 6 planted with Carex comosa, 6 with Eleocharis palustris, and 6 have no plants, were deployed in a section of the Milliron Research Wetlands. Physical and chemical parameters are being monitored bi-weekly. The AFI systems are constructed using PVC pipes to provide buoyance, EVA foam mats as platforms, and nylon nets to cover the system. Each AFI unit has nine luffa sponges, inserted in the foam mat, to hold aquatic plant seedlings, keep the moisture of roots, and enlarge the surface area for bacterial biofilm development. Since nutrient removal from the wetland is affected by numerous natural processes, a mesocosm experiment was set up to assist the quantification of nutrient removal due specifically to the presence of AFIs. The mesocosm experiment mimics the natural experiment at the wetland and contain 12 equal-size tanks containing water pumped directly from the wetland, 3 of which have AFIs with Carex comosa, 3 have Eleocharis palustris, 3 have no plants, and 3 contain just water from the wetland. Physical and chemical measurements (as well as sample collections) are performed weekly in the tanks. Water in the tanks are exchanged bi-weekly. Preliminary results show that the AFI systems quickly developed large root systems and extensive bacterial biofilms. The effects of the associations between plant biomass, biofilm development, and changing chemical and physical conditions will be investigated as the experiment progresses.
Lake sedimentation rate represents a synthetic metric of ecosystem functioning. Many localized studies have reported a significant association between land use/land cover changes and lake sediment mass accumulation rates, with a few global syntheses echoing these findings at larger scales. In the literature, studies evaluating lead-210 (210Pb) for establishing sediment chronologies will report at least one of three dating models, but the constant rate of supply (C.R.S.) model is the most widely used. However, it is often unclear how or why this model is selected, despite its influence on the interpretation of many subsequent analyses about ecosystem dynamics and functioning. It would thus be advantageous to design an objective and semi-automated way of choosing among dating models. We measured radioisotopic activities in 37 sediment cores across four ecozones of eastern Canada and developed an approach to assess model fit for the three commonly applied dating models. The derived chronologies were then used to evaluate the spatial and temporal variation in sedimentation rates across four ecozones in Canada (covering a surface area of 2.2 x 10^6 km2). We observed a recent increase in lake sedimentation rates across most lakes, as has been observed globally, albeit with significant differences in the magnitude of sedimentation rates across ecozones. Across all lakes, we found that regional human population counts and mean annual air temperatures were significant temporal predictors of variation in mass accumulation rates. Overall, this analytical framework offers an objective approach for assessing fit and selecting among sediment age models, which contributes to a more robust quantification of sedimentation rates. With this first application, we provide a quantitative assessment of how lake sedimentation rates vary across a northern lake-rich region and have responded to environmental change.
Due to its substantial role on the Earth’s biogeochemical cycles and human health, nitrogen is recognized as one of the major water quality indicators of Sustainable Development Goal 6.3.2. Quantifying these potential impacts in large spatial scales still appears to be a grand challenge because of the high computational demand required by the distributed physically based global models and their intensive data requirements for calibration and validation. The former prevents a comprehensive analysis of the full spectrum of the model behavior under different conditions, and the latter impinges on the reliability of model-based inference. To tackle this problem, we developed a data-driven model using a spatio-temporal Random Forest algorithm to predict levels of nitrogen at 0.5-degree spatial resolution from 1992 to 2010 across the world. Several variables representing livestock, climate, hydrology, topography, etc. have been selected as predictors. The response variable of interest was nitrate–nitrite, which is responsible for the high risk of infant methemoglobinemia. Our results indicate that changes in the nitrogen concentration is mainly driven by cattle and sheep population, fertilizer application, precipitation, and temperature variability, implying livestock population, climate change, and anthropogenic forces can be important risk factors for global water quality deterioration. Furthermore, using the predicted levels of nitrogen, we characterized large-scale water quality patterns, and thus identified a few major ‘hot spots’ of water quality. The proposed model can also help assess potential impacts of future scenarios (e.g., livestock production or land use change) on global water quality conditions for better development of effective policy strategies.