• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.10 no.1
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• pp.56-68
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• 2005

We measured nitrate and other nutrients in sediment pore waters retrieved from three sites at the southern upper-tidal flats of the Ganghwa Island. Denitrification rate is estimated by applying a simple 1-D model to the nitrate profiles. Results from Jangwha and Dongmak sites are $7.8{\sim}9.4{\times}10^{-7}{\mu}mol{\cdot}cm^{-2}{\cdot}sec^{-1}$, and $1.4{\sim}3.6{\times}10^{-7}{\mu}mol{\cdot}cm^{-2}{\cdot}sec^{-1}$, respectively. Rates are comparable to those reported around the world in an order of magnitude. Denitrification was lower in summer. The rates were about 1.5 times higher at site where the surface sediments consist of relatively coarser particles. This implies that particle size would control the reactant supply to the subsurface sediment. One may claim the denitrification as an evidence of the biogeochemical purification function of tidal flat. However, the purification seems not a general attribute of a tidal flat when whole system is scrutinized by a thermodynamic criterion. Currently the term 'tidal flat' is used when describing the diverse coastal wetlands such as salt marshes, sandy tidal flats and muddy tidal flats, which exhibit quite different ecological functions. Thus it is worthy of mentioning that the classification of coastal wetlands on the basis of sedimentological characteristics and biogeochemical functions should facilitate our understanding.

• Journal of The Korean Society of Agricultural Engineers
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• v.51 no.6
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• pp.53-62
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• 2009

The EFDC (Environmental Fluid Dynamics Code), a numerical model for simulating three-dimensional (3D) flow, transport, and biogeochemical processes in surface water systems including rivers, reservoirs, and estuaries, was applied to assess the effect of sea water and fresh water exchange rates ($Q_e$) on the mixing characteristics of a conservative pollutant (tracer) induced from upstreams and salinity in Saemangeum Lake, Korea. The lake has been closed by a 33 km estuary embankment since last April of 2006, and now seawater enters the lake partially through two sluice gates (Sinsi and Garyuk), which is driving the changes of hydrodynamic and water quality properties of the lake. The EFDC was constructed and calibrated with surveyed bathymetry data and field data including water level, temperature, and salinity in 2008. The model showed good agreement with the field data and adequately replicated the spatial and temporal variations of the variables. The validated model was applied to simulated the tracer and salinity with two different gate operation scenarios: RUN-1 and RUN-2. RUN-1 is the case of real operation condition ($Q_e=25,000,000\;m^3$) of 2008, while RUN-2 assumed full open of Sinsi gate to increase $Q_e$ by $120,000,000\;m^3$. Statistical analysis of the simulation results indicate that mixing characteristics of pollutants from upstream can be significantly affected by the amount of $Q_e$.

• Journal of Korean Society on Water Environment
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• v.24 no.5
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• pp.603-610
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• 2008

A mathematical model was developed to understand how the presence of plants affects vertical profiles of electron acceptors, their reduced species, and trace metals in the wetland sediments. The model accounted for biodegradation of organic matter utilizing sequential electron acceptors and subsequent chemical reactions using stoichiometric relationship. These biogeochemical reactions were affected by the combined effects of oxygen release and evapotranspiration driven by wetland plants. The measured data showed that $SO_4{^{2-}}$ concentrations increased at the beginning of the growing season and then gradually decreased. Based on the measured data, it was hypothesized that the limitation of the solid phase sulfide in direct contact with the roots may result in the gradual decrease of $SO_4{^{2-}}$ concentrations. With the dynamic formulation for the limitation of the solid phase sulfide, model simulated time variable sulfate profiles using published model parameters. Oxygen release from roots produced divalent metal species (i.e. $Cd^{2+}$) as well as oxidized sulfur species (i.e. $SO_4{^{2-}}$) in the sediment pore water. Evapotranspiration-induced advection increased flux of divalent metal species from the overlying water column into the rhizosphere. The increased divalent metal species were converted to the metal sulfide with sufficient FeS around the rhizosphere, which contributed to the decrease of bioavailability and toxicity of divalent metal activity in the pore water. Since the divalent metal activity is a good predictor of the metal bioavailability, this model with a proper simulation of solid phase sulfide plays an essential role to predict the dynamics of trace metals in the wetland sediments.

• Journal of Korean Society on Water Environment
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• v.33 no.3
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• pp.326-333
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• 2017

The lakes and reservoirs have been reported as important sources of carbon emissions to the atmosphere in many countries. Although field experiments and theoretical investigations based on the fundamental gas exchange theory have proposed the quantitative amounts of Net Atmospheric Flux (NAF) in various climate regions, there are still large uncertainties at the global scale estimation. Mechanistic models can be used for understanding and estimating the temporal and spatial variations of the NAFs considering complicated hydrodynamic and biogeochemical processes in a reservoir, but these models require extensive and expensive datasets and model parameters. On the other hand, data driven machine learning (ML) algorithms are likely to be alternative tools to estimate the NAFs in responding to independent environmental variables. The objective of this study was to develop random forest (RF) and multi-layer artificial neural network (ANN) models for the estimation of the daily $CO_2$ NAFs in Daecheong Reservoir located in Geum River of Korea, and compare the models performance against the multiple linear regression (MLR) model that proposed in the previous study (Chung et al., 2016). As a result, the RF and ANN models showed much enhanced performance in the estimation of the high NAF values, while MLR model significantly under estimated them. Across validation with 10-fold random samplings was applied to evaluate the performance of three models, and indicated that the ANN model is best, and followed by RF and MLR models.

• Korean Journal of Ecology and Environment
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• v.36 no.4 s.105
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• pp.434-446
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• 2003

The fate and transport of many trace metals, metalloids, and radionuclides in porous media is closely linked to the biogeochemical reactions that occur as a result of organic carbon being sequentially degraded by different microorganisms using a series of terminal electron acceptors. The spatial distribution of these biogeochemical reactions is affected by processes that are often unique and/or characteristic to a specific environment. Generic model formulations have been developed and applied to simulate the fate and transport of arsenic in two hydrologic settings, permanently flooded freshwater sediments, namely non-vegetated wetland sediments and vegetated wetland sediments. The key physical processes that have been considered are sedimentation, effects of roots on biogeochemistry, advective transport, and differences in mixing processes. Steady-state formulations were applied to the sedimentary environments. Results of numerical simulations show that these physical processes significantly affect the chemical profiles of different electron acceptors, their reduced species, and arsenate as well as arsenite that will result from the degradation of an organic carbon source in the sediments. Even though specific biological transformations are allowed to proceed only in zones where they are thermodynamically favorable, the results show that mixing as well as abiotic reactions can make the profiles of individual electron acceptors overlap and/or appear to reverse their expected order.

• Proceedings of the KSRS Conference
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• 2004.10a
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• pp.152-153
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• 2004

The growth rate of atmospheric $CO_2$ undergoes significant interannual variability, largely due to temporal variability of partitioning of $CO_2$ between terrestrial biosphere and ocean. In the present paper, as a follow-up to the work by Lee et al. [1], we estimated the year-to-year variability in net global air-sea $CO_2$ fluxes between 1982 and 2003 from observed changes in wind speed and estimated changes in ${\Delta}pCO_2$ Changes in $pCO_{25W}$ were inferred from global records of sea surface temperature (SST) anomalies and seasonally varying SST dependence of $pCO_{25W}$. The modeled interannual variability of $\pm0.2\;Pg\;C\;yr^{-1}\;(1{\sigma})$ from the present work is significantly smaller than the values deduced from atmospheric observations of $^{1.3}CO_2/CO_2$ in conjunction with different atmospheric transport models, but it is closer to the recent estimates inferred from a 3-D ocean biogeochemical model and atmospheric transport models constrained with extensive observations of atmospheric $CO_2$.

• Journal of Korean Society on Water Environment
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• v.26 no.1
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• pp.133-139
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• 2010

Global human activities associated with the use of fossil fuels have aggravated climate change, increasing air temperature. Consequently, climate change has the potential to alter surface water temperature with significant impacts on biogeochemical cycling and ecosystems in natural water body. In this study, we examined temporal trends on historical records of surface water temperature, and investigated the air temperature/water temperature relationship and the potential water temperature change from an air temperature scenario developed with regional climate model. Although the temporal trends of water temperature are highly variable site-by-site, surface water temperature was highly dependent on air temperature, and has increased significantly in some sub-watersheds over the last two decades. The results presented here demonstrate that water temperature changes are expected to be slightly higher in river system than reservoir systems and more significant during winter than summer for both river and reservoir system. Projected change of surface water temperature will likely increase $1.06^{\circ}C$ for rivers and $0.95^{\circ}C$ for reservoirs during the period 2008 to 2050. Given the potential climatic changes, every $1^{\circ}C$ increase in water temperature could cause dissolved oxygen levels to fall every 0.206 ppm.

• Atmosphere
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• v.28 no.4
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• pp.393-402
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• 2018

This study investigated future changes in the Arctic permafrost features and related biogeochemical alterations under global warming. The Community Land Model (CLM) with biogeochemistry (BGC) was run for the period 2005 to 2099 with projected future climate based on the Special Report on Emissions Scenarios (SRES) A2 scenario. Under global warming, over the Arctic land except for the permafrost region, the rise in soil temperature led to an increase in soil liquid and decrease in soil ice. Also, the Arctic ground obtained carbon dioxide from the atmosphere due to the increase in photosynthesis of vegetation. On the other hand, over the permafrost region, the microbial respiration was increased due to thawing permafrost, resulting in increased carbon dioxide emissions. Methane emissions associated with total water storage have increased over most of Arctic land, especially in the permafrost region. Methane releases were predicted to be greatly increased especially near the rivers and lakes associated with an increased chance of flooding. In conclusion, at the end of $21^{st}$ century, except for permafrost region, the Arctic ground is projected to be the sink of carbon dioxide, and only permafrost region the source of carbon dioxide. This study suggests that thawing permafrost can further to accelerate global warming significantly.

• Journal of Korean Society on Water Environment
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• v.30 no.5
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• pp.491-502
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• 2014

As meteorology is the driving force for lake thermodynamics and mixing processes, the effects of climate change on the physical limnology and associated ecosystem are emerging issues. The potential impacts of climate change on the physical features of a reservoir include the heat budget and thermodynamic balance across the air-water interface, formation and stability of the thermal stratification, and the timing of turn over. In addition, the changed physical processes may result in alteration of materials and energy flow because the biogeochemical processes of a stratified waterbody is strongly associated with the thermal stability. In this study, a novel modeling framework that consists of an artificial neural network (ANN), a watershed model (SWAT), a reservoir operation model(HEC-ResSim) and a hydrodynamic and water quality model (CE-QUAL-W2) is developed for projecting the effects of climate change on the reservoir water temperature and thermal stability. The results showed that increasing air temperature will cause higher epilimnion temperatures, earlier and more persistent thermal stratification, and increased thermal stability in the future. The Schmidt stability index used to evaluate the stratification strength showed tendency to increase, implying that the climate change may have considerable impacts on the water quality and ecosystem through changing the vertical mixing characteristics of the reservoir.

• Journal of Korean Society on Water Environment
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• v.36 no.1
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• pp.14-28
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• 2020

Future climate change may affect the hydro-thermal and biogeochemical characteristics of dam reservoirs, the most important water resources in Korea. Thus, scientific projection of the impact of climate change on the reservoir environment, factoring uncertainties, is crucial for sustainable water use. The purpose of this study was to predict the future water temperature and stratification structure of the Soyanggang Reservoir in response to a total of 42 scenarios, combining two climate scenarios, seven GCM models, one surface runoff model, and three wind scenarios of hydrodynamic model, and to quantify the uncertainty of each modeling step and scenario. Although there are differences depending on the scenarios, the annual reservoir water temperature tended to rise steadily. In the RCP 4.5 and 8.5 scenarios, the upper water temperature is expected to rise by 0.029 ℃ (±0.012)/year and 0.048 ℃ (±0.014)/year, respectively. These rise rates are correspond to 88.1 % and 85.7 % of the air temperature rise rate. Meanwhile, the lower water temperature is expected to rise by 0.016 ℃ (±0.009)/year and 0.027 ℃ (±0.010)/year, respectively, which is approximately 48.6 % and 46.3 % of the air temperature rise rate. Additionally, as the water temperatures rises, the stratification strength of the reservoir is expected to be stronger, and the number of days when the temperature difference between the upper and lower layers exceeds 5 ℃ increases in the future. As a result of uncertainty quantification, the uncertainty of the GCM models showed the highest contribution with 55.8 %, followed by 30.8 % RCP scenario, and 12.8 % W2 model.