• Journal of Korea Water Resources Association
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• v.45 no.12
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• pp.1293-1307
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• 2012

This study is to evaluate the RHESSys (Regional Hydro-Ecological Simulation System) simulated streamflow (Q), evapotranspiration (ET), soil moisture (SM), gross primary productivity (GPP) and photosynthetic productivity (PSNnet) with the measured data. The RHESSys is a hydro-ecological model designed to simulate integrated water, carbon, and nutrient cycling and transport over spatially variable terrain. A 8.5 $km^2$ Seolma-cheon catchment located in the northwest of South Korea was adopted. The catchment covers 90.0% forest and the dominant soil is sandy loam. The model was calibrated with 2 years (2007-2008) daily Q at the watershed outlet and MODIS (Moderate Resolution Imaging Spectroradiometer) GPP, PSNnet and 3 year (2007～2009) daily ET data measured at flux tower using the eddy-covariance technique. The coefficient of determination ($R^2$) and the Nash-Sutcliffe model efficiency (ME) for Q were 0.74 and 0.63, and the average $R^2$ for ET and GPP were 0.54 and 0.93 respectively. The model was validated with 1 year (2009) Q and GPP. The $R^2$ and the ME for Q were 0.92 and 0.84, the $R^2$ for GPP were 0.93.

• Proceedings of the Korea Water Resources Association Conference
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• 2016.05a
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• pp.209-210
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• 2016

우리나라의 연평균강수량은 약 1362 mm이며, 총강수량의 약 30% 이상이 증발산을 통해 손실되고 있다고 추정되어지고 있다. 증발산은 물 수지 분석에 있어 매우 중요한 성분이며, 많은 부분을 차지하지만 다른 요인들에 비해 직접적인 관측이 어려워 과거에는 경험식을 사용하거나 단순하게 가정에 의해 결정해 왔다. 또한 기상자료로부터 증발산량을 추정하거나 증발접시나 추정식으로 잠재증발산을 추정하고 있다. 또한 최근 기후변화의 가속화에 따른 홍수의 가뭄의 강도와 빈도가 높아지고 있으며, 이에 따라 수자원 관리에 있어서 기초수문조사 항목에 많은 변화를 요구하고 있다. 그 결과 2007년 4월 하천법 개정으로 증발산량 및 토양수분량이 기초수문조사 항목으로 추가되었으며, K-water 연구원에서는 용담시험유역에 플럭스타워를 설치하였고 현재 운영 중에 있다. 덕유산 플럭스타워는 용담시험유역 내에 위치한 금강 수계 구량천 상류부의 덕곡제 유역 내에 설치하였으며, 2011년 4월부터 실제 증발산량을 관측하고 있다. 동경 $127^{\circ}$42'23" ~ $127^{\circ}$44'53", 북위 $35^{\circ}$50'47" ~ $35^{\circ}$52'50"사이로 중부지방에 위치한 유일한 증발산관측 타워이다. 유역 면적은 9.27 km2으로 유로연장 3.48 km, 유역 평균폭 2.66 km, 형상계수는 0.77이며, 덕곡제플럭스 타워 주변의 토지이용은 대부분 산림으로 구성되어 있으며, 침활 혼효림과 낙엽송림으로 임상 분포가 이루어져 있다. 주요 관측기기로는 3차원 풍향 풍속계, $CO_2/H_2O$ 기체분석기, 순복사 측정 센서, 지중열플럭스 측정 센서 등이 있다. 2011년부터 측정된 자료를 바탕으로 에디공분산 방법을 이용하여 증발산량을 측정하였으며, 30분간의 데이터 18,000개 중 취득률 90 % 이상의 데이터를 대상을 분석을 실시하였다. 2011 ~ 2015년도 증발산량 분석 결과는 아래의 표와 같다. 증발산의 패턴은 1월부터 서서히 증가하지만 활발하지는 않고, 4월부터 매우 활발해져 8월에 최대치에 이른다. 10월부터 증발산량은 급격히 감소하기 시작하며 11, 12월에는 증발산이 거의 발생하지 않는 공통적인 경향을 보였다. 2013년 8, 9월은 다른 해와 다른 경향을 보이고 있는데, 이는 2013년 8, 9월에 강우가 많이 발생하여 증발산량이 감소하였기 때문으로 판단된다. 2015년 8월은 다른 년도와 비교했을 때, 매우 높은 증발산량을 보이는데 이는 2015년 8월에 많은 강우에도 식생이 활발하게 작용하였기 때문으로 판단된다.

• Journal of Wetlands Research
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• v.23 no.4
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• pp.277-286
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• 2021

This study compares the long-term hydrological cycles of the Seolmacheon and Cheongmicheon basin by applying the structural equation model (SEM). These two basins are found different especially in their land-use pattern. Both basins have the actual evapotranspiration data measured by the eddy-covariance method as well as the rainfall and runoff data. The length of the data considered in this study is nine years from 2010 to 2018. The structure of the SEM is determined by considering the correlations among the data as well as the general knowledge on the hydrological cycle. As a result, a total of three SEMs are applied sequentially to analyze their fittings. As irony would have it, two basins are found to be similar in the application of one SEM, but different in the application of another. Especially, when considering the feedback process between precipitation and evapotranspiration, two basins are found to be very different. That is, the feedback process between precipitation and evapotranspiration is found to be significant in the Cheongmicheon basin where the portion of agricultural area (i.e., paddy) is more than 40%.

• Journal of The Korean Society of Agricultural Engineers
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• v.63 no.6
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• pp.1-16
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• 2021

Global warming due to climate change is expected to significantly affect the hydrological cycle of agriculture. Therefore, in order to predict the magnitude of climate impact on agricultural water resources in the future, it is necessary to estimate the water demand for irrigation as the climate change. This study aimed at evaluating the future changes in water demand for irrigation under two Shared Socioeconomic Pathways (SSPs) (SSP2-4.5 and SSP5-8.5) scenarios for paddy rice in Gimje, South Korea. The APEX-Paddy model developed for the simulation of paddy environment was used. The model was calibrated and validated using the H₂O flux observation data by the eddy covariance system installed at the field. Sixteen General Circulation Models (GCMs) collected from the Climate Model Intercomparison Project phase 6 (CMIP6) and downscaled using Simple Quantile Mapping (SQM) were used. The future climate data obtained were subjected to APEX-Paddy model simulation to evaluate the future water demand for irrigation at the paddy field. Changes in water demand for irrigation were evaluated for Near-future-NF (2011-2040), Mid-future-MF (2041-2070), and Far-future-FF (2071-2100) by comparing with historical data (1981-2010). The result revealed that, water demand for irrigation would increase by 2.3%, 4.8%, and 7.5% for NF, MF and FF respectively under SSP2-4.5 as compared to the historical demand. Under SSP5-8.5, the water demand for irrigation will worsen by 1.6%, 5.7%, 9.7%, for NF, MF and FF respectively. The increasing water demand for irrigating paddy field into the future is due to increasing evapotranspiration resulting from rising daily mean temperatures and solar radiation under the changing climate.

• Journal of Korea Water Resources Association
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• v.54 no.10
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• pp.779-793
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• 2021

Water stress and environmental drivers are important factors to explain the variance of gross primary production (GPP). Environmental drivers are used to generate GPP in Moderate Resolution Imaging Spectroradiometer (MODIS) algorithm and process-based model. However, MODIS algorithm only consider the vapor pressure deficit (VPD) data while the process-based biogeochemical model also uses limited data to express water stress. We compared the relationship between environmental drivers and GPP from eddy covariance method, MODIS algorithm, and Community Land Model 4 (CLM 4) simulation in normal years and drought years. To consider water stress specifically, we used VPD and evaporative fraction (EF). We evaluated the effects from environmental drivers and EF towards GPP products using the structural equation modeling (SEM) in South Korea. We found that GPP products from MODIS algorithm and model simulation results were not restricted from VPD data if VPD was underestimated. We also found that in the cropland area, irrigation effects can relieve VPD effects to GPP. However, GPP products derived from MODIS and CLM 4 had limitation to explain the irrigation effects to GPP. Overall, these results will enhance the understanding of GPP products derived from MODIS and CLM 4.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.67-67
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• 2022

국가 물관리 측면에서 증발산량과 토양수분량은 자연계 손실로서 국내 수자원 총량의 약43%(563억 m³/년)를 차지하며, 수자원의 계획과 개발, 물순환 과정 규명 및 다양한 수재해 분석 등을 위한 수문 요소이다. 정부는 2005년 「수문조사 선진화 5개년 계획」과 2008년 「제1차 수문조사기본계획(2010~2019년)」을 통해 2019년까지 증발산량과 토양수분량 관측소 확대(각각 25개 지점) 기반을 마련하였고 「수자원의 조사·계획 및 관리에 관한 법률」에 따라 매년 공인 수문 자료로 증발산량과 토양수분량을 측정하고 있다. 증발산량과 토양수분량은 댐 유역의 정밀한 물순환 해석에도 매우 중요한 정보로서 현재 K-water에서의 관측은 일부 시험유역(용담댐 유역)의 flux tower에 의한 에디공분산법(Eddy Covariance Method) 및 토양수분 센서(TDR, Time Domain Reflectometery)에 의한 지점 자료의 생산만 각각 이루어지고 있다. 본 연구에서는 K-water 댐 유역의 증발산량 및 토양수분량 관측 현황과 그간 관측된 자료의 특성을 각종 경향성 분석 등과 함께 소개하고자 한다, 증발산량의 경우는 2개소의 flux tower를운영(덕유산 지점 2011년 이후, 용담 지점 2017년 이후)하고 있으며, 토양수분량은 총 7개소(계북, 천천, 상전, 안천, 부귀, 주천 지점 2013년 이후, 장계 지점 2017년 이후)에 TDR센서를 설치, 계측 운영 중이다. 이렇게 관측된 자료는 매년 홍수통제소 주관 관련 전문가 공인심사를 통해 일자료 기준으로 한국수문조사연보에 수록되고 있으며, K-water에서도 연보를 통해 공개된 자료를 기준으로 공공데이터포털(data.go.kr) 등과 연계하여 온라인 자료 서비스 중이다. 한편, 최근 2020년 「제2차 수문조사 기본계획(2020~2029년)」에서는 수자원 위성 개발연구와 연계하여 위성을 활용한 증발산량과 토양수분량 산정 연구의 필요성이 강조되고 있다. 하지만 본 연구에서 살펴본 지점 자료만으로는 댐 유역을 포함한 광역단위의 시계열 공간정보를 생산하기 한계가 있으며, 댐 유역과 국내 전 지역의 공간 시계열 증발산량 및 토양수분량 자료 산정과 활용 방안에 대해 정립하고, 나아가 위성영상을 활용한 댐 유역 증발산량·토양수분량 관측 가이드라인 마련 등을 위해서는 국가적으로 많은 재원의 투입과 노력이 필요한 상황이다.

• Korean Journal of Agricultural and Forest Meteorology
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• v.22 no.1
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• pp.26-46
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• 2020

An irrigated-maize agroecosystem is viewed as an open thermodynamic system upon which solar radiation impresses a large gradient that moves the system away from equilibrium. Following the imperative of the second law of thermodynamics, such agroecosystem resists and reduces the externally applied gradient by using all means of this nature-human coupled system acting together as a nonequilibrium dissipative process. The ultimate purpose of our study is to test this hypothesis by examining the energetics of agroecosystem growth and development. As a first step toward this test, we employed the eddy covariance flux data from 2003 to 2014 at the AmeriFlux NE1 irrigated-maize site at Mead, Nebraska, USA, and analyzed the energetics of this agroecosystem by scrutinizing its radiation, energy and entropy exchange. Our results showed: (1) more energy capture during growing season than non-growing season, and increasing energy capture through growing season until senescence; (2) more energy flow activity within and through the system, providing greater potential for degradation; (3) higher efficiency in terms of carbon uptake and water use through growing season until senescence; and (4) the resulting energy degradation occurred at the expense of increasing net entropy accumulation within the system as well as net entropy transfer out to the surrounding environment. Under the drought conditions in 2012, the increased entropy production within the system was accompanied by the enhanced entropy transfer out of the system, resulting in insignificant net entropy change. Drought mitigation with more frequent irrigation shifted the main route of entropy transfer from sensible to latent heat fluxes, yielding the production and carbon uptake exceeding the 12-year mean values at the cost of less efficient use of water and light.

• Korean Journal of Agricultural and Forest Meteorology
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• v.20 no.1
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• pp.57-72
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• 2018

Time series analysis of tower flux measurement can be used to build quantitative evidence for the achievement of climate-smart agriculture (CSA). In this study, we have assessed the first objective of CSA (regarding ecosystem productivity and efficiency) for rice paddy-dominated heterogeneous farmland. A set of quantitative indicators were evaluated by analysing the time series data of carbon, water and energy fluxes over the Haenam farmland site in Korea (HFK) during the rice growing seasons from 2003 to 2015. Four different varieties of rice were cultivated during the study period in chronological order of Dongjin No. 1 (2003-2008), Nampyung (2009), Onnuri (2010-2011), and Saenuri (2012-2015). Overall at HFK, gross primary productivity (GPP) ranged from 800 to $944g\;C\;m^{-2}$, water use efficiency (WUE) ranged from 1.91 to $2.80g\;C\;kg\;H_2O^{-1}$, carbon uptake efficiency (CUE) ranged from 1.06 to 1.34, and light use efficiency (LUE) ranged from 0.99 to $1.55g\;C\;MJ^{-1}$. Among the four rice varieties, Dongjin No. 1-dominated HFK showed the highest productivity with higher WUE and LUE, but comparable CUE. Considering the heterogeneous vegetation cover at HFK, a rule of thumb comparison suggested that the productivity of Dongjin No1-dominated HFK was comparable to those of monoculture rice paddies in Asia, whereas HFK was more efficient in water use and less efficient in carbon uptake. Saenuri-dominated HFK also produced high productivity but with the growing season length longer than Dongjin No.1. Although the latter showed better traits for CSA, farmers cultivate Saenuri because of higher pest resistance (associated with adaptability and resilience). This emphasizes the need for the evaluation of other two objectives of CSA (i.e. system resilience and greenhouse gas mitigation) for complete assessment at HFK, which is currently in progress.

• Korean Journal of Agricultural and Forest Meteorology
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• v.20 no.1
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• pp.88-100
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• 2018

Methane emissions from rice paddies are the largest source of greenhouse gases in the agricultural sector, but there are significant regional differences depending on the surrounding conditions and cultivation practices. To visualize these differences and to analyze their causes and characteristics, the methane emissions from each administrative district in South Korea were calculated according to the IPCC guidelines using the data from the 2010 Agriculture, Forestry and Fisheries Census, and then the results were mapped by using the ArcGIS. The nationwide average of methane emissions per unit area was $380{\pm}74kg\;CH_4\;ha^{-1}\;yr^{-1}$. The western region showed a trend toward higher values than the eastern region. One of the major causes resulting in such regional differences was the $SF_o$ (scaling factor associated with the application of organic matter), where the number of cultivation days played an important role to either offset or deepen the differences. Comparison of our results against the actual methane emissions data observed by eddy covariance flux measurement in the three KoFlux rice paddy sites in Gimje, Haenam and Cheorwon showed some differences but encouraging results with a difference of 10 % or less depending on the sites and years. Using the updated GWP (global warming potential) value of 28, the national total methane emission in 2010 was estimated to be $8,742,000tons\;CO_2eq$ - 13% lower than that of the National Greenhouse Gas Inventory Report (i.e., $10,048,000tons\;CO_2eq$). The administrative districts-based map of methane emissions developed in this study can help identify the regional differences, and the analysis of their key controlling factors will provide important scientific basis for the practical policy makings for methane mitigation.

• Korean Journal of Agricultural and Forest Meteorology
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• v.11 no.1
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• pp.39-47
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• 2009

The importance of securing water resources and their efficient management has attracted more attention recently due to water deficit. In water budget analysis, however, evapotranspiration(${\lambda}E$) has been approximated as the residual in the water balance equation or estimated from empirical equations and assumptions. To minimize the uncertainties in these estimates, it is necessary to directly measure ${\lambda}E$. In this study, using the eddy covariance technique, we have measured ${\lambda}E$ in a mixed forest in the Seolmacheon catchment in Korea from September 2007 to December 2008. During the growing season(May-July), ${\lambda}E$ in this mixed forest averaged about $2.2\;mm\;d^{-1}$, whereas it was on average $0.5\;mm\;d^{-1}$ during the non-growing season in winter. The annual total ${\lambda}E$ in 2008 was $581\;mm\;y^{-1}$, which is about 1/3 of the annual precipitation of 1997 mm. Despite the differences in the amount and frequency of precipitation, the accumulated ${\lambda}E$ during the overlapping period(i.e., September to December) for 2007 and 2008 was both ${\sim}110$ mm, showing virtually no difference. The omega factor, which is a measure of decoupling between forest and the atmosphere, was on average 0.5, indicating that the contributions of equilibrium ${\lambda}E$ and imposed ${\lambda}E$ to the total ${\lambda}E$ were about the same. The results suggest that ${\lambda}E$ in this mixed forest was controlled by various factors such as net radiation, vapor pressure deficit, and canopy conductance. In this study, based on the direct measurements of ${\lambda}E$, we have quantified the relative contribution of ${\lambda}E$ in the water balance of a mixed forest in the Seolmacheon catchment. In combination with runoff data, the information on ${\lambda}E$ would greatly enhance the reliability of water budget analysis in this catchment.