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Dive into the research topics where Hyojung Kwon is active.

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Featured researches published by Hyojung Kwon.


IEEE Transactions on Geoscience and Remote Sensing | 2006

Evaluation of remote sensing based terrestrial productivity from MODIS using regional tower eddy flux network observations

Faith Ann Heinsch; Maosheng Zhao; Steven W. Running; John S. Kimball; Ramakrisbna Nemani; Kenneth J. Davis; Paul V. Bolstad; Bruce D. Cook; Ankur R. Desai; Daniel M. Ricciuto; Beverly E. Law; Walter Oechel; Hyojung Kwon; Hongyan Luo; Steven C. Wofsy; Allison L. Dunn; J. W. Munger; Dennis D. Baldocchi; Liukang Xu; David Y. Hollinger; Andrew D. Richardson; Paul C. Stoy; M. Siqueira; Russell K. Monson; Sean P. Burns; Lawrence B. Flanagan

The Moderate Resolution Spectroradiometer (MODIS) sensor has provided near real-time estimates of gross primary production (GPP) since March 2000. We compare four years (2000 to 2003) of satellite-based calculations of GPP with tower eddy CO2 flux-based estimates across diverse land cover types and climate regimes. We examine the potential error contributions from meteorology, leaf area index (LAI)/fPAR, and land cover. The error between annual GPP computed from NASAs Data Assimilation Offices (DAO) and tower-based meteorology is 28%, indicating that NASAs DAO global meteorology plays an important role in the accuracy of the GPP algorithm. Approximately 62% of MOD15-based estimates of LAI were within the estimates based on field optical measurements, although remaining values overestimated site values. Land cover presented the fewest errors, with most errors within the forest classes, reducing potential error. Tower-based and MODIS estimates of annual GPP compare favorably for most biomes, although MODIS GPP overestimates tower-based calculations by 20%-30%. Seasonally, summer estimates of MODIS GPP are closest to tower data, and spring estimates are the worst, most likely the result of the relatively rapid onset of leaf-out. The results of this study indicate, however, that the current MODIS GPP algorithm shows reasonable spatial patterns and temporal variability across a diverse range of biomes and climate regimes. So, while continued efforts are needed to isolate particular problems in specific biomes, we are optimistic about the general quality of these data, and continuation of the MOD17 GPP product will likely provide a key component of global terrestrial ecosystem analysis, providing continuous weekly measurements of global vegetation production


Korean Journal of Agricultural and Forest Meteorology | 2009

Standardization of KoFlux Eddy-Covariance Data Processing

Jinkyu Hong; Hyojung Kwon; Jong-Hwan Lim; Young-Hwa Byun; Johan Lee; Joon Kim

The standardization of eddy-covariance data processing is essential for the analysis and synthesis of vast amount of data being accumulated through continuous observations in various flux measurement networks. End users eventually benefit from the open and transparent standardization protocol by clear understanding of final products such as evapotranspiration and gross primary productivity. In this paper, we briefly introduced KoFlux efforts to standardize data processing methodologies and then estimated uncertainties of surface fluxes due to different processing methods. Based on our scrutiny of the data observed at Gwangneung KoFlux site, net ecosystem exchange and ecosystem respiration were sensitive to the selection of different processing methods. Gross primary production, however, was consistent within errors due to cancellation of the differences in NEE and Re, emphasizing that independent observation of ecosystem respiration is required for accurate estimates of carbon exchange. Nocturnal soil evaporation was small and thus the annually integrated evapotranspiration was not sensitive to the selection of different data processing methods. The implementation of such standardized data processing protocol to AsiaFlux will enable the establishment of consistent database for validation of models of carbon cycle, dynamic vegetation, and land-atmosphere interaction at regional scale.


Korean Journal of Agricultural and Forest Meteorology | 2009

Seasonal Variations of Evapotranspiration Observed in a Mixed forest in the Seolmacheon Catchment

Hyojung Kwon; Jung-Hoon Lee; Yeonkil Lee; Jinwon Lee; Sung Won Jung; Joon Kim

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() 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 . In this study, using the eddy covariance technique, we have measured in a mixed forest in the Seolmacheon catchment in Korea from September 2007 to December 2008. During the growing season(May-July), in this mixed forest averaged about , whereas it was on average during the non-growing season in winter. The annual total in 2008 was , which is about 1/3 of the annual precipitation of 1997 mm. Despite the differences in the amount and frequency of precipitation, the accumulated during the overlapping period(i.e., September to December) for 2007 and 2008 was both 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 and imposed to the total were about the same. The results suggest that 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 , we have quantified the relative contribution of in the water balance of a mixed forest in the Seolmacheon catchment. In combination with runoff data, the information on would greatly enhance the reliability of water budget analysis in this catchment.


Physical Geography | 2008

The Relationship Between Soil Moisture and NDVI Near Barrow, Alaska

Ryan Engstrom; Allen Hope; Hyojung Kwon; Douglas A. Stow

Ice-wedge polygons and drained thaw lake basins cover large portions of the Barrow Peninsula of Alaska. These landforms lead to variations in topography in the relatively flat coastal plain. These variations in topography, while small, lead to large variations in soil moisture, which have a substantial impact on carbon, water, and energy fluxes in these systems. In this study, we hypothesize that the spatial patterns of soil moisture control variations in vegetation production, and hence remotely sensed greenness. We investigate the relationship between soil moisture and a remotely sensed measure of vegetation greenness, the normalized difference vegetation index (NDVI). Soil moisture was measured in the summer of 2000 in the fetch of an eddy flux tower (0.5 km2 footprint), and NDVI data were collected using an aerial digital multispectral camera system. Results indicate that the relationship between soil moisture and NDVI was significant in areas dominated by high-centered polygons and troughs, and where the general slope of the terrain promoted the redistribution of soil moisture. However, in areas with little to no relief, the correlation between soil moisture and NDVI was not significant.


Tree Physiology | 2012

Size-mediated tree transpiration along soil drainage gradients in a boreal black spruce forest wildfire chronosequence

J. L. Angstmann; Brent E. Ewers; Hyojung Kwon

Boreal forests are crucial to climate change predictions because of their large land area and ability to sequester and store carbon, which is controlled by water availability. Heterogeneity of these forests is predicted to increase with climate change through more frequent wildfires, warmer, longer growing seasons and potential drainage of forested wetlands. This study aims at quantifying controls over tree transpiration with drainage condition, stand age and species in a central Canadian black spruce boreal forest. Heat dissipation sensors were installed in 2007 and data were collected through 2008 on 118 trees (69 Picea mariana (Mill.) Britton, Sterns & Poggenb. (black spruce), 25 Populus tremuloides Michx. (trembling aspen), 19 Pinus banksiana Lamb. (jack pine), 3 Larix laricina (Du Roi) K. Koch (tamarack) and 2 Salix spp. (willow)) at four stand ages (18, 43, 77 and 157 years old) each containing a well- and poorly-drained stand. Transpiration estimates from sap flux were expressed per unit xylem area, J(S), per unit ground area, E(C) and per unit leaf area, E(L), using sapwood (A(S)) and leaf (A(L)) area calculated from stand- and species-specific allometry. Soil drainage differences in transpiration were variable; only the 43- and 157-year-old poorly-drained stands had ∼ 50% higher total stand E(C) than well-drained locations. Total stand E(C) tended to decrease with stand age after an initial increase between the 18- and 43-year-old stands. Soil drainage differences in transpiration were controlled primarily by short-term physiological drivers such as vapor pressure deficit and soil moisture whereas stand age differences were controlled by successional species shifts and changes in tree size (i.e., A(S)). Future predictions of boreal climate change must include stand age, species and soil drainage heterogeneity to avoid biased estimates of forest water loss and latent energy exchanges.


Journal of Hydrometeorology | 2012

On Estimating Wet Canopy Evaporation from Deciduous and Coniferous Forests in the Asian Monsoon Climate

Minseok Kang; Hyojung Kwon; Jung Hwa Cheon; Joon Kim

AbstractContinuous and direct measurement of evapotranspiration (ET) by the eddy covariance (EC) technique is still a challenge under monsoon climate because of a considerable amount of missing data during the long rainy periods and the consequential gap-filling process. Under such wet canopy conditions, especially in forests, evaporation of the intercepted precipitation (EWC) contributes significantly to the total ET. To quantify the role of EWC, leaf wetness has been measured at multiple levels in the canopy simultaneously with eddy covariance measurements at the KoFlux Gwangneung deciduous and coniferous forests for the entire year from September 2007 to August 2008. In this study, the measured EWC and the controlling mechanism during the wet canopy conditions have been scrutinized. Based on the evaluation of the four different algorithms of EWC estimation, that of the variable infiltration capacity (VIC) land surface model (LSM) has been adopted. All the missing EWC data are then recalculated by using...


Korean Journal of Agricultural and Forest Meteorology | 2009

Estimation of Net Radiation in Three Different Plant Functional Types in Korea

Hyojung Kwon

Net radiation () is a driving force of biological and physical processes between the surface and the atmosphere and its knowledge is critical to weather forecasting and water resource management. The measurement of is, however, scarce and it is typically estimated from an empirical relationship. This study presented two different methods of estimation over three major plant functional types (i.e., a deciduous forest, a coniferous forest, and a farmland) in Korea. One is a linear regression method between and solar radiation and the other is a radiation balance method. The two methods were examined using the data collected in 2008 at the three sites. Based on the linear regression method over a year, was 70% of the incoming shortwave radiation () for a deciduous forest, 79% for a coniferous forest, and 64% for a farmland, indicating that the relationship was plant functional type-specific. For the radiation balance method, the inclusion of longwave radiation component slightly improved estimations. Overall, there was a good agreement between the observed and the estimated from both methods, indicating a reliable applicability of the two methods in estimating .


Korean Journal of Agricultural and Forest Meteorology | 2010

On Estimation of Zero Plane Displacement from Single-Level Wind Measurement above a Coniferous Forest

Jinkyu Hong; Hyojung Kwon; Jong-Hwan Lim; Joon Kim

Zero plane displacement (d) is the elevated height of the apparent momentum sink exerted by the vegetation on the air. For a vegetative canopy, d depends on the roughness structure of a plant canopy such as leaf area index, canopy height and canopy density, and thus is critical for the analysis of canopy turbulence and the calculation of surface scalar fluxes. In this research note, we estimated d at the Gwangneung coniferous forest by employing two independent methods of Rotach (1994) and Martano (2000), which require only a single-level eddy-covariance measurement. In general, these two methods provided comparable estimates of (where is the canopy height, i.e., ~23m), which ranged from 0.51 to 0.97 depending on wind directions. These estimates of were within the ranges (i.e., 0.64~0.94) reported from other forests in the literature but were sensitive to the forms of the nondimensional functions for atmospheric stability. Our finding indicates that one should be careful in interepreation of zero plane displacement estimated from a single-level eddy covariance measurement that is conductaed within the roughness sublayer.


Remote Sensing | 2016

Evaluation of a Phenology-Dependent Response Method for Estimating Leaf Area Index of Rice Across Climate Gradients

Bora Lee; Hyojung Kwon; Akira Miyata; Steve Lindner; John Tenhunen

Accurate estimate of the seasonal leaf area index (LAI) in croplands is required for understanding not only intra- and inter-annual crop development, but also crop management. Lack of consideration in different growth phases in the relationship between LAI and vegetation indices (VI) often results in unsatisfactory estimation in the seasonal course of LAI. In this study, we partitioned the growing season into two phases separated by maximum VI ( VI max ) and applied the general regression model to the data gained from two phases. As an alternative method to capture the influence of seasonal phenological development on the LAI-VI relationship, we developed a consistent development curve method and compared its performance with the general regression approaches. We used the Normalized Difference VI (NDVI) and the Enhanced VI (EVI) from the rice paddy sites in Asia (South Korea and Japan) and Europe (Spain) to examine its applicability across different climate conditions and management cycles. When the general regression method was used, separating the season into two phases resulted in no better estimation than the estimation obtained with the entire season observation due to an abrupt change in seasonal LAI occurring during the transition between the before and after VI max . The consistent development curve method reproduced the seasonal patterns of LAI from both NDVI and EVI across all sites better than the general regression method. Despite less than satisfactory estimation of a local LAI max , the consistent development curve method demonstrates improvement in estimating the seasonal course of LAI. The method can aid in providing accurate seasonal LAI as an input into ecological process-based models.


Rangeland Ecology & Management | 2018

Biophysical Factors and Canopy Coupling Control Ecosystem Water and Carbon Fluxes of Semiarid Sagebrush Ecosystems

David E. Reed; Brent E. Ewers; Elise Pendall; Kusum J. Naithani; Hyojung Kwon; Robert D. Kelly

ABSTRACT The sagebrush-steppe ecosystem covers much of western North America, and its productivity is sensitive to warming and increasingly variable precipitation. Interannual variation in precipitation has been shown to be the most significant factor controlling biogeochemical cycling while both soil and atmospheric drought are dominant factors of ecosystem fluxes. We show that plant canopies can also act to limit water losses through stomatal and aerodynamic control. We use 4 data-yr from 2 sites (2 069 and 2 469 m above sea level elevation, respectively) to evaluate control of carbon and water fluxes and to calculate the degree to which the ecosystem canopy and atmosphere are decoupled. Environmental conditions were similar between the two sites, although the lower elevation site was slightly warmer (1.8°C higher temperature) and drier (0.2 kPa higher vapor pressure deficit). Ecosystem responses of net ecosystem exchange (NEE) and evapotranspiration (ET) to environmental drivers were similar between sites and years, with the wet site-yr 2009 having the largest ET and NEE fluxes. Canopy leaf area led to divergent behavior of the canopy-atmosphere decoupling parameter under high (> 11% by volume) soil moisture conditions. During low (< 11%) soil moisture periods, both sites had tight ecosystem stomatal control on ET with little NEE activity. This study highlights how semiarid ecosystems can alter their canopy leaf area in order to control how decoupled semi-arid canopies are to the atmosphere, potentially moderating impacts of climate change.

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Joon Kim

Seoul National University

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Walter C. Oechel

San Diego State University

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Allen Hope

San Diego State University

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Ryan Engstrom

George Washington University

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