Measurements of regional sensible heat flux over Mongolian grassland using large aperture scintillometer
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With the flux data observed by Eddy Covariance system(EC) and Large Aperture Scintillometer(LAS) at Xiaotangshan(Changping district,Beijing) in 2002 and 2004,and based on an analytical footprint model,the scale relationship of sensible heat flux between EC and LAS measurement was analyzed.The results showed that the level of surface homogeneity or heterogeneity had a direct influence on the sensible heat flux difference measured by eddy covariance system and large aperture scintillometer.After analyzing the relationship between flux differences of EC and LAS observation and the footprint ratio and size ratio of overlap zone of source areas of the two sensors over homogeneous and heterogeneous surfaces,a scale transfer equation was constructed to composite the sensible heat flux measured by eddy covariance system at each patch into the area-averaged sensible heat flux.
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The eddy correlation (EC) method has been recommended as one of the standard methods for measuring actual evapotranspiration. One of the challenges in interpreting eddy correlation measurements is that the balance between various energy components measured and the total energy received can sometimes be off by up to 30%. An independent measurement of sensible heat flux by scintillometry is often used to resolve this energy balance closure problem. An EC system was set up at a 47 ha commercial corn field in southeastern North Dakota in 2008. Net radiation, latent heat flux, sensible heat flux, soil heat flux and other weather parameters were measured continuously at a 30-min interval during the growing season. A Scintec Boundary Layer Scintillometer 900 (BLS900) was temporarily deployed for seven days when clear sky conditions were satisfied. During the synchronous experiment, both systems were adjusted to at least 1 m above the canopy and collected data at an increased frequency of every minute. The EC system was placed at approximately the midpoint of the scintillometer path. The comparison of sensible heat fluxes measured by the two methods showed good agreement. The results will be used to validate the eddy correlation system in an attempt to reconcile the closure of energy balance.
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Sensible and latent heat fluxes are important parameters for understanding energy/mass interactions. At present, there are a number of methods available to estimate these fluxes, with varying degrees of difficulty and success. In the present study, we explore the use of the flux variance method and the surface renewal method to estimate the sensible and latent heat fluxes in two agricultural sites in China. The results of these two methods were compared to direct measurements using eddy covariance. The study sites, a wheat field in a semi-arid area and a rice paddy in a humid area provide an opportunity to examine two extreme conditions. In the arid conditions, both estimation methods provided similar sensible heat flux estimations but higher latent heat flux estimations with respect to the eddy covariance measurements. The sum of the estimated sensible and latent heat fluxes was in good agreement with measurements. In the rice paddy, sensible heat flux estimated by both methods showed similar results, whereas latent heat fluxes estimation were 1.2 times greater than eddy covariance measurements. The surface renewal method underestimated the Bowen ratio over the rice paddy, but overestimated it over the wheat field. In conclusion, both methods appear more appropriate for the estimation of sensible heat flux rather than for latent heat flux, in particular in humid environments.
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Scintillometers are becoming increasingly popular for the validation of satellite remote sensing sensible heat-flux estimates due to the comparable spatial resolutions. However, it is important to gain confidence in the accuracy of the sensible heat-flux measurements obtained by the scintillometer. Large aperture scintillometer (LAS) and eddy-covariance (EC) measurements were collected over a homogeneous, dry and semi-arid region near Las Cruces, New Mexico, USA, where the homogeneity allowed direct comparison of the two instruments despite their differences in footprint sizes. The differences between the sensible heat-flux measured by both LAS and EC systems fall within the differences between two EC systems. We conclude that the large aperture scintillometer is a reliable system for measuring sensible heat flux in a dry semiarid region.
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Eddy covariance and scintillometer techniques are effective ways to measure the sensible heat flux at local and regional scales.As part of ChinaFLUX,continuous sensible heat flux was measured by using the Eddy Covariance(EC) technique at two and three canopy heights of a subtropical pinus plantation on the red earth hilly region in southeastern China.Meanwhile,continuous sensible heat flux was also measured by Small Aperture Scintillometer(SAS),which was installed in the direction of east by south,with about 70m distance between the laser emitter and receiver.There were consistence between the EC systems at two and three canopy heights and SAS system.At the diurnal and seasonal scales,sensible heat flux by SAS was higher than that by the EC systems at two and three canopy heights.However,sensible heat flux by SAS was lower than that by the EC systems at two and three canopy heights.There showed the strongest correlation in the direction of from south to south by west and from north by east to east by north,which were perpendicular to the dominating wind direction.During the daytime,the correlation of sensible heat flux between the EC systems at two and three canopy heights and SAS system were higher than that during the nighttime.The difference of sensible heat flux between the EC systems at two and three canopy heights and SAS system were related with the disparity in their footprint and disadvantages.
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This study examined the relationship between LAS (Large Aperture Scintillometer) and EC (Eddy Covariance technique) under different crop conditions in term of the sensible heat flux using the data collected from a typical cropland in the Weishan Irrigation District,Liaocheng city,Shandong province.The results showed that the measurements of LAS and EC are in good agreement in this cropland.But the estimated sensible heat flux by LAS is slightly higher than that by EC,which might be due to either the disagreement in their heterogeneous source areas or the underestimation of EC measurement.LAS could provide a reliable way of measuring area-average sensible heat flux
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Large Aperture Scintillometers (LAS), Bowen ratio and eddy covariance measurements were employed to study sensible heat flux over homogeneous bare soil surface from March 20/sup th/ to April 20/sup th/, 2002, at XiaoTangshan area, Beijing. The diurnal variation of sensible heat flux from LAS is analyzed in this paper, and the relation between sensible heat flux and weather conditions is discussed. Further, test comparisons of the scintillometer flux measurements with the measurements of Bowen ratio and eddy correlation methods show good agreement; The correlation coefficients are over 0.8.
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