Two energy balance closure approaches： applications and comparisons over an oasis-desert ecotone

Studies of energy balance that rely on eddy covariance（EC） are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux（LE） and sensible heat flux（Hs）. The Bowen ratio（BR） and energy balance residual（ER） approaches are two widely-used methods to correct the LE. A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments. In this study, two energy balance approaches（i.e., BR and ER） were compared to correct the LE measured at six EC sites（i.e., three vegetated, one mixed and two non-vegetated sites） in an oasis-desert ecotone of the Heihe River Basin, China. The influences of meteorological factors on those two approaches were also quantitatively assessed. Our results demonstrated that the average energy closure ratio（（LE＋Hs）/（Rn–Gs）; where Rn is the surface net radiation and Gs is the surface soil heat flux） was approximately close to 1.0 at wetland, maize and village sites, but far from 1.0 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites. After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site. The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference. Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types. We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas.

A Diagnostic Approach towards the Causes of Energy Balance Closure Problem

Hydrometeorological models are often evaluated and optimized on the basis of micrometeorological measurements. However, it has been known for more than three decades that surface measurements of sensible and latent heat energy (LE) are systematically underestimated. We studied this problem using six years of eddy-correlation measurements for four fields (corn, soybean, and prairie) in central Iowa, USA. We recorded major components of the energy equation (i.e. net radiation, sensible heat flux, LE, and soil heat flux, photosynthesis), and indirectly estimated most of the minor components of energy balance (namely storage in the soil, canopy and air). Storage in the canopy was related to leaf area index (LAI) acquired from Moderate Resolution Imaging Spectrometer (MODIS). In this paper, a diagnostic approach is investigated where systematic error is identified first. Three dimensional (3D) plots of the residual of energy equation vs. potential variables indicated the imbalance was largest mainly during the cold non-growing season when the soil was dry. Correlations between energy balance residual (EBR) and energy components showed that soil storage was not precisely estimated. Finally, an a-posteriori analysis (constrained linear multiple regression (CMLR)) was conducted to quantify the contribution of major/minor components of the energy equation towards EBR. The result highlights that the contribution of pertinent components of energy to EBR is mainly controlled by prevailing monthly hydrometeorological conditions;however, precise quantification of causes of imbalance is site-specific. A comparison between the a-posteriori analysis technique and the Bowen-ratio method demonstrates that the Bowen-ratio basically presumes a higher level of underestimation in LE. The results obtained in this study suggest that a-posteriori analysis may offer a superior methodology to correct measured eddy-correlation measurements. Furthermore, the overall trends in the correction of LE measurements suggest that there is a potential for rough monthly corrections of LE, irrespective of the type of crop.

Spatial variation of net radiation and its contribution to energy balance closures in grassland ecosystems

Introduction:Low energy balance closure(EBC)at a particular eddy-covariance flux site has increased the uncertainties of carbon,water,and energy measurements and has thus hampered the urgent research of scaling up and modeling analyses through site combinations in regional or global flux networks.Methods:A series of manipulative experiments were conducted in this study to explore the role of net radiation(Rn)in the EBC in relation to spatial variability of vegetation characteristics,source area,and sensor type in three sites of the Inner Mongolian grassland of northern China.Results:At all three sites,the residual fluxes of EBC peaked consistently at 110 W m^(-2).The spatial variability in net radiation was 19 W m^(-2)(5%of R_(n))during the day and 7 W m^(-2)(16%)at night,with an average of 13 W m^(-2)(11%)from eight plot measurements across the three sites.Large area measurements of Rn significantly increased by 9 W m^(-2)during the day and decreased by 4 W m^(-2)at night in the unclipped treatments.Net radiation decreased by 25 W m^(-2)(6%of Rn)at midday and 81 MJ m^(-2)(6%)during a growing season with heavier regular clipping than that in unclipped treatments.The Rn was lower by 11–21 W m^(-2)(~20–40%of Rn)measured by CNR1 than by Q7.1 at night,while there was only 6 W m^(-2)(~1–2%of Rn)difference during the daytime between these two types of commonly used net radiometers.Conclusions:Overall,the inclusion of the uncertainty in available energy accounted for 65%of the~110 W m^(-2)shortfalls in the lack of closure.Clearly,the unclosed energy balance at these three grassland sites remains significant,with unexplored mechanisms for future research.

Radiation and energy balance on a hillslope forest: horizontal versus slope-parallel installation of radiometer

Radiation is a major driver to the carbon,water, and energy exchanges of an ecosystem. For local radiation balance measurements, one essential question is whether the measurement systems should be installed horizontally or parallel to inclined slope surface. With a case study over a temperate deciduous forest on a moderate inclined(9°) northwest-facing slope, we quantified the slope effect on net radiation(Rn) and its components and the energy balance closure measured by an eddy covariance(EC) system.Compared with the slope-parallel radiometer, the horizontal sensor overestimated the incident solar radiation(SR) by 7%, the incoming photosynthetically active radiation(PAR) by 1.5%, and the incoming near-infrared radiation(NIR) by 10%;while underestimated the reflected shortwave radiation(SR)by 4% and NIR by 5%. The influence of radiometerorientation on incoming longwave radiation(LR) was about 3%, while that on outgoing LR was negligible.Summing all these components, horizontal sensor overestimated the Rn by 9%. Converting the horizontally-measured incident radiation to slopesurface reduced a half of the biases on incoming SR and Rn. Measuring the Rn with slope-parallel radiometer and correcting the slope-effect on horizontally-measured incident SR improved the energy balance ratio(EBR) by 8% and 5%,respectively. A mini-review indicated that, the horizontal sensor underestimated(overestimated) the EBR on north-facing(south-facing) slopes in temperate zone in the Northern Hemisphere, with an inclination angular sensitivity of EBR as high as 1.17%per degree of inclination angle. We recommend measuring radiations on inclined terrains with slopeparallel radiometers, or correcting at least for the incident SR in energy balance studies.