| The measurements and analysis of the Earth's surface energy balance components including net solar radiation(R_n),soil heat flux(G),sensible heat flux(H),and latent heat flux(LE)are essential to develop land surface and climate models for meteorological,environmental,agricultural,social and economic issues.Specifically,the processes of surface energy and water exchange are significantly necessary to the forming and evolution of regional climate/climate change.The net solar radiation(R_n)is the primary source of energy on the Earth surface,which is divided as;one part is used for the warming of the air called the sensible heat flux(H),the other part is consumed for evaporation and transpiration of water from water bodies and living tissues due to the water vapor flux or latent heat flux(LE)also known as evapotranspiration(ET)and the remaining part is transferred to the soil which is known as the soil heat flux(G).Apart from these,there are also some advection and heat storage in the upper soil and vegetation cover in reasonably small amount.Collectively all these components add up to a phenomenon called the surface energy balance.It is very important to maintain the standard of measured surface flux data.The widely accepted flux observation techniques are eddy covariance(EC)method and the scintillometry(Large Aperture Scintillometer,LAS)method.The most critical issues and challenges in this topic are 1)dynamics of surface energy balance components,2)the surface energy imbalance problem,which is still unsolved,3)energy partitioning in an ecosystem,and 4)the validation problem of satellite derived surface fluxes in regional and global scales.Based on these research problems and issues,the main objectives of this thesis can be outlined as-1)to contribute to the literature review on the measurement and estimation of surface energy balance components using ground and satellite instruments;2)to assess the seasonal and annual variation of the energy and water vapor fluxes using the long-term flux observation and micro-meteorological data over a semi-arid ecosystem in north China;3)to make some scientific contribution in the evaluation of the energy balance closure(EBC)for identifying and delineating the causes behind the surface energy imbalance problem;4)to investigate the energy partitioning with respect to the weather conditions and seasonality;and finally 5)to estimate and validate the satellite(TERRA-MODIS)derived surface fluxes in regional scale with the LAS,EC and other meteorological instruments over three distinct land covers in northwest China for investigating the influences of land surface characteristics on the estimation.The literature review of this paper discussed the critical reviews of prior and recent advances in the estimation methods and models by focusing their advantages,disadvantages and recommendations.Uncertainties associated with satellite estimations and some key directions for further studies were also discussed,including the status of ground-based measurement in regional and global scale and the advent of new satellite technologies for quantifying the surface energy balance components more accurately.This existing literature study inferred that the further advances in the satellite remote sensing and worldwide ground-based measurement networks will enhance the capabilities for the potential estimation of the surface energy balance parameters as well as monitoring the global water and energy cycles to develop significant environmental studies for the betterment of living on the Earth.In the operational(data collecting,processing and analyzing)aspect,this study assessed the seasonal and annual variations of the surface energy fluxes,the surface energy balance closure(EBC),the reasons behind the surface energy imbalance,and energy partitioning based on the two years(2014-2015)meteorological flux observation datasets over a semi-arid agricultural region in north China.The EBC analysis was carried out using the ordinary least square(OLS)regression model of the dependent turbulent fluxes against the independently available energy,and the energy balance ratio(EBR).The results revealed that the EBC ranged from 0.63 to 0.78 at all respects with an overall EBC of 0.74,and the daily EBC was better than 30-min averages.The identified main reasons behind this imbalance were 1)the ignorance of the heat storage terms(soil and canopy),which improved the EBC by approximately 7%;2)the vegetation and surface heterogeneity negatively affected the EBR;and 3)the occurrence of advection broke the energy balance by decreasing EBC;the last reason was the novel finding in the way of solving the imbalance problem.The energy partitioning was investigated using the residual method and two ratios(sensible heat flux/net radiation and latent heat flux/net radiation).The results showed that the sensible heat flux was dominating during dry periods and the latent heat flux was dominating during the wet periods,and the rate of dominance was controlled by the availability of water,vegetation dynamics and weather conditions.The results of energy partitioning also implied that the study area was out of danger to drought.In the applicational(model driving and validating)aspect,this study focused on the validation and performance evaluation of the satellite derived H and R_n based on a single source remote sensing model over three distinct landscapes in northwest China during May to September 2015 using data from TERRA-MODIS and the meteorological observations.The spatial pattern of H and R_n was also estimated in regional scale.The results indicated that the remote sensing model with MODIS scenes performed well for the purpose of H estimation but the influence of different landscapes was noticeable.The satellite derived H(MODIS?H)was compared with the LAS measured H(LAS?H)and EC measured H(EC?H)over the three different land surfaces.The root mean square errors(rmse)of MODIS?H with respect to LAS?H were 31.63 W/m~2 over alpine grassland,44.07 W/m~2 over croplands,and 112.98 W/m~2over mixed forests.The aggregated values(from May to September)of mean and standard deviation showed that the MODIS?H was moderately over-estimated with fewer fluctuation over alpine grasslands,slightly overestimated with moderate fluctuation over croplands,and highly over estimated with higher fluctuation over mixed forests.The larger rmse and over estimations could be explained by the vegetation heterogeneity,the high wind speed,the complicated thermodynamic states,and the approximated calculation of surface emissivity.The feasibility of estimating the net radiation(R_n)was also tested based on a remote sensing surface energy balance model using satellite(TERRA/MODIS)data.The results revealed that the estimated net radiation flux from satellite data agreed well with the field observations over three different surfaces with mean relative error of 9.33%over upstream superstation,13.95%over middle stream superstation,and 11.63%over downstream superstation in the study area.These findings revealed that the regional distribution of net radiation flux over the study region was in good agreement with ground observation.All these information are significantly important for the assessment of surface energy balance and energy partitioning in an ecosystem,and also for the estimation and validation of satellite derived surface fluxes over different surfaces. |
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