| Evapotranspiration(ET)is a critical component of the regional water and surface energy balances.The traditional observation methods for agroecosystems are limited to the application requirements of single-point observation scales and homogeneous topography,thus making it difficult to obtain ET from complex heterogeneous land surface at greater spatial scale.The optical-microwave scintillometer(OMS)system is advantageous in the estimation of ET over complex heterogeneous land surface depends on its observation level of kilometer meters scale,which is combined by the large-aperture scintillometer with the microwave scintillometer.The integration of OMS system and the eddy covariance(EC)system based on micrometeorological methods with its observation scale that over hundred meters and the optical-microwave scintillometer system,which has good prospects and development potential for the estimation of ET over a complex heterogeneous land surface.The agroecosystem of complex heterogeneous land surface which mainly planted winter wheat is the subject of this paper.Based on the turbulent flux observation data of EC system and OMS system from October 2019 to June 2020 in Xinxiang comprehensive experimental base of Chinese Academy of Agricultural Sciences,variations in energy balance,dynamics of heat fluxes,characteristics of ET,influences of environmental factors and spatial representativeness of observation sites in agroecosystems over the complex heterogeneous land surface are analyzed,the main findings are as follows:(1)The energy closure of typical time scale and two typical space scale were analyzed:In the flux observation of the winter wheat,the energy balance ratio(EBR)at hundred meters scale in the whole growth stage was 0.61,and in the early growth stage and middle late growth stage the EBR was 0.79 and0.54 respectively.While in the early and middle growth stage,the EBR at kilometer meters scale was 1.25and 1.06 respectively,which showing the state of over-closed.The degree of EBR varies significantly among different growing stages,day or night:the EBR was higher in the non-peak period than in the peak period,the variation in EBR at night is relatively large while less in the daytime.(2)The variation rules and influencing factors of water and heat fluxes at two typical spatial scales were described:The diurnal variation characteristics of energy distribution in different growth stages of winter wheat were obvious at hundred meter scale and kilometer scale,the general trend of latent heat flux and net radiation increased firstly then decreased,the intensity of latent heat flux oscillate at zero in the nighttime and reaches its peak near midday;For half-hour timescale,the latent heat at hundred meter scale is mainly affected by net radiation,vapour pressure deficit,relative air humidity and wind speed.The latent heat at kilometer meter scale is mainly affected by net radiation,vapour pressure deficit,relative air humidity;For day timescale,the latent heat at hundred meter scale is mainly affected by net radiation,wind speed and relative air humidity.The latent heat at kilometer meter scale is mainly affected by vapour pressure deficit,wind speed,net radiation,relative air humidity and temperature.(3)The characteristics of flux source distribution at two typical spatial scales were clarified:The range of source areas in average wind direction at different growing stages of the winter wheat were obtained from the Flux Footprint Prediction(FFP)model,the source area ranges from 5053 to 13140 m~2 under the unstable atmospheric condition,the source area ranges from 13250 to 19350 m~2 under the stable atmospheric condition;From the Flux Source Area Model(FSAM),the source area for the directions of0°?90°,90°?180°,180°?270°and 270°?360°were obtained,under the unstable atmospheric condition the source area is 3394.40,127012.86,6166.61 m~2 and 7151.00 m~2 respectively,under the stable atmospheric condition the source area is 4968.08,5804.82,34607.51 m~2 and 16280.19 m~2 respectively;The proportion of flux contribution of different land types was determined from the Kormann and Meixner(KM)method,the mean value of flux contribution at different growing stages within the land division reached 96.77%,while the mean value of field fluxes was 94.07%.At different fertility stages of the winter wheat,the results of three models showed similar trends,the flux source area at hundred meter scale and kilometer scale both exhibit the distribution characteristic that the source areas of unstable atmospheric condition are larger than the source areas of the stable atmospheric condition.There are some differences between the model results:the consistency between FFP and KM was good(R~2=0.96),the consistency between KM and FSAM was slightly poor(R~2=0.60),the consistency between FFP and FSAM was poor(R~2=0.34).The combination of FFP model and KM method has a good effect on obtaining the source area range and flux contribution of different land type.(4)The variation rules of ET at timescale of half-hour timescale,day and growth stage of winter wheat at hundred meter scale and kilometer scale were expounded:The total ET at hundred meter scale was436.13 mm over the whole growing stage of winter wheat,its daily average ET was 1.86 mm.From January 1,2020 to May 25,2020,the ET at hundred meter scale was 331.46 mm,the ET at kilometer meter scale was 331.46 mm.The ET at hundred meter scale and kilometer scale under different weather conditions,the order of peak values of ET at half-hour timescale was sunny>rainy>cloudy>overcast>snowy,the order of values of ET at day timescale was sunny>rainy>overcast>cloudy>snowy;With the development of growth stage,ET at hundred meter scale and kilometer scale decreased first then increased,and reached the lowest in overwintering stage and the highest in filling stage. |
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