摘要
通过分析2007年9月～2008年8月年在淮河流农田下垫面采用涡动相关法(Eddy Covariance,简称EC)测得的地气间湍流通量以及辐射平衡分量和廓线资料,研究淮河流域农田地气相互作用日、季变化特征,分析了农田不同下垫面以及作物不同生长期地表反照率、水热通量、CO2通量、总体输送系数以及能量平衡比率等的变化规律,并且将涡度法与梯度法测得的感热、潜热和动量通量进行对比分析,以期能够全面深入了解农田生态系统的湍流输送特征。得到的主要结果有:
(1)不同高度的风速、温度和湿度日、季变化均很明显。风速夜间小于白天,风速随高度的增加而增大;温度廓线夜间呈逆温状态,白天温度随高度的增加而减小;湿度基本随高度的增加呈递减趋势,但也出现了逆湿现象。10 cm以上土壤温度有明显的日、季变化特征,而10 cm以下土壤温度只存在明显月变化。农田土壤湿度与降水及下垫面状况有关。
(2)农田下垫面近地层辐射平衡各分量和能量通量随着季节、天气和下垫面的变化,其变化的幅度、形态存在着明显的差异。淮河流域热量资源较丰富,太阳辐射年总量约为4878.8 MJ·m-2;净辐射在各能量间的分配和地表反照率主要受下垫面状况的影响,感热通量在下垫面为裸地时最大,占净辐射比例可达30%～50%;潜热一直是农田能量消耗的主要形式,占净辐射比例为50%～80%,土壤热通量占净辐射的比例一般均小于10%;反照率变化表现为:裸地<麦田<稻田<雪地。农田下垫面的总体输送过程中动力因子的作用大于热力作用;CO2通量受下垫面的影响较大,在不同下垫面以及作物不同的生长期,CO2通量均有明显的变化,CO2净吸收在作物抽穗期最大可达2.86 g/(m2·h),研究表明近中性条件是最有利于水热和CO2通量垂直传输的大气状态。
(3)地表能量平衡分析表明:农田能量平衡处于不闭合状态,全天能量闭合率平均为0.89,各下垫面的能量平衡闭合率表现为:裸地>小麦>水稻>雪地,裸地闭合率最高可达0.91,而雪地只有0.46,白天能量平衡闭合程度明显大于夜间,白天能量闭合率平均为0.78,夜间平均仅有0.24;农田能量不闭合与夜间能量不平衡、下垫面部分能量吸收项的忽略以及湍流强度对湍流通量输送的影响有关,整个观测期涡度相关仪器直接观测的湍流能量(LE+H)始终小于有效能( Rn ? G),涡度相关仪器直接观测的湍流通量可能有被低估的趋势。
(4)农田中梯度观测和涡度法观测的感热通量有较好的一致性,两方法所观测通量的复相关系数均超过了0.6,且通过信度检验,可认为梯度法与涡度法总体具有较好的相关性。
(5)对采用MDV方法插补前后的涡度数据在显著性α=0.05水平上进行Z检验,除CO2通量正值区插补前后数据的分布差异显著外,其它数据插补前后差异均不显著。
The features of surface albedo, water and heat flux, CO2 flux, bulk transfer coefficients and energy balance in different underlying surface and growing period have been investigated based on the data of turbulent fluxes observed by Eddy Covariance method, radiation balance components and profile data over cropland in Huaihe River Basin from Sept.2007 to Aug.2008 in order to complete insight into the features of turbulent transfers over the cropland ecosystem. The seasonal and diurnal variation of the interaction between surface and atmosphere were also elucidated from these data. Furthermore, the sensible heat, latent heat and momentum fluxes are calculated by the eddy correlation and vertical gradient method are compared. The main results can be listed as follows:
(1)It is found that diurnal and seasonal variation features of wind speed, temperature and humidity in different height is obvious, Wind velocity at daytime is larger than night which increase with elevate observation height. The air temperature decrease as elevate observation height at daytime and temperature inversion state appear at night, Humidity decrease frequently as elevate observation height, but humidity inversion state is also observed. The diurnal and seasonal variation features of soil temperature from 0cm layer to 10cm layer is obvious, however, only monthly variation regulation exist surpass that depth. The soil humidity of cropland is dependent on precipitate and underlying surface state.
(2)The analysis of radiation balance components and energy flux in Huaihe River basin shows that: the range and pattern variation of radiation balance components and energy flux are influenced by the change of season, weather and underlying surface state. Annual total solar radiation amount in this area reaches around 4878.8 MJ·m-2, indicated that the Huaihe River basin has an abundance heat resource. We also found that allocation of net radiation among various energy and surface albedo were strongly influenced by underlying surface state. The heat fluxes of sensible and latent are mainly budget items of net radiation. The largest ratio of the sensible heat flux to the net radiation is about 30% to 50% on bare soil. The Latent heat flux is a prevailing item of the net radiation over the cropland, which is 50% to 80% of net radiation, whereas soil heat flux account for less than 10%. The results from the surface albedo investigation suggest that: bare field< wheat field< paddy field < snow field. The bulk transfer coefficients of momentum is larger than the bulk transfer coefficients of sensible heat at the same observation height. To some extent, CO2 flux is determined by underlying surface state, and the variation of CO2 flux with different underlying surface and crop growing stage is obvious. The net CO2 assimilation reached maximum value of 2.86 g/(m2·h) in jointing stage during the crop growing period, and study further shows that water, heat and CO2 flux vertical transportation was in favor of near neutral conditions.
(3)The analysis of the surface energy balance shows that the phenomenon of energy imbalance exists over cropland. The average energy closure rate was 0.91 in bald field, whereas only 0.46 in snow field and the study shows that: bare field> wheat field> paddy field> snow field during the observed period. The average energy closure rate was 0.89 during the observed period, which 0.78 at daytime and 0.24 at night. The energy in-closure of cropland was influenced by energy imbalance at night, the ignorance of some energy item and turbulent intensity impact on turbulent transfers. During the observation period, the turbulent fluxes(LE+H)that observed by eddy covariance method always less then available energy( Rn ? G),which indicated that turbulent fluxes might be underestimated.
(4)Sensible heat, latent heat, momentum flux observed by vertical gradient method was consistent with eddy covariance method. The multiple correlation coefficients between data acquired by these two methods were greater than 0.6 and reliability test indicated that there was a fine correlation between two methods.
(5)Using Z-test at 0.05 level to analysis the difference of EC data before and after interpolate by MDV method, it shows that the difference of CO2 positive value region was obvious.
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