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Evaluation on Energy Balance of Farmland in Shouxian County of Anhui Province
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摘要: 为了评估安徽省寿县国家气候观象台新增的近地层通量观测系统业务运行状况，利用2007年9月—2008年8月寿县近地层通量系统观测资料，分析了农田下垫面能量平衡情况。结果表明：全年能量平衡比率平均为0.89，但不同条件下存在差异：白天明显大于夜间，春、夏、秋季明显大于冬季，裸地和麦、稻田明显大于雪地，晴、昙、阴、雨天气情况下差异不大。总体来说，能量不平衡是通量观测中较为普遍的现象，一般认为不平衡程度在10%~30%为合理范围，寿县国家气候观象台全年的能量不平衡程度 (11%) 恰好属于这个范围，并接近不平衡程度的下限，说明涡度相关法在淮河流域农田生态系统通量观测中可靠性较高。Abstract: Energy balance means the balance between the sum of latent heat, sensible heat flux observed by Eddy Covariance (EC) technique and the variance of net radiation flux, soil heat, canopy heat. According to the first law of thermodynamics and basic assumption of EC observation, the ratio of energy balance can be used as an effective evidence to evaluate data quality and system performance of an observation system theoretically. Energy balance ratio of the field has been investigated based on the data of flux observation system on near surface layer observed at the representative station of Huaihe River Basin, Shouxian National Climate Observatory from September 2007 to August 2008 in order to assess operation status of this new system. The results show that the annual average energy balance ratio is 0.89 during the period, and it is higher in the day than that in the night, higher in spring, summer, autumn than that in winter, higher over the bare surface than that over the cropland surface, while different weather conditions make little differences. The largest energy balance ratio is 0.92 over the bare surface, and smaller ratio is 0.9 over wheat field and paddy field, while it's only 0.50 over snow surface. It also shows that energy balance is influenced by the negligence of heat reserve during exuberant growing period to some extent and absorption tem, such as thaw, freeze and sublimation. During the observation period, the observed turbulent fluxes are always less than available energy, indicating that turbulent fluxes might be underestimated. The energy balance during daytime seems better than that at night because of stronger turbulence. It can be concluded that the applicability of EC method is best in flat, bare field, better in the day than that in the night, better on the underlying surface with low vegetation than that covered with snow. All in all, the phenomenon of energy imbalance is fairly common in flux observation and it often reaches 10% to 30%. The annual energy balance ratio (0.89) of Shouxian National Climate Observatory is close to the lower limit of ranges, so the EC method of flux observation over farmland ecosystem in Huaihe River Basin is reliable.
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图 1 湍流通量 (EL+H)、有效能 (Rn-G) 和能量平衡比率 (REB) 的日变化
Fig. 1 Diurnal variations of (EL+H), (Rn-G) and REB
图 2 湍流通量 (EL+H)、有效能 (Rn-G) 和能量平衡比率 (REB) 的月变化
Fig. 2 Monthly variations of (EL+H), (Rn-G) and REB
表 1 寿县通量观测系统主要观测仪器的基本技术性能[22]
Table 1 Basic technical characteristics of primary observation instruments of boundary layer in Shouxian[22]
观测仪器 准确度 分辨率 平均时间 自动采样频率 190SB光合有效辐射传感器 5% 1 W/m2 1 min 60次/min CNR1净辐射传感器 15%~20% 1 W/m2 1 min 60次/min CSAT3三维超声风温仪 风速水平分量：4.0 cm/s；
风速垂直分量：2.0 cm/s风速水平分量1 mm/s；风速垂直
分量0.5 mm/s；虚温0.025℃30 min 10 Hz LI-7500红外H2O/CO2分析仪 0.3 mg/kg (CO2)；
0.15 mmol/mol (H2O)0.1 mg/kg (CO2)；
0.1 mmol/mol (H2O)30 min 10 Hz HFP01土壤热通量仪 ±20% 1 W/m2 30 min 1次/min 
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表 2 不同条件下能量平衡比率的比较
Table 2 Energy balance ratios in different conditions
条件 分类 样本数 REB 不同时段 全天, 白天, 夜间 17568, 8333, 9235 0.89, 0.78, 0.24 不同季节 秋, 冬, 春, 夏 4368, 4368, 4416, 4416 0.91, 0.77, 0.92, 0.90 不同下垫面 裸地, 雪地, 小麦, 水稻 5856, 1488, 5328, 4896 0.92, 0.50, 0.90, 0.87 不同天气 晴, 昙, 阴, 雨 3600, 5568, 1968, 6432 0.85, 0.88, 0.89, 0.91
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表 3 不同下垫面观测的对应时段
Table 3 Periods corresponding to different underlying surface
下垫面 对应时段 裸地 (半裸地) 2007年10月—2008年1月上旬和6月上、中旬 雪地 2008年1月中旬—2月上旬 麦地 2008年2月中旬—5月 稻田 2008年6月下旬—9月
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