辽东山区天然次生林能量平衡和蒸散

Energy balance and evapotranspiration in a natural secondary forest in Eastern Liaoning Province,China

基于辽宁冰砬山森林生态系统定位研究站2012年森林内、外微气象观测数据,采用波文比-能量平衡法(BERB)研究了辽东山区天然次生林能量平衡组分及蒸散特征。结果表明,天然次生林全年获得净辐射能量(Rn)为1.63×109J/m2,其中生长季Rn占全年的71%。Rn月均值呈单峰状季节变化;5月份Rn最大,达101.73 W/m2;12月份最小,仅为-2.38 W/m2。Rn在晴朗天气的日变化呈单峰型,峰值出现在12:00前后,Rn在日出后0.5 h至日落前1.5 h为正值,其它时间为负值。潜热通量(LE)、感热通量(H)在晴朗天气呈单峰型日变化规律。LE呈单峰型季节变化,7月份最大。H呈双峰型季节变化,峰值在4月份,次峰值在9月份。波文比(β)近似呈"U"字型季节变化,非生长季β均值为1.50,即H占有效能量的60%,生长季β均值为0.43,即LE占有效能量的70%。生长季土壤热通量(G)为能量支出项,约占有效能量的2.5%,晴朗天气呈单峰型日变化。非生长季G为能量收入项,约占有效能量的6.8%,1月份几乎没有日变化。辽东山区天然次生林全年蒸散(E)总量为541.8 mm,占全年降水总量的70.3%,蒸散耗水是该森林生态系统最主要的水分支出项。

Evapotranspiration is one of the major components of forest water budgets, and therefore, must be estimated accurately. Accordingly, it is essential to thoroughly understand the relationships among evapotranspiration and the components of energy balance fluxes to enable development of forest hydrology and forest meteorology, which are the foundation of management and utilization of water resources. This study was conducted in a natural secondary forest in a mountainous area of Eastern Liaoning Province, China. Components of energy balance fluxes and characteristics of evapotranspiration were investigated at the Bingla Mountain Forest Ecological Station （4235＇ N, 12503＇ E） during 2012, by using the Bowen ratio-energy balance method （BERB）. The monthly variation in net radiation （Rn） exhibited a single peak, with the maximum value （101.73 W/m2） occurring in May and the minimum value （-2.38 W/m2） in December. Additionally, during fine weather, the diurnal peak variation of Rn occurred at noon, indicating a distinctive positive trend from 0.5 h after sunrise to 1.5 h before sunset and a negative trend for the remaining time. Latent heat flux （LE） and sensible heat flux （H） exhibited the same diurnal single peak pattern. However, seasonal variations of LE and H showed different trends, with LE exhibiting a single peak in July and H showing double peaks, the highest in April and the second highest in September. The seasonal characteristics of the Bowen-ratio （β） values generated an approximate U-shaped pattern. The average value of β was 0.43 in the growing season （May to Sep）, indicating that latent heat flux accounted for 70% of the effective energy. In contrast, the average value of β was 1.5, indicating that sensible heat flux accounted for 60% of the effective energy during the non-growing season. Soil heat flux （G） showed different diurnal variations in different periods. During the growing season, the diurnal variation of G showed an obvious single peak, and G accounted for 2.5% of the effective energy of energy expenditure. However, during the non-growing season, G showed no apparent diurnal variation and accounted for 6.8% of the effective energy as energy deposition. The annual total evapotranspiration was 541.8 mm in natural secondary forest in mountainous areas, accounting for a large proportion （70.3%） of the total annual precipitation （771 mm）. The total evapotranspiration in the growing season was 398.3 mm, accounting for 61.8% of the annual precipitation （644.4 mm） in the same period. Taken together, these findings indicate that forest evapotranspiration was the most important expenditure for natural secondary forest in the Eastern Liaoning Mountainous Region. The evapotranspiration value assessed using the BERB method in this study was equivalent to those reported by similar studies, demonstrating that this method is reliable and accurate in natural secondary forest. The results of this study implied that evapotranspiration was not only influenced by precipitation but also by energy flux, roughness of the underlying surface, soil thermal properties, soil moisture, presence of plants and litter, and canopy density. Owing to its complexity and variability, the response mechanisms for energy balance and evapotranspiration in this region should be further investigated.