溪頭柳杉人工林微氣象特性與能量收支之研究

Micrometeorological Characteristics and Energy Budget in a Cryptomeria japonica Plantation at Xitou area

林宜蓁(I-Chen Lin)

指導教授 ： 陳明杰

國立臺灣大學/生物資源暨農學院/森林環境暨資源學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.00464

摘要

本研究的目的為探討柳杉人工林於不同高度的氣溫、水汽壓等之季變化和日變化特性，不同高度層能量收支的乾濕季變化，以及降雨對能量收支日變化的影響等。研究地點位於臺大實驗林溪頭營林區第3林班173號柳杉人工林，收集2014年1月22日至2015年1月21日通量塔的降雨量、淨輻射通量、土壤熱通量、氣溫、相對濕度、大氣壓力等監測資料，並利用包溫比法估算淨輻射通量分配於顯熱通量與潛熱通量等。資料整理分析時，定義春季為3月~5月、夏季為6月~8月、秋季為9月~11月與冬季為12月~2月，以及定義濕季為4月~9月、乾季為10月~3月。
林分之微氣象及能量收支特性因不同高度層而異。研究結果顯示，四季皆於枝葉密集之樹冠上層（高度23.3 m）監測到最高的氣溫及水汽壓，且高度23.3 m以下的氣溫及水汽壓垂直變化隨著高度下降而降低。然而，於林下植物層上方（高度2.3 m），因為植物的蒸發散與土壤蒸發作用，使得此高度於四季皆有較高的水汽壓值。
樹冠層上方（高度32.5 m）之淨輻射通量全年平均為68.97 W m-2，於夏季最高78.84 W m-2，冬季最低60.31 W m-2，受到樹冠層遮蔽、反射與吸收的影響，樹冠上層（高度24.8 m）及林下植物層上方（高度3.8 m）的淨輻射通量僅佔樹冠層上方之10.1%及6.0%。此外，由於季節、地形與太陽高度角較低等因素影響，利用Beer-Bouguer Law計算淨輻射通量經過樹冠層的消散係數，冬季為1.11最高，春季為1.00最低。土壤熱通量全年平均為–0.68 W m-2，於濕季時為吸熱的正值，於乾季時轉變為放熱的負值。
利用包溫比法估算能量分量結果，樹冠層上方的顯熱通量大多時候為正值，但於降雨、陰天或有霧的天氣型態時，有時會出現負值的情形，即顯熱通量的能量向下傳遞。研究期間夏季之降雨天數為53天，平均顯熱通量為–4.21 W m-2。由於濕季時淨輻射通量高且蒸發散作用旺盛，所以樹冠層上方有較高的潛熱通量，佔淨輻射通量之95.5%，反之，於淨輻射通量與降雨量皆較低的乾季時，潛熱通量佔淨輻射通量之61.6%。
研究期間，樹冠層上方與樹冠上層皆以潛熱通量的形式為主，分別佔淨輻射通量之79.7%與102.1%，顯熱通量次之，土壤熱通量分別佔–1.1%與–13.5%；樹冠中層因周圍枝葉較稀疏，故以顯熱通量為主要能量消耗的型式，其佔淨輻射通量66.5%，而土壤熱通量佔–10.1%；林下植物層上方受到植物蒸發散與土壤蒸發作用的影響，潛熱通量佔淨輻射通量之190.9%，為主要消耗能量的型式，且土壤熱通量佔–15.8%。由結果發現，土壤熱通量對於離地表較近的高度層之能量收支有較大的影響。
天氣型態對淨輻射通量的影響很大，無降雨期間，因霧或陰天等天氣型態的不同，使得淨輻射通量與晴天的差異很大。並且，受到陰天或霧的影響，顯熱通量有時於白天出現負值。夏季時，降雨期間由於淨輻射通量下降，潛熱通量降低，冬季時，無降雨期間的顯熱通量高於潛熱通量，但於降雨期間出現潛熱通量高於顯熱通量。

關鍵字

柳杉人工林 ； 微氣象因子 ； 能量收支 ； 包溫比能量平衡法

並列摘要

This research was to investigate the seasonal and diurnal change of air temperature and water vapor pressure at different height, the energy budget at different height layer in dry and wet season, and the diurnal change of energy budget during rainy day in a Japanese cedar (Cryptomeria japonica) plantation. The study site was located at the third compartment in Xitou tract, NTU Experimental Forest. Rainfall, net radiation flux, soil heat flux, air temperature, relative humidity and barometric pressure were collected derived from flux tower during 22 January 2014 and 21 January 2015. Bowen ratio energy balance method (BREB) was used to estimate sensible heat flux and latent heat flux, and discuss the net radiation flux assigned to sensible heat flux and latent heat flux. In data analysis, defined spring was during March to May, summer was during June to August, autumn was during September to November and winter was during December to February. In addition, wet season was during April to September and dry season was during October to March.
The micro-scale meteorological factors and energy budget characteristics were different at different height layer. In this study, the monitored highest air temperature and water vapor pressure appeared in the upper-canopy layer (23.3 m height) in four seasons. Air temperature and water vapor pressure down from 23.3 m height were lower with reduced height. However, owing to the evapotranspiration by understories and soil evaporation, the water vapor pressure increased above the under-stories layer (2.3 m height).
The average net radiation measured above-canopy (32.5 m height) was 68.97 W m-2. It was the highest in summer (78.84 W m-2) and the lowest in winter (60.31 W m-2). The radiation could be sheltered, reflected and absorbed by canopy, so the ratio of net radiation measured at upper-canopy layer (24.8 m height) and above under-stories layer (3.8 m height) were 10.1% and 6.0% to above the canopy. Furthermore, the extinction coefficient calculated by Beer-Bouguer Law that was influenced by season, topography and solar altitude angle, was highest in winter (1.11) and lowest in spring (1.00). Besides, the average soil heat flux was –0.68 W m-2, and it was endothermic and exothermic in wet season and dry season.
By using BREB method to estimate energy flux above canopy, the value of sensible heat flux was positive at most of the time. And it was negative in rainy, cloudy or foggy day, which usually occurred in wet season. Particularly, 53 rainy days occurred in summer, so sensible heat flux was –4.21 W m-2. In addition, impacted by sufficient net radiation and rainfall, latent heat flux ratio was 95.5% to net radiation flux in wet season, otherwise, it was only 61.6% in dry season.
In research period, the energy was mainly took advantage of latent heat flux at above-canopy and upper-canopy layer, which was 79.7% and 102.1% to net radiation flux while the soil heat flux was –1.1% and –13.5%. Because the leaves were few at the middle-canopy layer, the sensible heat flux became the main type to consume energy (66.5%) while the soil heat flux was –10.1%. Above under-stories layer, latent heat flux and soil heat flux were 190.9% and–15.8%. The result also revealed that the emitted soil heat flux could mainly affect energy budget of nearing forest floor.
The weather condition played an important role in net radiation flux. The net radiation flux was different between foggy, cloudy and sunny day. And sensible heat flux was negative in rainy day. Furthermore, because the net radiation flux was decreased in rainy day, the latent heat flux was diminished in summer. In winter, latent heat flux was higher than sensible heat flux in rainy day, which was different from sunny day.

並列關鍵字

Japanese cedar (Cryptomeria japonica) plantation ； micro-scale meteorological factors ； energy budget ； Bowen ratio energy balance method (BREB)

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