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Study on the Microclimatic Characteristics of a Natural Machilus zuihoensis Stand
|關鍵字:||Machilus zuihoensis;香楠;Microclimate;Heat budget;Interception;微氣候;熱收支;截留量||出版社:||水土保持學系||摘要:||
從香楠林熱收支解析得知，地中傳導熱量、顯熱、潛熱及植被儲存熱量分佔淨輻射量之平均比例為：5.2、27.1、42.6及25.1%。而潛熱隨淨輻射量增加趨勢並非線性，且由潛熱估算一天之香楠林蒸發散量，結果為0.53~3.3 mm，平均為1.4 ± 0.46 mm。又香楠林植冠儲存熱量經估算為0.84 ~ 1.72 J×℃-1×m-3。
全天之香楠樹冠表層葉溫（TL）多低於其上方氣層溫度，表示香楠樹冠仍能吸收太陽輻射。在冬季低溫狀態下之林內地面之最低日平均地溫仍在18 ℃以上，且香楠林內之地表至其下深度40 cm範圍內，地溫自9時開始遞增，且持續至午夜才又遞降，地表下深度40至100 cm範圍內，地溫幾已形成一上下溫差僅0.5 ℃之等溫分布。在林內之相對濕度均大於80 %，且愈近地表其值甚或可達95 %以上。而當TL小於林內氣溫（Ti）時，則林內相對濕度將隨高度遞降，又當TL< Ti時，林內相對濕度將隨高度略增。
氣流進入試驗集水區內，受地形、地物影響，常使風速減弱及風向轉變，而形成小尺度之渦流或擾動，造成試驗集水區內部處於弱風及降雨集中之狀態，則試驗集水區內部降雨量略大於其區外者。鬱閉林分下，樹冠厚度可作為香楠林樹冠結構之特徵，亦可作為估測樹冠截留量之根據。香楠林最大單位樹冠厚度之截留量（ψ）為3.2 mm/m。若以ψ乘以樹冠厚度，可知該天然香楠林之最大樹冠截留量將不會超過27.4 mm。而平均截留量與林外降雨量間具有非線性二次式之關係，據此推知，截留量隨降雨量增加將有其極限。
This study aimed at observation on the microclimate in a natural Machilus zuihoensis stand. A series of observations for the heat flux budget, air temperature, leaf temperature and earth temperature, relative humidity, wind speed and rainfall were performed from May 1995 to September 2001. The experimental site was located in the east of experimental watershed of National Pingtung University of Science and Technology, Taiwan (elevation = 90m; latitude = 22º39'N; longitude = 120º36'E). The results showed as follows:
The analytical heat budget in Machilus zuihoensis stand to find the average proportion of soil heat flux, atmosphere heat flux, latent heat flux and canopy heat storage in proportion to net radiation were as 5.2, 27.1, 42.6 and 25.1%. Regressive relationship between latent heat and net radiation showed a non-linear curve. To estimate evapotranspiration by the latent heat would indicate mean value was 1.4 ± 0.6 mm day-1. Then the estimated canopy heat capacity was between 0.84 ~ 1.72 J×℃-1×m-3.
The leaf temperature(TL)was less than air temperature above canopy in whole day, and then solar radiation was still absorbed by canopy. As to the mean temperature under ground surface was higher than 18℃ in winter. Consequently, the tendency of earth temperature was documented, 0 to 40cm depth increased at 9 a.m. and decreased in midnight. However, an isothermal layer which existed in 40 to 100 cm surrounding temperature difference was less than 0.5 ℃. The relative humidity (R.H.) under canopy was more than 80 % but nearby ground surface was up to 95 %. The correlation between R.H. and height were showed positive at TL< Ti (air temperature under canopy) and negative at TL>Ti.
Air current flows into the experimental watershed that is restricted by the geomorphologic surface and ground cover, thus the wind speed will be reduced and results in wind rotation. The small scale of turbulence and eddy are sometimes occurred and then resulted in the concentrated rainfall in this area. The rainfalls in experimental watershed were more than outside. The canopy depth of the stand could be reasonable represents its canopy structure and thus could be used as an indicator of canopy interception. Based on the relationships between gross rainfall and the ratio of interception to canopy depth(Dc)indicated that maximum I/Dc ratio would reach a value of 3.2 mm/m. Regressive relationship between interception and gross rainfall also showed a non-linear curve and implied the canopy interception would impliedly have a limited value as the gross rainfall increased. The maximum value of 3.2 mm/m times the canopy depth might indicate that maximum canopy interception of the studied woods is 27.4 mm.
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