Surface Flux Measurements Above a Mountainous Cryptomeria Forest

地表之可感熱、潛熱以及二氧化碳通量，常以渦流相關法估測於近似理想的地形，但如何在複雜地形中合適地使用，仍是一大挑戰，而檢測單一測點是否能有效代表複雜地形中的流體特性是必須的。本研究選擇的實驗地位於台灣南投的溪頭實驗林，是一57年生的人工柳杉山林地，海拔高度介於800~2000公尺高，且山地坡度為13.6度。研究結果顯示，雖然實驗地地形複雜，空間的變異係數幾乎都小於15%，另外，白天之變異係數小於晚上的變異係數，可見白天有較好的均質特性，而地表通量的空間變異性比較中，潛熱會大於可感熱及二氧化碳通量，原因可能是水氣有較複雜的源或匯存在。在大氣於不穩定的狀況下，垂直與水平風速會符合Monin-Obukhov相似理論，但相似常數因位置不同而有不同；在純量部分，以溫度最符合Monin-Obukhov相似理論，二氧化碳普通，而水氣並不符合。Under ideal or near-ideal conditions, the eddy-covariance (EC) method is a widely used technique in the measurement of sensible heat, water vapor, and CO2 fluxes. Nowadays, adapting the EC method to complex terrain measurements is a challenging topic. It is necessary to examine whether measurements from single eddy covariance system can represent the flow properties of turbulence at complex terrain. The experimental site in this study is located in a valley covered with 57-year-old Cryptomeria plantation at Sitou, Taiwan. The weather is warm and humid through the whole year. The elevation of this area is from 800 to 2000 m above sea level, and the averaged slope is 13.6 degree. The results showed that almost all the coefficient of spatial variation (CV) values for surface fluxes in this study are less than 15%. Moreover, the values of CV in the daytime are smaller than those in the nighttime. This indicates that the flow variables are more homogeneous during daytime. It is noted that the spatial variability of water vapor flux is larger than those for sensible heat and CO2 fluxes; this may due to the complex sources/sinks distribution of water vapor in this site. Under unstable condition, vertical and horizontal wind velocities meet the Monin-Obukhov similarity theory (MOST) predictions, though the similarity constants at different locations are not the same. For scalar variances, temperature follows MOST well, CO2 meets it fairly, but water vapor does not follow the MOST predictions for all the three locations