The Study of Energy and Water Exchanges above an Evergreen Forest in Taiwan

2006

Yi-Yi Chen1, Ming-Hsu Li2, and Gu-Zhou Wei3

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Abstract. Energy and water exchanges above forest ecosystems are fundamental processes for characterizing land-atmosphere interactions in earth hydrological cycles. The objective of this study is to improve our understanding of the influence of atmospheric forcing on the rate and magnitude of forest energy and water fluxes. The Lien-Hua-Chih (LHC) observation site (23o55’52’’ N, 120o53’39’’ E, 773 m elevation) was established in the summer of 2006 in a natural evergreen forest. It is located inside an experimental watershed (No.5 watershed, 8.39 ha) managed by the Taiwan Forestry Research Institute and the averaged canopy height is about 17 m. Soil moistures/temperatures were measured at -10 cm, -30 cm, -50 cm, -70 cm, and -90 cm. Soil heat flux plate was placed at -5 cm. A drainage gauge was installed at -50 cm to collect infiltrated water. Temperature and relative humidity sensors were placed every 5 m from ground surface to the top of the tower at 20 m, where net radiation and wind speed/directions were also installed. Long-term data of low response instruments were recorded every 30-minute averaged from 10-minute samplings. A nearby weather station provides daily pan evaporation and precipitation data. Prior to the construction of observation tower, soil moistures/temperatures at multiple depths of three different sites were measured since the summer of 2004. By neglecting horizontal soil water flow (e.g., small surface gradient) and infiltration (e.g., normally 2~3 days after rainfalls), the loss of soil water is equivalent to the amount of evapotranspiration (ET). For those days right after rainfalls cease, the ET is estimated by potential ET due to high soil moisture content. Since the response of soil water variations is relatively slow to the fluctuations of atmospheric forcing, only daily ET is estimated from daily soil water loss. The annual precipitation (P) of 2005 was 2674 mm and the annual ET estimated from soil water losses was 664 mm.
The amount of winter ET is larger than that of winter P and the ET/P ratio of spring is 28%. For wet seasons of summer and autumn, the ratios are 16% and 17%, respectively. Although the ET/P ratios of summer and autumn are low, the amounts of ETs are higher than that of spring due to high precipitation of typhoons and strong radiations in summer and autumn. In additional to low frequency instruments, an eddy covariance (EC) system, including a 3-D sonic anemometer Young 81000 and a Krypton Hygrometer KH20, were periodically practiced for LH and SH measurements above canopy at 25 m. During wet seasons (summer and autumn), fogs and afternoon thunderstorms often caused failures of the EC system. For those days right after rainfall, the ETs estimated by EC are often larger than those estimated from soil water losses due to the contributions of substantial amounts of ETs from interceptions.

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Keywords: Evapotranspiration; Eddy Covariance; Latent heat flux; Sensible heat flux

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1, Corresponding author, Graduate Research Assistant, Institute of Hydrological Sciences, National Central University, Jung-Li, 320, Taiwan, E-mail: spancer_hot@hotmail.com
2, Associate professor, Institute of Hydrological Sciences, National Central University, Jung-Li, 320, Taiwan, E-mail: mli@cc.ncu.edu.tw
3 ,Graduate student, Institute of Hydrological Sciences, National Central University, Jung-Li, 320, Taiwan, E-mail: opel88885@hotmail.com