Estimates of Energy and Water Exchanges at Short Grass with Low- and High-frequency Monitoring Systems

Ming-Hsu Li1, Yi-Yi Chen2, and Gu-Zhou Wei3

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Abstract. Land-atmosphere interactions involve complicated physical, chemical, and biological processes and play an important role in earth hydrological cycles. Energy and water exchanges are fundamental processes for characterizing land surface responses to the atmospheric forcing, as well as ecosystem feedbacks. A vertically integrated micrometeorology and land hydrology monitoring system was established over a short grass site at the National Central University, Taiwan. Long-term data of solar radiation, wind direction/speed, relative humidity, air temperature/pressure, precipitation, pan evaporation, ground heat flux, and soil temperature/moisture at multiple depths were recorded every 10-minute averaged from 1-minute samplings. Under the assumptions of without horizontal soil water flow (e.g., flat terrain) 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 close to 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 first observation year was 2904 mm and the annual ET estimated from soil water losses was 735 mm. The highest ET/P ratio is 73% of winter and the lowest one is 12% of spring. For wet seasons of summer and autumn, the ratios are 30% and 19%, respectively. The annual precipitation (P) of second observation year was 2056 mm and the annual ET estimated from soil water losses was 686 mm. Although the ET/P ratios of summer and autumn are low, the amounts of ETs are higher than those of spring and winter due to high precipitation of typhoons and strong radiations in summer and autumn. Micrometeorology data were further applied into two types of potential ET equations, including the Penman-Monteith and the Priestly-Taylor, to compute daily potential ET. By comparing potential ETs with ETs from soil water losses in associated with soil moisture contents, a correction factor for estimating ET out of potential ET using soil moisture contents as references was established. In additional to low frequency instruments, an eddy covariance (EC) system, including a 3-D sonic anemometer, Young 81000, and a Krypton Hygrometer, KH20, was periodically practiced for latent heat (LH) and sensible heat (SH) measurements. At low soil water content of Jdays 267, 2006, the LH measured by EC was equivalent to an ET depth of 1.56 mm/day corresponding to a soil water loss of 1.85 mm/day. However, soil water losses under wet soil conditions did show significant deviations from EC responses at current studies. Further investigations such as footprint analysis are required to reveal these discrepancies.

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

 

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