Indirect Measurement of Evapotranspiration from Soil Moisture Depletion

2007

Ming-Hsu Li1 and Yi-Ying Chen2

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Abstract: Direct and in situ measurement of evapotranspiration (ET), such as the eddy covariance (EC) method, is often expensive and complicated, especially over tall canopy. In view of soil water balance, depletion of soil moisture can be attributed to canopy ET when horizontal soil moisture movement is negligible and percolation ceases. This study computed the daily soil moisture depletion at the Lien-Hua-Chih (LHC) station (23o55’52’’ N, 120o53’39’’ E, 773 m elevation) from July, 2004 to June, 2007 to estimate daily ET. The station is inside an experimental watershed of a natural evergreen forest and the canopy height is about 17 m. Rainfall days are assumed to be no ET. For those days with high soil moisture content, normally 2~3 days after significant rainfall input, ET is estimated by potential ET. Soil moistures were measured by capacitance probes at -10 cm, -30 cm, -50 cm, -70 cm, and -90 cm. A soil heat flux plate was placed at -5 cm. In the summer of 2006, a 22 m tall observation tower was constructed. Temperature and relative humidity sensors were placed every 5 m from ground surface to 20 m for inner and above canopy measurements. Net radiation and wind speed/directions were also installed. A drainage gauge was installed at -50 cm to collect infiltrated water. Continuous measurements 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. Since the response of soil water variations is relatively slow to the fluctuations of atmospheric forcing, only daily ET is estimated from daily soil moisture depletion. The annual average precipitation is 2902 mm and the annual average ET is 700 mm. The seasonal ET patterns of the first two water years are similar. The third year has a higher ET because soil moisture was recharged frequently by rainfall In order to examine the applicability of this approach, an EC system, including a 3-D sonic anemometer (Young 81000) and a krypton hygrometer (Campbell KH20), were practiced for LH and SH measurements at the top of the tower. During wet seasons (summer and autumn), fogs and afternoon thunderstorms often caused failures of the EC system. The ETs directly measured by the EC are slightly higher than those estimated from soil moisture depletions. Spatial soil moisture heterogeneity is suspected to be the cause of this discrepancy.

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Keywords: Evapotranspiration; Precipitation; Eddy Covariance

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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