Effects of Evapotranspiration and Earth Tides on the Water-Level Fluctuation in a Forest Catchment in Gwangneung

Closing energy and water budgets in terrestrial ecosystems still remains as a challenge, particularly for those with heterogeneous vegetation in complex terrains. This paper presents an attempt to quantitatively estimate parts of water-level fluctuation due to evapotranspiration (ET) and earth tides, and their seasonality at the Gwangneung forest catchment, Korea. The long-term water level was monitored from a piezometer at 5-min intervals from February 2006 to July 2007. For the seasonal variation, summer and winter sub-datasets were analyzed using the power spectral analysis. Cyclic movements with frequencies of 1 cycle / 24.0 +/- 0.5 hr were extracted from the water-level monitoring data. Daily fluctuation of water levels ranged from 0.070 +/- 0.015 m in the winter to 0.130 +/- 0.061 m in the summer of 2006. In winter, the water-level fluctuated on average 13.9 mm, of which 94% was attributed to the effect of earth tide and the remainder to ET. In summer, the fluctuation component associated with ET was on average 25.9 mm(66% of the total fluctuation), which is equivalent to a daily ET of similar to 2.9 mm. When compared against the ET observed from the eddy covariance flux tower at the same site for the same periods, the ranges of seasonal ET inferred from the water-level fluctuations were virtually the same.