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 from0.070±0.015min the winter to 0.130±0.061 min 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 ~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.