Abstract: This paper studied the water temperature changes caused by the Earth tide, based on 356 water temperature monitoring wells from the Groundwater Monitoring Network of China. First, 35 wells were selected since they had tidal responses from water temperature data. Then, harmonic analysis was performed to the water temperature data to derive Earth tide constituents (M₂, S₂, K₂, O₁, P₁, S₁, K₁), using the Baytap-G software. The result showed that the amplitudes were 0.00017~0.01825℃, the phase lags were -149.98°~61.47°, the amplitude ratios were 0.01~10℃/10^-6m·s^-2, for the M₂ Earth tide constituent which was low contaminated. The tidal variations of water temperature were result from tidal variations of water level, since all the water levels that showed tidal variations and the amplitudes of water temperature in different depths of one well had similar order. Besides, the tidal variations of water temperature were influenced by the sun-radiation heat, water flow from the aquifer and geothermal gradient. The tidal variations of water temperature near the aquifer had greater amplitudes. The amplitudes of tidal variations of water temperature were proportional to the geothermal gradients. The tidal variations of water temperature could be more easily recorded in artesian wells than non-artesian wells, as the sun-radiation heat was an interference of tidal variations of water temperature and it had less influence on the changes of water temperature in artesian wells than non-artesian wells. The tidal variations of water temperature could be more easily recorded in the eastern China than the west, since the influence depths of sun-radiation heat in the gorge and mountain areas were deeper than plains.

Key words: Wellwater temperature, Water level, Earth tide, Phase lag, Mainland China