Effects of vadose zone on groundwater table fluctuations in unconfined aquifers

- Groundwater table fluctuations in unconfined aquifers were examined.
- New governing equation satisfies the mass conservation principle.
- Analytical solution was derived to quantify the effect of vadose zone.
- Groundwater table fluctuations are affected by the ground surface (i.e., vadose zone thickness).

SummaryAbove a shallow unconfined aquifer, a considerable amount of water is stored in the vadose zone. Through water exchange with the underlying unconfined aquifer, the vadose zone affects the groundwater table dynamics and overall behavior of the aquifer. In this paper, we examine tide-induced groundwater table fluctuations in unconfined aquifers influenced by vadose zone of finite thickness. Under the condition of small aquifer thickness (D) compared with the groundwater wavelength (L) (i.e., μ2=D/L2≪1) and small boundary oscillation amplitude (a) (i.e., ε=a/D≪1) (where μ2 and ε are two parameters), an approximate analytical solution was derived to quantify systematically the vadose zone effects, with a particular consideration of capping by the ground surface, i.e., the upper boundary of the vadose zone. Depending on the extent to which the capillary rise is truncated by the ground surface, the vadose zone enhances the groundwater table fluctuations in an unconfined aquifer. However, the mean groundwater table height and exchange between surface water and groundwater are reduced due to the presence of the vadose zone. These effects are intensified with increased capillary rise, but weakened as the vadose zone thickens. This study provides a criterion for assessing the importance of vadose zone in modulating the response of unconfined aquifers to low-frequency forcing oscillations such as tides.

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