Effect of rainfall intensity and distribution on groundwater level fluctuations

A mathematical equation describing the relationship between the groundwater level increment and the effective accumulated rainfall amount is proposed in this study. The groundwater level measurements were taken from the Donher well station in Central Taiwan and the rainfall data were collected from seven rain-gauge stations nearby the well station. To analyze the relationship, we first employed the BAYTAP-G program to filter out the response of the groundwater level to non-rainfall effects, such as barometric pressure and earth tide. Because both the present and antecedent rainfall events give an impact on the groundwater level increment, an effective accumulated rainfall amount was defined to account for their influences using the exponential-decay weighting method. The resulting groundwater level, conventionally termed the residual groundwater level, was found to linearly depend on the effective accumulated rainfall amount. It was also shown that the effective accumulated rainfall amount calculated from the exponential-decay weighting method with a decay parameter of 0.25 reveals a highly-correlated relation to the residual groundwater level increment. Lastly, we demonstrated that rainfall occurring beyond the surface watershed within which the Donher well station locates indeed affects the residual groundwater level. The effect can be incorporated into our proposed equation.