Ming-Hsu Li, I-Chieh Tseng, and Shao-Yiu Hsu
National Central University, Graduate Institute of Hydrological and Oceanic Sciences, Jhongli, Taoyuan City, Taiwan
During rainfall events, infiltration leads to increase of soil moisture and subsequently recharges groundwaters which are well known hydrological processes as described in numerous hydrological models for simulations and predictions. This study presents two-year continuous observations of soil moistures with high temporal and vertical resolutions, along with measurements of rainfalls, soil temperatures, pore water pressures, and groundwater levels, over a grass pilot site to investigate effects of increased soil moisture on changes in groundwater levels.
During rainfall events, persistent groundwater gradients, due to flat terrain, were observed that limiting influences of horizontal groundwater flows to changes in groundwater levels. Responses of groundwater levels have two different regimes. At the early stage, the increase of near surface hydrostatic pressures due to infiltrated water not only increased soil moisture, but also induced fast responses in rising groundwater levels. Responses of groundwater levels were 9.5 hours in average earlier than the increase of soil moisture above groundwater tables. The second effect is the well-known direct recharge through infiltrated water. This hypothesis is supported by changes in groundwater temperature were slower than changes in soil moisture above groundwater tables. Accumulated wetting in soil moisture will induce fast responses in groundwater levels can be explained by changes in near surface hydrostatic pressure caused by infiltrated water. Unfortunately, none hydrological models available up to dates are capable of capturing such fast response which may lead to overestimations of groundwater recharges.
High spatial/temporal hydrological observations presented in this study revealed the role of soil moisture dynamics in changes of groundwater levels during rainfall events, which require the rethinking of rainfall infiltration processes to groundwater recharges. Further investigations are needed to test robustness of our hypotheses of such fast responses in groundwater levels to changes in surface soil moisture.