| dc.description.abstract | Evapotranspiration (ET) and rainfall infiltration (RI) are primary fluxes of water exchanges between land surface and atmosphere. The objectives of this study is to investigate how ET and RI affect soil moisture and groundwater variations. Integrated hydrometeorology measurements were established at the meteorology station inside the campus of National Central University to provide continued observations for calculations of potential ET, real ET, and RI values. During rainfall wetting period, tensiometers and soil moisture sensors provide estimations of RI and parameters of soil characteristic curves. During non-rainfall drying periods, amounts of real ET were estimated by losses of soil moisture. Changes of soil moisture and groundwater levels were analyzed to investigate connections of soil moisture and groundwater during wetting and drying processes.
Data selected from 2016/12/9 to 2018/9/31 were separated into wet events and dry events. Persistent groundwater flow gradients were observed. For wet events, averaged groundwater inflow and outflow velocities are 0.102 mm/hr and 0.117 mm/hr, respectively. For dry events, averaged groundwater inflow and outflow velocities are 0.1 mm/hr and 0.09 mm/hr, respectively. Long-term averaged (for example: after the rain has just stopped, or intermittent rain events, that is, it is impossible to determine whether it is a dry or wet process event.) groundwater inflow and outflow velocities are 0.041 mm/hr and 0.042 mm/hr, respectively. Regional groundwater inflow and outflow do not have significant effects on changes of groundwater levels.
During rainfall events, the rise of groundwater levels were caused by two different effects. The first one is due to the increase of near surface hydrostatic pressures as the increase groundwater levels were early than that of soil moisture at -400 cm. This hypothesis was supported by insignificant differences between groundwater inflow and outflow velocities. The later second effect is the recharge of infiltrated water. Rainfall events observed between 2016/12/9 and 2018/9/31, the rise of groundwater levels are 9.5 hours earlier than the increase of soil moisture at -400 cm. After the infiltrated water reach saturated zones, significant differences between groundwater inflow and outflow velocities were observed. Based on rainfall events analyzed in this study, 40 mm of accumulated rainfall is capable to induce sufficient infiltration to cause the increase of soil moisture at -400 cm and reach saturated zone subsequently. For dry events, declines of groundwater levels were earlier than decreases of soil moisture at -400 cm. It is suspected that the declines of groundwater levels were induced by the decrease of soil moisture due to reduces of near surface (i.e., above -400 cm) hydrostatic pressure. This hypothesis was supported by differences between groundwater inflow and outflow velocities were insignificant during dry events. Drying or wetting in near surface soil moisture will induce changes in groundwater levels to quickly reflect changes of near surface hydrostatic pressures caused by variations of soil moistures due to ET and RI. | en_US |