| Summary: | Groundwater and soil moisture are key components of the terrestrial water cycle. The equilibrium state of soil moisture and water table is important to understand water balance and has been investigated on regional scales and a global scale. However, the equilibrium state of soil moisture and water table interaction has never been investigated on a global scale. Hence, this study focuses on explaining the equilibrium state of soil moisture and water table as influenced by climate on a global scale. The concept of ‘climatological equilibrium state’ in hydrology was proposed and define it as the situation when a similar pattern of climate continues for a long time. Within the modeling framework of one dimensional coupled land surface and water table dynamics, a 30 years climate forcing has been repeated to create a 1020 year-long synthetic climate dataset. The offline simulations under wet and dry initial condition and with and without effect of frozen soil are conducted. The simulated water table depth, soil moisture and runoff are compared with observations. The relationship between soil moisture, water table depth and climate(precipitation, evapotranspiration, budyko’s dryness index) were analyzed. The results revealed that ～20％ of land surface area does not reach the climatological equilibrium state, mostly in arid and semi-arid regions while ～80％ of land surface area reach the climatological equilibrium state. The ‘climatological e-folding time’( simulation years to reach the equilibrium) is ～300 from 600 years at arid and semi-arid regions and ten times longer than assumed 30 years as climate. It suggests that response of water table is slow to climate change and water table is transient at those regions under the current climate because in reality, climate will change before water table reach the equilibrium. The initial condition (existence of ice, water table depth, soil moisture) remain for a long time at permafrost area and it is important to know past climate and hydrological state for understanding ...
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