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Abstract:
Water table dynamics, a basic hydrologic process, was traditionally not considered in
global-scale land surface models (LSMs). In this study, a representation ofwater table dynamics
is integrated into a global LSM to address the shortcomings of existing global-scale modeling
studies and the appropriateness of specifying certain parameters as globally constant.
Evaluation of model simulation using globally varying parameters against river discharge
observations in selected large rivers shows improvements when the water table dynamics is
included in the model. The mechanisms by which the water table dynamics affects land surface
hydrologic simulation are then investigated by analyzing the sensitivity simulation of
groundwater (GW) capillary flux. The result indicates that global mean evapotranspiration (ET)
increases by ~9% whenGWcapillary flux is considered. The semiarid regions with marked dry
season have the largest increase (~25%), while the humid and high-latitude regions with
sufficient moisture but limited radiation energy have the smallest increase. Increase in ET is
more pronounced in dry season when GWrecharge becomes negative (upwardmoisture supply
from the aquifer), but its magnitude depends on the water table depth (WTD). On the other
hand, a deeper WTD caused by the GW capillary flux is found to decrease runoff throughout
the year in regions with a large increase in ET in dry season only. Based on ourmodeling result,
about 50% of global land area (especially in humid and high-latitude regions) is simulated to
have the mean WTD shallower than 5 m, which emphasizes the significance of representing water table dynamics in global-scale LSMs.
