Predicting the effects of climate change on the vadose zone is a new and important application of the analysis of simultaneous heat and water movement between the atmosphere and the vadose zone. The HYDRUS-1D software system, with meteorological submodels that evaluate the surface water and energy balance, has been reported to be a useful tool for predicting field thermal and moisture conditions in semiarid regions. Application of the physical model to a humid climate and all types of soils is necessary because this climate has pronounced biological activity and thus significantly contributes to the global environment. The purpose of this study was to evaluate the usefulness of HYDRUS in predicting annual changes in soil moisture and temperature in the vadose zone under a monsoon climate. The model was evaluated using observed data from a western suburb of Tokyo. The soil properties utilized in the simulation were obtained using laboratory experiments. Climate data for the meteorological submodels were obtained from open-access climate records. The observed surface heat flux agreed well with the predicted surface heat flux produced by the meteorological submodels. The numerical results produced by HYDRUS using the meteorological submodels represented annual changes in soil thermal and moisture conditions fairly well. Our results show that the meteorological submodels of HYDRUS are appropriate to produce thermal and water boundary conditions under a monsoon climate. This suggests that HYDRUS has the potential to simulate the physical conditions of the vadose zone by using forecast climate data coupled with global circulation models.

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