This study is carried out seeing that it is important to understand the interacting components of the Earth system and global climate changes including seasonal changes and its variability in Indonesia. Unfortunately, until now there are only few researches into numerical study on atmospheric, hydrological and coastal circulation topics in coastal city in Indonesia, especially researches which consider the dynamic interactions among the major components of the Earth system, as atmosphere, watershed and the estuary. The objective of this study is to evaluate and validate the applicability of non-hydrostatic meteorological model (MM5), watershed simulation model (HSPF), and three-dimensional hydrodynamic with sediment transport model for estuary and coastal ocean (ECOMSED) in Semarang, Indonesia as part of the Regional Environment Simulator (RES). The simulated results showed that the modeling approach appeared to be a powerful tool to define relevancies between each component in the Earth system. For atmospheric simulation, although some biases were found by comparing the value of observed data and simulated results, trend of precipitation that occured at these meteorological stations shows a similar pattern. In hydrologic modeling, performance of simulation has reflected a good computation by numerical model and results met a good criteria with correlation coefficient (R) of 0.89, determination coefficient (R2) of 0.79 and Nash-Sutcliffe Efficiency (NSE) criteria resulted in values of 0.68. The simulation which combines the hydrology model (HSPF) and the meteorological model (MM5) has shown better results than observation data. Model computation for coastal estuary modeling was also performed. The simulation provided a time series of sea water level, temperature, salinity, and distribution of sediment concentration in the Garang estuary, Semarang. The simulation result of sea surface elevation changes agree very well with the determination coefficient (R2) of 0.83 if compared with Semarang station, and 0.85 compare with Tanjungmas Port station. The result also showed that circulation in estuary has been dominated by river discharge rather than sea tide. For further research, it is suggested to consider the interaction of meteorology-hydrology-coastal/estuary components by using the coupler system in the Earth system modeling (RES).