Surface and groundwater dynamic interactions in the Upper Great Chao Phraya Plain of Thailand: semi-coupling of SWAT and MODFLOW

Surface and groundwater dynamic interaction in the Upper Great Chaophraya Plain of Thailand is explored. The surface-soil water model SWAT and the groundwater model MODFLOW are semi-coupled to determine the various hydrological components and their relevant flow behaviour. The coupled models are executed by running the simulations individually using the monthly river-groundwater interactions and the groundwater recharge to generate the surface-groundwater dynamic interactions. The calibrated results from coupled simulations show that the coupling-method improves both the streamflow and the groundwater simulations. From the water balance analysis a quantitative picture of the local surface and subsurface water interaction is obtained. The semi-coupled models result in streamflowand groundwater level estimates that are improved by 12% and 2.3%, respectively, especially in the dry season. The dynamic simulations reveal that the interaction is seasonally dependent The improvement of the water balance analysis suggest that coupled modelling is a key to clarify the relationship between surfaceand subsurface water, as well as their interaction.