Mars water cycle The first work package will be dedicated to the water exchange between the regolith and the lower atmosphere. The 1-D model will be developed for the low atmosphere, including the key processes to make the model fully independent, i.e. vertical eddy diffusion transport, radiative transfer, ice cloud formation with radiative feedbacks. The resulting model will then be coupled to a subsurface model recently developed [Hu, 2019], which describes the transport, diffusion, condensation, and adsorption of H 2 O and HDO in the regolith. The diurnal cycle of water vapour resulting from the regolith-atmosphere interaction will be studied. Finally, the subsurface transport model will be integrated into the GEM-Mars 3-D GCM developed at BIRA-IASB to investigate the influence of the regolith on the water cycle on a seasonal basis.