We have written a one‐dimensional numerical model of the exchange of H₂O between the atmosphere and subsurface of Mars through the planetary boundary layer (PBL). Our goal is to explore the mechanisms of H₂O exchange and to elucidate the role played by the regolith in the local H₂O budget. The atmospheric model includes effects of Coriolis, pressure gradient, and frictional forces for momentum: radiation, sensible heat flux, and advection for heat. The model differs from Flasar and Goody by use of appropriate Viking‐based physical constants and inclusion of the radiative effects of atmospheric dust. The pressure gradient force is specified or computed from a simple slope model. The subsurface model accounts for conduction of heat and diffusion of H₂O through a porous adsorbing medium in response to diurnal forcing. The model is initialized with depth‐independent H₂O concentrations (2 kg m⁻³) in the regolith and a dry atmosphere. The model terminates when the atmospheric H₂O column abundance stabilizes to 0.1% per sol. Results suggest that in most cases, the flux through the Martian surface reverses twice in the course of each sol. In the midmorning, the regolith begins to release H₂O to the atmosphere and continues to do so until midafternoon, when it once more becomes a sink. It remains an H₂O sink throughout the Martian night. In the early morning and late afternoon, while the atmosphere is convective, the atmosphere supplies H₂O to the ground at a rapid rate, occasionally resulting in strong pulses of H₂O into the ground. The model also predicts that for typical conditions, perhaps 15–20 sols are required for the regolith to supply an initially dry atmosphere with its equilibrium load. The effects of surface albedo, thermal inertia, solar declination, atmospheric optical depth, and regolith pore structure are explored. Increased albedo cools the regolith, so less H₂O appears in the atmospheric column above a bright surface. The friction velocity is higher above a dark surface, so there is more diurnal H₂O exchange; relative humidities are much higher above a bright surface. Thermal inertia I affects the propagation of energy through a periodically heated homogeneous surface. Our results suggest that higher thermal inertia forces more H₂O into the atmosphere because the regolith is warmer at depth. Surface stresses are higher above a low I surface, but there is less diurnal exchange because the atmosphere is dry. The latitude experiment predicts that the total diurnal insolation is more important to the adsorptively controlled H₂O column abundance than the peak daytime surface temperature. Fogs and high relative humidity will be far more prevalent in the winter hemisphere. The dust opacity of the atmosphere plays a very significant role; the PBL height, column abundances, relative humidity, and surface stresses all increase very strongly as the optical depth approaches zero. The dust opacity of the atmosphere must be considered in subsequent PBL models.