A novel non-gradient model is formulated for estimating gas fluxes using single-level time-series data of near-surface gas concentration over land surfaces. When the vertical turbulent transport process in the atmospheric surface layer is described by a one-dimensional diffusion equation, a gas flux is expressed as a weighted integral of the time-history of single-level gas concentration. The eddy-diffusivity may be parameterized as a function of sensible heat flux based on the Monin-Obukhov similarity theory without explicit dependence on wind speed and surface roughness. Sensible heat flux may be estimated from net radiation and surface temperature using the maximum entropy production model. Case studies at six sites with diverse vegetation covers, geographic and climatic conditions at sub-daily scale demonstrate the model’s ability to simulate diurnal variations of water vapor and CO₂ fluxes using fewer inputs than other models. Good performance of the model at seasonal scale suggests that the proposed model is a promising tool for the assessment of annual water and carbon budgets over vegetated land surfaces.