A theoretical study of thermal convection in a fluid-saturated horizontal porous layer is reported for the case of isothermal heating from below. The onset time of natural convection and convective heat transport for large Darcy-Rayleigh number systems are analyzed, based on the propagation theory involving temporal dependence of perturbed quantities. Also, an overall feature of heat transport is discussed in connection with the Forchheimer modification of the Darcy flow model.