An equation is presented for computing infiltration rates from a study of temperatures observed near the land surface. The boundary conditions of heat and liquid movement assumed are (1) a sinusoidal temperature fluctuation of constant amplitude at the land surface and (2) a constant and uniform percolation rate normal to the land surface in a homogeneous medium. In natural mediums of average heat properties, percolation rates of the order of 2 cm/day or greater can be detected with ease by analyzing the temperature profile resulting from diurnal temperature fluctuations. Percolation rates of the order of 0.1 cm/day can be detected by analyzing the temperature profile resulting from, annual temperature fluctuations at the land surface. With the most favorable conditions of low heat conductivity, large amplitude of temperature at the surface, and careful measurements of temperature, analysis of diurnal temperature fluctuations may yield accurate detection of velocity to a minimum of 0.3 cm/day.