A climate model is used to simulate, for an idealized Pangean continent, the changes of temperature, wind, and precipitation produced by changes in earth’s orbital parameters. The calculation uses an orbit of moderate eccentricity (0.025). Enhanced summer and winter monsoon circulations and increased summer rainfall and runoff are simulated for the hemisphere where perihelion occurs in summer and aphelion in winter. The increases in rainfall and runoff are about 25% or more, relative to the average climate. These increases occur in the tropical and subtropical belt extending from the equator to about 40° latitude in some regions. These same regions would experience 25% or more reductions in rainfall and runoff in the opposite phase of the precession cycle. These model results suggest that rainfall and runoff would undergo cyclic changes of ± 25% or more with periods of 23,000 years over a substantial fraction of the Pangean tropics and subtropics.