The effects of long-wavelength (>100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (Δrm) have root-mean-square (RMS) values for 1994--1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for horizontal deformation. We compare ΔrM with observed Global Positioning System (GPS) heights (ΔrO) (which include adjustments to remove estimated effects of atmospheric pressure and annual tidal and non-tidal ocean loading) for 147 globally distributed sites. When the ΔrO time series are adjusted by ΔrM, their variances are reduced, on average, by an amount equal to the variance of the ΔrM. Of the ΔrO time series exhibiting a strong annual signal, more than half are found to have an annual harmonic that is in phase and of comparable amplitude with the annual harmonic in the ΔrM. The ΔrM time series exhibit long-period variations that could be mistaken for secular tectonic trends or postglacial rebound when observed over a time span of a few years. ¿ 2001 American Geophysical Union