Continental water storage is a key variable in the Earth system that has never been adequately monitored globally. Since variations in water storage on land affect the time dependent component of Earth's gravity field, the NASA Gravity Recovery and Climate Experiment (GRACE) satellite mission, which will accurately map the gravity field at 2--4 week intervals, may soon provide global data on temporal changes in continental water storage. This study characterizes water storage changes in 20 drainage basins ranging in size from 130,000 to 5,782,000 km2 and uses estimates of uncertainty in the GRACE technique to determine in which basins water storage changes may be detectable by GRACE and how this detectability may vary in space and time. Results indicate that GRACE will likely detect changes in water storage in most of the basins on monthly or longer time steps and that instrument errors, atmospheric modeling errors, and the magnitude of the variations themselves will be the primary controls on the relative accuracy of the GRACE-derived estimates. ¿ 1999 American Geophysical Union