NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by the National Aeronautics and Space Administration under Award No. NNX-07AT39G, the National Science Foundation under Award No. EAR-0634914, the Department of Interior–U.S. Geological Survey under Award No. 06HQGR0183, the Department of Defense–National Geospatial-Intelligence Agency under Award No. HM158207P0006, the Department of Defense–United States Naval Observatory under Award No. NNG07HS32P (through NASA), and the Department of Commerce – National Oceanic and Atmospheric Administration’s National Geodetic Survey under Award No. NA08NOS4000319. The opinions, findings, and conclusions or recommendations contained in this document are those of the authors and do not necessarily reflect the views of the organizations or agencies that provided support for the project. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Government.

Cover: Design by Francesca Moghari. Image courtesy of Sean Swenson, The National Center for Atmospheric Research. The image depicts the mean annual amplitude of terrestrial water storage for the period 2002-2010 derived from GRACE time-variable gravity field data.