A global water cycle reanalysis (2003–2012) reconciling satellite gravimetry and altimetry observations with a hydrological model ensemble

We present a global water cycle reanalysis that reconciles water balance estimates derived from the GRACE satellite mission, satellite water level altimetry and off-line estimates from several hydrological models. Error estimates for the sequential data assimilation scheme were derived from available uncertainty information and the triple collocation technique. Errors in four GRACE storage products were estimated to be 11–12 mm over land areas, while errors in monthly storage changes derived from five global hydrological models were estimated to be 17–28 mm. Prior and posterior estimates were evaluated against independent observations of river water level and discharge, snow water storage and glacier mass loss. Data assimilation improved or maintained agreement overall, although results varied regionally. Uncertainties were greatest in regions where glacier mass loss and sub-surface storage decline are both plausible but poorly constrained. We calculated a global water budget for 2003–2012. The main changes were a net loss of polar ice (−341 Gt yr−1) and mountain glaciers (−185 Gt yr−1), with an additional decrease in seasonal snow pack (−19 Gt yr−1). Storage in lakes increased by +77 Gt yr−1, due to new reservoir impoundments (+87 Gt yr−1), water level change in the Caspian Sea (−27 Gt yr−1) and net increases in the remaining lakes combined (+17 Gt yr−1). There was no change in subsurface storage, because groundwater depletion (−90 Gt yr−1) was offset by increased water storage in the seasonally wet tropics of South America and southern Africa (+87 Gt yr−1), which agrees with observed and predicted changes in the tropical monsoon.