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[1] Freshwater discharge along continental margins is a key Earth system variable that is not well monitored globally. Here we propose a method for estimating monthly river basin outflows based on the use of new GRACE satellite estimates of terrestrial water storage changes in a coupled land-atmosphere water balance. Using GRACE land water storage changes (which include changes in groundwater storage) in the water balance method results in more holistic estimates of basin discharge, which we call total basin discharge, that include not only streamflow, but the net of surface, groundwater and tidal inflows and outflows. The method was tested on the Amazon and Mississippi river basins, and could ultimately be applied to the major drainage regions and river basins of the globe. Estimated Amazon total basin discharge was well correlated with observed streamflow, but with a phase lag and underestimation of low flows. Estimated total basin discharge in the Mississippi river basin had a greater annual amplitude than observed streamflow, but showed good temporal covariance. Results for both basins highlight important differences between estimated total basin discharge and observed streamflow, at least part of which can be attributed to groundwater storage changes. Atmospheric moisture data and methods of GRACE data processing also contributed to the differences.