Presented at the 2014 Ocean Sciences MeetingLinking information from sea surface salinity (SSS) to the amount of freshwater that leaves or enters the ocean via evaporation, precipitation, and runoff (FWF), can improve our understanding of the global hydrological cycle, but the problem is challenging because the relationship between SSS and FWF can depend on complex upper-ocean dynamic processes. One way around this issue is to efficiently combine information from SSS measurements with dynamical and physical constraints of ocean models. Here we compute data errors necessary for assimilation of Aquarius and SMOS data into ocean state estimates. The resulting data error variance is spatially complex, and ranges from less than 0.05 psu^2 in the open ocean to 1-2 psu^2 along the coasts and high latitudes. Derived errors are less than mission accuracy requirements in low and mid latitudes, but exceed in situ data error, particularly at high latitudes. Overall, we find that Aquarius and SMOS constraints could measurably affect SSS values from modern state estimates in several ocean regions, including near western boundary currents and in the tropics, and thus influence estimates of FWF.