A simulation of monthly global gravity coefficients has been created that represents time-variable hydrology, ice mass loss and global mean sea level rise. Hydrology variations are from a numerical model, whereas the ice mass variations are based on recently observed rates and patterns of melt for glaciers, Greenland and Antarctica. A eustatic sea level variation consistent to balance the mass variations over land is added. This simulation is then used to test the capability of recovering trends in ocean mass, continental water storage and Greenland and Antarctica melting, using methods used to determine these from GRACE data. We find that ocean mass trends can be significantly biased low by the large melting rates from Greenland, Antarctica and glaciers, unless data within 300 km of continents is ignored. Any smoothing of the data beyond a truncation to degree/order 60 will also slightly bias the result. Trends of continental water storage and Antarctica mass loss also tend to be biased due to the sea level rise signal leaking into the estimate. Greenland melting rates are not affected.