Variations of atmospheric angular momentum are dominated driving forces behind the length-of-day change, while mass redistribution in the oceans and hydrosphere is expected to cause the length-of-day to change as well. We estimate atmospheric, oceanic, and hydrological contributions to the length-of-day variations during 1993--2004 using the National Centers for Environmental Prediction reanalysis atmospheric model, the Estimating the Circulation and Climate of the Ocean consortium's data assimilating ocean general circulation model, and the U.S. Climate Prediction Center's land data assimilation system model. A coherent mass balance among the atmosphere, ocean, and continental water is implemented. At seasonal timescales the mass balance among the atmosphere, land, and oceans is very important and nearly cancels all the contributions to the length-of-day variation predicted by the oceanic and hydrological models, although at intraseasonal timescales, good correlations exist between observed length-of-day variations unaccounted for by the atmosphere and contributions from the ocean and continental water. This study indicates that in a fully mass balanced Earth system the combined seasonal oceanic and hydrological contributions to the length-of-day variation are too small to explain the residual length-of-day variations unaccounted for by the atmosphere. The discrepancies between observed length-of-day variations and atmospheric contributions appear more likely caused by the errors of the atmospheric models, in particular of the wind fields.