A new study in Nature Geoscience says that the rest of the water comes from human activity on land, mostly pumping groundwater and then letting it run off into our rivers and sewers:
Depletion of groundwater reserves has more than doubled in recent decades as a result of population growth and the increased demand on groundwater reservoirs for drinking water and the irrigation of croplands. Most of the water pumped up from deep pools is not replenished; it evaporates into the air or flows into river channels, feeding into the seas. Artificial reservoirs, such as the Hoover Dam on the Colorado River and the Three Gorges Dam on the Yangtze River in China, have the opposite effect, locking up water that would otherwise flow into the seas. Scientists once speculated that the effects cancelled each other out, but this study and other recent ones have shown that groundwater depletion has a larger net effect.