Atmospheric and river basin water balance..if we consider or accommodate only these two broad groups of water parcels..I think more than a half features/issues are get well encompassed ...Question has very big domain....so a better and depthfull insights are inevitable ..but  Omar ..one thing is very much sure that global-water balance studies needs very specific acts, tools, visions  and methods , in contrast to water balance at micro levels ( catchments , commands or region basis)...

Nice question it is , to the my way of thinking....

Thanks a lot to your useful answer Gaur