Global Drying? Or where is the missing water?

Globally, average sea levels have risen by about 4 inches in the past fifty years. Recently reported research suggests that construction of dams has masked this effect and that without large reservoirs, sea levels would have risen another inch and half. Artificially constructed reservoirs worldwide store about 2,600 cubic miles of water — nearly equal to the volume of Lake Superior. What is puzzling is the source of the increased water volume in the oceans. As currently understood, melting ice and thermal expansion account for only about two inches of increase. Where did the other 2-3.5" of water come from?

One possibility is that urbanization, agriculture, and other changes in land-use have dramatically decreased the amount of time it takes for preciptation falling over land masses to return to the sea. On very localized watershed scales, we know that increases in the extent of impermeable surfaces, loss of wetlands, irrigation, and reductions in mass of plant cover have greatly increased run-off rates. If such localized effects are multiplied across the broader landscape perhaps in aggregate they might account for the otherwise unaccountable sea level increase. If so, such "global drying" could exacerbate the impending freshwater supply crisis. (Update: Just found this paper which suggests that on average terrestrial landscapes have been "drying out" though not at a rate sufficient to account for the observed increase in sea level).