From Daily Coffee News:
"More than 99% of coffee’s water footprint is the water for “growing the coffee plant,” according to the 2003 coffee study. The problems is that the WF methodology does not sufficiently recognize the basics of the water cycle (or hydrological cycle), and specifically the role of green water. WF mistakenly presumes that any water utilized by crops through evapotranspiration is “consumed,” in the same way that water is consumed when it’s pumped from aquifers.
This conceptual flaw becomes extremely important for rainfed systems (such as coffee) where virtually all the water used is green water. Technically, hydrologists would say that green water is utilized through evapotranspiration, but this is not the same as being consumed. The water vapor from coffee trees cycles through the hydrological cycle naturally, becoming precipitation once again someplace else, rather quickly.
Depending on context, the utilization and cycling of green water through a landscape often contributes very positively to ecosystem services, such as mitigating floods and runoff. By changing this one assumption in the WF methodology, the water footprint for coffee would be considerably reduced."