Global Reporting Initiative+Total water withdrawals, GRI 303-3-a (formerly G4-EN8-a)+Starbucks Corporation+2014+Discussion

Page number 19. The answer was 18.72 gal and had to convert it to cubic meters.

p19. 18.73gal(0.070900763m3) is water use per square foot per month. Per year amount is 224.76gal(0.850809153m3) per square foot. However, total square feet is unknown. Therefore, total water withdrawal amount is unknown. 

p19. The report states that the total water use in 2014 is 23.1% decrease from 2008.