In Australia, they're based off the highest median water intake according to the 1995 National Nutrition Survey. So basically, the recommendation aligns with what many people already drink each day.

Research uncovers a novel way of measuring feed consumption on pastures “We’re really focused on DMI (dry matter intake) as a way to calculate RFI, an efficiency metric for an animal so that people ...

How much vitamin C you need per day depends on your age, gender, whether or not you're currently pregnant, and if you smoke.