I am reliably informed that a 70kg man needs 3 litres of liquid intake in 24 hours. This does not, of course, mean coffee, tea, coke, alcohol etc. as they are all diuretics.
Less than that and, in a dry, low-pressure atmosphere you are more at risk than those with their feet on the ground of problems such as DVT (leading to possibilities of pulmonary embolism, also kidney stones (very painful!) and, possibly worse than the rest (particularly for people like me!) generally poor mental functioning, poor concentration etc.
Mainly, the requirement is to be sensible about it. Drinking nothing but coffee on a 10-hour trip don't qualify!
