It works out about a pint of water an hour of hard exercise, with some form of electrolyte replacement (6 pints a day typically and NO, beer and coffee don't count!
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Don't think of rehydration as camel like storage. Water is used by the body to flush out toxins. The harder you work, the more toxins build up, the more flushing is required. Properly hydrated, your urine should be nearly clear. If it isn't at the end of the day, more flushing is required - you're dehydrated. Better to keep a steady influx of water through the day. If there are 16oz to a pint, a 90 oz bladder should keep you going through the day, with a pint before and 1 after work. This isn't just for hot weather - winter respiration loses just as much as sweating.
Food wise, 4,000 calories for hard exercise is required, possibly more for long strenuous days. That works out at about 2kg/4.4lbs of food a day.
Using these as base lines for a hard days work, you won't go far wrong. It doesn't matter what the food is constituted from to get through a hard working day - using the weight principle you should find enough calories. If your out of lunch or forgotten it, go and find something heavy from the grocers like short bread, fruit loaf, biscuits, marzipan, cheese etc. Probably explains why so many climbers are addictd to chocolate! Obviously, long term, minerals and vitamins with carbo/protein/fat balance need to be considered.
Don't just rely on the working day to keep fit - cardio-vascular and flexibility need additional work to balance the effects of a typical tree work day.
Rotating tasks through the day at best, week at worst, will help a lot toward reducing fatigue - refusals are often management issues. Acceptance of the 'athletic arborist' or 'industrial athlete' in the work place really needs targeting by the trade associations at both worker and employer. Wo/manpower is the mainstay of arboriculture - pity we don't maintain brain and body the same as lesser resources like computer and machinery...
