The required amount of water in the PHB mirrors the US Army requirements, assuming very hard work in hot conditions. See Department of the Army Field Manual, Survival, No. 3-05.70:
https://fas.org/irp/doddir/army/fm3-05-70.pdf
Some quotes below for those interested, but not interested enough to download a 676-page PDF. But before the quotes, here is how I handle water in my games.
1. GOLDEN RULE: This is a fantasy role-play game, not a prepper survival course. When it doesn't add to the fun, hand wave.
But if resource management makes sense, adds to the sense of challenge and tension, and is something the players enjoy, then ...
2. Stick to RAW PHB for the most part. Unless you really think arguing over medical requirements of fantasy races in various climates is a fun way to spend your time.
3. If you want more verisimilitude, use the PHB guideline for HARD work in HOT conditions. Halve it for cooler weather and moderate activity. Halve it again if you are riding in a covered cart in mild weather.
4. Mounts can allow you to carry a lot more water, but the mounts themselves need water. Average IDLE horse take 5 to 10 gallons per day. A horse on the trail needs 10-30 gallons a day. Hard working horses will obviously need much more. Much of horses water intake is metabolic and they can get a lot of their water needs met from fresh pasture. For wilderness horse camping in temperate areas, the recommendation for a day ride is to carry a watering can with 5 gallons of water for the horse.
5. Water is heavy. 5 gallons weighs 41.7 pounds. When hiking in the wilderness, it is important to plan your route so that you have access to water. You are not going to carry 5 gallons, except when using mounts. I've done a lot of wilderness hikes including stints with the Student Conservation Corps, living in the wilderness for weeks at a time. We would generally carry two 1 liter bottles, one on each side of our frame pack and we had a number of 5-gallon collapsible containers for use when we made camp.
6. Waterborne disease, like giardia, can be a real threat, and can give players another opportunity to benefit from having proficiency in survival.
If you've never spent much time making long treks in the wilderness, it can be easy to take water for granted. Working in survival skills and resource management into your game can be fun on occasion, especially if there is a chase or time limit or something else you are trying to overcome that adds to the thrill of the story. Most of the time, however, I handwave it.
What the US Army Survival Field Guide says about water requirements:
"The body requires a certain amount of water for a certain level of activity at a certain temperature. For example, a person performing hard work in the sun at 43 degrees C (109 F) requires 19 liters (5 gallons) of water daily."
Page 13-9 has a chart showing Daily Water Requirements for Three Levels of Activity: A. Hard Work in Sun, B. Moderate Work in Sun, and C. Rest in Shade.
"For example, if one is doing 8 hours of hard work in the sun . . . when the average temperature for the day is 50 degree C . . ., one's water requirement for the day will be approximately 25 liters."
Looking at the chart, if you are resting in the shade when the temp is an average of 15 degrees C (59 F), you need only 2-3 liters (.5 to .8 gallons)
It recommends that to avoid "voluntary dehydration" that at temps below 38 degrees C (100 F) drink 0.5 liter every hour. Above that, drink 1 liter every hour.
R the origin of the myth is a 1945 nutrition recommendation that stated that most people needed 2.5 litres of water a day... completely ignoring the following sentence from that recommendation, which stated that the majority of that would come from food (which contains a lot of water).
