Question: I've run a number of races, and there are almost always a number of water stations. I don't usually stop for water on the short races, but I'm not sure how long of a race I should do that for - when should I start drinking water? How much and often should I drink it?

Answer: 
Hydration when racing has a lot of variables, all of which need to be taken into consideration for longer races - which I would say is anything longer than a 10k:

* difficulty of the course (hills, headwind, altitude)
* temperature and humidity
* and your acclimatization to those weather conditions
* your effort, this is somewhat of a double edged sword: run fast and you're done quicker, run slow and you're out there longer, and often you'll be out in the warmer parts of the day more than the early finishers.
* your body: everyone sweats different amounts. Some don't drink much during a 1/2 marathon, some drink a liter or more.
* for longer races, your nutrition intake also adds a fluid component: you need water for your body to digest the nutrition.
* pre-race hydration: it's not uncommon for racers of >4 hour races to adjust up their electrolyte and hydration in the days leading up to a race.

First - try to do a sweat test in an environment as close as you can to the race, and get a good idea of what you generally need, and then consider the other variables.

At the end of a race you will almost never hear people saying "oh dang, I drank too much", but go by the medical tent after a marathon, and you will see racks of people wishing they had taken hydration more seriously.
It's really much simpler than this. The marathon doctors have been recommending the following for a few years now, with the risk of hyponatremia in mind:

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> If you're thirsty, drink.
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Citations:

* *Runner's World* magazine, 2005: ["How much should you drink during a marathon?"](http://www.runnersworld.com/drinks-hydration/how-much-should-you-drink-during-marathon) (references, in particular, Tim Noakes, author of the widely-respected *Lore of Running* and author of the IMMDA statement below)
* Georgetown University Medical Center, 2007: ["Runners: Let thirst be your guide"](http://explore.georgetown.edu/news/?ID=25347)
* British Journal of Sports Medicine, 2013: ["Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat"](http://bjsm.bmj.com/content/early/2013/09/20/bjsports-2013-092417.abstract)

Additionally, [this 2001 statement](http://www.usatf.org/groups/Coaches/library/2007/hydration/IMMDAAdvisoryStatement.pdf) from the International Marathon Medical Directors Association (IMMDA), which concludes:

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> "Accordingly perhaps the wisest advice that can be provided to athletes in marathon races is that they should drink *ad libitum* and aim for ingestion rates that never exceed about 800 ml per hour."
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