I always wondered how much temporary population growth occurs as a result of big events like Stampede that attract outside tourists. My suspicion is the number is a lot lower than it might appear - particularly because it's not really in anyone's interested to be accurate (i.e. more reasonable) when estimating the economic and tourism benefits. Convention centres have a common bias in their demand forecasting where they always over count out-of-town visitors, when most convention centre traffic ends up being local attendees (and therefore the hotel and tourism spin-offs are less than forecast because locals don't need those things).
For example, if 100,000 people are at the Stampede grounds each day, are 50,000 from out of town? 10,000? How many Calgarians are out of town that week on holidays of their own? The equation on if we are going to use less or more water because of a limited-term tourism event is happening seems complex - probably most obvious is the 1,000+ cows/horses that clearly don't live here full-time - assuming cows drink tap water when in the city, they don't usually in pasture.
It gets more complex too - not all tourists are here the whole time, and it's the individual daily net population gain we care about when related to water. Do tourists even use as much water as a household? I don't think there's much laundry going on for short vacations. Calgarians are leaving and returning from their own vacations all the time too.
Lots of variables at play, I wonder if anyone has estimated a realistic "current day population" during stampede = current population of Calgary + tourists visiting - Calgarians out of town.