Water usage, and particularly, water waste is becoming an issue for many communities as drought and shortages become more commonplace. Conserving water is an environmentally responsible thing to do and measuring your water usage is a way to show you are doing your part for the community.
If you're using bottled water, count the number of bottles you provide athletes in the course of the event. If you're using tap water, you can meter water usage with a gauge you attach to the end of a hose. (Search online for "personal water meters.")
On the course, don't fill the cups up all the way as much of the water is spilled. Instead, fill them half or two-thirds full. For a large race, this simple tactic can dramatically reduce water waste.
One caveat: Providing adequate water on the course and after the race is paramount to the safety of your event participants. Do not compromise your event's safety.