List all the ways you have used water so far today. Now stretch your brain and list the ways that you used water last week-end. Now comes the tough part. Keep a water journal for today. Write down every time you use water, guess about how much you use, and what you used it for.
Toilets vary but they usually take about two gallons to flush.
When you have finished, be prepared to advise your commander on how much water a colonists should be allowed each day.