if you are measuring the amount of water evaporating from a swimming pool would gallons or quartz give you a more precise measurement?

That completely depends on the size of the pool, and on length of time

the evaporation continues.

If you're talking about the amount of water that evaporates from a

kiddie pool in the next half hour, then cups or quarts might producer

a convenient number.

If, on the other hand, you're willing to wait longer, then the day will

eventually arrive when an entire olympic-size pool will go bone-dry.

If it's precision you're concerned with, then smaller units are typically

used for measurements of greater precision.