Creditable primary source on amount of water intake

My wife and I have a 4 year-old son.My wife frequently suffers from UTI (urinal tract infection) so she drinks lots of water. Due to her countless first hand experiences, she also worries about our son and makes him drink lots of water.The problem is, he doesn't always want to drink all that water. The whole "eight cups of water a day" mantra. My son will drink when he's thirsty, but when he isn't thirsty, forcing him to drink lots of water when he doesn't want to often leads to massive tantrums.I kind of side with my son, because if I were in his shoes, I'd find it very annoying if I am force fed water every 30 minutes,I know my wife means all the good in the world, worrying about my son getting UTI and stuff, however I really think the whole drinking water thing is over the top because: 1. He is a boy. Boys have longer urinal tract and therefore less susceptible to UTI. 2. I recently learned that the whole "eight cups of water a day" thing is actually false. You are fine as long as you drink when you are thirsty. Unfortunately it is through a pop science channel and I don't think it'll be convincing enough. 3. My wife's UTI usually happens after her period. I don't think her situation is applicable to our son.Do any of you know some creditable primary source on the subject of appropriate amount of water intake?I really want my wife to clam down and let our son be if he doesn't want to drink all that water. She gets mad and my son starts a tantrum over how much water to drink every day. I really want this silly argument to stop.Thank you. via /r/Parenting http://bit.ly/2RbwYAh