Daily water intake for younger cows is from 3 to 8 gallons of water per day while older cows drink as much as 30 to 40 gallons of water per day.
A good thing to do if you have a cow or cows is to have a water trough that automatically fills back up on it's own as the water level drops so that your cow will always have access to fresh clean water.
Cows are a lot of work and require lots of food as well to keep fattened up and healthy.