Each 10 x 10 area of grass needs around 62 gallons of water so to water a football field which is around a 360 x 160 area that would mean you would need around 10,000 to 20,000 gallons of water.
It all depends on the size of the football field because some football fields are larger than some others but you can expect the football field to take at least 8,000 to 10,000 gallons of water to water the entire football field.
Watering grass properly takes around 1/2 gallon of water per square foot so the larger the area the more gallons of water it takes to water the grass and the smaller the area the less amount of water you need to water the grass.