For every 1 square foot there are 6 gallons in 1 inch of rainfall.
So if you have 1000 square foot and there is 1 inch of rain falling down over those 1000 square feet then you would have 6,000 gallons of water.
For example if you were collecting rainwater from a roof and your roof was 1000 square foot and it rained 1 inch of rain over those 1000 square feet of roof you would be able to collect at least 6000 gallons of water.
When it rains even 1 to 2 inches of rain over an entire town, city etc you get millions of gallons of water that goes into the ground eventually.
So while it may not seem like much water in 1 to 2 inches there are a lot of gallons of water from all that rain.
1 to 2 inches of rainfall produces much more water than 1 to 2 inches of snowfall.