"Relatively easy" is brushing over an awful lot. https://xkcd.com/793/ https://xkcd.com/793/ You can get a rough approximation, and that's been done at great

"Relatively easy" is brushing over an awful lot. https://xkcd.com/793/ https://xkcd.com/793/ You can get a rough approximation, and that's been done at great effort for the entire US at the FEMA website: https://msc.fema.gov/portal/home https://msc.fema.gov/portal/home. It's not a simple problem to solve. Some of the caveats are discussed in this Practical Engineering video: https://www.youtube.com/watch?v=EACkiMRT0pc https://www.youtube.com/watch?v=EACkiMRT0pc Volume of outflow through a stream is equal to rainfall times watershed area. Seems intuitive, right? Except what about water that gets absorbed into the ground, or captured in a temporary holding pond, slowed by vegetation, or accelerated in a storm drain...I think Grady put it well when he pointed out that the silly Imperial units of outflow in cubic feet per second, rainfall in inches per hour, and watershed area in acres multiply together to 1.008, which is off by about 1% but close enough that civil engineers just throw away the constant entirely because it's such an imprecise approximation that the error is irrelevant.