I bought the Perfect Pond book and I've gotten to the section where it talks about calculating your drainage area and the annual precipitation, etc. It's led me to two questions:

1) In measuring the drainage acres on a topo map, do you measure to the highest counter line or do you measure to the middle of the highest flat? For example if the high point is a gentle sloping ridge doesn't it make sense to measure to the middle of the ridge?

2) There's a formula in the book using annual precipitation to calculate the gallons of water needed in a pond and what would be available for runoff. I live in Iowa, I wonder if average snowfall should be factored in as well as the precipitation? If so, how do you convert inches of snow to an equal equation of inches of rain?