Calculating rainwater drainage can have both practical and academic functions. If you are installing a rainwater tank, then working out the amount of rainwater falling on your roof will allow you to choose the right sized tank. Cities calculate rainwater drainage when building storm drain systems to prevent widespread flooding. Working out the rainwater drainage is a series of calculations using easily attainable data. Working it out will help you plan for bad weather, perhaps by building irrigation channels or by another method.
Measure the area of land you are calculating. Measure the length and the width in feet, and multiply together to get the area in square feet. If your land is an irregular size, divide it up into smaller squares and rectangles and add the areas together to get an overall size.
Find out the average annual rainfall for the area you are measuring. The website "Weather" (see Resources) allows you to access this information by inserting your zip code or town name. When you find the average rainfall for your area, convert it into feet from inches by dividing by 12. For example 24 inches of rain is 2 feet. Multiply this by the area you are covering. For example, 2 feet of rain per year on an area of 100 square feet is 200 cubic feet of water.
Work out how much of the rain is surface runoff. If the area you are considering is paved ,or is hard, 100 per cent of it will be surface runoff. If the area is a lake or pond, or a rain garden which collects water, the surface runoff will be 0 per cent. A normal lawn will absorb 40 per cent of rainwater, with 60 per cent becoming surface runoff.
Work out how much rain will be absorbed for your given area. For example, if half of your 100 square foot area is paved, and half is lawn, then half of the 200 cubic feet will become surface runoff from the paved area, and 60 per cent of the other half will be surface runoff from the lawn. The amount absorbed will be 40 per cent of half of 200 cubic feet, or 40 per cent of 100 cubic feet. This means that 40 cubic feet of water will drain into the lawn per year, with 160 cubic feet being surface runoff.