Measuring water drainage is important in calculating the levels of runoff you have on your property. Drainage calculations involve determining how much water your lawn or yard will absorb and how much will remain as surface water. You will need to factor this in when building drainage systems or storm drains. Measuring water drainage is a series of simple calculations.

Divide the area you are measuring into three different types. Measure the paved areas, the areas of lawn or garden and the areas of water or water gardens. Separately calculate the total area of each of these three areas in square feet by multiplying length by width.

Find out the average annual rainfall for your area. Use resources such as the National Weather Service or an online source like the Weather Channel. These will give you the statistics in inches per year. Divide this number by 12 to find feet per year. For example, 36 inches is three feet.

Multiply the rainfall per year with the area you have measured for each surface. If you receive three feet of rain per year and your paved area is 100 square feet, this area will receive 300 cubic feet of rain. Calculate the amount of water for each of the three areas.

Determine the amount of water that will drain using the average drainage figures. A paved area will absorb 0 per cent of the water, meaning 100 per cent of it will be surface runoff. A lake or pond will absorb 100 per cent of the water, with 0 per cent surface runoff. A lawn or garden will absorb 40 per cent of the water, with 60 per cent surface runoff. Multiply the cubic feet of water on a garden by 0.4 to find the amount that will drain, or 0.6 to find the amount which will be surface runoff.

Add the surface runoff. You can now use this number to calculate drainage or irrigation system needs.

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