How to Calculate Surface Water Drainage

Divide your property into areas of like drainage potential. Paved areas, hills and valleys, well-drained, and clay soils are examples. Each area will drain differently. More water from precipitation and irrigation stays in depressions, loamy soil and sand. Paved areas will shed nearly all of the water that falls on them.

Compute the area for each drainage potential segment; break areas into rectangles, squares and circles. The area of rectangles and squares equals length multiplied by width; divide that in half to find the area of a triangle. Multiply the distance from the center of a circle to its edge by itself, then multiply that “squared” number by pi -- approximated at 3.14 -- to find a circle’s area. Finally, compute the total square area of the property itself.

Determine the volume of runoff per area. Paved and built areas might absorb only 10 percent and shed 90 percent of the water that falls on them, but take up only 20 percent of the property. Other areas may absorb up to 90 percent of the water that falls on them.

Calculate the average runoff by proportionally adding runoff percentages and dividing by the number of areas. For example add 90 times 2 (20 percent) for the paved area above plus 60 times 4, or 60 percent runoff and 40 percent of total area, plus 20 times 4, or a runoff rate of 20 percent on 40 percent for other hypothetical areas. Divide 180 plus 240 plus 80 by 10, giving a rough average of 50 percent runoff for the entire property.

Set out several rain gauges or bury tin cans in the yard to measure rainfall or irrigation. After each precipitation event, record the reading in each measurement device. Add the totals together and divide by the total number of devices for an average surface water value per event.

Compute the surface water in cubic feet, so it can be easily used to compute how many cubic feet fell -- and drained -- per square foot. For example; a 1.5-inch rainstorm would yield 0.009 cubic feet of water per square foot of property. A 35,000 square foot suburban lot would receive 315 cubic feet of water, 157.5 cubic feet of which would run off, or drain away from the area on which it fell.