How to Set a Basement Depth Based on the Water Table

Determine the water depth for your area. Check the U.S. Geological Survey's website --water.usgs.gov/ogw -- for a state map of water table depths. Select your state and then locate your county from the list provided. The water depth is indicated in feet across from your county's listing.

Determine the seasonal fluctuations in the water table in your area. Your county extension service might have this information. Otherwise, calculate an estimate by taking 25 percent of the average annual precipitation for your area and divide it by 0.3, which is the average effective porosity for non-cohesive soils. For example, if the average precipitation in your area is 29 inches, 25 percent of this is 7.25. Divide 7.25 by 0.3 to equal approximately 24 inches or two feet. The water table in this area might fluctuate up or down by two feet.

Combine the estimated seasonal fluctuation with the water table depth in your area. For example, if the USGS lists the water table in your area as 20 feet below the land-surface datum (approximate land surface), and your seasonal fluctuation is an estimated two feet, the estimated water table depth ranges from 18 feet to 22 feet.

Set the basement depth above the seasonally adjusted water table depth. The exact depth of the basement in comparison to the water table is unimportant. As long as the lowest slab of the basement is above the maximum water table elevation, the basement should not flood.