How Much Water is in a Drilled Well?

Homeowners use private wells to supply water to their homes and gardens.

Water in the Well Casing

Well casings hold water that replenishes from the surrounding aquifer.Well casings hold water that replenishes from the surrounding aquifer.
The design of the well, pore space within the aquifer and precipitation levels affect the flow rate of the water to the well. The casing of the well can hold a volume of water that will replenish as water travels from the surrounding aquifer into the well.

The diameter and depth of the well along with the static water level determine the amount of water readily available in the casing. To determine the amount, you must measure the depth to water and the total well depth. Subtracting the depth to water from total well depth will give you length of the water column. Multiply the length of water column in feet by the gallon volume per given diameter. For example, a 4-inch-diameter well has 0.65 gallons of water per foot and a 6-inch diameter well has 1.47 gallons per foot. For example, a 4-inch diameter well with 10 feet of water will contain 6.5 gallons of water in the well's casing.

Water in the Aquifer

Aquifers situated below the water table have the ability to transmit water. Water enters these aquifers through infiltration from rain, water bodies or upper aquifers. The available, connected pore space in the aquifer determines how much water the aquifer can transmit. Aquifers with a large volume of open pores that connect from one to another will transmit the most water. Well pumps pull water from within the casing allowing more water from the surrounding aquifer material to enter casing. The amount of water in the well is limited by the amount of water in the aquifer, which can vary.

Seasonal Well Variations

Aquifers rely on infiltration from rainfall, water bodies and upper aquifers for water; however, the seasonal variation in precipitation can affect the available water in the area. Times of low rainfall and drought will reduce the amount of water in the aquifer, which can reduce the amount of water available in the drilled well. As the water level in the aquifer drops, the water level in the well also drops. This decrease reduces the amount of water in the casing and the available water stored in the aquifer.

Well Competition and Dry Wells

When the water level in a well drops below the level of the pump intake, the well is effectively dry. A dry well can result from a reduction in precipitation, which reduces the amount of water in the aquifer; however, it can also occur if there are nearby wells installed in the same aquifer that compete for the same water supply. Wells that pump at a rate that exceeds the rate that the water enters the well casing or the rate that precipitation replenishes the water supply in the aquifer can also cause a well to become dry.

About the Author

Tracy Barnhart is an earth science expert. A professional geologist with over 16 years of technical writing experience, she has expanded her writing skills to include instructional articles on business, parenting, finance and science. She has Bachelor of Science and Master of Science degrees in geology from Furman University and the University of South Carolina.