Water in the Well Casing
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.