A water well is a vertical shaft drilled into the earth to access groundwater, providing an independent source for domestic use, agriculture, or industry. Calculating the volume of water within the shaft is important for any well owner, as it directly influences decisions about pump size and storage needs. This calculation provides an instantaneous snapshot of the stored resource, helping ensure household water security and preventing the consequences of over-pumping or installing inadequate equipment. The physical dimensions of the well establish the container that holds the water supply.
Understanding Well Anatomy and Water Levels
The calculation of water volume relies entirely on two primary physical measurements: the well casing’s internal diameter and the height of the water column. The well casing is the cylindrical pipe inserted into the borehole, and its inner diameter determines the width of the water storage area. Total depth is the measurement from the ground surface to the bottom of the well shaft, defining the maximum possible container size for water.
The static water level (SWL) is the natural, undisturbed level of the water surface inside the well when the pump has been off for an extended period and the water has fully recovered. This measurement, taken from the top of the casing down to the water, represents the equilibrium point of the groundwater system. This static level often fluctuates seasonally due to changes in rainfall and regional groundwater use.
A different measurement, the pumping water level (PWL), is the water level while the pump is actively running. The difference between the static water level and the pumping water level is known as drawdown. Drawdown is a temporary lowering of the water surface caused by the rate of water extraction exceeding the rate at which water flows into the well. Tracking both the static and pumping water levels over time is important for monitoring the overall health and performance of the well.
Calculating Usable Water Volume
The volume of water stored within the cylindrical well casing is calculated by determining the height of the water column and multiplying it by the area of the casing’s cross-section. To find the height of the water column, you subtract the static water level measurement from the well’s total depth. This calculation isolates the vertical length of water standing in the pipe.
Since the well casing is essentially a vertical cylinder, the volume calculation uses the radius of the casing and the water column height. Because well dimensions are often measured in feet and inches, and water volume is measured in gallons, a conversion factor is necessary to simplify the calculation. This conversion allows for a simple gallons-per-foot value based solely on the casing’s internal diameter.
For instance, a common residential well with a 6-inch internal casing diameter holds approximately 1.47 gallons of water for every foot of water column. To find the total stored volume, multiply the water column height in feet by this gallons-per-foot factor. This calculated volume represents the instantaneous reserve of water available for immediate use. It is important to remember that this figure only represents the water physically stored in the well pipe.
Factors Determining Sustainable Water Supply
The total volume of water stored in the well casing is only a small part of the overall water picture; the sustainable supply depends on the surrounding aquifer. The aquifer is the underground layer that holds the groundwater and feeds the well. The long-term viability of the water source depends on how quickly the aquifer can replenish the water that is pumped out.
The well yield, or flow rate, determines the amount of water the well can deliver over a specific time, typically measured in gallons per minute (GPM). This yield is not a function of the casing volume, but rather the characteristics of the aquifer, such as its permeability and the number of fractures in the rock. Pumping tests are performed to determine the maximum sustainable yield of a well.
Drawdown is a measure of the well’s performance under stress. Excessive drawdown indicates that the rate of water extraction is too high for the aquifer to keep up. The rate at which the water level returns to the static water level after the pump stops is known as the recovery rate, which indicates the aquifer’s ability to recharge the well.
Matching the pump’s capacity to the well’s yield is necessary for sustainability. If a pump is sized to extract water faster than the aquifer can supply it, the water level will drop significantly, potentially leading to the pump running dry and causing mechanical failure. Monitoring the static water level over months and years provides data on the long-term recharge rate, revealing whether the groundwater supply is being sustained.