In the United States, the average domestic water use is about 300 L/day per person (Dieter and Maupin, 2017). Using this amount as an example, a domestic well serving a single-family, four-person household needs to provide 1,200 L/day and must be able to provide this in a two-hour period to meet peak water demands. This can be achieved with a combination of well yield and water storage. The storage component can be supplied from both the standing water in the well and/or a storage tank. Wells with higher yields need less water storage to meet water quantity requirements compared to wells with lower yields.
The amount of available water stored in the well can be calculated from the well depth, well diameter, static water level, and the pump intake setting allowance from the bottom of the well. For example, consider a 20 m deep, 152 mm diameter drilled well with a static water level of 5 m below the ground surface, a pump intake setting allowance of 5 m (i.e., an available height of the water column of 10 m and a radius of 0.076 m), and a 9 L/min well yield will meet the 1,200 L/day requirement as illustrated in Figure Box 1-1 and using calculations as outlined in Box Table 1-1. However, if that same well had a lower yield of 6 L/min, then it would need to be at least 40 m deep to meet the 1,200 L/day requirement. The additional water stored in the casing of the 40 m deep well will make up for the lower well yield during peak demand. Figure Box 1-1 shows examples of well yields and depths that can meet the 1,200 L/day requirement and Table Box 1-1 shows the calculations used to prepare the figure.
|Well Depth (m)||Well Yield (L/min)||Water Volume from 24-Hour Well Yield (L)||Water Volume from 2-Hour Well Yield (L)||Water Volume from Well Storage (L)||Total Water Volume Available from 2-Hour Well Yield and Well Storage (L)|
1. The target water supply volume of 1,200 L/day assumes a four-person household with each person using 300 L/day.
2. It is assumed that the entire water volume of 1,200 L/day will need to be supplied during a two-hour period to meet the peak demand. In addition, the well yield must be able to replenish this 1,200 L volume within 24 hours on an ongoing daily basis. To satisfy these requirements, the columns in this table named “Water Volume from 24-Hour Well Yield” and “Total Water Volume Available from 2-Hour Well Yield and Well Storage” must both meet or exceed the target volume of 1,200 L/day.
3. The “Water Volume from Well Storage” calculation assumes a 152 mm diameter well and that the available drawdown in the well is equal to the well depth minus 10 m (i.e., the static water level in the well is 5 m below ground surface and a 5 m allowance at the bottom of the well is used for a pump setting of 3 m off the bottom and a pump submergence of 2 m). The formula is as follows:
Water Volume from Well Storage (L) =
(Well Depth (m) – 10 m) π r2 (1,000 L/m3) = (Well Depth (m) – 10) (3.146) (0.152/2)2 (1,000).