Well Definitions & Glossary
Functional Flow - One way to determine functional flow is by the amount of water that you can get from the water system at the fixtures. This is true whether it is a public or private water system. Determine whether or not there is an adequate amount of water by turning on two or more faucets. Turn on 1, 2 or 3 faucets until you get the maximum flow from each faucet, then determine if the water coming from the highest fixture has adequate flow. If possible, the fixtures being checked should not have flow restrictors.
For example, go to the highest bathroom and turn on the water at the bathtub spout, the bathroom sink and flush the toilet. The reason you use the tub spout is because the supply pipe is not restricted, which means that you could be drawing 3 or more gallons of water per minute from the tub spout, and possibly 1 gallon a minute from a water-saving shower head.
Functional flow, which is often confused with pressure, is related to altitude. 1 psi will lift water 2.31 inches or about 28 inches. If the bathroom is on the second floor and the water service is in the basement, it may take about 7.5 to 8 psi just to lift the water to the second floor fixtures. For water to flow out of the fixtures, more altitude pressure is needed. When you turn on these fixtures, you should see adequate flow out of the bathroom sink, because that is the highest fixture, as compared to the height of the toilet and the bathtub. This is a dependable indicator, however, if there is not adequate flow at the sink, be sure it is not related specifically to the sink. To get a sense of the systemic flow in the house, check the fixtures throughout the house.
Yield - Yield is the amount of water that flows into the well from the aquifer. This is provided by the well driller. The well driller will use a high volume pump to draw down the well, and then measure how fast the water flows into the well from the aquifer. A yield test is not conclusive; it is only to be used as a guide as to how much water the well may yield. This is because it is a snapshot of the well, not a long-range test. There are many variables, such as the amount of rainfall the area has recently received; the level of the water table at that time; the type of aquifer; and the specific usage or demands on the well. A yield test is expressed in “gallons per minute.” A flow rate of 1 gallon per minute would yield only 60 gallons per hour. Therefore, to meet the requirement of the average household, you would need a storage tank with a capacity of approximately 150 gallons. Most jurisdictions have a minimum flow rate requirement.