Definition of terms

Definition of terms

Static Level

This is the level at which water stands in a well that is not being pumped. It is generally expressed as the distance from the ground surface (or from a measuring point near the ground surface such as the top of the well casing) to the water level in the well. For a well which flows at the ground surface, the static water level is above the ground surface. It is measured by valving off or shutting off the flow of the well and measuring the head in feet. This is sometimes referred to as the shut-in head.

When we say the static level in a well is 80 feet, it means that the water stands 80 feet below the measuring point in the well when there is no pumping. When we say that a well has shut-in head of 10 feet we mean that the water would rise 10 feet above the measuring point in a pipe extended above that point. Static water levels should always be made before pumping a well.

Pumping Level

This is the level to which the water is lowered during pumping. It is generally expressed as the distance from the measuring point at the ground surface to the water level in the well.

Drawdown

Drawdown in a well means the extent of lowering of the water level during pumping operations. It is simply the difference between Static Level and the Pumping Level in the well.

We should understand that the drawdown represents the head in feet of water, that causes water to flow through the aquifer material toward a well at the rate that water is being taken from the aquifer. A highly permeable sand and gravel formation would require less head or "drawdown" to produce a given quantity of water than a less permeable sand which contained streaks of clay - both formations being of the same thickness.

Residual Drawdown

After pumping is stopped, water levels rise and approach the static water level observed before pumping started. During such a recovery period, the distance that the water level is found to be below the initial static water level is called residual drawdown.

Well Yield

Yield is the volume of water per unit of time discharged from a well, either by pumping or by free flow. It is measured commonly as the pumping rate in gallons per minute (gpm). Other units employed are gallons per hour (gph) for small yields, and cubic feet per second (cfs) for large yields. The well yield is sometimes referred to as the Pumping Rate.

Specific Capacity

The specific capacity of a well is its yield per unit of drawdown. It is commonly expressed as the "gallons per minute per foot of drawdown."

For example: If a well was producing 200 gallons per minute and had a drawdown of 10 feet after a period of pumping, the specific capacity would be 200 divided by 10 or 20 gallons per minute per foot of drawdown.

Since normal wells will continue to drawdown with time, it is well to determine the specific capacity from measurements taken after a given period of time. A commonly used period of time is 24 hours.