Understanding Well Discharge and Drawdown

When a well is pumped, water is removed from the aquifer. The rate at which water is pumped is called the discharge. As water is removed, the water level in the well drops below the static (non-pumping) water level. This drop in water level is called drawdown.

Defining Specific Capacity

The relationship between the discharge rate from a well and the resulting drawdown is a key indicator of the well's performance and the aquifer's ability to supply water. This relationship is specifically defined by the term specific capacity.

Specific capacity is the discharge rate per unit of drawdown observed in a well. It is typically expressed in units like cubic meters per day per meter of drawdown ($m^3/day/m$) or gallons per minute per foot of drawdown ($gpm/ft$).

The formula for specific capacity is:

Specific Capacity $ = \frac{\text{Discharge}}{\text{Drawdown}} $

A higher specific capacity generally indicates a more efficient well or a more permeable aquifer capable of yielding more water with less drawdown.

Analyzing Other Options

Let's briefly look at the other options to understand why they are not the correct answer in this context:

• Specific yield: This term relates to unconfined aquifers and is the ratio of the volume of water that drains from a saturated rock or soil due to gravity to the total volume of the aquifer material. It's about storage properties, not well performance related to discharge and drawdown.
• Safe yield: This refers to the maximum amount of water that can be withdrawn from an aquifer over a long period without depleting the supply or causing undesirable environmental impacts. It's a sustainability concept, not a measure of instantaneous well performance per unit drawdown.
• Specific storage: This term relates to confined aquifers and is the volume of water released from storage per unit volume of aquifer per unit change in hydraulic head. Like specific yield, it describes aquifer storage properties, not the well's discharge efficiency relative to drawdown.

Based on the definitions, specific capacity is the term that precisely describes the discharge per unit drawdown at a well.