What is the equation for well efficiency?

• A. WE = (theoretical drawdown/actual drawdown) x 100%
• B. WE = (actual drawdown/theoretical drawdown) x 100%
• C. WE = (theoretical drawdown - actual drawdown) x 100%
• D. WE = (theoretical drawdown/actual drawdown) / 100%

Answer: (A)

Well efficiency is about how closely the observed pumping response matches what the aquifer theory predicts, expressed as a percent. It compares the drawdown the aquifer would produce under the pumping rate (theoretical drawdown) with the drawdown actually observed in the well (actual drawdown). The standard formula is WE = (theoretical drawdown / actual drawdown) × 100%. If losses inside or around the well are small, the actual drawdown is close to the theoretical value, giving a high efficiency near 100%. If there are significant losses, the actual drawdown becomes larger than the theoretical, and the efficiency drops below 100%. For example, if the theoretical drawdown is 10 ft and the actual drawdown is 12 ft, the efficiency is about 83%. The other forms either invert the ratio, use a difference, or apply an extra division by 100, which don’t correctly express the percent efficiency.