Water Source and Storage Calculations - Basic Mathematics for Water and Wastewater Operators - page 221

W ell y ield
Well yield is the volume of water per unit of time that is produced from the well
pumping. Usually, well yield is measured in terms of gallons per minute (gpm) or
gallons per hour (gph). Sometimes, large flows are measured in cubic feet per second
(cfs). Well yield is determined by using the following equation:

Well yield (gpm) = Gallons produced ÷ Duration of test (min) (15.2)

■ Example 15.3
Problem: When the drawdown level of a well was stabilized, it was determined that
the well produced 400 gal during a 5-min test. What was the well yield?

Solution:

Well yield = Gallons produced ÷ Duration of test (min) = 400 gal ÷ 5 min = 80 gpm

■ Example 15.4
Problem: During a 5-min test for well yield, a total of 780 gal was removed from the
well. What was the well yield in gpm? In gph?

Solution:

Well yield = Gallons produced ÷ Duration of test (min) = 780 gal ÷ 5 min = 156 gpm

Then convert gpm flow to gph flow:

156 gpm × 60 min/hr = 9360 gph

s peCifiC y ield
Specific yield is the discharge capacity of the well per foot of drawdown. The spe-
cific yield may range from 1 gpm/ft drawdown to more than 100 gpm/ft drawdown
for a properly developed well. Specific yield is calculated using Equation 15.3:

Specific yield (gpm/ft) = Well yield (gpm) ÷ Drawdown (ft) (15.3)

■ Example 15.5
Problem: A well produces 260 gpm. If the drawdown for the well is 22 ft, what is the
specific yield in gpm/ft?

Solution:

Specific yield = Well yield (gpm) ÷ Drawdown (ft) = 260 gpm ÷ 22 ft = 11.8 gpm/ft