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98 Chapter 3 Water Sources: Groundwater
production well. The curve is extended to cover the maximum available drawdown. For
a well receiving water from more than one aquifer, the resultant specific capacity is the
sum of the specific capacities of the aquifer penetrated, reduced appropriately for par-
tial penetration.

3.15.3 Maximum Yield
The following procedure is carried out to estimate the maximum yield of a well:
1. Calculate the specific capacity of the fully penetrating well having the proposed
diameter from Eq. 3.15 or Eq. 3.18.
2. Reduce the specific capacity obtained above for partial penetration. This can be
done by using Eq. 3.41.
3. Adjust the specific capacity for the desired duration of pumping from Eq. 3.40.
4. Calculate the maximum available drawdown.
5. Compute the maximum yield of the well by multiplying the specific capacity in
step 3 by the maximum available drawdown.

EXAMPLE 3.9 COMPUTATION OF THE SPECIFIC CAPACITY OF A WELL AND ITS MAXIMUM YIELD
A well having an effective diameter of 12 in. (305 mm) is to be located in a relatively homogeneous
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artesian aquifer with a transmissivity of 10,000 gpd/ft (124.18 m /d/m) and a storage coefficient of
4
4 l0 . The initial piezometric surface level is 20 ft (6.1 m) below the land surface. The depth to
the top of the aquifer is 150 ft (45.72 m) and the thickness of the aquifer is 50 ft (15.24 m). The well
is to be finished with a screen length of 20 ft (6.1 m). Compute the specific capacity of the well and
its maximum yield after 10 days of pumping. Neglect well losses.
Solution 1 (U.S. Customary System):
The specific capacity of a l2-in., fully penetrating well after 40 days of pumping can be calculated
from Eqs. 3.15 and 3.16:
s 114.6 QW(u)>T (3.15)
2
u 1.87(S>T)(r >t) (3.16)
4
4
2
u 1.87(4 l0 >10 )(0.5 >10) 1.87 l0 9
From Table 3.2: W(u) 19.52
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(Q>s) T>114.6 W(u) l0,000>(114.6 19.52) 4.47 gpm/ft (80 m /d/m)
The percentage of aquifer screened is K p 20>50 40%
The slenderness of the well factor is b>r w 50>0.5 100
The value of F p from Fig. 3.12 is 0.65
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The expected specific capacity of the well 0.65 4.47 2.9 gpm/ft (51.87 m /d/m)
The maximum available drawdown 150 20 130 ft (39.62 m)
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The maximum yield of the well 130 2.9 377 gpm (2,055 m /d)
In view of the approximations involved with the evaluation, the actual maximum yield will be
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between perhaps 300 and 450 gpm (1,635 and 2,453 m /d).