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3.15 Yield of a Well 97
stable well. A decrease in C for higher discharges may indicate development of a well
during testing; an increase in C may denote clogging. The acceptance test for a well
should include a step-drawdown test, because such a test permits the characteristics of the
aquifer and of the well that govern the efficient performance of a water supply system to
be evaluated.

3.15 YIELD OF A WELL
For the optimal design of a well (or a well field), the combination of discharge and draw-
down that gives the lowest-cost solution is sought. Both the capital outlays and the opera-
tion and maintenance costs need to be considered over the economic life of the structure.
The interrelationship of pumping rate, drawdown, and specific yield serves as a basis for
the selection of optimal design capacity. We have shown previously that the specific capac-
ity decreases as the pumping rate is increased. Hence the earlier increments of drawdown
are more effective in producing yields than the later ones; that is, each additional unit of
yield is more expensive than the previous one. Increasing the yield of a well by one unit is
economically justified only if the cost of developing this unit from alternate sources, such
as another well or surface supplies, is higher.
The yield obtainable from a well at any site depends on (a) the hydraulic character-
istics of the aquifer, which may be given in terms of a specific capacity drawdown rela-
tionship; (b) the drawdown at the pumping well; (c) the length of the intake section of
the well; (d) the effective diameter of the well; and (e) the number of aquifers penetrated
by the well.

3.15.1 Maximum Available Drawdown

The maximum available drawdown at a well site can be estimated by the difference in ele-
vations between the static water level and a conservation level below which it is
undesirable to let the water levels drop. The conservation level is controlled by hydrogeo-
logic conditions (type and thickness of the aquifer and the location of the most permeable
strata), maintenance of the efficiency of the well, preservation of water quality, and pump-
ing costs. In an artesian aquifer, good design practice requires that the drawdown not result
in the dewatering of any part of the aquifer. Hence the maximum allowable drawdown is
the distance between the initial piezometric level and the top of the aquifer.
In a water table aquifer, the pumping level should be kept above the top of the screen.
The yield-drawdown relationship of homogeneous water table aquifers indicates that opti-
mum yields are obtained by screening the lower one-half to one-third of the aquifer. A
common practice is to limit the maximum available drawdown to one-half to two-thirds of
the saturated thickness. In very thick aquifers, such as artesian or water table aquifers, the
limiting factor in obtaining yields is not the drawdown but the cost of pumping. In some
locations, the available drawdown may be controlled by the presence of poor quality water.
The maximum drawdown should be such as to avoid drawing this poor quality water into
the pumping well.

3.15.2 Specific Capacity–Drawdown Curve

A graph of specific capacity vs. drawdown is prepared from the data on existing wells
in the formation if such data are available. Specific capacities should be adjusted for
well losses and partial penetration and should be reduced to a common well radius and
duration of pumping. If no data are available, a step-drawdown test is conducted on the