surface area by the average depth. The answer is
cubic feet. Multiply by 7.5 to obtain gallons at the
water source. An example of this is a pond with
an average depth of 7 feet and a surface area of
2,864 square feet. It is calculated as shown
Q= Ax D x 7.5
Q = 2,864 x 7 x 7.5
Q = 150,360 gallons
Lakes and ponds are usually located within
the water table, and the hydraulics of the water
feeding the lake or ponds are similar to that of
wells. Therefore, a drawdown test, using a
method similar to the one described below for
wells, may be used to calculate the quantity of
water. To perform the test, you should draw
down the static water level to 1 or 2 feet and then
record the recovery time. Also, devise a method
to discharge the water being pumped so it does
not return to the source during the test.
To calculate the quantity of water that can be
supplied from newly constructed or existing
wells, you must make a drawdown test. To
perform this test properly, you must understand
the hydraulics of a well.
Before being pumped, the level of water in a
well is the same as the level of the water table in
the water-bearing formation in which the well is
completed. This is called the static level in the
well and in the foundation. (fig. 9-1). The depth
from the ground surface to the static water level
should be measured and this distance used to
describe its position. Thus if the water in the well
Figure 9-1.Static water level before pumping.
is 25 feet below ground, the static water level is
said to be 25 feet for this well. Elevation of the
static water level above mean sea level can also be
used to describe its position.
When a well is pumped, the water level drops.
After several hours of pumping at a constant rate,
it stabilizes itself in a lower position. This is called
the pumping level or dynamic water level for this
rate of pumping (fig. 9-2).
The distance the water is lowered by pumping
is called the drawdown. It is the difference between
the static level and the pumping level. The
drawdown in the well, resulting from pumping,
lowers the water pressure in the well, but the
surrounding water-bearing formation retains its
original pressure. In response to this difference in
pressure, water flows out of the pores of the
formation into the well.
The water-bearing formation does not furnish
its water all at once to the well being pumped. The
flow of water into the well is held back by the
frictional resistance offered by the formation to the
flow of water through its pores. The resistance
varies in each formation and is developed in direct
proportion to the rate of movement or velocity of
the water in the formation. The rate of flow,
resulting from a given pressure difference, depends
on the fictional resistance to flow developed in the
formation. The term used to describe this
characteristic of a porous material is permeability.
For a particular type of well, the yield of the
well for any given drawdown is
Figure 9-2.Pumping level.