Water Source and Storage Calculations - Basic Mathematics for Water and Wastewater Operators - page 219

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Water Source and
Storage Calculations

Approximately 40 million cubic miles of water cover or reside within the Earth.
The oceans contain about 97% of all water on Earth. The other 3% is freshwater: (1)
snow and ice on the surface of the Earth contain about 2.25% of the water, (2) usable
groundwater is approximately 0.3%, and (3) surface freshwater is less than 0.5%. In
the United States, for example, average rainfall is approximately 2.6 ft (a volume of
5900 km 3 ). Of this amount, approximately 71% evaporates (about 4200 km 3 ), and
29% goes to stream flow (about 1700 km 3 ).
Beneficial freshwater uses include manufacturing, food production, domestic and
public needs, recreation, hydroelectric power production, and flood control. Stream
flow withdrawn annually is about 7.5% (440 km 3 ). Irrigation and industry use almost
half of this amount (3.4%, or 200 km 3 /yr). Municipalities use only about 0.6% (35
km 3 /yr) of this amount. Historically, in the United States, water usage has been
increasing (as might be expected); for example, in 1900, 40 billion gallons of fresh-
water were used. In 1975, usage increased to 455 billion gallons. Estimated use in
2000 was about 720 billion gallons.
The primary sources of freshwater include the following:

• Captured and stored rainfall in cisterns and water jars

• Groundwater from springs, artesian wells, and drilled or dug wells

• Surface water from lakes, rivers, and streams

• Desalinized seawater or brackish groundwater

• Reclaimed wastewater

WATER SOURCE CALCULATIONS
Water source calculations covered in this section apply to wells and pond or lake
storage capacity. Specific well calculations discussed include well drawdown, well
yield, specific yield, well casing disinfection, and deep-well turbine pump capacity.

W ell d raWdoWn
Drawdown is the drop in the level of water in a well when water is being pumped
(see Figure 15.1). Drawdown is usually measured in feet or meters. One of the most
important reasons for measuring drawdown is to make sure that the source water
is adequate and not being depleted. The data collected to calculate drawdown can
indicate if the water supply is slowly declining. Early detection can give the system

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