Page 397 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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Chapter 11

Hydrology: Rainfall and Runoff

11.1 BACKGROUND
Hydrology is the science of water in nature: its properties, distribution, and behavior.
Statistical hydrology is the application of statistical methods of analysis to measurable
hydrologic events for the purpose of arriving at engineering decisions. By the introduc-
tion of suitable statistical techniques, “enormous amounts of quantitative information can
often be reduced to a handful of parameters that convey, clearly and incisively, the under-
lying structure of the original or raw data. These estimates are important for engineers
and economists so that proper risk analysis can be performed to influence investment de-
cisions in future infrastructure and to determine the yield reliability characteristics of
water supply systems (Fair, Geyer, and Okun, 1966).
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The total water resource of Earth is approximately 330 10 mile or 363 10 9
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billion gallons (BG) or 1374 10 billion liters (BL) about 95% in the oceans and seas
and 2% in the polar ice caps. However, the 35 g/L of salt in seawater and the remoteness
as well as fundamentally ephemeral nature of the polar ice caps interfere with their use.
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This leaves as the potential freshwater resource of Earth no more than 10.9 10 BG
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(41.3 10 BL) in lakes, streams, permeable soils, and the atmosphere; only about 3%
is in the atmosphere, the remainder being split almost equally between surface and
ground.
Fortunately, the hydrosphere is not static; its waters circulate (Fig. 11.1). Between
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110 10 and 130 10 BG (416 10 and 492 10 BL) of water fall annually from
the skies, about a quarter onto the continents and islands and the remainder onto the seas;
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10 10 to 11 10 BG (37.8 10 to 41.6 10 BL) return to the oceans as annual
runoff. In overall estimates about a third of the land mass of Earth is classified as well
watered, the remainder as semiarid and arid.
Within the contiguous United States, the 100th meridian (it matches the western
boundary of Oklahoma and splits North Dakota, South Dakota, Nebraska, Kansas, and
Texas) is the general dividing point between annual rainfalls of 20 in. (508 mm) or more to
the East and, except for the Pacific slopes, less than 20 in. (508 mm) to the West. The total
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length of surface streams is about 3 10 miles (4.83 10 km) and the five Great Lakes,
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four of them shared with Canada, hold 7.8 10 BG (29.5 10 BL), and constitute
geographically the largest surface storage of freshwater on Earth.
As shown in Fig. 11.1, water is transferred to Earth’s atmosphere (a) through the
evaporation of moisture from land and water surfaces and (b) through the transpiration of
water from terrestrial and emergent aquatic plants. Solar radiation provides the required
energy. Internal or dynamic cooling of rising, moisture-laden air and its exposure to cold at
high altitudes eventually lower the temperature of ascending air masses to the dew point,
condense the moisture, and precipitate it on land and sea. Overland and subsurface flows
complete the hydrologic cycle.

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