not life could exist on any other planet, they must measure its presence there. In
short, all life is dependent upon water and its unique properties.
3 Hydrology
Hydrology is the science of the phenomena of water in all its states; its distribution
and occurrence in the earth’s atmosphere, on the earth’s surface, and in the soil and
rock strata; and the relationship of these phenomena to the life and activities of
humankind. It is concerned with both the quality and the quantity of water available.
Since water is essential to the preservation of life on earth, we need a good
understanding of hydrology.
The amount of water present on the earth today is essentially the same as that
which was present when the earth was formed. It may be in a different physical state
and some minor additions may have occurred through chemical reactions and
cosmic particle bombardment, but for the most part, the total quantity of water on
the earth has not changed. It does, however, undergo many physical transformations
from the vapor phase to the liquid and the solid phases. This is a constant and
ongoing cycle, commonly called the hydrologic cycle. Figure 6.1, from Osborn and
Harrison [2], depicts the various phases and locations of water on the earth. It may be
seen that water vapor occurs in the atmosphere, condenses, and forms precipitation
that falls over the face of the earth, both the land and the water. A portion of the water
evaporates in falling; another amount evaporates after striking objects on earth such
as trees or buildings. Another portion flows along the ground, forming streams
and/or lakes, whereas still another part infiltrates the soil to become ground water.
Both surface water and ground water flow toward the lowest level, ultimately
reaching the ocean. Here a large amount of the evaporation occurs. Evaporation
occurs from all moist ground surfaces and all open waters. In addition, transpiration
occurs from all plant life. Usually in evaluating the amount of water vapor formed,
evaporation and transpiration are combined as evapotranspiration since it is difficult
to differentiate one from the other on the earth’s surface.
Tremendous amounts of water pass through the hydrologic cycle. Some idea of
this can be gained from Fig. 6.2 [3]. Large amounts of energy are utilized in the
evapotranspiration process in converting water to water vapor. Also, the amount of
precipitation onto the land and the ocean is similar when one considers that approximately 75% of the earth is covered by the ocean. This does not imply, however, that
the distribution of rainfall over the earth’s surface is uniform. There are many
localized factors that control the distribution of precipitation across the earth’s
surface.
The main source of energy for evapotranspiration is the sun. Small increases in
evaporation may occur as a result of human activities, particularly in the use of heat
for power generation. However, this total amount is small compared to the energy of
the sun and is significant only in localized areas. When water evaporates, the vapor is
lighter than air; therefore, it rises and expands as the atmosphere rarifies. Expansion
236
D. B. Aulenbach et al.
short, all life is dependent upon water and its unique properties.
3 Hydrology
Hydrology is the science of the phenomena of water in all its states; its distribution
and occurrence in the earth’s atmosphere, on the earth’s surface, and in the soil and
rock strata; and the relationship of these phenomena to the life and activities of
humankind. It is concerned with both the quality and the quantity of water available.
Since water is essential to the preservation of life on earth, we need a good
understanding of hydrology.
The amount of water present on the earth today is essentially the same as that
which was present when the earth was formed. It may be in a different physical state
and some minor additions may have occurred through chemical reactions and
cosmic particle bombardment, but for the most part, the total quantity of water on
the earth has not changed. It does, however, undergo many physical transformations
from the vapor phase to the liquid and the solid phases. This is a constant and
ongoing cycle, commonly called the hydrologic cycle. Figure 6.1, from Osborn and
Harrison [2], depicts the various phases and locations of water on the earth. It may be
seen that water vapor occurs in the atmosphere, condenses, and forms precipitation
that falls over the face of the earth, both the land and the water. A portion of the water
evaporates in falling; another amount evaporates after striking objects on earth such
as trees or buildings. Another portion flows along the ground, forming streams
and/or lakes, whereas still another part infiltrates the soil to become ground water.
Both surface water and ground water flow toward the lowest level, ultimately
reaching the ocean. Here a large amount of the evaporation occurs. Evaporation
occurs from all moist ground surfaces and all open waters. In addition, transpiration
occurs from all plant life. Usually in evaluating the amount of water vapor formed,
evaporation and transpiration are combined as evapotranspiration since it is difficult
to differentiate one from the other on the earth’s surface.
Tremendous amounts of water pass through the hydrologic cycle. Some idea of
this can be gained from Fig. 6.2 [3]. Large amounts of energy are utilized in the
evapotranspiration process in converting water to water vapor. Also, the amount of
precipitation onto the land and the ocean is similar when one considers that approximately 75% of the earth is covered by the ocean. This does not imply, however, that
the distribution of rainfall over the earth’s surface is uniform. There are many
localized factors that control the distribution of precipitation across the earth’s
surface.
The main source of energy for evapotranspiration is the sun. Small increases in
evaporation may occur as a result of human activities, particularly in the use of heat
for power generation. However, this total amount is small compared to the energy of
the sun and is significant only in localized areas. When water evaporates, the vapor is
lighter than air; therefore, it rises and expands as the atmosphere rarifies. Expansion
236
D. B. Aulenbach et al.