1290 WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE
Evaporation from the land, 6.21 10 19 gms per year, is
dependent on the soil and climatic conditions. It is strongly
influenced by the relative humidity. For a given atmospheric
water content, the relative humidity increases and the evapora-
tion decreases as the temperature falls. Wind enhances evapo-
ration by continually removing the vaporized water and thus
locally lowering the relative humidity. With all other factors
equal, sea water containing 3.5% salts will evaporate 3.5%
more slowly than fresh water. Barometric pressure has little
direct effect on the rates of evaporation.TranspirationWater loss from land is enhanced by the transpiration process
by which plants and other photosynthetic organisms draw
solutions containing essential growth nutrients from the soil
to provide for maintenance and growth. The excess fluid is
exuded to the atmosphere through the mesophyll cells. From
97 to 99% of the water which enters plants from soil is lost by
evaporation from the leaves. The amount of dry matter pro-
duced in graphs per 1000 grams of water transpired is called
the transpiration efficiency. Odum has estimated typical effi-
ciencies to be 2 or less, with drought-resistant crops such as
sorghum and millet having only slightly higher values. An
acre of red maple trees in a soil with ample moisture may
lose water sufficient to cover the acre with 0.7 meter of water
during a single growing season. Transpiration is also affected
by the factors governing evaporation, as well as light and soil
conditions.Atmospheric WaterThe water in the atmosphere amounts to 1.29 10 19 grams
or approximately one-tenth of that in all fresh water lakes
and is responsible for the pressure differences which create
weather. It has been estimated that without this atmospheric
water blanket, radiational cooling would reduce the earth’s
average surface temperature from 15C to about 0C. Low
humidity desert environments often experience temperatures
of 65C during the day and below freezing at night. Under
ordinary conditions the water vapor in the air allows only
20% of the earth’s surface heat to radiate back through the
atmosphere directly. As previously mentioned it is through
the atmosphere that fresh water evaporated from the sea is
transported over the global land masses.PrecipitationPrecipitation of atmospheric moisture occurs only when the
temperature of an air mass is lowered to or below the point
of saturation, the dew point. Air temperature is lowered by
combinations of the following phenomena:1) heat loss by infrared radiation,
2) adiabatic cooling as large air masses expand
under reduced pressure after they have risen or
been lifted to higher altitudes and
3) temperature decrease due to mixing with cooler air
masses.During the condensation of water the heat of vaporiza-
tion, 540 cals. per gram is released. Various mechanisms of
coalescence account for the different forms of precipitation
such as rain, snow, hail and sleet. Condensation at ground
level is responsible for the formation of dew, frost, and rime
or hoarfrost.
The unequal worldwide distribution of precipitation,
which varies from 0 to 10 meters per year, is determined to a
large extent by the movement of large air masses or weather
systems as directed by the profiles of mountain ranges.
The total precipitation, its annual distribution and the bal-
ance between precipitation and evapo-transpiration are the
chief limiting factors that determine the resulting climax biotic
communities for terrestrial ecosystems. The following average
annual precipitations are general biotic determinants: 0 to 10
inches, deserts; 10 to 30 inches, grass lands, savanna or open
woodland; 30 to 50 inches, dry forest; and greater than 50
inches, wet forest. Thus, the supply of water not only severely
limits man’s own agricultural, domestic and industrial activi-
ties but also dictates the products of nature. As ecologists have
pointed out, the availability of water, more than any other factor,
determines the ultimate population of a geographic province.
The influence on the earth is affected not only by the
quantity of water which reaches the land masses from the
sea but also by the natural processes which follow. Some
precipitation is intercepted directly by the flora and soon
evaporates. Some rainfall may never reach the ground. For
example, in hot deserts rain on its downward flight through
warm dry air often completely re-evaporates.Runoff and Ground WaterPrecipitation reaching the surface at rates greater than those
of infiltration, evaporation and surface retention, will flow
overland in drainage systems which may eventually reach the
sea. This overflow is termed runoff. Although the 8.33 10 21
grams of ground water which exists is a massive quantity, it
moves too slowly to act as a significant transport process in
the over-all hydrologic cycle. In fact, rapid withdrawal from
the ground at a given site can cause serious local depletion.
Water from the hydrocycle which infiltrates into the
ground water systems extracts soluble mineral matter from
soil, sand and rock. According to King, “Water partakes of
and is conditioned by the character of the land on which it
falls, and through which it flows.”Hydrologic WeatheringThese surface and subsurface waters weather natural geo-
logic formations through physical, chemical and biologi-
cal means. Physical weathering of formations is the initial
mechanical weathering action which, through cycles of
freezing and thawing, mechanical stress reversals, and swell-
ing and contracting, exposes rock and mineral surfaces to
chemical weathering processes.
Chemical weathering results from changes in the chemi-
cal environments surrounding the minerals; i.e., from environ-
ments conducive to their formation to those in which they areC023_004_r03.indd 1290C023_004_r03.indd 1290 11/18/2005 11:12:31 AM11/18/2005 11:12:31 AM