Geoscience Reference
In-Depth Information
state. In the case of ice, the latent heat of fusion
(0.33
evaporation as well as by plant factors such as the
stage of plant growth, leaf area and leaf tempera-
ture, and also by the amount of soil moisture (see
Chapter 12C). It occurs mainly during the day,
when the
stomata
(small pores in the leaves),
through which transpiration takes place, are open.
This opening is determined primarily by light
intensity. Transpiration naturally varies greatly
with season, and during the winter months in
mid-latitudes conifers lose only 10-18 percent
of their total annual transpiration losses and
deciduous trees less than 4 percent.
In practice, it is difficult to separate water
evaporated from the soil,
intercepted
(liquid or
solid), remaining on vegetation surfaces after
precipitation and subsequently evaporated or
sublimated, and transpiration. For this reason,
evaporation, or the compound term
evapotran-
spiration,
may be used to refer to the total loss.
Over land, annual evaporation is 52 percent due
to transpiration, 28 percent soil evaporation and
20 percent interception.
Evapotranspiration losses from natural
surfaces cannot be measured directly. There are,
however, various indirect methods of assessment,
as well as theoretical formulae. One method of
estimation is based on the moisture balance
equation at the surface:
10
6
Jkg
-1
) is needed to melt the ice to water
at 0°C. The same amount of heat is released
during freezing. Sublimation/deposition of ice
directly to vapor, or vice versa, involves the sum
of the two latent heats (i.e., 2.83
×
10
6
Jkg
-1
) and
therefore sublimation is less common than
evaporation. Nevertheless, in dry windy climates
15-30 percent of the annual snow pack may be lost
through sublimation in situ combined with the
more important sublimation of blowing snow.
The diurnal range of temperature can be
moderated by humid air, when evaporation takes
place during the day and condensation at night.
The relationship of saturation vapor pressure to
temperature (
Figure 2.14
) means that evapora-
tion processes limit low-latitude ocean surface
temperature (i.e., where evaporation is at a
maximum) to values of about 30°C. This plays an
important role in regulating the temperature of
ocean surfaces and overlying air in the tropics.
The rate of evaporation depends on a number
of factors. The two most important are the
difference between the saturation vapor pressure
at the water surface and the vapor pressure of the
air, and the existence of a continual supply of
energy to the surface. Wind velocity also affects
the evaporation rate, because the wind is generally
associated with the advection of unsaturated air,
which will absorb the available moisture.
Water loss from plant surfaces, chiefly leaves,
is a complex process termed
transpiration
. It
occurs when the vapor pressure in the leaf cells is
greater than the atmospheric vapor pressure. It is
vital as a life function in that it causes a rise of plant
nutrients from the soil and cools the leaves. The
cells of the plant roots can exert an osmotic
tension of up to about 15 atmospheres upon the
water films between the adjacent soil particles. As
these soil water films shrink, however, the tension
within them increases. If the tension of the soil
films exceeds the osmotic root tension, the
continuity of the plant's water uptake is broken,
and wilting occurs. Transpiration is controlled
by the atmospheric factors that determine
×
P - E= r +
Δ
S
Δ
S is the storage change in the block of soil and this
term may also include the storage of water in the
snow pack. This equation may be applied to a
gauged river catchment, where precipitation and
runoff (r) are measured, or to a block of soil. In the
latter case we measure the percolation through an
enclosed block of soil with a vegetation cover
(usually grass, but occasionally a large area with
tree cover) and record the rainfall upon it. The
block, termed a
lysimeter
, is weighed regularly so
that weight changes unaccounted for by rainfall or
runoff may be ascribed to evapotranspiration
losses, provided the grass is kept short! The
technique allows the determination of daily
evapotranspiration amounts. If the soil block is
