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Chapter XI. Evaporation from Wen-Watered Ecosystems

Chapter XI. Evaporation from Wen-Watered Ecosystems

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T R A N S P I R A T I O N OF W A T E R F R O M L E A V E S



275



In comparison with a water body, a vegetation-soil ecosystem has

small heat storage, as good absorption of solar radiation, and a better

linkage with the atmosphere by means of convective exchanges

because it is rougher. These likenesses and differences suggest a little

about the process of evaporation from ecosystems, the energy sources

that power it and the external conditions that govern it.

TRANSPIRATION OF WATER FROM LEAVES



Transpiration plays a role in the operations of vegetation that is

somewhat more than just a leakage of water that happens to occur

when cells are opened to receive CO, from the environment. It

represents an ”energy subsidy” (Odum, 1971, p. 43) to the ecosystem

from outside. This subsidy powers the intake of nutrients from the

soil and thus is a mechanism for nutrient conservation in the soilvegetation system (Odum, 1971, p. 95). Also, by cooling the leaves it

helps to “reduce the respiratory heat loss (i.e.,the ’disorder pump-out’)

necessary to maintain the biological structure” (Odum, 1971, p. 43).

While this book tends to deal more with the circulations of water

outside plant tissue in ecosystems-delivery

of rain, interception,

infiltration, and so on-we cannot forget the important role of water

within the tissues of the plants in the system.

While transpiration is a part of the functioning of life processes of

plants, it is also at the same time a physical process. The usual number

of kilojoules of energy are converted to latent heat, drawing on one or

another source of energy, when a certain mass of water is vaporized;

water has to be delivered to the site where vaporization can take place;

and the vapor has to be carried away.



Resistance to the Diffusion of Water Vapor

One characteristic of evaporation in an ecosystem not found in other

evaporating systems is that the wet cell walls, the sites of vaporization, are not directly exposed to the free air, which is the ultimate sink

for the vapor molecules. They open instead on intercellular spaces

within the leaf, which interpose an additional resistance to molecular

diffusion and tend to slow down the vaporization process.

The resistance of vapor moving from leaf to air is as much as lo7

times the resistance to movement in 1-m length of stem, and lo6 times

the resistance to movement of water into a root. This internal resistance is added to the resistance located between leaf outer surface and

air, which depends on wind speed and other factors.



276



XI.



E V A P O R A T I O N FROM WELL-WATERED ECOSYSTEMS



Photosynthesizing cells in leaves must extract CO, from a medium,

the atmosphere, in which it exists at a very low concentration. This

extraction requires an extensive area of wet cell walls to which CO,

molecules may diffuse in the intercellular air spaces and through

which, dissolved in water, they can move into the cells. A square

meter of crop or forest land supports several square meters of leaf

surface area." This area is multiplied by the reticulation of airways and

interstices between cells of the leaf mesophyll, a net of air-filled spaces

opening from each stoma.

Movement of C O , inward through these spaces to the moist cell

walls, and movement of water vapor in the opposite direction, takes

place by diffusion. Resistance to diffusion r1 is a kind of inverse

conductivity term, and can be so entered in the gradient equation for

transpiration E as



where pl and pa indicate specific humidity or water vapor concentration at the cell wall and in the air, respectively (Gates and Hanks,

1967). The resistance terms are expressed in units of seconds per

centimeter, y1 being that within the leaf while r , represents resistance

to diffusion of vapor from the leaf surface into the free air.

The resistance functions provide a means of separately analyzing the

path of vapor diffusing in the turbulent atmosphere (air resistance

index, which is a reciprocal of the turbulent diffusivity of the airstream) and the path of vapor diffusing through the interstices of

leaves from the wet cell walls (stomata1 resistance), or "external" and

"internal" resistances, respectively. (Resistances are used here, rather

than conductivities, for the same reason they are used in figuring

thermal insulation of a house wall made oi several materials of

different conductivity, namely, that they can be combined by simple

addition.)

EVAPOTRANSPIRATION FROM PLANT COMMUNITIES



Several approximations enter into the generalization that wellwatered vegetation acts as a wet surface. In broad terms, however, the

generalization is useful and includes a process, evaporation from the

soil, that is difficult to separate from transpiration. It is linked with an

* The ratio between leaf surface area and ground area is called the "leaf area index." It

is a useful parameter of a plant canopy, indicating differences among species and

changing as a plant goes through its life cycle.



EVAPOTRANSPIRATION FROM P L A N T COMMUNITIES



277



energy concept, potential evapotranspiration, which is the rate of

moisture conversion obtaining at a vegetation-covered surface having

these idealized characteristics:



(1) Plants short and densely spaced, growing actively, having

access to unlimited soil moisture;

( 2 ) Vegetation surface uniform, covering the soil, and infinite in

extent.

This characterization lays stress on plant foliage en m u s e and hence

on the features common to different species of plants, i.e., the physical

exchanges of energy and moisture that occur at the surfaces of all

leaves. It reduces emphasis on the differences between species,

internal resistance or other changes under biological control, and soil

effects, in order to concentrate on the physical process of vaporization.

Uniformity of cover is essential, as is completeness of the coverage

of soil. The characteristic of infinite extent tends to reduce local

differences that might result from proximity to a boundary. The most

basic characteristic, of course, is that of unlimited soil moisture.



Sources of Energy

Under the conditions stated for potential evapotranspiration, what

forms of energy are important? What are their sources?



(1) During the hours of daylight, when stomata are open and

transpiration is in progress, little energy is likeiy to come from the

substrate, which at these hours is taking in energy-not giving it out.

Moreover, the air space contained and immobilized beneath a mantle

of vegetation tends to insulate the soil from the outer active surface,

which is now elevated from the soil surface to the upper part of the

vegetation canopy. Little heat is transported across this space and the

substrate plays only a minor role in the energetics of the situation,

quite in contrast with the water bodies considered in the preceding

chapter .

(2) Small amounts of energy are released by respiration in plants,

and by decomposition of organic matter in the soil. This heat, not

readily transported to the upper part of the plant canopy, has minor

importance.

( 3 ) 5ensible-heat flux H from air to vegetation is minimized as a

source of energy to support evapotranspiration by the criterion,

stated earlier, of infinite extent. Air moving over such an infinite

plane sooner or later comes to equilibrium with the underlying



278



XI.



E V A P O R A T I O N FROM WELL-WATERED ECOSYSTEMS



surface, and any movement of sensible heat during the hours of

radiation surplus and active evapotranspiration is upward.

(4) The sources that remain as important sources of energy for

evapotranspiration are the fluxes of radiation. Potential evapotranspiration therefore represents a rate of energy conversion that approaches

the summation of all the radiation fluxes, short wave and long wave,

incoming and outgoing R . All fluxes are substantial during the hours

when evapotranspiration is in progress.

The radiation flux that is most important, because its variations are

echoed in the regimes of the other fluxes, is the incoming short-wave

flux from the sun. This flux also contains the particular wavelengths

that have photochemical efficacy.

The daily regimes in photosynthesis, i.e., operation with open

stomata, and in thermal energy supply, are both aspects of the daily

regime of solar radiation. Except in certain night-active epiphytes,

they produce parallel regimes in solar radiation and evapotranspiration. A nondimensional representation of the typical daily regime is

shown in Fig. XI-1 (Fleming, 1970, Fig. 3), for clear days. Note the

slight lag after solar noon.

The general success of approximating methods in estimating evapotranspiration from well-watered vegetation cover indicates the central



i"



-AVERAGE



HISTOGRAM



I



06



l



l



08



in



HOUR



Fig. XI-1. Nondimensional form of diurnal regime of solar radiation on clear days

(Fleming, 1970).



EVAPOTRANSPIRATION FROM P L A N T COMMUNITIES



HOUR OF DAY



2 79



EVAPOTRANSPIRATION

(mm hi')



Fig. XI-2. (a) Diurnal marches of solar radiation (solid line) and evapotranspiration

from meadow (dashed line). (b) Relation between hourly solar radiation and hourly

evapotranspiration from meadow during the day. Before noon, solid line; after noon,

dashed line (Koziov, 1959).



role of radiative energy in evaporation from this type of surface cover.

This role is well expressed in the daily cycie.



Radiation and Evapotranspiration in the Diurnal Cycle

The relative importance of radiant energy and air temperature as

factors in the daily regime of evapotranspiration from a meadow is

shown by comparing Figs. XI-2 and XI-3 (Kozlov, 1959). In Fig. XI-2



(mm



Fig. XI-3.



hF')



(a) Diurnal marches of air temperature (solid line) and evapotranspiration

from meadow (dashed line). Note how evapotranspiration decreases in the afternoon

although the air stays warm. (b) Relation between hourly values of air temperature

(ordinate) and hourly evapotranspiration. Before noon, dashed line; after noon, solid

line (Kozlov, 1959).



280



XI.



E V A P O R A T I O N F R O M WELL-WATERED E C O S Y S T E M S



the daily regime of short-wave radiation is seen to have about the

same shape as the regime of evapotranspiration, except that radiation

starts an hour earlier and stays about an hour ahead of the curve of

evapotranspiration, illustrating a kind of warming-up process. Fig. XI3 stands in contrast. While the rising limb of the daily march of air

temperature coincides with the rising limb of the evapotranspiration

curve fairly well, there is no resemblance between the two curves after

noon. Evapotranspiration begins to decline at a time when air temperature is still climbing. It continues to decline through the afternoon

hours, while air temperature is still as high as it was at noon; and it

reaches zero while the air is as warm as it had been at 1100.

When evapotranspiration at each hour is plotted against the corresponding value of solar radiation or air temperature, we obtain the

graphs in Figs. XI-2b and XI-3b. In such graphs, it is not uncommon

to find that the rising and falling hours plot along separate lines to

form a hysteresis loop, but the loops in these two graphs differ from

one another. In the graph of radiation versus evapotranspiration the

width of the loop is equivalent to about 0.04 mm hr-l, that is,

evapotranspiration for the same value of solar radiation is 0.04 mm

hr-' greater in the afternoon than in the morning. Since little heat is

going into the soil in the afternoon, this difference is reasonable. In

contrast, the loop in the graph of evapotranspiration versus air

temperature (Fig. XI-3) is much wider. For example, at an air temperature of 17°C at 1100 in the morning, the evapotranspiration rate is 0.25

mm hr-', but at the same temperature in the afternoon (reached at

1930), it is only 0.04 mm hr-'. In energy-flux terms, the width of the

hysteresis loops is 150 W mp2for the air temperature relation and only

35 W m-' for solar radiation.

Clearly, the air-temperature regime is a poor predictor of the

evapotranspiration regime. Much of whatever association might be

found between them results from the circumstance that both are

responses to the forcing function of solar radiation; but each makes its

response to this external function in its own way. Evapotranspiration

is more directly related, as we have seen, to variations in incoming

solar energy than to the dependent environmental parameter of air

temperature.



The Net Surplus of Whole-Spectrum Radiation Instead of using incoming solar radiation alone, it is common to include information on

the energy transferred to and from the vegetation surface by the other

fluxes of radiation. Vegetation reflects a fraction of the incoming solar

radiation; it absorbs long-wave radiation emitted by the air and



EVAPOTRANSPIRATION FROM PLANT COMMUNITIES



281



clouds, and in accordance with its regime of leaf temperature it emits

long-wave radiation also.

The summation of these several fluxes-called the net surplus of

whole-spectrum radiation, or sometimes just "net radiation"-represents all the radiative transfers of energy R . By definition it must

therefore be the same as the sum of all the nonradiative transfers. Of

these, the conversion of energy in vaporizing water E is by far the

largest in the situation under study here; the exchanges of heat with

the soil G and air H are minimized by the conditions set under the

definition of potential evapotranspiration, as noted earlier.

There is, therefore, a tendency for many investigators to equate

evapotranspiration with the net surplus of whole-spectrum radiation

R , a quantity that is easily although not always accurately measured.

Often only the daylight period, or the somewhat shorter period of

radiative surplus, is examined. Rouse (1970) found high correlations of

radiation surplus with two-week sums of evapotranspiration at sites

on Mt. St. Hilaire, near Montreal.

Another way to look at this situation is to begin by writing the

whole energy-budget equation



R ~ +JX L J + R s R t +XLRt +R,,,t



+ E + H + G =O,



in which subscripts refer to short-wave (S) and long-wave (L) radiation, and subscripts R and E to reflected and emitted radiation,

respectively. Subscripted arrows represent the direction; in all terms a

flux toward the ecosystem surface is taken as +, a flux away from it as

-. Applying this statement to a wet surface of great extent and small

capacity to store heat, terms H (sensible-heat flux) and G (soil-heat

flux) drop out, leaving



Rearranging terms, we can group on the right side all the energy

fluxes that are independent of the temperature of the ecosystem

surface, thus



The new group on the right is equivalent to the fractions of shortwave and long-wave radiation absorbed by the ecosystem, and can be

regarded as a loading, or, more positively, as an intake of radiant

energy RA* (Miller, 1972). As this energy intake changes, so does

surface temperature. In response to changes in surface temperature,

* As referred to in the opening pages of Chapter X.



282



XI. E V A P O R A T I O N F R O M W E L L - W A T E R E D E C O S Y S T E M S



the two left-hand fluxes, both temperature dependent, also must

change, E in response to surface vapor pressure, R L E in

~ accordance

with the Stefan-Boltzmann fourth-power law.

Finally,



E



=



-RA



-



RLE~.



The right-hand side is equivalent to net radiation, but expresses the

fact that a temperature-independent and a temperature-dependent

component are mmbined in it.

In some situations it is not desirable to neglect the soil-heat flux G ,

especially if the study is restricted to daytime hours when G is

prevailingly downward. It may be measured, or simply estimated as

0.1-0.2 of the net radiation surplus. Because this method still leaves

out the sensible-heat flux, it is sometimes found to overestimate

evapotranspiration (Davies and McCaughey, 1968) by as much as 1

mm day-'.

The outflow of sensible heat from vegetation, as from a water

surface, while minimized by the definition of potential evapotranspiration, might still be brought into the picture by the use of the Bowen

ratio, a number that depends on the relative coolness and dryness of

the air overlying the warm, moist vegetation mantle. In the postulated

condition of ample soil-moisture supply, infinite areal extent, and

uniformity, the fraction of the net surplus of whole-spectrum radiation

converted into sensible heat is not large, about 0.1-0.2.

It can be included by using the Bowen ratio (beta) to partition the



05



o



1



z



I



I



4



6



I



a



I



1



1



1



1



I



1



1



10



12



14



16



la



20



22



2.1



HOUR OF DAY



Fig. XI-4. Measurements of hourly evapotranspiration (mm hr-l) from unirrigated

Themeda grassland at Krawarree i n the southern tablelands of New South Wales in early

summer. Readings are made by the energy-partition evaporation recorder (solid line)

and weighing lysimeter (dashed line) (McIlroy, 1971).



EVAPOTRANSPIRATION FROM P L A N T COMMUNITIES



283



Fig. XI-5. Hourly energy fluxes (expressed in equivalents of mm hr-' evaporation,

i.e., 1 mm hr-' = 700 W m-') at field of alfalfa and brome grass on Plainfield sand in

Wisconsin, 4 September 1957. R,, is the net surplus (or deficit, at night) of all-wave

radiation fluxes, E the evapotranspiration, H the sensible-heat flux (relatively small),

and S the soii-heat flux (Tanner, 1960). Signs reversed for nonradiative fluxes. Reprinted

with permission from the Soil Science Society of America.



value of R (or of R - G) into H and E , H being equal to beta E . In the

right conditions the Bowen ratio can be computed from measurements

of atmospheric temperature and vapor pressure at two heights above

the evaporating surface. These measurements, made every few minutes, can be fed into a small field computer that calculates the ratio.

McIlroy (1971) has carried this method one step farther, by hooking up

the Bowen-ratio meter and computer to a recording net radiometer in a

system called EPER (energy partition evaporation recorder). Figure XI4 shows the record from this instrument over rough grazed pasture of

Themeda grass at a CSIRO site at Krawarree, N.S.W. in early summer.

The comparison is with lysimeter measurements of water loss from

this soil-plant ecosystem, which is typicaI of much upland pastoral

country in southern Australia. The peak rate of evaporation is reached

at noon and is equivalent to 0.47 mm hr-l. Good comparisons were

also found with the sums of carefully measured R , G, and H in one of

the few sites where H has actually been measured (Dilley, 1974, p. 23).

Figure XI-5 (Tanner, 1960) shows the tendency of the flux of latent



284



XI.



E V A P O R A T I O N FROM WELL-WATERED ECOSYSTEMS



heat from pasture in Wisconsin to approach the net surplus of the

radiation fluxes. In this situation, soil-heat and sensible-heat fluxes

are small.



The Combination Equation McIlroy’s combination equation (1971)

for potential evapotranspiration is a more exact statement. It shows

how atmospheric dryness, expressed as the depression D below air

temperature of a wet-bulb thermometer can be used to modify the

basic relation to net radiation less soil-heat flux ( R - G):

(Pot ET)(L) = (s/(s



+ y))(X



-



G)



+ hD.



The first term on the right side of the equation varies with air

temperature, and includes the change in the dependence of vapor

pressure on temperature (s) and the psychrometric constant (y).* A

coefficient of heat transfer between air and foliage h is related to

roughness of the vegetation canopy, i.e., the resistance to diffusion of

vapor molecules and sensible heat.

Even this improved formula does not always exactly express the rate

of evapotranspiration from well-watered vegetation. Experiments suggest that a degree of biological control is sometimes exerted (Shepherd, 1972), although its effect is small in annual totals.

Advective Factors



Wind Effect The reader might have noticed that no consideration of

wind appears in the foregoing discussion of potential evapotranspiration. Wind speed has been found, in fact, to have little or no

influence.

Potential evapotranspiration tends to approach the net surplus of

whole-spectrum radiation, which is made up of radiation fluxes that

are little affected by wind speed. Even in the combination equation,

which brings in the factor of atmospheric dryness, it is unnecessary to

introduce any index of air movement.

In this respect, potential evapotranspiration is not analogous to

evaporation from a body of water. Stirring of a water body by wind

action increases the movement of heat from lower layers to the surface,

but stirring of a vegetation canopy by wind makes available little

additional heat, assuming the air to be no warmer than the leaves.

An increase in wind speed increases the coefficient of turbulent

exchange, but this is counterbalanced by a decrease in the leaf-to-air

* A dimensional conversion between vapor-pressure gradient and temperature gradient.



E V A P O T R A N S P I R A T I O N FROM P L A N T C O M M U N I T I E S



285



gradients of both temperature and vapor pressure. As more sensible

heat is removed from the leaves they cool, and their vapor pressure

decreases. The fluxes of latent and sensible heat remain about the

same as before.

In conditions that do not meet the criterion of infinite extent of

vegetation, however, the heat-supply situation is different. An airstream coming over a small area of transpiring vegetation does not

immediately attain equilibrium with it but remains warmer or drier,

or both, depending on the condition of the upwind area it came from.

Instead of accepting a small flux of sensible heat from the underlying

transpiring surface, as shown in the foregoing graph of Tanner's

experiments, it may convey sensible heat to it. Advection of hot air

accelerates evapotranspiration by providing additional energy to ecosystems situated in dry surroundings.



Local Tvanspovt of Heat A few years ago, when researchers were

changing their thinking about wind as the all-powerful factor in

evapotranspiration and were beginning to look to radiation to take its

place in the scheme of things, they were occasionally disturbed by

anomalous results from field experiments. Small plots of irrigated

corn, for example, transpired at rates significantly higher than could

be accounted for by radiant energy.

It was found that these plots were receiving appreciable amounts of

sensible heat carried by the wind from unirrigated, hotter lands lying

upwind. The fetch of the airstream over the irrigated crop was far too

short to allow the airstream to come to equilibrium with the underlying moist surface. Moreover, if the irrigated plants were tall and

widely spaced, projecting above the surrounding surfaces, even

greater heat transfer from the hot air was found to occur.

These additions of a nonradiative heat flux to the existing surplus of

radiant energy were thus identified. The first was named the "oasis"

effect, and the second the "clothesline" effect. The oasis effect in

particular has considerable interest to us on a micro- and mesoclimatic

scale. Table I suggests the size of the nonradiative energy intake by a

transpiring stand of tall grass (1-m height) in Arizona under two

conditions (Bavel et al., 1963).

(1) On 23 July the lysimeter grass was in the midst of grass of the

same height and density. It was transpiring actively, obviously with

the aid of some supplementary heat from the warm (42°C maximum)

air. This supplement averaged approximately 100 W m-2 over the

0600-1800 period, being especially strong in the late afternoon.



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