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III. Validity of Evapotranspiration Data

III. Validity of Evapotranspiration Data

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studies in which deep percolation does not complicate the measurements

of ET.



Most of the data available on fertilizers in relation to evapotranspiration come from small-container experiments, lysimeters, or small-plot

field experiments, which vary in their extent of control of border effects.

Most container experiments are conducted without the benefit of a

surrounding crop so that the plant is not confined over the area of the

soil in the container. Opportunities to intercept heat and radiation from

the surroundings are great, and the yields and water-use data cannot

be projected to an area basis in the field. Lysimeters may be subject

to this error unless precautions are taken, and even these may not be

sufficient. Van Bavel (1961) gives a good discussion of lysimeter

techniques for obtaining data on evapotranspiration. Even small plots,

differentially treated, are exposed to different opportunities for evapotranspiration. This effect of advection of energy is, or may be, a serious

consideration in experimental data because the prime effect of fertilizer

response is an increase in crop yield, which may be apparent in plant

height, leaf area, and sometimes in leaf or crop color.

This problem can best be understood through the concept of the

heat budget and its significance to evapotranspiration (see Penman, 1948;

Tanner, l957,1960a, b ) . The heat budget or energy balance approach is

based on the fact that the net radiation (difference between the incoming

radiation and the radiation losses from soil and crop surfaces) can be

partitioned into components that either evaporate water; heat the air,

the soil, or the plant; or can be used in photosynthesis. The following

simplified expression of the heat budget equation, in which all terms

represent vertical heat flux, is taken from Peters (1960):



where R, znet radiation; S = soil heat; A = sensible heat (air); E =

evaporation; P =photosynthesis. A term C for heat storage in the crop

should be added. Over a growing season, energy going into S and C

is small. Energy going into P is often ignored, but recent data show that

it may be higher than commonly supposed. For short time periods, in

per cent of total radiation it can be 4 per cent for sugar beets (Gaastra,

1958; Blackman and Black, 1959), 5 per cent for corn (Lemon, 1960),

and 5.5 per cent for barley (Kamel, 1959).

In the data available on fertility-evapotranspiration relationships,

interest centers on how crop size and color affect net radiation and

sensible heat transfer. On small plots and lysimeters, energy available for

evapotranspiration can be increased by the turbulent transfer of sensible




heat downward from surrounding drier areas. Lemon et al. (1957) call

this the oasis effect, and Halstead and Covey (1957) call it turbulent


Small plots, inadequately guarded lysimeters, and containers are

subject to another source of upward bias of evapotranspiration-the

horizontal movement of sensible heat moving through the crop by wind

movement from hotter and drier areas. This is called the “clothesline”

effect by Tanner (1957), who states that on the upwind sides of fields

this horizontal transfer is much greater than the turbulent exchange

from above. He further states that all of a small test plot is subject to

the clothesline effect, All investigators agree that these effects are greater

in hot and climates than in cool humid ones and that they are greater

the hotter and drier is the day.

Some of the extreme effects of advection may be illustrated as

follows: Lemon et al. (1957), working with a continuous cover of wellwatered cotton 5 days after a general rain of 3 inches, found that after

2 P.M. energy for evapotranspiration greatly in excess of net radiation

was gained from a drier area upwind even though identical cotton was

growing in that direction for 10 miles. This is the oasis effect, and Lemon

et al. state, “It is of interest that such a marked effect can be found in

an oasis of this large size.” Of particular interest to the problem of

fertilizer effects on plant height are observations of Lemon et al. (1957)

on water loss of a continuous cover of cotton on plots on which the

cotton grew to different heights influenced by preceding moisture treatments. Plant height did not affect net radiation. When the mean integrated soil moisture tension was in the range of 0 to 2 atmospheres, the

relative evapotranspiration rates for cotton 40, 25, and 18 inches high

were 1, and about 0.8 and 0.7, respectively. They state: “This feature

must have been due to horizontal heat transfer brought about by a plant

characteristic, namely, plant height and/or leaf area difference.” Tanner

(1957,1960b) shows that on an unusually hot, dry day in Wisconsin with

north wind sweeping over a dry area, heat derived from the air contributed as much as 25 per cent to the total evapotranspiration of an

alfalfa-bromegrass-ladino clover hay field. For alfalfa the same day,

evapotranspiration was 1.1 times the net rediation. Lemon et al. (1957)

studied evapotranspiration and net radiation on five large blocks

(1150 by 120 feet oriented in the direction of the prevailing wind) at

College Station, Texas. Three of the blocks were irrigated at 1-, 12-,

and 16-day intervals, respectively, and two were nonimgated. When

measurements were made, the plots had a continuous cover of cotton

and plant heights varied from 40 to 23 inches. The net radiation was

essentially the same over all five blocks regardless of moisture tension



or plant height. A dryland block ( N 1 ) was badly wilted and evapotranspiration was essentially zero, but on the block with a moisture

tension of almost zero the evapotranspiration was more than 2.5 times

the net radiation.

Lysimeters may be especially subject to advective heat influences,

particularly of the “clothesline” type. Tanner (1957) cites some data

of Moldenhauer (1952) showing that the evapotranspiration of reed

canarygrass in 2.4-foot lysimeters surrounded by mowed grass exceeded

the net rediation by 2.5 times for a 2-week period in August.




Fertilizer does affect plant size, total leaf area, and often the color

of the foliage. As mentioned above, Lemon et al. (1957) found no

effect of moisture tension at which plants were grown and their heights

on net radiation. His measurements were made when plants varied from

full turgor to wilted, so there should have been differences in leaf temperature. Chlorophyll content and color of plants vary with nutrient

supply. In fact, reflectance of red radiation at 625 mp has been proposed

as a reliable measure of the chlorophyll content of the leaf (Benedict

and Swidler, 1961). Energy in the visible spectrum amounts to about

40 per cent of the total solar radiation. Possibly, the visible differences

in color and reflectance observed are compensated by differences in

longwave radiation because of color of the radiating leaf surface.

Aubertin and Peters (1961) studied the effect of 20- and 40-inch row

spacings of corn, each at stand densities of 15,600 and 31,300 plants grown

on a dark Brunizem soil and found differences in net radiation in midsummer. Net radiation was greatest when more soil could “see” the sky.

On light-colored, dry soils net radiation, because of increased reflection,

could be lower, the lower the soil cover. Bahrani and Taylor (1961)

reported a marked decrease in net radiation from about 400 to 240 cal.

cm.-2 day-1 when alfalfa was harvested, accompanied by an increase in

mean soil temperature at the 4-inch depth from 24 to 33” C. After

irrigation soil temperature dropped and net radiation rose to 320 cal.

cm.-2 day-l. This removal of vegetative cover and irrigation of a noncovered soil surface is far more drastic than that fertilizers would bring

about, except as fertilizer aided in establishment of vegetative cover.

The extent to which fertilizers may affect net radiation of a crop must

be considered to be an unsettled question at this time.




The problem posed by advected heat, whether it be by horizontal or

vertical transfer, is the extent to which container or small plot experiments




can be relied on to predict the evapotranspiration and the evapotranspiration-yield relations of a large field. Tanner (196Ob) states, “Many

of the evapotranspiration data that have been gathered from greenhouse

pots and small plots are not suitable for field interpretation.” In an

earlier paper Tanner (1957) stated: “When small plots, lysimeters, etc.,

are employed, they should be ‘guarded’ for some distance by a crop of

identical size, maturity and moisture conditions. When moisture or

fertility variables are inserted in an experimental field, they often introduce different crop heights on the different areas, and the ‘clothesline

effect’ will cause different amounts of evapotranspiration on these areas.

The usual small plot and lysimeter layouts, and also experimental strips

across a large field, cannot be used to obtain evapotranspiration data

representative of a large field.” Robins and Haise (1961) caution: “Plant

height differences between small plots due to fertilizer or other treatments

appreciably alter the turbulent air flow pattern and thus evapotranspiration rate, especially where advected energy is a problem. To the author’s

knowledge, no suitable procedure and, in fact, no general understanding

of the magnitude of such effects is now available. Some difference in

evapotranspiration is inevitable, however, and this factor makes comparisons of such treatments in field experiments difficult.”

Under conditions of low advection in humid regions, fertilizers may

have much smaller or no effects on evapotranspiration. Van Bavel

( 1961) summarized data from Coshocton, Ohio; Seabrook, New Jersey;

and Raleigh, North Carolina, showing that such diverse crops as corn

and meadow had about the same evapotranspiration rates when grown

adjacently in lysimeters.

De \Vit (1958) has reinterpreted some of the data of Briggs and

Shantz (1913a, 1914), Dillman (1931), and Miller (1916, 1923) and

has developed the relation P = mW/E,, in which P is production per

container, \V is kilograms water transpired, E , is evaporation in millimeters from a sunken U. S. Bureau of Plant Industry pan, and m is a

constant characteristics of a species and holds for the bright sunshine

conditions of the Great Plains from Dalhart, Texas, to Mandan, North

Dakota. He says m is independent of weather, nutrient supply (if not

too low), and water availability (if not too high). It is implicit that

some variations in production ( P ) must come from differences in fertility

status. The ratio W / E e may be a measure of advection. From actual

alfalfa hay yields obtained in five western USA irrigation experiments,

De \\’it calculated the evapotranspiration by the use of the rn values

from the container experiments and found that they agreed closely with

the water used. He states, “From the agreement between measured

and calculated slope, it may be concluded that these experimental plots



of half an hectare of less obtained considerable amounts of heat by

advection.” De Wit points out that the transpiration rate of alfalfa in

the field would have to be twice the free-water surface evaporation rate

if m from containers was applied. So he concludes that “on fields, su%ciently supplied with water, the relation between transpiration and production can only be the same as in containers, if the fields are so small

that sufficient energy is obtained by advection.”

De Wit (1958)also examined the data available from northen Europe

on dry matter production of oats, peas, and beets in relation to transpiration and found the relation P = nW, where P is dry weight, W is

water transpired, and n is a constant for a species. The constant n derived

from containers could not be used to predict evapotranspiration rates of

large fields with high yields. Evapotranspiration of large fields was

always lower because of lack of advected energy.

This discussion of the heat budget approach and heat advection is

not redundant to a discussion of fertilizers, crop yields, and evapotranspiration. The data now available come from experiments in which

this iduence was either ignored or minimized, and in all cases, not

evaluated quantitatively. At present no one has designed an experiment

that meets what appear to be the requirements for an unequivocal

answer to the effects of fertilizers on evapotranspiration. On the other

hand, it must be remembered that in the natural situation in the field

the crop is subject to advective transfer of heat. In arid and semiarid

climates it is universally conceded that this influence is greater than in

humid climates. Furthermore, the effects of advection can be great for

short periods even in humid climates and yet be of little quantitative

significance over a longer period of time such as the entire growing

season of a crop. The best the author can do in reviewing the data

available is to convey the pertinent details as given by the investigators


IV. The Effects of Fertilizers on the Relationship of

Evapotranspiration and Yield

At least six possible situations may exist for evapotranspiration ( E T )

and yield ( Y ) as Y is changed by fertilization which lead to six

possible relations between water-use efficiency (Y/ET) and Y. Diagrammatic models of these are presented in Fig. l. For some cases no

data are available, and therefore only hypothetical curves and lines can

be given, The yield increase produced by fertilization usually follows a

decreasing increment function of some sort (see Mason, 1956, for

examples). This means that each succeeding equal increment a level of



nutrient availability is reached at which yield increases become infinitesimal. Therefore, the points derived from fertilizer experimentation on a

plot of ET vs. 2’ draw closer together as 2’ increases if fertilizers were

applied in equal increments of a nutrient. Overfertilization may produce

yield decreases that are ignored in the models.

In these models it is presumed that water is available for evapotranspiration and that the water conductivity of the soil and the capacity

of the root system to absorb water are not seriously limiting the ability

FIG.1. Six possible models of the relation between evapotranspiration (E T ) and

yield of dry matter (Y) (top part of each diagram), and water-use efficiency (Y/ET)

reiative to yield (lower part). These models presume that water is nonlimiting for

yield or evapotranspiration and that fertilizer is applied in equal increments

resulting in declining increments of yield.

of the soil-plant system to meet the evaporative demand. Data presented

in this section are chosen with this criterion in mind, but there is always

some question as to the extent to which it is fulfilled as the soil dries

and the moisture tension increases. Increase in moisture tension affects

not only the capillary flow of water to the soil surface for evaporation,

but also the flow to the root surface (see Kelley (1954), Veihmeyer and

Hendrickson (1950), Hagan (1955), and Russell (1959) for recent

reviews on the plant availability of soil water at various tensions, and

contents at equal tensions). Tanner (1960a, b ) drew attention to difficulties of interpreting field experiments relating plant growth to soil

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III. Validity of Evapotranspiration Data

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