PEL 214
ATOMIC ENERGY
BOARD
THE CHANGE IN C O N C E N T R A T I O N OF
T R I T I U M IN WATER D U R I N G EVAPORATION
A N D THE
POSSIBILITY OF U S I N G IT TO DETERMINE
EVAPORATION F R O M WATER, SOIL A N D
PLANT SURFACES
Bv
M. VAN DER WESTHUIZEN
and
MAVIS J . SMITH
PELINDABA
PRETORIA
•0
REPUBLIC OF SOUTH AFRICA
SEPTEMBER 1971
™
r
I ' I
ATOMIC: ENERGY HOARD
TIIK CHANGE IN CONCKNTRATION OF TRITIUM IN WATER DURING
EVAPORATION AND THli POSSIBILITY OF USING IT TO DKTERMINI
EVAPORATION FROM WATKR, SOIL AND PLANT SURFACI.S
by
M. van der Westhuizen*
and
Mavis J. Smith*
"TSOTOPKS AND RADIATION
DIVISION
PK LINDA BA
September, 1971
POSTAL ADDRESS:
Private Bag 256,
PRETORIA
SUMMARY
SAMKVATTINC
Die verandering in tritiumkonsentrasie van oop wateroppervlakke gedurende verdamping word bespreek en 'n aanral proefnemings word beskryf om die formule van
Craig en Gordon te toets. Die eerste aantal proefnemings is uitgevoer om die onbekende
konstante te bepaal en die daaropvolgende proefnemings is toe gebruik om die formule as
sodanig te toets. Die ooreenstemming tussen gcmete en berekende waardes was goed,
soos deur die korrelasiekoeffisiente getoon wat beter as 0,95 was.
Die verandering in tritiumkonsentrasie in die verskillende grondlae gedurende
verdamping vanaf die grond se oppervlak word bespreek en die teorie van Zimmerman
et ai word verstrek. Twee proefnemings is gedoen en in albei gevalle was die berekende
waardes laer as die gemete waardes.
Die moontlike verandering in die tritiumkonsentrasie in plante word bespreek. Vier
proefnemings is uitgevoer waarin die tritiumkonsentrasies in water, grond, wortels en
blare bepaal is. In die blare was die konsentrasie laer as in die bygevoegde water, en dit
kom voor asof die verhouding van die konsentrasie in die blare tot die in die water lineer
Die toepassing van hierdie metode om verdamping vanaf groot water-, grond- en
plantoppervlakke te bepaal, is tans nog ietwat onprakties. Verdere navorsing behoort dit
moontlik te maak om die verdamping van 'n boom of 'n groep borne met hierdie metode te
bepaal. Met'ndeeglikestudie van hierdie metode sal 'n duideliker bee Id van die meganisme
van verdamping vanaf water-, grond- en plantoppervlakke verkry kan word.
o $(:9Cc
OCÁ
discussed and a number of experiments to test the evaporation formula of Craig and
Cordon is described. The first few experiments were done to determine the unknown
constant and the subsequent experiments were then used to test the formula as such. The
agreement between measured and calculated values was good, as indicated by the correla­
tion coefficients which were better than 0,95.
The change in tritium concentration in the different soil layers during evaporation
from the soil surface is discussed and the theory of Zimmerman et ai is given. Two
experiments were done and in both cases the calculated values were lower than the
measured values.
The possible changes in the tritium concentration in plants are discussed. Four
experiments were done in which the tritium concentrations in the water, soil, roots and
leaves were determined. The concentration in the leaves was lower than that in the added
water, and it seems that the ratio of the concentration in the leaves to that in the water is
linearly related to the measured evaporation rate.
met die gemete verdampingstempe verband hou.
IS&SJ
The change in tritium concentration of open water surfaces during evaporation is
4
The application of this method for determining evaporation from large water, soil
and plant surf aces is somewhat impracticable at the present stage. With more research, it
should be possible to determine the evaporation from a tree or a group of trees using this
method. A thorough study of this method will throw light on the mechanism of evaporation
from water, soil and plant surfaces.
1. INTRODUCTION
!!.,<)
In South Africa, with its limited water supply, the
a1
P.1,<)
I,L'lo
conservation of water is of utmost importance. According to
a Report of thí Commission of Investigation into Water
matters
, 27% of the country's stored water is lost by
evaporation. The development of methods to curb the
CONTENTS
1. INTRODUCTION
where P ^ vapour pressure.
As IITO is more common than I o, further discussion.
will be confined to $ only.
evaporation of water from lakes, dams and rivers is of
The raiio or is known as the equilibrium fractionation
great importance. Before such preventative methods can be
factor. As the value of a is greater than unity, we can
evaluated, accurate methods for determining the amount of
expect from basic principles that 11./) will evaporate faster
water which evaporates must be available.
lhan HTO and that the tritium concentration in water will
5
5
The prevention of evaporation from a free water s u r ­
increase with evaporation. In the equilibrium stateamixture
2.1 Theory
5
face is not the only matter of prime importance; research
of HTO and IUO obeys Raleigh's I aw which can be written
2.2 Procedure
6
into the reduction of evaporation from ground and plant
as follows:
2. WATER SURFACES
surfaces (evapotranspiration) without having an adverse
2.2.1 Experiment 1
0
2.2.2 Experiment 2
7
effect on the size of the harvest is just as important. Know­
2.2.3 Experiment 3
g
ledge of the amount of evapotranspiration is also imperative
2.2.4 Experiment 4
9
for optimum irrigation.
2.2.5 Experiment 5
JQ
Numerous methods of determining the evaporation from
R
= initial tritium concentration
JQ
large surfaces have been developed. A summary of the
V
= volume of mixture at any given time
V
= initial volume of liquid
a
= equilibrium fractionation factor
2.3 Conclusion
3.1 Theory
|j
existing methods is given in a report of the World Meteoro(2)
logical Organization . All the existing methods have
3.2 Procedure
U
shortcomings and the search goes on for new methods. A
3.3 Conclusion
12
further method worth investigating is based on the fact that
4. PLANT SURFACES
,
the concentrations of the different isotopic species of water
(3)
(HTO, T O , HDO, D O , etc.) change during evaporation .
3. SOIL SURFACES
u
2
4.1 Theory
j
4.2 Procedure
12
Such an investigation can also help to improve the basic
13
knowledge of the mechanism of evaporation - of which very
4.3 Conclusion
5. GENERAL CONCLUSIONS
6. REFERENCES
1 4
2
little is known.
R
V
o
o
where R
= concentration of tritium at any given time
Very little investigation has been done on the concentra­
tion changes of the heavier isotopic forms of water during
evaporation under natural conditions (i.e. evaporation from
open containers in the atmosphere).
The first recorded
research is that of Craig et al
in 1963. They investigated
18
the changes in D and
0 in ordinary water under natural
circumstances. Their findings were that the enrichment
The evaporation from water, ground and plant surfaces
will be discussed separately. In each case basic discussions
will be given, followed by experimental details.
stopped after a loss of about 60% of the volume and that the
concentration of D tends towards a constant value. This
result does not obey Raleigh's Law.
Craig and Gordon
2. WATER SURFACES
2.1 Theory
then set to work from basic
principles and in 1965 they came up with a model of evapora­
tion. The model is too long to be discussed here so only the
Evaporation is normally a function of vapour pressure.
final equation will be given. According to this model, the
Although it is known that the vapour pressure of H.O is
tritium concentration of the nett flow of water which leaves
higher than that of HTO and T O , the ratios of these vapour
the surface is given by
pressures (or and a. respectively) are not exactly known at
(4)
different temperatures. Jacobs
has summarized all the
R.
R
E
KaO-h)
hR
K(l-h)
(1)
known values and plotted a graph of a and a against
temperature. The points on this graph are somewhat
scattered, but at a temperature of 20°C the average value?
r
R
of a and or. are
= tritium concentration in water vapour In the
atmosphere
H 0
2
1,100
*HTO
where R . = tritium concentration in the reservoir after
E
evaporation has taken place
h
* relative humidity of the atmosphere, normalized
to the water surface temperature
!
PI.I
!
i
l.'-J -
214-6
a
= equilibrium fractionation factor
K = a facmr which is dependent on the ratio of the
diffusion coefficient of UK) am! H
v
().
Cordon and Craig were originally of the opinion that
outflow or inflow, and where it is assumed that a, h and R
a
appears that K could possibly be a function of the relative
box. All the water vapour present in the box during the
equation (1) and (2) will render
experiment was the result of evaporation from the water pan.
K
R
h R
1-h
_v_ r » , - i 8
\, L "o ' "a J
=
where V
humidity.
The model suggested that the change in the isotopic
concentration of the water can be caused in two ways, i.e.
(a) fractionation and \b) molecular exchange between the
water molecules in the air and those in the liquid. Depending
on the relative concentration in the liquid and in the air, the
liquid will either be enriched or depleted in the concentration
of the heavier isotope. When the concentration in the air is
()
R
place. If the concentration in the air is lower than that in
the water, molecular exchange will b? dominant and the
water will be depleted in tritium.
KB
h
U)
V
= volume of water at any time after evaporation
= concentration of tritium in the water at any
LI = the sum of the different components of inflow
LI' = the sum of the different components of outflow
K
= mean evaporation rate
P
= total amount of rain during &t.
tion in the water, because only ratios of concentration
surface.
1
appear in equation (4). The standard deviations of counting
1
+
From this the fraction which was lost through evapora­
o
Results of Experiment 2
o
From equation (4) it is evident that y
R.
calculated if R, . R, , R and h are measured and or and K
O L a
are known. The first aim of this study was therefore to test
K , K., iv , R , R , = concentration of tritium in the
reservoir, the inflow, the out­
flow, the evaporation and the
precipiialion respectively
= decay constant for tritium =
0,55 year" .
()
The change in the tritium concentration with volume is given
R
-2- = 0,886
()
TAHI.E 1
R
equation (4). A number of experiments were done to deter­
RESL'I TS OF E X P E K1MENT 2
h = 0,541
A
When the values above, together with a value of a =
h
1,100 and various values of K were substituted in Equation
V
(measured)
T
O
(4), the best value of K = 1,050 was obtained. For this value
V
of K, the measured and calculated values of \T g ^
0,973
0,726
0,691
0,934
0,964
0,712
0,014
0,869
An enrichment of 8,6% in the tritium concentration
0,956
0,696
0,634
0,803
The experimental layout shown in figure 1 was used in
took place. This change in concentration of the tritium is
0,940
0,682
0,643
0,724
the Laboratory. A container with water was placed in an
relatively small compared with the large change in the
0,926
0,677
0,641
0,632
airtight perspex box with dimensions 100 cm x 80 cm x 100 cm.
volume of the water. Subsequent experiments were planned
0,888
0,674
0,656
0,487
To improve the counting statistics, the water was enriched
which would result in adepletion of the tritium concentration.
0,846
0,663
0,680
0,408
with tritium to a concentration of 0,013 u.Ci per ml. The
A greater numerical change in the concentration would re­
0,800
0,652
0,701
0,355
water temperature was measured with a conventional ther­
sult and therefore more accurate results would be obtained.
0,784
0,642
0,700
0,276
a
r e e
f)
mometer and the air temperature and humidity were
determined with a thermohygrograph. The air was drawr
in Figure 2. A volume change of 70% resulted in a 20%
2.2.2 Experiment 2
out of the box, passed through a freeze trap where the water
If the period under investigaiion is of the order of days
vapour was frozen out, and the c'ry air was returned to the
or weeks, the last term on the right-hand side of equation (3)
box. It was thus a closed system and the air in the box was
can he neglected.
at no stage mixed with atmospheric air. Before the experi­
In the special case «rf a reservoir where there is no
The results of Experiment 2 are summarised in 1 able I.
= 1,086
can be
2.2.1 Experiment 1
where
described in Experiment 1.
the following tritium ratios were measured.
v
2.2 Procedure
(3)
other details of the experiment were identical to those
Results of Experiment 1
When approximately 75% of the water had evaporated
The tritium balance equation for the sam" reservoir
p
THE EXPERIMENTAL ARRANGEMENT
the water level was measured and samples were taken. The
a, K and h have already been defined.
the accuracy of equation (4).
K P - \h XLT
FIGURE I
depletion of the evaporating water took place. At intervals
results were better than 0,1%.
h
done where this calculated value of K was used to determine
Kj LX* V&Rj = TLK,! - S R I ' - R K"! ^t •
THERMO
HYGROGRAPH
of the relative tritium concentrations were made by means
of the atmosphere above the evaporating water
mine the value of K and after that more experiments were
can be written as:
Samples of the water in the evaporation pan were
not necessary to determine the absolute tritium concentra­
v
where £.V = volume change
COLO TRAP
R - concentration of tritium in the water vapour
together with the balance equation of a reservoir^). The
(2)
stances. The water collected in the freeze trap was used to
of a Packard Tricarb Liquid Scintillation Counter. It was
To test and apply this equation it must be viewed
= IE I - II! - I-1 k
enrichment of the water took place under these circum­
taken at the beginning and end of the experiment. Analyses
tion is given by
mass balance of a reservoir during a period ilt is given by:
B
JL
remained.
time after evaporation has commenced
B
1
imately one quarter of the water in the evaporation pan
= initial concentration of tritium
has commenced
R
AIR FLOW
INTEGRATOR
determine R . The experiment was continued until approx­
initial volume of water
o
approximately the same as that in the water, fractionation
will be dominant and enrichment of the water will take
in such a way that no atmospheric water vapour entered the
are constant, integration of equation (3) and substitution of
a
the factor K was a constant. From more recent work it
A known volume of water was placed in the evaporation pan
ment began, all the water vapour in the box was frozen out.
change in the tritium concentration. The best collective
In this experiment the dry air (see Figure 1) was not
value of K for the nine sub-experiments of Experiment 2
returned to the box, but replaced with atmospheric air.
was obtained with the use of a computer program by the
Because of the low tritium concentration in atmospheric
X^ method and was found to be 1,055. The measured and
V
calculated values of TT for this value of K and with a *
air, molecular exchange was the dominant process and thus
v
o
PEL 214-9
PKl 214 - 8
A standard American Type A evaporation tank, 120 cm
clear that a volume change of 60% caused a concentration
the measured and calculated values is 0,9584 and the y}
diameter and 25 cm deep, was chosen as the evaporation
change of 30%. The best combined value of K for the seven
value is 0,0978. Both of these tests are very significant.
container. A Stevenson screen with athermohygrographwas
sub-experiments was once again determined with the use of
placed a few metres from the tank. Air was pumped from a
point 3 centimetres above the water surface. The water
a computer program and found to be K = 1,038. In Figure 5
V
the calculated and measured values of y are given for this
vapour was collected by means of a freeze trap. Water
value of K and for a value of 1,100 for or.
TABLE 3
RESULTS OF EXPKRIMENT 4
R
determined as accurately as possible. The water used for
per ml. The concentration of tritium in the samples was
o.o
Oil
The results are given in Table 2. hi Figure 4 the
change in the tritium concentration in the water corresponding to the volume change is shown. From die graph it is
0.Ï
4
o.t
o
0,814
0,955
0,329
0,993
0,950
0.878
0.914
0.327
0.418
0.916
0.846
0,858
0,324
0,428
0,857
0,747
0,870
0,323
0,469
0,845
0,798
0.845
0,321
0,447
0,525
0.767
0,861
0,320
0,459
0.815
0,791
0.816
0,317
0,452
0,755
0.687
0,820
0.316
0.464
0.734
0.792
0.798
0,314
0,453
0,708
0.645
0.790
0,313
0,487
0,698
0,631
0,775
0,310
0.465
0,664
0,609
1 MEASURRJ
0.759
0,308
0,473
0,623
0,598
FIGURE S
0.740
0,307
0,513
0,621
0,597
« • MOO
• /
• j
0.4
/
.•
/
/
f%
0.S
o.t
TABLE 2
0.1
RESULTS OF EXPERIMENT 3
«•
FIGURE 2
R
RELATIVE CHANGE OF TRITIUM CONCENTRATION IN WATER DURING
EVAPORATION
L
R
o
R
A
h
w
0,261
0,541
1,100 are given in Figure 3. From this figure it can be seen
0,985
0,245
0,560
0,905
that the distribution of points around the 1:1 line is relatively
0,913
0,254
0,538
0,779
small.
0,862
0,262
0,477
0,714
0,823
0,262
0,502
0,609
OH
e - 1.100
k • I.OM
0,1
/ *
-/•
s
0»
0*
u
»1*
—
i
i
i
0.7 U
U
V
I,'
CALCULATED AND MEASURED VALUES OF ^ FOR C 1.100 AND
0,468
0,506
0,659
0,260
0,494
0,405
(4) is relatively accurate provided that the correct value of
the three values and tested in two further experiments. The
weighted average was found as follows:
m
s
1,050 • (9 x 1,055) + (7 x l,04O)
17
02
^r
k • 1.041
f
0.1
similar. A weighted average value of K was calculated from
,:l
« • 1.100
0.9
K is known. The three determined values of K are fairly
at
0,0978
»•»
0,263
•
0,9584
Conclusions drawn from the first three experiments
0,772
• S
OJ
r
.2
k • 1.038
The first three experiments have shown that equation
• f
0.7
i
V
0,996
o
| _1_ I
1 1 I
11 0.1 0.3 0,3 0.4 OJ M
v
« * - (measured)
O
0,954
R
o
V
— (calculated)
O
0.974
o.s
H>"
— (measured)
O
0,358
o.o
CUL
Results of Experiment 2
h
0.329
k • 10M1
<
R
A
0,940
the experiment originally contained about 0,001 p,Ci tritium
determined as in Experiment 1.
R
L
R
O
samples were taken at intervals and the water level was
0 7
0
•
/ • •
zs
III
A • r . 0.SSI4
f
X *0JS7t
5 o.i
2
3
o.s
<
°
0.4
-
0.1
0
•
0.1 0J
1,049
o j o* e> ojt o,T o* o* u
^0,1
0,2
-*• MEASURED
FIGURE 3
CALCULATED AND MEASURED VALUES OF*
-
o.i
2.2.4 Experiment 4
FOR* . 1.100 AND K. 1,055
-
Z_L _ L
This experiment was conducted in exactly the same
1
1
1 1 1 1 1
0.1 0.2 0,3 0,4 0,5 OjB 0,7 0$ 0 * 1J0
manner as Experiment 3.
-jf»
2.2.3 Kxperiment 3
Results of Experiment 4
As the previous experiments gave good results when
atmospheric air (and not dry air) was present above the
evaporating surfaces, it was decided that further experiments
should be done under field conditions.
MEASURED
»0
FWURI *
CHARM M REUIIVt TRITIUM CONCENTRATION M
The results of Experiment 4 are shown in Table 3. The
v
measured and calculated values of y for a value of K 1,049 are shown in Figure 6. The correlation coefficient for
FIGURE 6
THE MEASURED AND CALCULATED VALUES OF
1
FOR * « 1,100 AND k > 1,049 ( k OBTAINED
FROM PREVIOUS EXPERIENCE )
0
IM.I
111
21-4 - 1 0
1.0
L'.LV'' E x p e r i m e n t 5
tc * 1.100 AND k . 1.049
/
concerning the presence of the micro layer between the
0.9
This experiment wui conducted in exactly the same
manner as Experiments 3 and 4.
liquid and the atmosphere and during the process of
••Jr
-
evaporation in general.
o 0.7
Results of experiment 5
1,100 and K = 1,049 are shown in Figure 7. In this case the
correlation coefficient between the measured and calculated
vmues was 0,9642 and the x~ value was 0,07642.
3 0.5 "
<
H>°
V
done by determining the evaporation from the river by
r « 0.9642
X « 0.07642
another method and then calculating the concentration
2
/
0.4 h
change caused by the evaporation.
3. SOIL SURFACES
0.1
1
'/,
1
1
1
1
1
1
0.1 0.2 0,3 0.4 0.5 0,6 0.7 0.6 0.9 1.0
v
\
MEASURED
When evaporation takes place from a soil surface,
water with a certain tritium concentration moves from the
deeper layers to the surface where fractionation and exchange
0,980
0,993
0,397
0,900
0,974
0,372
0,871
0,861
0,334
0,845
0,863
the surface will diffuse to the deeper layers until an equili­
0,382
0,814
0,837
brium state is attained. According to Zimmerman, Ekhalt
0,934
0,319
0,788
0,793
0,928
0,341
0,773
0,776
0,930
0,359
0,739
0,781
0,936
0,377
0,708
0,799
0,373
0,671
0,822
0,360
0,619
0,555
0,363
0,881
0,600
0,850
0,335
0,575
0,565
0,829
0,262
0,511
0,536
0,446
0,446
0,501
Concerning the practical application of this method in
the case of natural waters, it can be stated that the differ­
ence between K_, R, , and R will be small in the case of
O
L
a
0,801
0,264
0,425
0,453
0,808
0,266
0,394
0,467
0,260
0,964
0,951
0,946
0,250
0,264
0,854
0,864
0,823
0,246
0,258
mean i 0,256
0,9642
K = 1,049
0,0762
Therefore by determining the tritium concent rat ion in
soil samples taken at different depths, it should IK- possible
to ascertain the evaporation rare. Conversely, il"the evapora­
tion rate is known, it should be possible to predict the strati­
and the measurement of soil water flow.
0,505
0,668
. If this gradient is then
3.1 Theory
1,001
(>
has a gradient of
In ' and \
surfaces. This may possibly be of value for age analyses
/
— (calculated)
O
1,003
R
therefore
7
<>
fication of the tritium concentration in the uppermost soil
0.3
RESULTS OF EXPERIMENT 5
— (measured)
O
. ' /
0.2
TABLE 4
h
canals where tritium i s used as a tracer. This can be
0.6 --
In i: = in C - ^
<> I)
The graph of the relationship Ivtwecn
determined, and p and I) are known, I can l>e calculated.
(3) To correct current flow measurements of rivers and
Ui
The results of experiment 5 are shown in Table 4. The
V
measured and calculated values of ~ " for values of a =
(2) A study of this method can offer much scientific insight
.'14-11
take place.
FIGURE 7
THE MEASURED AND CALCULATED VALUES OF
1
FORfc=1.100 AND k =1.049
the evaporation can successfully be applied in practice. This
is confirmed to a certain degree by two publications which
appeared after the abovementioned experiments were c o m (8)
pleted. According to Gat
this method of determining the
Depending on the relative concentration of
tritium in the water and in the water vapour of the atmos­
phere, the water in the uppermost soil layers will either be
enriched or depleted. If it enriches, the excess tritium on
and Munnich
, the downward flux of the excess tritium in
this state i s equal to the mass upward flow of the water.
Therefore
surface was maintained. The water diffused through the
column to evaporate on the surface. The water was enriched
with tritium to a concentration of approximately 0,02 u,Ci/m I.
The concentration of tritium in the water vapour of the
atmosphere is lower, therefore depletion took place at the
surface.
After evaporation had been continued for four weeks,
rate of each layer i s plotted against the corresponding
«.000
D = self diffusion constant of water in soil
short evaporation periods. Because of the low levels of
x
- distance from the soil surface
tritium
w = real upward velocity of water in soil.
in natural water in the Southern
to determine these small differences with any accuracy. The
(5)
where
= amount of excess tritium
Hemisphere (approximately 0,2pCi/mi) it would be difficult
prepared and a constant water level at 50 cm from the upper
the tritium concentration determined. In I'igure 8 the count
c
concentration
laboratory. A column of soil with a diameter of 10 cm was
distillation, the water was extracted from each layer and
D-r- = + wc = c
dx
will not be better than 40%.
To test this theory an apparatus was set up in the
the soil was divided into layers. By means of quantitive
,^dc
D-r- = -wc
dx
evaporation in practice can be applied with an accuracy of
(9)
10%, but according to Zimmerman and Ekhalt
the accuracy
3.2 Procedure
The relationship between w and the evaporation rate E i s
given by
value of the relative humidity must also be determined with
reasonable accuracy. The other methods of determining
2.3 Conclusion
evaporation have equally serious shortcomings, s o it may
From the results of Experiments 4 and5 it is concluded
perhaps be worthwhile to test this method on a large scale.
that equation (4) can be used with reasonable accuracy for
This method can be of considerable value for three
the determination of evaporation from free water surfaces
further reasons:
provided that a suitable value for K is used. From all the
(1) On comparing
= pw
p
= porosity of the ground
where
The solution of Equation (5) gives
C. exp(--)
different methods of determining the
experiments it is also apparent that the value of K is in the
evaporation (which is a necessity) it appears that this
region of 1,050.
method can very well be used as a substandard provided
The experiments show that this method of determining
E
that the water i s slightly enriched.
o
(6)
1.000
J_
20
40
0EPTH (em)
where
C 3 concentration of e x c e s s tritium at the surface,
o
The equation can be written as
FIGURE S
CHANGE OF TRITIUM
THE SOIL COLUMN
CONCENTRATION WITH DEPTH IN
60
ri I
Pi:i
2 H - 1:
3.3 Conclusion
parts. The water from the leaves, roots and soil was
from that of the water used for maintaining a constant water
It is evident that there was an appreciable difference
extracted by means of quant at ive distillation. Samples were
level, and these values are plotted against the depths on
between the measured and calculated values in the first
also taken from the added water. Four experiments were
semi-log paper, the> form a straight-line giaph in accord­
experiment. In the second experiment techniques were
done - three in the perspex box previously described, and
ance with Equation (4). This is shown in Figure 9. In deter­
improved and the calculated value was determined more
one in the laboratory outside of the box. The fourth specimen
mining the position of the straight line, the emphasis was
accurately. However, doubt still exists concerning the value
was therefore subjected to completely different environ­
placed on the concentration values of the upper layers of
of D. Much more experimental work is necessary before a
mental conditions compared with the other three.
the soil column. The value of I) was taken from the literature
decision can be taken as to whether this method of measuring
as 1) = l,88cm /d. The porosity ft was determined by
evaporation from soil surfaces is accurate enough. The low
standard methods and found to be p - 0,565. The average
concentration of natural tritium and the relatively small
evaporation was calculated from the gradient of the graph
change with depth hampers the practical use of this method.
2
4.1 Theory
woo -
&
The results are given in Table 5. lr can be seen from
0.S
An important problem in botany, forestry and agricul­
where, for some unknown reason', the concentration in the
ture is that of determining the total transpiration of a large
leaves is much higher than that in the soil. It is interesting
growing tree. The methods currently being used are either
to note that the concentration in the leaves was, in three
very expensive or not very accurate. The relationship
experiments, higher than that in the roots, indicating that
between the water already present in the soil, added water,
some enrichment must have taken place. In the last column
I
X
0.2
X
0.3
0.4
0.5
FIGURE 10
VARIATION OF CONCENTRATION RATIO OF LEAVES/
WATER WITH EVAPORATION RATE
the water in the roots and the water in the leaves and trunk
TABLE 5
transpiration of a tree. The transpiration could possibly
-
X
0.1
EVAPORATION RATE
also lower than that in the soil, except in Hxperiment 2
Tritium concentration in dpm/ml
also be determined by using the same principles as those
500
0.6
concentrations is shown. The concentration in the leaves is
of the tree could possibly give an indication concerning the
1000
u
o
u
than that in the water. In column 7 the ratio between these
4. PLANT SURFACES
10.000
Results
the table that the concentration in the leaves is always lower
in Figure 9.
I
- i:i
was divided into layers corresponding to the separate root
depths. When the count rate of each layer is subtracted
§
L'U
used in the case of evaporation from soil surfaces. Water
Position
Experiment
Inside
box
Concentra­
tion of
Evaporation
leaves -r
rate
concentracm/d^y
tion of
water
Water
Soil
Roots
Leaves
1
23081
19337
20302
18701
0,81
0,143
Inside
box
2
23611
13369
17014
19467
0,83
0,219
Inside
box
3
23366
22025
15930
21589
0,93
0,364
Outside
box
4
23211
16026
13153
15007
0,65
0,490
moves from the soil, through the roots, to the trunk and to
the leaves where enrichment will take place. The surplus
tritium will then diffuse back to the trunk, etc. By determi­
I
100
ning the tritium concentration at different positions, for
example by taking a few leaves at different heights, the
transpiration rate could possibly be calculated.
Contrary to the case of evaporation from water and
soil surfaces, the use of tritium-enriched water for deter­
mining evaporation from plants could well find practical
use. In many botanical and agricultural experiments it is
imperative that the transpiration of a single tree of a few
FIGURE 9
CHANGE IN TRITIUM DEFICIT IN EACH IAVEW WITH DEPTH/
trees should be known for the successful completion or
evaluation of the experiment. In such a case the use of
tritium-enriched water definitely has practical possibilities.
the measured evaporation rate is given. In Figure 10 the
ratio of the concentration in the leaves to that in the water.
ratio of the concentration of tritium in the leaves to that
The graph complies with the theoretical expectation, namely,
in the water is plotted against the evaporation rate (cm /day)
the higher the evaporation rate, the lower the backward
for the three experiments subjected to similar conditions in
diffusion and the smaller the change of molecular exchange.
the perspex box. The fourth experiment was carried out
However, the data are insufficient to make a definite con­
under altogether different conditions and shows a greater
4.2 Procedure
Results
evaporation rate than the other three.
With the aim of testing this possibility, geraniums
In the first experiment the measured evaporation was
were planted in the soil columns previously described.
0,16 cm/day, and the calculated value was 0,11 cm/day. In
After about two months the above-ground part of the plant
the second experiment the measured value was 0,13 cm/day
was divided into separate horizontal parts, the roots were
and the calculated evaporation was 0,104 cm/day.
divided into separate parts and the soil around the roots
4.3 Conclusion
From the graph in Figure 10 it appears that there is
a linear relationship between the evaporation rate and the
clusion. A thorough study would require a large number of
plants of different sizes under different conditions in glass
houses. The Atomic Energy Board possesses no such
amenities and therefore such a study cannot be undertaken.
The results obtained from these few ex,eriments point to a
very interesting field of ótudy which could lead to exceedingly
useful practical results.
PEL 214 - 14
IM I
6.
REFERENCES
5. (;I-:NI-:KAI. CONCI HSIONS
diate practical application, they offer s o much towards basic
JH
IS
CRAIG, 11. and I.I. Gordon: Deuterium and <'xNgcn-IS
variations in the ocean and the marine atmosphere.
Report of the Commission of Investigation into Water
Stable isotopes in oceanographic studies and paleotem-
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research and practical application in future that a thorough
Matters. Department of Water Affairs. Pretoria(1970).
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study of the concept of tritium concentration changes could
W.M.O. Measurement and estimation of evaporation and
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rewardingly be undertaken by institutions with the necessary
evapotranspiration.
field. Although these concepts may possibly have no i m m e ­
facilities.
Meteorological Organization, Geneva (1966).
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VAN DER WESTHU1ZEN, M.: Die aanwending van tri-
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Note No. 83,
World
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l.A.E.A. Guidebook on Nuclear Techniques in Hydro log).
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9.
ZIMMERMAN, I', and D.li. I khali: The use of stable
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