Application of Tracers in Arid Zone Hydrology {Proceedings of the Vienna Symposium,
August 1994). IAHS Publ. no. 232, 1995.
329
Utilization of oxygen-18 and deuterium in stem flow
for the identification of transpiration sources: soil
water versus groundwater in sand dune terrain
E. M. ADAR & I. GEV
The Jacob Blaustein Institute for Desert Research, and Department of Geology,
Ben Gurion University of the Negev, Sede Boker Campus, Sede Boker 84990,
Israel
J. LIPP
GSF-Institute fur Hydrologie, Ingolstadter Landstr. 1, PO Box 1129,
D-85758 Oberschleissheim, Germany
D. YAKIR & J. GAT
Department, of Environmental Sciences and Energy Research, Weizmann Institute
of Science, Rehovot, Israel
Y. COHEN
Institute for Soil and Water, Volcani Center, Bet Dagan 50250, Israel
Abstract Forestation and reforestation in an arid sand dune terrain raise
the issue as to what extent they affect the local groundwater system: that
is, the effects of transpiration versus groundwater recharge. In general,
sand dunes are considered to be the most efficient zone for groundwater
recharge in deserts. This is especially true for arid zones with a limited
amount of rainfall where most of the precipitation infiltrates into the
dunes and a high percentage of the deep percolated soil water reaches the
groundwater reservoir. In the late 1940s, a tamarisk forest was planted
in the sand dune desert at the north west of the Negev desert. Tamarisk
roots were found within the shallow aquifer 16 m below surface as well
as within the top 2 m of the sand. As ôl&0 and <5D composition is significantly different in soil water versus groundwater, the main research
objective was to determine the relative role of soil water versus shallow
groundwater as sources for transpiration. The distribution of oxygen-18
and the deuterium composition in the topsoil and in groundwater were
related to the spatial isotopic distribution along 15 m of double root
system, stem flow and twigs of two tamarisk trees. One tree is situated
within a small forest and its root length was determined by means of a
20 m borehole. The second is a single tree located on top of a sand dune,
and exposure of its double root system was made by cutting into the dune
to 12 m below surface. In the forested area, the total flux of transpiration
was measured as stem flow using a heat pulse method. Water was extracted by azeotropic and vacuum distillation from twigs, small roots and
from phloem and xylem taken from a stem core 0.5 m above the surface.
Similar cores were also obtained along the main root connecting the deep
and shallow roots from the exposed single tree on the dune. Best results
were obtained with oxygen-18 composition as long as the xylem was
E. M. Adar et al.
330
separated from the phloem. Isotopic mass balance expressions were
solved to elaborate on the relative water contribution from groundwater
and soil water to the total rate of transpiration in winter and summer.
Sets of water, ô180 and SD mass balance expressions were solved to
identify the relative contribution of transpired water. Results indicate that
groundwater serves as the main source of transpiration, even though a
sufficient amount of water exists in the topsoil. This is perfectly correlated with the soil water budget obtained with a soil neutron emission
moisture detector. The greater the soil moisture level, however, the less
water is pumped from groundwater. This phenomenon is explained by
the higher level of energy required to extract soil water from the high soil
matrix potential compared to the energy needed to lift saturated groundwater to the surface.
INTRODUCTION
Reforestation in sand dune terrain is well established practice. The dunes are not only
stabilized but also utilized as productive land for timber and firewood.
Sand dune terrain is also known to be the most effective region for groundwater
recharge. In fact the lack of stream channels in sand dunes suggests minimal runoff
leaving the sand basins and, as a consequence, a high rate of infiltration. A combination
of high infiltration rate and low water-holding capacity infers fast, deep percolation,
which is related to groundwater recharge. However, due to the high rate of effective
pores and high topsoil temperatures, potential diffusion of vapour also implies a high
rate of soil evaporation. Though the amount of rainfall is small, a relatively high percentage reaches the local aquifer.
Therefore, when trees are introduced into sand dunes, one has to consider how this
interferes with the local water balance and how it will affect the groundwater reservoir
through the rate and magnitude of recharge. These aspects have been studied in a small
forest of tamarisk trees which was planted in the late 1940s and early 1950s over sand
dunes in the northern Negev desert, Israel (Fig. 1). Shallow groundwater was found 1517 m below the surface. The tamarisk trees, like many other desert plants, are known
to have a dual root system: shallow roots within the top sand, and deep roots just above
the saturated water table. The main objective was to identify the extent of water transpiration from the shallow groundwater, as compared to how much, if any, is extracted
from the top unsaturated sand. It is assumed that most of the moisture in the topsoil
eventually evaporates. Therefore, transpiration that relies on that source of water seems
to minimally effect the rate of recharge. This paper describes a method that enables
identification of the relative portion of groundwater versus soil moisture in the sap flow
through the tree stem. It relies on the significant isotopic differences between groundwater and soil moisture which prevail in extreme arid conditions.
METHODS
Sampling and analysing for ô180 and SD in groundwater, soil moisture and sap
flows
Thefirstattempt to extract water from the tree was performed on crushed wood obtained
Oxygen-18 and deuterium in stem flow for identification of transpiration sources
331
340
50
100km
- Mediterranean
Sea
.AVIV
• 32°
jUERUSALEN
Israel
ARETAMIMv-v-g
v ••! 5-'=i V:f-.:V Se'de Boker
NEGEV
ELAT,
Fig. 1 Location of research area: the northern Negev Desert, Israel.
with a motor drill. Table 1 shows the ô180 values obtained from liquid extracted from
a crushed wood sampled with a motor drill rather than core wood. Separating of the
xylem from the phloem was possible only for cuts of the core wood. Mixing of xylem
and phloem revealed inaccurate results as the up-flowing sap via the stem toward the
canopy is found only in the xylem. From the needles, the phloem transports liquid which
is enriched with heavy isotopes. It is also possible that the drill introduces heat which
cause some isotopic fractionation.
At the first stage we examined the hypothesis that (a) the sap flow in the trunk maintains the isotopic values as in groundwater; and (b) that the liquid in the top root system
represents the isotopic composition of the topsoil moisture. A tamarisk tree growing on
Table 1 Comparison of S180 values obtained from liquid extracted from crushed wood sampled with a
motor drill with values from core wood and twigs.
Sample source
Twigs
Stem wood
Core wood
Crushed wood
Electric drill
Hand drill
Electric drill
16 April 1993
16 April 1993
24 August 1993
24 August 1994
-3.05
-0.84
1.66
-1.98
-2.42
-2.75
-4.37
-1.69
-2.31
-1.47
Sampling method
Date
18
ô 0 values
E. M. Adar et al.
332
top of a sand dune was isolated near an open sand quarry. Half of the dune was mechanically excavated down to 12 m exposing a dual root system with the major root connecting the top with the lower roots. Cores of wood from the tamarisk tree were taken from
the main trunk, stem and main branches where the xylem was separated immediately
from the phloem into different vials. The core holes were then filled and sealed with
silicon to prevent damage to the tree. Thin roots from the upper root system were also
sampled for water extraction. Water samples were extracted by evaporation in a closed
vacuum line. Water from twigs and needles was obtained by an azeotropic distillation
with petroleum ether at 122°C.
Stable isotopes of soil water and groundwater were used to elaborate on the role of
each source in the tamarisk transpiration process. Groundwater and soil samples were
taken in the vicinity of the tree on the same day. Soil samples were taken with an auger
drill at 25-50 cm intervals along the topsoil. Soil water was extracted by azeotropic
distillation.
Measurement of sap flow utilizing the heat pulse method
Total sap flow through the stem is equivalent to the amount of water utilized by the tree
for transpiration. Sap flow was determined using the calibrated heat pulse method for
flux measurement in the stem (Cohen et al., 1981). The method relies on measuring the
convective velocity of a heat wave, v (m s"1) along the stem. Heat is produced by heating
a wire inserted radially into the stem, and is detected by a temperature sensor located a
few cm from the heater. If r (m) is the distance between the temperature detector and
the heater, tm (s) is the time of occurrence of the maximum temperature at the detector
since the heat pulse was released and k (m2 s"1) is the thermal diffusivity in wet wood,
the convective heat velocity is expressed by the following equation:
y
_ ^-Aktm)
(1)
where k (m2 s"1) is calculated from equation (1) for zero convective flux (v = 0);
* = ?IMm.
The sap flow density Jt (m s"1) is expressed as:
where p and pt (kg m3) and c and ct (MJ kg"1 "C 1 ) are the density and the specific heat
of the liquid and the wet wood respectively. For an infinitesimal stem wood crosssectional area as the total rate of flux through the stem F (m3 s 4 ) is calculated with the
following integral:
F = [ Jtds
(3)
where S is the total wet wood area. A comprehensive description of this method and a
way to assess the aforementioned parameters are elaborated in Cohen et al. (1985).
Oxygen-18 and deuterium in stem flow for identification of transpiration sources
333
A heating element was inserted in the stem below the bark. A temperature sensor
consisting of micro thermistors was installed 10 cm above the heating element. Heat
pulses were given every hour for several days in winter, spring and summer.
THEORY
Assessing the relative portion of groundwater in the total rate of transpiration relies on
the fact that both soil moisture and groundwater are mixed in the sap flow passing
though the stem. Also, due to intensive evaporation, the isotopic composition of soil
moisture is significantly enriched in heavy isotopes as opposed to that of local shallow
groundwater. The following simplified model is illustrated in Fig. 2.
The total rate of transpiration from the canopy TV is equal to the rate of sap flow Fst
Tr = Trst
Trst = Vs + Vow
STEM FLOW
Vst D s t 1 8 O s t
Upper Root
SOIL WATER
VS D S 1 8 0 S
GROUND WATER
V
n
18
'
•tlllllliliiiiiiiiiiiii
Tr s t C s t = VSCS+VGWPC
,8
Tr st l/ O st l = r v s l t
<U n l
TrsJ L J K J
J
D
S '
s
J
Tv
V
1 r ,8 o
GW ' „ !
^GW
LD G
Fig. 2 Illustration of the simplified model for the assessment of the partial contribution
of soil moisture versus groundwater acting in the transpiration process.
334
E. M. Adar et al.
through the stem (st), which is combined from water extracted from soil moisture Vs and
groundwater V Trs[ can be replaced by F (equation (3)) such that F = Vs + Vgw. Similarly, the isotopic composition of the liquid passing through the stem Cs[ is a linear
combination of the average isotopic value in soil moisture Cs and in groundwater Cgw.
A simple mass balance approach implies the following expression:
Therefore, for ô180 and ÔD one obtains a set of two mass balance equations with Vs and
V as the only unknowns:
TrstôlsO = Vsô™Os+Vgwôl*Ogw
(5)
TrstÔD = VsÔVs + Vgw8Dgw
For a particular observation the measured rate of sap flow (Trst) and the isotopic
composition of groundwater are quite constant. However, the ô180 and SD values vary
along the soil moisture profile. Therefore, the accurate composition of moisture entering
and dominating the isotopic composition of the upper root system is not precisely
known. Only one xylem sample from the stem was available for ô180 and SD analyses.
OBSERVATIONS AND RESULTS
The first stage was to examine possible isotopic fractionation during the sampling
process between soil moisture and the upper root system and from groundwater to the
lower trunk root (main root). This was done with ô180 in August 1993 when sap liquids
samples were extracted along a tamarisk tree with partially exposed root system as illustrated in Fig. 3. At the same time, groundwater and fresh soil samples from the upper
root system were taken for comparison with that of the wet wood. Groundwater was
sampled in three observation holes in the vicinity (300-900 m) of the tree.
Wherever the phloem could be separated from the xylem it appears that the liquid
in the phloem is heavily enriched compared to the sap flow in the xylem. The ô180 in
the sap flow at the lowest exposed part of the main root (~ 12 m below surface) is in
fairly good agreement with that of the average groundwater. The same values (±0.2 %o)
were obtained in two more samples below the main root confluence with the upper root
system. The topsoil profile revealed considerably enriched ô180 values. Compared to
that measured in the lower root, the sap flow in the main upper root and in the stem
clearly shows significant enrichment which remains more or less constant along the main
branches and the twigs. A sharp enrichment in ô180 was observed in the green needles.
As long as liquid from the needles or the phloem is excluded, the above-mentioned
findings indicate that <5I80 and ô18D in the sap flow (as measured at the stem) might be
used to quantify the relative contribution to the transpiration attributed by the two potential water sources. Where both ô180 and ô18D can be evaluated, equation (5) can be
solved for Vs and V respectively.
Calculation of the relative contribution of water for transpiration from topsoil
moisture and groundwater was attempted in a nearby forested area (Fig. 1) where sets
Oxygen-18 and deuterium in stem flow for identification of transpiration
sources
335
Needles
Lower main trunk
Xylem Phloem
Soil water 24/8/93
Depth fml
0.5
1.0
1.5
2.0
0-18
D
3.4
2.95
-1.51
1.79
16.1
3
-19.2
-4.1
~£&6
Aug 1992
Q-18
D
R15 -4.11 -22.3
R16 -4.34 -25.8
R17 -4.6
-25.4
Ground-water
Average -4.30 -23.95
Oct/Nov 1992
Q-18
D
R15 -3.93 -22.3
-20.8
R16
R17 -4.53 -22.3
Fig. 3 Stable isotopes distribution in soil moisture, groundwater, and along an exposed
tamarisk tree.
of wet wood from stems and main branches were sampled for stable isotopic composition. Groundwater and soil moisture extracts were also analysed for ô180 and ô18D. The
average moisture content in the topsoil was assessed using the neutron moisture detection method (Gev, 1995). The average values for ô180 and ô18D in the topsoil layer were
taken as the representative values for the upper root system. Results from the forested
area for winter 1993 are presented in Fig. 4. During January 1993, the soil moisture
content was 98 mm/2 m and the rate of sap flow was 24.61 day"1. In this month the soil
moisture contribution to the total sap flow was 14.4 1 day"1 (58%) compared to only
19.3 1 day"1 (42%) from groundwater.
In mid April, the rate of transpiration was slightly higher and reached an average
value of 27.21 day"1. The total topsoil moisture content was 96.7 mm/2 m, similar to
that found in the winter; groundwater content dropped, however, by 14 cm (Gev, 1995).
In this month the soil moisture contribution to the total sap flow decreased to 8.741 day"1
(32%) with 18.421 day"1 (68%) from groundwater (Fig. 5).
Figure 6 illustrates the ô180 and ô18D distribution in August 1993. The topsoil
moisture content dropped to 74 mm/2 m. Though the deep root obviously reached
groundwater, the average daily transpiration, as measured by heat pulse method in
tamarisk stems, was only 9.341 day"1. Solving equation (5) for Vs and Vgwrevealed 3.81
(41%) and 5.53 1 day"1 (59%) from soil moisture and groundwater respectively.
E. M. Adar et al.
336
DISCUSSION
During three years of hydrological study in this region, we have gathered evidence from
16 soil profiles in which the isotopic composition of the soil moisture was always significantly enriched relative to the local shallow groundwater. Examples are presented in
Figs 4, 5 and 6. This should, therefore, be reflected in the isotopic values of the sap
flow in both upper and lower root systems. This was confirmed only when proper core
wood samples were obtained manually and when the water extract was obtained from
the xylem only.
In view of the winter versus spring results, the rate of transpiration increased due
to climatic conditions though the topsoil moisture remained almost the same. Thus the
lower root system became more active extracting water from the shallow aquifer (42%
in winter and 68% in summer). This was also reflected by a 14 cm lowering of the local
groundwater level (Gev, 1995). Winter and summer results revealed that the transpiration rate dramatically decreased to 9.34 1 day"1. As soil moisture content at the upper
root system decreased, the relative contribution of groundwater to the total rate of transpiration increased (41% to 59%, respectively). Groundwater, however, though it is at
least 16 m below the surface, always serves as the major source of transpiration. This
might be attributed to the fact that due to soil water salinity (which affects the osmotic
[L.
' "~
Time
11:00
Main branch
0-18
-3.94
-3.78
-3.86
-3.78
D
-29.9
-29.7
-27.6
22.38=*
24.6 * -3.84
=
24.6
-22.38
V
S * -3..53 + G W * -4.22
-23.8
-21.12 V
- GWvs
V
V s = 14.4 (58%)
VGW= 10.3 (42%)
Fig. 4 Stable isotopes distribution and relative groundwater and soil moisture contribution in tamarisk stem flow in winter 1993.
Oxygen-18 and deuterium in stem flow for identification of transpiration sources
337
Fig. 5 Stable isotopes distribution and relative groundwater and soil moisture contribution in tamarisk stem flow in spring 1993.
potential) and the matrix potential (affected mainly by the content of water and clay) the
total hydraulic potential required to lift groundwater to the surface is still lower than to
extract soil moisture.
Results indicate a clear pattern and confirm the applicability of this method to elaborate on the relative rate of water extraction by each root system. However, since ô180
and ÔD values vary along the soil moisture profile, the accurate composition of moisture
entering and dominating the isotopic composition of the upper root system is not
precisely known. Special care should also be given to proper core wood sampling and
accurate separation of xylem from phloem. During each sampling mission only one
xylem sample from the stem was taken for ô180 and ÔD analyses. It might be, however,
that each ring in the xylem is dominated by sap flow originating at a specific root section
with different isotopic values. These uncertainties may impose some noise into the
model which decreases the accuracy of the assessed fluxes.
Isotopes can be used for quantitative assessment of the fractions of flow components
by sampling distribution in the stem, provided that the water samples are properly
extracted from the sap flow within the xylem only. If twigs are used, only early morning
sampling is acceptable before the phloem becomes active with downward flux.
E. M. Adar et al.
•Twigs p 0 A.M.) Needles
Main branch
0-18
0-18 D
-3.83 -21.2
Trunk ( Xylem) (Phloem)
0-18
0-18
-3.43
D
1.18 -16.5
0-18
D
3.57
9.3
Average -3.63 -21 ..2
9.5 "
Ground-water
Aug 1992
Oct/Nov 1992
lAverage _
R15
R16
R17
_
^4.30_ -2JS.95_|
ksi ,f3J3l [Vs] t2,28| J W » [-4-3 ]
Vs
*3.8tWay(4l&)
V<jW=5.53H/day<59%>
Fig. 6 Stable isotopes distribution and relative groundwater and soil moisture contribution in tamarisk stem flow in summer 1993.
Acknowledgement This research was carried out under the framework of the
"Hydrological implications of reforestation in arid sand dune terrain" grant, supported
by the Alton Jones Foundation of Charlottesville, Virginia, USA.
REFERENCES
Cohen, Y.,Fuchs,M. & Green, G. C. (1981) Improvement of the heat pulse method for determination of sapflowin trees
Plant CellEnviron. 4, 391-397.
Cohen, Y., Kelliher, F. M. & Black, T. A. (1985) Determination of sap flow in Douglas-fir tree using the heat pulse technique. Can. For Res. 15, 422-428.
Gev, I. (1995) The effect of reforestation in arid sand dune terrain on groundwater recharge and flow system. PhD dissertation, Ben Gurion University of the Negev, Dept of Geology and Mineralogy, Beer Sheva, Israel (submitted).