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Evapotranspiration From Saltcedar
Along the Lower Colorado River 1
Lloyd
w.
Gay2
Abstract.--Bowen ratio ET was measured periodically in a
stand of saltcedar on the Lower Colorado River during two
growing seasons. Rates ranged up to 12 mm/day in July. The
yearly ET totals at this site were estimated to be 1727 mm.
INTRODUCTION
exchange between the surface and the air (H), and
latent heat which is used to evaporate water
(LE). The flows represent flux densities (energy
per unit area per unit time); fluxes to the
surface are positive, and away, negative. The
sum of these four components in the energy budget
equation will always equal zero for the time
period of evaluation (s, min, day
etc.):
A variety of measurement methods have been
proposed
during
the
many
years
that
evapotranspiration (ET) has been under study
throughout the world. Most of these methods are
based upon Some form of the water budget. The
water budget, however, is a rather insensitive
method for estimating ET because of difficulties
in estimating soil moisture. This becomes a more
serious problem when plant roots reach the water
table, as in most riparian stands.
Q* + G + H + LE = O.
(1)
The flux densities Q* and G are readily
measured directly with a net radiometer and a
soil heat flux disk, respectively. The sensible
and latent heat flux densities (H and LE) are
estimated with the Bowen ratio model.
The most promising alternative to the water
budget method at this time appears to be the
Bowen ratio energy budget analysis (BREB), ,which
combines measurements
of certain atmospheric
variables (gradients of temperature and vapor
concentration) with an assessment of available
energy (net radiation and changes in stored
thermal energy) to yield estimates of ET.
LE and H can be expressed in terms of
measured temperature
and vapor concentration
gradients if equation
(1) is first divided
through by LE, and then solved to yield the Bowen
ratio model (Bowen, 1926),
This paper will describe the Bowen ratio
energy budget model and specialized equipment
used in the field measurements, and analyze the
results obtained from application to a dense
stand of saltcedar (Tamarix chinensis, Lour.).
LE =
-(Q* + G)/(l +
B)
( 2)
with B (Bowen's ratio) given by
B
= H/LE
AP(Kh/Ke)(dT/dz)(de/dz)
(3)
where AP is the psychrometric constant, with A =
0.00066/C and P (mb) being atmospheric pressure
at the measurement site (AP = 0.66 mb/C at sea
level), Kh and Ke are the turbulent diffusivities
for sensible energy and for vapor (the ratio
Kh/Ke is assumed to equal 1.0), dT/dz is the
gradient of potential temperature in the air
layer just over the canopy, and de/dz is the
gradient of vapor pressllre.
BOWEN RATIO MEASUREMENTS
The BREB method is well known and described
in many texts.
Tanner (1960) and Spittlehouse
and Black (1980) thoroughly review the method and
discuss field applications. The model is based
upon an energy budget analysis of the gains and
losses of thermal energy at the evaporating
surface. There are four major energy flows at
the surface: net radiation (Q*), change in stored
energy in soil and vegetation (G), sensible heat
Both gradients are measured over the same
dz, which is typically 0.5 to 1 m for crops, and
to 3 m for forests, and located in the air
layer just above
the canopy or evaporating
surface.
The gradients are small, and precise
measurements are needed to evaluate LE with
equations (2, 3).
Some evidence has emerged
(Verma, et al., 1978) to suggest that the assumed
equality-of-t"he diffusivities does not hold when
the
atmosphere
is
stable
(i.e.,
during
Paper
presented at The
First North
American
Riparian
Conference.
Riparian
Ecosystems and Their Management:
Reconciling
Conflicting Uses. [University of Arizona, Tucson,
April 16-18, 1985]
2 Lloyd W.
Gay, Professor of Watershed
Management, School of Renewable Natural Resources
University of Arizona, Tucson, AZ 85721
171
advection), but no consistent relationships have
been proposed for corrections.
RESULTS AND DISCUSSION
The Measurement Site
The Measurement System
The field measurements were carried out on
the floodplain of the Colorado River (425 km west
of Tucson) at a site approximately 50 kilometers
south and downstream
of Ehrenberg, AZ, and
Blythe, CA. The region is one of the driest in
Arizona, with the annual rainfall at Ehrenberg
totalling only 3.5 in~hes (Sellers and Hill,
1974). The average rainfall for the month of May
is 0.02 inches, and measureable precipitation in
the month of June fell only once during the 30
year record from 1942 throllgh 1971. The dry
Summers
are
also
exceptionally
warm.
Temperatures in excess of 115 F are frequently
recorded, and the highest temperature of record
is 122 F.
The regulated flow of the Colorado
River maintains a high, relatively constant water
table beneath the floodplain, which has led to
the
development
of
extensive
stands
of
preatophytes.
Intense heat, low humidity, dense
vegetation and high water tables combine to
provide ideal conditions for rapid ET rates.
The generator powered BREB system used in
this study has been described by Gay (1979).
It
consists of a set of specialized sensors) a data
acquisition system and a microcomputer.
The
field sensors are linked by long (80 m) cables to
the data acquisition and processing equipment,
which operate in an air conditioned van that
serves as a mobile laboratory. Data obtained in
this study were sampled with a high quality
digital data system (Acurex Autodata 9), and then
transmitted to a microcomputer (Tektronix 4051)
which transformed, analyzed and stored data in
real time and printed the reslllts.
The key sensors in the AZET system are the
unique psychrometers which combine together a
ceramic wetbulb element, high output resistance
thermometers and a new signal circuit to yield
exceptionally precise measurements of temperature
and humidity (Hartman
and Gay,
1981).
The
psychrometers are used in pairs to measure the
vertical gradients of
temperature and vapor
concentration. The excellent performance of the
psychrometers
is
enhanced
for
gradient
measurements
by interchanging
the pair
of
psychrometers between readings to eliminate from
the gradients any small biases that may exist
between sensors.
The BRET measurements were made on the
Cibola National Wildlife Refuge near the western
edge of a vast saltcedar thicket of Some 10
square km in area.
Th~ height of the vegetation
was about 6 m in the vicinity of the measurement
site; the canopy was closed and rather uniform in
this area. The site was about 3 km west of the
main channel; fetch to the desert edge was about
1 km to the west and north, and about 3 km to the
south and east.
The ET analysis
is completed every 12
minutes and tnese values are combined to yield
hourly and daily totals of ET. As a refinement,
the system is usually operated with two separate
sets of energy budget sensors simultaneously, and
their agreement is monitored to insure that the
system is functioning properly.
The floodplain
soils were
sandy, with
numerous pockets of coarser material laid down
when the river channel meandered through the
floodplain in the past.
The water table depth
remained nearly constant at 3.3 m during the two
summers of measurement,
as high runoff and
storage levels upstream resulted in higher than
average flows for the lower Colorado Riv~r.,
Measurement Precision
The generally accepted precision of water
budget ET or energy budget ET measured elsewhere
is about 15 or 20 percent.
The AZET system,
however, has detected differences in the daily ET
from two adjacent sites of as little as 0.5
percent (Osmolski and Gay, 1983), based upon
hourly and daily totals from a four day series of
careful measurements over irrigated alfalfa near
Tucson.
ET Totals
Measllrements were obtained for periods of
2-4 days
duration on
each of
8 separate
expeditions during the growing seasons of 1980
and 1981. The results are described in detail by
Gay3, and summarized by Gay and Hartman (1982).
Good data were obtained for
21 days (daytime
periods of positive net radiation) and for
11
nights. The daytime water use ranged from about
2 mm/day in spring and faU, up to about 12
mm/day in midsummers while night loss rates were
quite low, ranging from as little as 0.08 mm/day
up to 0.6 rnm/day in midsummer. The night data
were interpolated as needed to obtain 21 sets of
24-hour ET totals extending over the season.
Only a few tests of the method are available
for riparian communities. For example, Gay and
Fritschen (1979) compared S-day mean Sowen ratio
ET against values from adjacent, constant level
lysimeters at the U S Bureau of Reclamation
saltcedar research site on the Rio Grande near
Bernardo, NM. For low, sparse stand conditions,
Bowen ratio ET was 7.4 mm/day and the lysimeter
values were 6.6. Fo~ denser stands, the Bowen
ratio
measurement was
8.8 mm/day
and the
lysimeter showed 9.1. Th~ over all mean for both
classes of vegetation was ~.1 mm/day for the
Bowen ratio, and 7.8 for the lysimeters.
The
agreement was judged to be excellent for this
clear, hot summer period.
The
totals at
the
---Tcay:--r:---w-.--
two
ma~ts
differed
by
1984.
Tile Effects of
Vegetation Conversion upon Water Use by Riparian
Plant Communities. Research Project Completion
Report (B-084-ARIZ). School of Renewable Natural
Resources) Univ. A~i~ona, Tucson 85721.
172
Means
Mean daily ET totals (in mm).
Table 1.
are for two masts and dates shown (Gay and
Hartman, 1982).
Rbout 5 percent on a daily and R seasonal basis.
This difference remained during several runs in
which the sensors
were interchanged between
sites, thus confirming that the difference was
site relRted, rather than instrumental.
Th~
estimates at the two sites were averaged to
obtain the best estimate of saltcedar ET, and
then the days during each run were averaged to
reduce the day to day variation associated with
variable climatic conditions. The values are
tabulated in table 1 and plotted in figure 1 as a
basis for evaluating seasonal saltcedar ET.
___--'------r-Apr
Apr
May
Jun
Jul
Aug
Sep
Oct
The spring greenup and fall dormancy dates
were set at March 23 and November 11 after
inspection of the sites and trends in table 1.
The growing season length was thus 233 d~ys.
6, 7
28, 29
28, 29, 30
26, 27, 28
28, 29
IS, 16, 17
12, 13
30-Nov 2
day:'
night_l
- 2.8
-0.1
-0.3
-0.4
-0.5
-0.6
-0.6
-0.5
-0.1
-
6.8
8.2
-10.5
- 9.0
- 8.4
- 6.9
- 1.8
24-hour
--_._-
-
2.9
7.1
8.6
-11.0
- 9.6
- 9.0
- 7.4
- 1.9
1 night column contains interpoiated data.
A simple trapezoid integration of the curve
in figure
1 yields a total ET for the 233 day
growing season of 1637 mm (1548 mm day, and 89 mm
night).
This total
should be increased to
account for precipitation, under the reasonable
assumption
that
all of
the
precipitation
evaporates. Th~ BRET totals are thus increased
by 42 mm during the growing season and 90 mm for
the entire yeRr, based upon mean rainfall records
at Ehrenberg, 50 km to the north. The best
estimate of saltcedRr ET at this site in the
lower Colorado River valley thus hecomes 1677 mm
for the growing season, and 1727 mm for the year.
4,593 ha of saltcedar on ceach number 4, which
extends from Blythe-Ehrenberg south to a point
just downstream from the BREB measurement site.
The Bureau estimated the annual water use to be
1359 mm, excluding
precipitation, using the
Blaney-Criddle formula adjusted for the density
of
vegetation
throughout the
reach.
The
agreement with the BREB estimate is rather good,
considering that the saltcedar at the BREB site
was quite dense, and the water use there should
be at a near maximum rate.
More
speculative
estimates from
other
regions range up to 2100 mm for the Gila River
near Phoenix (Horton and Campbell,
1974). The
climatic conditions on the lower Colorado are
warmer and drier than on the Gila, and it seems
unlikely that
vegetation density
and water
availability there could be favorable enough to
generate water use as high as the estimates of
Horton and Campbell.
Other estimates of ET for this region are
quite generalized.
For example, the U.
S.
Bureau of Reclamation 4 estimated that there wer~
12r--------------------------------------y
Ita
CONCLUSIONS
"fI>
1J
The energy budget technique is a well, known,
physically based method for evaluating energy
used to vaporize water at an evaporating surface.
The method gave cOllsistant, reproducible results
at the salt cedar site. The method has become
even more attractive with the recent development
of portable, battery
powered, energy budget
systems that will facilitate sampling a variety
of vegetative and environmental conditions (Gay
and Greenberg, 1985).
8
"-E
E
v
tW
6
It:
<t
a
w
u
t-
..J
4
<t
(f)
~ta
The BRET measurements reported here form a
unique data set for dense riparian vegetation,
with water freely available in a hot, arid
clim~te.
The growing season and yearly ET totals
estimated from these measurements, 1677 and 1727
mm respectively, including 90 mm of mean annual
precipitation, were considerably less than the
2100 mm projected for salt cedar on the Gila River
near Phoenix by Horton and Campbell (1974). Th~
Colorado River saltcedar water use was in general
agreement with water use by irrigated alfalfa.
The saltcedar
BRET measurements
were quite
consistant, and it is felt that an estimate of
1700 to 1750 mm yearly ET is reasonable for dense
saltcedar along the lower Colorado River.
1tata 120 14ta 160 180 200 2Z0 24ta 260 280 3ta0 320
DAY Of YEAR
Figure 1.
Mean ET rates at the salt cedar site.
Seasonal totals obtained by trapezoid rul~.
----4--u.--s:----Bureau of Reclamation.
1964.
Pacific
Southwest Water
Pl;:m.
Supplemental
Information Report on Water Salvage ProjectsLower Colocado Riv~r. USBR, Boulder City, NV.
173
Horton, J. S., and C. J. Campbell. 1974. Management of phreatophyte and riparian vegetation
for maximum multiple use values. Res. Paper
RM-117. USDA Forest Service, Rocky Mtn. Forest and Range Expt. Station, Fort Collins.
Osmolski, Z., and T~. W. Gay. 1983. Comparison
of Bowen ratio estimates from two sets of
sensors. Prl)c., 16th Conf. Agric., Forest
Meteor., pp. 77-78.
Amer. Meteor. Soc.,
Boston.
Sellers, W. D., and R. H. Hill.
1974. Arizona
Climate 1931-1972.
Untv. Arizona Press,
Tucson. 616 pp.
Spittlehouse, D.
L., and T. A.
Black. 1980.
Evaluation of the Bowen ratio/energy balance
method for determining forest evapotranspiration. Atmos.-Ocean 18:98-116.
Tanner, C. B. 1960. Energy balance approach to
evapotranspiration from crops. Soil Sci.
Soc. Amer. Proc. '24: 1-9.
Verma., S.I3.,~.
.J.
R08enberg and B.
L.
Dlda.
i~78.
Turbull.~nt
exchdng(!
coefficients for sensible heat and water
vapor under advective conditions. J. App!.
Meteor. 17:330-338.
LITERATURE CITED
Bowen, I. S. 1926. The ratio of heat iosses by
conduction and evaporation from any water
surfaces. Phys. Rev. 27:779-787.
Fritschen, 1..
J., and L.
W.
Gay.
1979.
Environmental
Instrumentation.
Springer
Verlag, New York. 216 pp.
Gay, L. W. 1979. A simple system for real-time
processing
of
energy
budget
data.
Proceedings,
WMO
Symposium
on
Forest
Meteorology, Ottawa.
Pp. 81-83.
WMO No.
527, Geneva.
Gay, L. W., lind L. J. Fritschen.
1979.
An
energy budget analysis of water use by
saltcedar.
Water
Resources
Res.
15:1589-1592.
Gay, L. W. , and R. J. Gt"eenberg.
1985. The
AZET battery powered Bowen ratio system.
Proc. 17th Conf. Agric., Forest Meteor.,
pp. 181-182. Amer. Meteor. Sl)c., Boston.
Gay, L.
W., ::I.nd
R.
K.
Hartman.
1981.
Evapotranspiration from irrigated alfalfa
and riparian saltcedar. Proc.
15th Conf.
Agric., Forest Metel)r., pp.
94-97. A~er.
Meteor. Soc., Eoston.
Gay, L.
W., and R.
K.
Hartmlin. 1982.
ET
measurements over riparian saltcedar on the
Colorado River.
Hydrol., Water Resources
Ariz. and Southwest. 12:9-15. Ariz. WRRC,
Univ. Arizona, Tucson.
Hartman, R.
K., and L.
W.
Gay.
1981.
Improvements in the design and calibration
of temperature measurement systems. P~oc.,
15th Conf.
Agric., Forest Meteor., pp.
150-151. Amer. Meteor. So~., Bo~ton.
Acknowledgements:
The work reported here was
supported in part by the Arizona Agricultural
Experiment Station, and in part by federal funds
provided by the U.
S.
Department of the
Interior, as authorized under the Water Research
and Development Act of 1978 (P. L.
95-467).
Approved for publication
as Paper No. 540,
Arizona Agricultural Experiment Station.
174