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Energy Balance Dynamics Within
a Shrub-Dominated Watershed
J. Ross Wight
Clayton L. Hanson
Gerald N. Flerchinger
communities and to compare model-simulated values with
field-measured values.
Abstract-Components of the surface energy balance were measured within a shrub-dominated watershed in southwestern Idaho
during the 1989, 1990, and 1991 growing seasons. On clear days, net
radiation averaged between 40 and 50% of the incoming solar
radiation with the highest values occurring during spring and early
summer before the grasses and forbs began to senesce. The ratio of
latent heat to net radiation varied from more than 0.75 in the spring
to less than 0.05 in the fall at the low elevation site. Albedo averaged
between 13 and 17% with little site, seasonal, or diurnal variation.
Simulated energy fluxes using the SHAW model were in good
agreement with field-measured values.
Methods and Procedures -----Energy Balance
We measured the components of a surface energy budget
over five sagebrush-grass plant communities (table 1) on the
USDA, ARS Reynolds Creek Experimental Watershed during the 1989, 1990, and 1991 growing seasons. These components included: incoming solar radiation (Rs), net radiation
(Rn), soil heat flux (G), and LE--all measured in (W/m2 ).
Sensible heat, also in W/m2, was calculated from the surface
energy balance equation. Albedo was also determined for
Nancy, Lower Sheep, and Reynolds Mountain.
Bowen ratio-energy balance systems as described by Wight
and others (1993) were used to measure the component
fluxes. The positive-head, ceramic-wick, aspirated psychrometer (PCAP) system was used in 1989, 1990, and on the
Nancy site in 1991. The cooled-mirror, dew-point hygrometer (CDH) system was used on the Lower Sheep and
Reynolds Mountain sites during 1991. Energy fluxes were
recorded at 12- and 20-minute intervals with the PCAP and
CDH systems, respectively. Representative clear days are
reported.
Components of the surface energy balance vary significantly in time and space within arid and semiarid ecosystems and have a direct impact on ecosystem processes. An
understanding of these fluxes, their magnitude and dynamics, is important to the study of plant and animal dynamics
and hydrological processes within these ecosystems. The
latent heat flux (LE) or evapotranspiration (ET) is a major
component of the hydrologic cycle and natural resource
models. Technology to measure these fluxes, including LE,
over arid and semiarid landscapes is now readily available
(Wight and others 1993). The purpose of this study was to
determine the energy balance dynamics of sagebrush-grass
Table 1-Site characteristics.
Site
Nancy
Lower Sheep
Upper Sheep (A)
Upper Sheep (8)
Reynolds Mountain
Elevation Slope Aspect
m
Percent
1,444
1,649
1,871
1,884
2,073
4
10
12
14
4
Growing
season
LAI1
3/20 -11/1
4/1-10/1
4/10-10/1
4/20-10/1
5/10-10/1
0.73
0.96
0.42
1.2
1.38
Average annual
precipitation
Dominant vegetation
mm
NE
NW
W
NE
S
295
338
386
491
748
'Leaf area index at peak standing crop.
In: Barrow, Jerry R.; McArthur, E. Durant; Sosebee, Ronald E.; Tausch,
Robin J., comps. 1996. Proceedings: shrubland ecosystem dynamics in a
changing environment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. Rep.
INT-GTR-338. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
J. Ross Wight is a Range Scientist, Clayton L. Hanson is an Agricultural
Engineer, Gerald N. Flerchingeris a Hydrologic Engineer, U.S. Department
of Agriculture, Agricultural Research Service, Northwest Watershed
Research Center, 800 Park Blvd., Plaza IV, Boise, 10 83712.
249
Wyoming big sagebrush Artemisia tridentata wyomingensis
Low sagebrush A. arbuscula
Low sagebrush A. arbuscula
Mountain big sagebrush A. tridentata vaseyana
Mountain big sagebrush A. tridentata vaseyana
Model Simulation
800
The model simulation study used energy balance data
collected during the 1990 growing season from the Upper
Sheep A and B sites. Model simulations were compared with
measured data on an hourly basis.
The energy balance was simulated using SHAW, a simultaneous heat and water model (Flerchinger and Pierson
1991). SHAW is a detailed, physical process model of a
vertical, one-dimensional canopy-snow-residue-soil system
which integrates the detailed physics of heat and water
transfer through a plant canopy, snow, residue, and soil into
one simultaneous solution. It provides daily and hourly
predictions of the surface energy fluxes plus runoff, soil
profiles of temperature and water, and snow and soil frost
depths. SHAW can run using either hourly or daily inputs of
solar radiation, air temperature, wind, and relative humidity. In this study hourly input data were used.
600
Reynolds Mountain (7/3/91)
_ _ _ Rn
t------I
N
~
G
"__________~H
LE
E
400
>- 200
en
a --________________ _
c
Q)
o
-200
)(
::::J
-
1..0-
-400
-600
a
2
E
~
8
10
12
14
16
18
20
22
24
_ _ _ Rn
600
t------I
G
~H
__________ LE
400
Results and Discussion
>- 200
Energy Balance
c
en
6
Lower Sheep Creek (7/3/91)
800
...
4
a __ -------_____ _
Q)
o -200
x
Di urnal variations in the energy balance for three study
sites for July 3, 1991, are illustrated in figure 1. Small
differences among the three sites are reflected in the LE and
Rn components which are associated with the amount of
vegetation and available soil water present. The lowest
values for these components were on the Nancy site which
has less vegetation and dries earlier in the season than the
other two sites. There was an increase in Rs going from the
Nancy and Lower Sheep sites (elevations of 1,444 and 1,649 m,
respectively) to the Reynolds mountain site (2,073 m elevation). Albedo remained relatively constant throughout the
daylight hours.
Seasonal variation in the energy balance for three sites
during a growing season is represented in figure 2. As
expected, Rs increased from spring to mid summer and then
decreased reflecting the change in the Earth's position
relative to the sun.
On clear days, Rn averaged between 40 and 50% of the
incoming solar radiation with the highest values occurring
during spring and early summer before the grasses and forbs
began to senesce. Net radiation was generally higher at
Reynolds Mountain than at the other two sites reflecting
slightly higher Rs and possibly small differences in site
vegetation characteristics.
Albedo was relatively constant throughout the growing
season and among sites averaging between 13 and 17%. The
highest values were recorded on the Nancy site which was
drier than the other sites. This site also contributed the most
energy to downwind advection.
The LE component varied throughout the growing season,
primarily a function of available soil water. The LE to Rn
::::J
-
1..0-
-400
-600
a
2
~
6
8
10
12
14
16
18
20
22
24
16
18
20
22
24
Nancy Gulch (7/3/91)
800
...
E
4
_ _ _ Rn
600
t------I
"
400
G
__________t) H
LE
>- 200
II)
c
0
Q)
0
-200
)(
::::J
-
1..0-
-400
-600
0
2
4
6
8
10
12
14
Figure 1-Diurnal dynamics of the energy balance
fluxes within three sagebrush-grass communities.
ratios varied from more than 0.75 in the spring to less than
0.05 in the fall at Nancy, the driest of the three sites.
Maximum daily ET rates of about 5 mm occurred at the
Reynolds Mountain site.
The G component increased slightly towards mid summer
and then decreased thereafter.
250
34
0
32
0
0
30
0
0
0
28
0
0
26
0
0
24
22
20
18
~
~
~
~
~
1
!
CIl
~
oS
~
~ 0\
!:::
ii=i
>.
~
~
i
z
~
~
j
~
CIl
~
oS
[ ~
~
~
e!
~
j
~
:9
CIl
~
J:
zIi
~
~
~
S
::E
!
oS
0
16
14
~
~
0
.a 12
e
~
10
0
0
0
0
0
0
0
0
8
6
0.3
4
11
11
2
11
11
11
11
11
11
11
0.2
0.1
0
-2
•
•
*
•
*
-4
*
•
•
•
•
*
-6
•
*
*
-8
120
•
*
140
160
180
200
220
240
Day of Year
Figure 2-Seasona~ dynamics of the energy balance
components within three sagebrush grass communities.
(0 = Rs; 0= Rn; ~ = albedo;. = G; *= LE)
Model Simulation
Rn was over 0.95 for both sites. Coefficient of efficiency for
hourly simulated latent heat ranged from 0.61 for the low
sagebrush site (Upper Sheep A) to 0.72 for the mountain big
sagebrush (Upper Sheep B). Measured and simulated ET
accumulated over approximately 25 days of measurement
were 41 and 44 mm, respectively, for the low sagebrush, and
74 and 69 mm for the mountain big sagebrush. Measured
and simulated cumulative ET were within 7%.
Simulated diurnal and daily energy fluxes compared quite
well with measured data. Comparison of measured and
simulated diurnal variation in each component of the surface energy balance is given in figures 3 and 4 for the Upper
Sheep sites. The coefficient of efficiency (for example, the
variation in measured values explained by the model and
analogous to the coefficient of determination) for simulated
251
-r------------------,
1
800
c:
(C
400
600
Flerchinger, G. N.; Pierson, F. B. 1991. Modeling plant canopy
effects on variability of soil temperature and water. Agric. and
Forest Meteor. 56(1991):227-246.
Wight, J. R.; Hanson, C. L.; Wright, J. L. 1993. Comparing Bowen
ratio-energy balance systems for measuring ET, p. 953-960. In:
Allen, R. G.; Neale, C. M. eds. Management of irrigation and
drainage systems: Integrated perspectives. Proc. National Conference of Irrigation and Drainage Engineering. ASeE, New
York, NY.
_::g ;--,--_._--.,--.......--......---......---,-----1
~
200
400
I
References -----------------------------
-20~
-400
--1---.--,---,-----,-----,-----,-----,---/
~
~ -20~
200
-400
-600 -t--......----...,..._-...,..._-....----....---....----r------!
200
o
(!)
-200
-400
-600
--
Simulated
......... Measured
~-...,...---...,...._-...,...._-....__-....__-....__--r---;
176 177 178
179 180 181
182 183 184
Day of Year for 1990
Figure 3-Comparison offield-measured and modelsimulated surface energy fluxes on the Upper Sheep
A low sagebrush site.
1
800,-----------------------~
c:
(C
400
600
_::g ~----r----...,..._-__,__-_._-__,_---r______l
200
400
I
~
-20~
-400 -t---....-----r---.,.--,.----.------r-----!
~ -20~
200
-400
-600
200
~
-t----r---,----,--~-...,...----.....,....----;
o
(!)
-200
-400
-600
- - Simulated
......... Measured
-t----,.---.---r----,----,---,-----!
184
185
186
187
188
189
190
191
Day of Year for 1990
Figure 4-Comparison of field-measured and
model-simulated surface energy fluxes on the Upper Sheep B mountain big sagebrush site.
252