S.G. Li et al. Agricultural and Forest Meteorology 102 2000 125–137 129
z
o
= 0.016u
2 ∗
g 10
where g is the gravitational constant. 2.3. Data processing
The following criteria were adopted for data se- lection and analysis. 1 Most data were observed in
fine days or in a bit cloudy days. 2 Wind directions were mainly southwest to south 180–240
◦
. In case of the ungrazed plot, wind angles were within 194–232
◦
. 3 Data obtained when wind speed was smaller than
0.5 m s
− 1
were rejected. 4 Missing meteorological data were filled via interpolation between earlier and
later measurements. 5 Mean values of 30 min in day- time were used for analysis.
3. Results
3.1. Reflectivity albedo As shown in Fig. 2 and Table 2, the reflectivity of
the unfenced grassland Plot E in 1991 was higher in May 25 due to lower vegetation coverage 20,
whereas it became lower in August when plants were growing densely with higher coverage 70. After
enclosure in 1992, albedo was slightly decreased at the ungrazed plot Plot A and the lightly grazed plot
Plot B, increased little at the moderately grazed plot
Fig. 2. Changes in albedo of the plots A, B, C, D, and E. An enclosure treatment was started in 1992.
Plot C, and increased substantially at the overgrazed Plot D. Consecutive observations after the substan-
tial rain events demonstrated that albedo decreased by 3–5 at Plot B, 18–23 at Plot D, and 14–17 at
Plot F in comparison with those before the rains. It took less time for the albedo to return to its pre-rainfall
values at Plots D and F than at Plot B when met a clear day with a bit strong wind after rain. Albedo was
satisfactorily expressed as negative exponential func- tions of aboveground biomass and vegetation coverage
Fig. 3, independent of the plots and the measurement time.
3.2. Heat budget components Table 3 shows daytime integrated values of energy
components and partitioning of the net available radi- ation into the sensible heat and latent heat fluxes. The
thermal properties of grassland surfaces demonstrated following changes:
1. Inter-seasonally, ratios of net radiation, R
n
, or net available energy, R
n
–G, to incoming solar radia- tion, R
s
, tended to decrease with increase of graz- ing intensity. For example, the fractions of the net
available radiation versus solar radiation in August 1994 were 55 at Plot A 4 August 1994, 53 at
Plot B 9 August 1994, 36 at Plot C 23 August 1994, and 20 at Plot D 11 August 1994, re-
spectively. Intra-seasonally, these ratios were lower at the beginning of plant growth May and June,
dry seasons and higher during rapid growth peri- ods July and August, rainy seasons.
2. In the heat budget, the contribution of latent heat exchange, expressed as evaporative fraction, i.e.,
lER
n
–G, tended to decrease with increasing in- tensity of grazing. For example, it was 37 at Plot
A 10 June 1994, 20 at Plot C 4 June 1994, and 15 at Plot D 2 June 1994, respectively. There
was only 0.3 mm rainfall in nearly 2 weeks before these measurements were done, indicating similar
soil surface conditions at these plots.
3. The mobile sand dune, which can be regarded as an extreme of the desertified grassland, had lower
values of R
n
R
s
or R
n
–GR
s
due to higher albedo as compared to grassland.
4. Removal of grazing or decrease of grazing in- tensity seemed to increase values of R
n
–GR
s
130 S.G. Li et al. Agricultural and Forest Meteorology 102 2000 125–137
Table 2 Days after rains Ds, data in parenthesis are amount of rainfall in mm, rainfalls within 10 days before measurements P
10d
in mm, mean albedo from 10 a.m. to 15 p.m. Ad in , mean canopy heights H
c
in cm, mean vegetation cover VC in , daytime mean soil temperatures at 0.01 m depth St
0.01
,
◦
C, daytime mean wind speeds at 2.5 m above the surfaces u
2.5
m s
− 1
, roughness lengths z
o
, cm and the drag coefficients in daytime C
d
at the measured sites are also shown for later discussion
a
Plots Dates
Ds P
10d
Ad H
c
VC St
0.01
u
2.5
z
o
C
d
days mm mm
cm
o
C m s
− 1
cm A
10 June 1994 3 0.6
0.7 22
45.0 65
31.2 3.7
3.26 0.0073
1 July 1993 3 24.5
30.3 18
40.0 85
28.0 5.3
4.34 0.0087
4 August 1994 2 0.9
6.6 19
65.0 90
32.1 4.9
12.86 0.0137
8 August 1993 3 19.6
98.3 18
55.0 90
26.8 2.6
1.61 0.0051
22 August 1992 1 15.9
15.9 20
29.0 90
2.14 3.3
26.8 0.0079
B 8 June 1994
1 0.6 0.6
22 35.0
60 30.1
4.9 5 August 1993
3 52.1 81.9
22 25.0
80 28.2
2.4 0.41
0.0033 9 August 1994
4 62.0 62.9
20 50.0
80 30.4
3.3 2.27
0.0062 10 August 1992
1 1.1 41.7
23 24.0
75 29.6
2.6 1.15
0.0055
C 4 June 1994
7 0.3 0.3
25 20.0
30 28.8
3.8 2.60
0.0064 8 August 1992
5 39.1 41.3
24 14.0
50 28.0
2.9 23 August 1994
9 13.6 45.9
21 25.0
65 35.6
3.7 0.53
0.0038 D
2 June 1994 5 0.3
0.3 30
5.0 10
31.3 4.5
0.19 0.0026
30 July 1993 1 32.1
b
32.1 18
12.0 20
29.3 2.6
6 August 1992 3 39.1
40.6 25
12.0 30
31.0 3.6
11 August 1994 2 7.4
70.3 31
10.0 10
30.1 2.2
0.32 0.0034
E 18 May 1991
2 4.2 13.7
25 7.5
20 25.8
2.8 0.45
0.0041 21 June 1991
5 4.8 59.0
22 15.0
40 32.5
4.4 0.98
0.0043 2 August 1991
3 15.2 62.7
20 15.0
70 32.1
3.7 0.86
0.0045 13 September 1991
6 1.9 102.6
20. 15.0
70 25.0
3.4 0.34
0.0035 F
12 June 1994 5 0.7
0.7 34
4.3 0.003
0.0019 16 July 1991
2 2.5 6.2
33 5
35.0 3.9
0.004 0.0024
2 August 1994 6 5.7
7.5 35
5 3.5
0.001 0.0029
18 August 1992 4 0.0
6.8 35
5 32.4
2.1 0.002
a
Plots A, B, C, D, E, and F represent the ungrazed plot, the lightly grazed plot, the moderately grazed plot, the overgrazed plot, the unfenced grassland, and the mobile sand dune, respectively.
b
There was also a 10.2 mm rainfall during the measurement.
and lER
n
–G compared with those values before enclosure.
5. These changes above-mentioned were subject to or affected by such factors as vegetation status and soil
moisture regime. Higher vegetation cover often in- creased values of R
n
–GR
s
and lER
n
–G through altering surface albedo. Rain increased R
n
R
s
and lER
n
ratios through wetting soil Fig. 4, especially at the mobile dune Plot F, not shown and at the
overgrazed plot Plot D. For example, the R
n
R
s
was increased about 3 at Plot B 1 day after rain 7 August 1994, and it was increased about 20
at Plot D 1 day after rain 14 August 1994. 3.3. Wind profiles
Fig. 5 shows daytime wind profiles over the grazing experimental plots and the mobile dune. Wind profiles
above the grassland were similar to those in neutral conditions, i.e., the ideal logarithmic-linear distribu-
tion shape, whereas the wind profiles over the dune were typical lapse ones in unstable conditions that
deviate significantly from the log-linear form. Wind regimes over the grassland were altered due to grazing.
Wind speeds near the surfaces increased with increase of grazing intensity that caused by changes of rough-
ness length. But this pattern of wind increase was not
S.G. Li et al. Agricultural and Forest Meteorology 102 2000 125–137 131
Fig. 3. Relationships between the vegetation coverage VC, and albedo AD, , and between the aboveground biomass
W
d
, dry weight, g m
− 2
and albedo. The vegetation coverage was eye-estimated with the quadrat method. The aboveground
biomass is monthly averaged value obtained by the quadrat har- vest method. Letters A, B, C, D, E, and F, represent the un-
grazed plot, the lightly grazed plot, the moderately grazed plot, the overgrazed plot, the unfenced grassland, and the mobile sand
dune, respectively. The solid lines shown are the lines of best fit with Ad=33.11 exp−0.0067VC n=25, r
2
= 0.90 and Ad=28.35
exp−0.0022W
d
n=23, r
2
= 0.57.
evident in June when the experiment started and the canopy heights were lower, and it was very obvious
in August when sustained grazing had occurred and vegetation was in well-grown status.
Linear relationships between mean wind speeds at the highest measurement point, u
5.0
m s
− 1
and fric- tion velocity, u
∗
m s
− 1
in daytime over the grazing experiment plots and the mobile dune are shown in
Fig. 6. Friction velocity linearly increased with in- crease of wind speed. For a given value of u
5.0
, friction velocity decreased with increasing grazing intensity.
As a result of changes of canopy structure, e.g., de- crease of vegetation cover and canopy height, caused
by grazing, slope of the regression equations decreased with increase of grazing intensity. This relationship
can also be expressed as the drag coefficient, which was defined as C
d
= u
∗
u
5.0 2
. C
d
decreased with in- crease of grazing intensity as shown in Table 2. For
example, C
d
was 0.0137 at Plot A 4 August 1994, 0.0062 at Plot B 9 August 1994, 0.0038 at Plot C
23 August 1994, and 0.0034 at Plot D 11 August 1994, respectively. C
d
of the mobile sand dune was lower than those of the grazing plots.
Also shown in Table 2 are the surface roughness lengths, z
o
of the measured sites, which decreased with increasing grazing intensity. For example, z
o
was 12.86 cm at Plot A 4 August 1994, 2.27 cm at Plot B 9 August 1994, 0.53 cm at Plot C 23 Au-
gust 1994, and 0.32 cm at Plot D 11 August 1994, respectively.
4. Discussion and conclusions
