J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21 9
Table 6 The decrease in transpiration and Ω for the 12 corn scenarios
a
Scenario description Vegetation type — corn
1 TR
Ω LSX
current
PSU
current
LSX
field
PSU
field
LSX
current
PSU
current
LSX
field
PSU
field
Standard 9–10
6–11 46–54
28–40 0.64–0.71
0.64–0.81 0.75–0.77
0.76–0.82 Medium soil water content
9–10 6–11
46–54 27–39
0.64–0.71 0.64–0.82
0.75–0.77 0.76–0.83
Low soil water content
b
8–10 5–10
44–52 26–39
0.66–0.75 0.66–0.83
0.76–0.79 0.77–0.84
High wind 10–12
6–14 50–57
32–45 0.57–0.66
0.50–0.80 0.73–0.75
0.74–0.80 Low wind
6–8 4–7
39–48 21–30
0.74–0.80 0.76–0.87
0.78–0.81 0.82–0.87
High surface layer humidity 9–10
6–9 46–55
29–37 0.55–0.67
0.69–0.80 0.75–0.77
0.78–0.82 Low surface layer humidity
9–10 6–14
46–54 29–41
0.61–0.65 0.60–0.82
0.76–0.77 0.75–0.83
Reduced solar irradiance 11–12
14–15 58–62
47–48 0.52–0.60
0.50–0.55 0.80–0.81
0.76–0.77 Coupled canopy
15–16 10–16
55–60 39–50
0.46–0.52 0.46–0.66
0.70–0.71 0.70–0.75
Decoupled canopy 6–7
4–6 31–46
19–27 0.76–0.81
0.77–0.86 0.79–0.85
0.84–0.88 Biomass increase
2–4 3–7
41–50 25–36
0.84–0.94 0.70–0.90
0.76–0.78 0.77–0.84
High surface layer temperature 7–8
1–10 43–50
18–37 0.74–0.76
0.67–0.95 0.77–0.78
0.77–0.88
a
The table presents ranges of the respective values — any important differences and diurnal trends are discussed in the text. The diurnal trend of the decrease in transpiration and Ω for LSX follow generally the same pattern during the day in all the scenarios. The
decrease in transpiration had its maximum value early in the morning and in the late afternoon with a minimum value during the middle of the day. The decoupling coefficient, Ω, obtained its maximum value during the middle of the day with minimums in the early morning
and later afternoon. In PSUBAMS, the decrease in transpiration and Ω also follow a similar diurnal trend in all scenarios. The decrease in transpiration had its maximum value early in the morning and decreased to its minimum value late in the afternoon. The decoupling
coefficient, Ω, had its lowest value early in the morning and increased to its maximum value late in the afternoon. Percent increases in stomatal resistance are not shown as the increase ∼30 did not vary between scenarios.
b
Indicates scenario was conducted under non-water stress conditions.
to 0.8 for the majority of a day with a rapid drop-off later in the afternoon towards zero.
A particularly useful yardstick in understanding changes in Ω under varying environmental conditions
is the magnitude of the ratio between the stomatal resistance to that of the total aerodynamic resistance
r
s
r
a
. This ratio is important for two reasons. First, the greater r
s
r
a
, the larger a given percent increase in stomatal resistance imposed to simulate doubled
[CO
2
] will impact upon the transpiration. This is so since an equivalent imposed percent increase in
stomatal resistance increases the absolute value of stomatal resistance r
s
if it is initially large, more than if it is initially small and therefore produces
more of a change in the total resistance from the leaf through the surface layer Jarvis and McNaughton,
1986; McNaughton and Jarvis, 1991; Steduto and Hsiao, 1998a–c. This idea is important when viewing
the differences between corn and soybeans using the field derived parameterizations where values for the
minimum stomatal resistance are very different. Sec- ond, r
s
r
a
varies substantially in the other scenarios where the environmental conditions alter r
a
so that the sensitivity of transpiration to an increase in stomatal
resistance varies. Therefore, the impact of this ratio is evident in Tables 6 and 7 when viewing the differences
in Ω between scenarios. For example, the sensitivity of transpiration is found to be different between high
and low wind conditions as a result of r
s
r
a
being greater in windy conditions lower r
a
than in calm conditions.
3. Results
3.1. Corn Fig. 1 illustrates Ω for all four cases. The LSX
current
and PSU
current
values demonstrate more sensitivity in the transpiration to a doubling of [CO
2
] than LSX
field
and PSU
field
. LSX
current
shows that Ω varies from a morning value of 0.64 to a maximum of 0.72 at
12:30 p.m. On the other hand, PSU
current
illustrates a gradual increase in Ω from 0.63 to 0.80 by late after-
10 J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21
J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21 11
Fig. 1. The diurnal variation in Ω for all four cases for corn.
noon which corresponds to a gradual lessening of the transpiration decrease during the same time. The field
derived cases produce a similar pattern with higher ranges of values of 0.75–0.77 LSX
field
and 0.77–0.82 PSU
field
. These values for Ω are within the range of values cited by Steduto and Hsiao 1998c for their
corn canopy measurements. What is most interesting between their study and the modeled results presented
here is the trend of Ω as the afternoon progresses. The modeled values of Ω either remain generally
constant LSX
current
and LSX
field
or gradually in- crease during the afternoon PSU
current
and PSU
field
while the measurements of Steduto and Hsiao 1998c show a distinct decrease in Ω during the afternoon
indicating a large degree of coupling later in the afternoon.
These time series of Ω are equivalent to a per- centage ratio in the range of 28–36 and 20–37
for LSX
current
and PSU
current
and, 23–25 and 18–23 for LSX
field
and PSU
field
. LSX
current
and PSU
current
was where the decrease in transpiration was the least, with magnitudes ranging from 9 to
10 LSX and 6 to 11 PSUBAMS. Although not shown, LSX transpiration remained essentially
constant during the day while PSUBAMS showed a gradual lessening in the transpiration decrease with
a minimum occurring in the late afternoon around 4:00 p.m. LSX
field
and PSU
field
, however, had a greater decrease in transpiration ranging from 46 to
55 LSX and 28 to 40 PSUBAMS with larger differences between both models. The decrease in
transpiration for LSX
field
and PSU
field
was greater than LSX
current
and PSU
current
because the stom- atal resistance increase from present day to doubled
[CO
2
], expressed as a percentage was much greater refer Table 2.
12 J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21
Fig. 2. The diurnal variation in Ω for all four cases for soybeans.
It is important to note that the trends in Ω, as previ- ously described, that occur over the course of the day
were equivalent in all the corn scenarios as outlined in Table 6. Only the change in the magnitude of Ω
changed and is of consequence. The results shown in these tables are consistent with the findings from Ste-
duto and Hsiao 1998a–c for varying environmental conditions as viewed from the departure from a stan-
dard set of conditions. For example, under simulated cloudy conditions reduced solar irradiance, modeled
values of Ω were lower than the standard scenario and ranged from 0.50 to 0.60. Low wind conditions simu-
lated values of Ω spanning 0.74–0.87 greater than the standard scenario while a ‘coupled’ canopy scenario
sparse canopy with small leaves under high winds — Table 5 indicated values of Ω ranging from 0.46 to
0.66 less than the standard scenario. 3.2. Soybeans
Fig. 2 illustrates the sensitivity of transpiration Ω for all four cases. The plot clearly shows that Ω is
greater in magnitude for the field derived SRP cases LSX
field
and PSU
field
than that for the cases using the current SRP’s LSX
current
and PSU
current
. LSX
current
and PSU
current
simulated ranges of Ω from 0.61 to 0.65 and 0.63 to 0.80, respectively, while for LSX
field
and PSU
field
the range was 0.86–0.87 and 0.80–0.87. These time series of Ω illustrate that the percentage ratio was
in the range of 35–39 and 20–39 for LSX
current
and PSU
current
and, 13–14 and 12–20 for LSX
field
and PSU
field
. The magnitude of the decrease in transpiration
shown in Fig. 3 for LSX
current
and PSU
current
ranged from 13 to 20 and 5 to 11, respectively. Fig. 3
J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21 13
Fig. 3. The decrease in transpiration for all four cases for soybeans.
is illustrated as it manifests some interesting diurnal differences that were not evident in the equivalent
corn transpiration plot. These trends are an increase in the transpiration decrease during the day in LSX
and a reduced decrease in transpiration during the middle of the day in PSUBAMS. For soybeans, the
decrease in transpiration in LSX increases during the day whereas the decrease remained more-or-less
constant for corn. For PSUBAMS, the systematic decrease after 11:00 a.m. observed for corn is not ap-
parent for soybeans which reverses this trend around 1:00 p.m. These trends in transpiration decrease are
due to marked differences in the stomatal resistance increase from present day to doubled [CO
2
] during the day. Such variations were not observed in the
corn simulation. The reason for such differences in stomatal resistance are described in the discussion
section. The decrease in transpiration for LSX
field
and PSU
field
range from 38 to 41 and 30 to 38, respec- tively, and are greater than LSX
current
and PSU
current
due to a larger stomatal resistance increase. This stom- atal resistance increase is much greater than the 32
increase in resistance observed solely from measure- ments of minimum stomatal resistance. It is caused
by the increase in f VPD and f T from present day to doubled [CO
2
], especially by f T refer Table 4. Under the initial conditions chosen for these
scenarios, the foliage temperatures were below 303 K 30
◦
C so that a large increase in f T and, therefore, in stomatal resistance occurred in the majority of these
scenarios when incorporating the field derived pa- rameterization into LSX and PSUBAMS. Wilson and
Bunce 1997 also showed large increases in stomatal resistance via an increase in f T for cool summer
days when using equivalent relationships for f T.
14 J.C. Gottschalck et al. Agricultural and Forest Meteorology 106 2001 1–21
4. Discussion
