Persistent Cold Air Drainage and Modeled Nocturnal Leaf Water
Potential in Complex Forested Mountainous Terrain
Lin Gray, Jason A. Hubbart, Kathleen Kavanagh, Timothy E. Link, Robert Pangle
University of Idaho, Moscow Idaho, USA.
Elevation (m)
1380
1340
1320
1300
1280
3
4
5
6
7
8
Air Temperature Sensor
Aspect
Categorized
N
N
N
N
N
N
N
N
ELR Inversion (88m)
Full Slope
(155m)
25.16
28.00
30.00
30.19
28.97
28.90
28.32
27.55
26.84
26.52
Departure from
AELR (degC)
31.66
34.50
36.50
36.69
35.47
35.40
34.82
34.05
33.34
33.02
D Full slope (155m)
50
0.5
0.4
Ni h
23:00
20:00
Figure 3. Nocturnal stomatal
conductance for Douglas-fir
trees numbered 2, 3, and 5.
The gray bar on the X-axis
represents nighttime hours.
n = 4 to 8; bars are SE.
0.0
0
-0.1
-10
-20
0:00
4:00
8:00
12:00
16:00
-0.2
20:00
Time
Figure 2. Vapor pressure deficit (D) and saturated
vapor pressure esat, based on ambient vapor
pressure (ea, kPa) and ambient air temperature
(Ta, °C).
Change in VPD (kPa) Bottom
to Top of Slope
Departure from
AELR (degC)
49.68
54.91
57.98
57.64
56.16
55.59
55.25
54.11
52.98
53.20
Full Slope
(155m)
0.40
0.50
0.50
0.50
0.40
0.40
0.50
0.40
0.40
0.40
4:00
8:00
Time (Hr)
Figure 4. The nocturnal rise in temperature and D
with elevation, and the return to closer to normal ELR
(i.e. -6.5 ºC) during daylight hours.
24 per. Mov. Avg. (Ave degC(1290-1445m))
24 per. Mov. Avg. (D (kPa) )
24 per. Mov. Avg. (Full Forest ea (kPa))
24 per. Mov. Avg. (Ave esat (kPa))
Inversion Layer
(88m)
0.40
0.50
0.50
0.50
0.40
0.40
0.50
0.40
0.40
0.40
ELR=environmental lapse rate (conventionally -6.5degC/1000m)
o
20.0
15.0
10.0
5.0
• Vapor pressure deficits
closely tracked
topographic variation in
temperature (Figures 2,
4, 5).
• On average, D increased
by at least 0.4 kPa from
the bottom to the top of
the slope during all
nocturnal hours (Figure 5).
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
25.0
192
202
212
222
232
242
1350
Figure 1. The Mica Creek Experimental Watershed (MCEW) in northern Idaho.
western larch
41.6 (3.7)a
1.34 (0.14)a
Douglas-fir
28.1 (3.5)b
0.93 (0.07)ab
western redcedar
12.9 (3.6)b
0.99 (0.05)b
5
19
0
20
5
20
0
21
5
21
0
22
5
22
0
23
5
23
0
24
Figure 6. Daily minimum vapor pressure
deficit (D) at three elevation bands.
1450
Max. Vapor Pressure Deficit July and Aug
LAOC
1400
LAOC
=-0.3
=-0.6
=-0.9
=-1.2
1350
1300
• Topographic shifts in D can influence
transpiration rate and thus the ability
of Ψpd to come into equilibrium with
Ψs if stomata are not closed.
1450
1400
PSME
PSME
=-0.3
=-0.6
=-0.9
=-1.2
1350
1300
1450
1400
THPL
THPL
=-0.3
=-0.6
=-0.9
=-1.2
15
20
0.5
1.0
-1.50
-1.25
-1.00
-0.75
-0.50
-0.25
-1.75
-1.50
-1.25
-1.00
-0.75
-0.50
-0.25
0.00
1.5
Mean Vapor Pressure Deficit (KPa)
o
Mean temp ( C)
Conclusions
• These results showed that at this scale (i.e., < 1km), the observed lapse
rates could lead to highly variable nocturnal transpiration.
• The increase in D due to changing elevation and inverted lapse rate leads to
the potentially erroneous conclusion that soil moisture might be more limited
at higher elevations relative to lower elevations.
1350
Literature Cited
20
30
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Max Vapor Pressure Deficit (kPa)
Max Temp ( C)
• Modeled Ψpd commonly Figure 5. Environmental lapse rates (ELR) based on
was lower than Ψs (model mean and maximum temperatures, and corresponding
set-point)
D for July and August 2004. Dashed line represents
the normal assumed ELR (-6.5 ºC/1000m).
Table 6. Minimum Ta, nocturnal environmental lapse
rate (ELR) and vapor pressure deficits (D).
Change in D (kPa)
ELR (ºC)/1000m
Time
20:00
21:00
22:00
23:00
0:00
1:00
2:00
3:00
4:00
5:00
-1.75
initial soil water potential and elevation.
10
o
KL (mmol m-2 s-1 MPa-1)
0
19
Day of Year
• Strongest temperature inversions
develop under cloudless conditions.
1300
10
gs-noc (mmol m-2 s-1)
5
18
Modeled Predawn Leaf Water Potential (MPa)
1300
Species
0.00
-2.00
1400
1400
• Modeled Ψpd became
even more negative with
increasing elevation
(Figure 7).
0.25
1300
0400
0800
1200
2000
1450
• Minimum nocturnal D
seldom fell to 0.0 kPa
(Figure 6)
• The shift in D, from the bottom to
the top of the slope on an average
night resulted in a - 0.11 MPa
decline in modeled Ψpd for
Douglas-fir and western larch, and
a - 0.06 MPa decline for western
redcedar (Figures 5, 6).
• Results imply that if one is estimating
Ψs from Ψpd, and assumes that
stomata are closed, Ψs may be
Figure 7. Modeled responses of predawn
incorrectly estimated due to
leaf water potential in western larch,
nocturnal transpiration.
Douglas-fir and western red cedar to
1450
VPD=vapor pressure deficit (kPa)
Table 3. Species and parameters used to assess disequilibrium between modeled Ψpd and Ψs. Nocturnal
stomatal conductance (gs-noc) and leaf specific
conductance (KL) for several conifer species at Priest
River Experimental Forest in N. Idaho. Data was
collected in July and August 2005. Numbers in
parentheses are standard errors and letters indicate
significant differences (p < 0.05) Kavanagh et al. In Press.
0.75
1350
AELR=assumed environmental lapse rate
• ELR’s were persistently
inverted (Table 2).
• ELR most strongly
inverted around 00:00,
±2hrs.
• The strongest cold air
layer persisted in the
lower 88 m of the slope,
which was on average
63% stronger than the
inverted lapse rates
calculated over the
whole slope (Figure 4).
1.00
Mean Vapor Pressure Deficit July and Aug
Day of Year
Elevation (m)
0.1
10
1.25
0.50
0
0.0
182
D (kPa)
0.2
20
1445m
1334m
1290m
1.50
25
0.3
30
1.75
Minimum D (kPa)
75
• Nocturnal conductance
is approximately 30% of
daytime maximum
(Table 3, Figure 3).
D Inversion (88m)
50
40
o
100
30.0
60
Inversion Layer
(88m)
43.18
48.41
51.48
51.14
49.66
49.09
48.75
47.61
46.48
46.70
125
Temperature ( C)
ELR Full Slope (155m)
• Average Ta for July and
August was 17.1 °C and
16.0 °C respectively.
Time
20:00
21:00
22:00
23:00
0:00
1:00
2:00
3:00
4:00
5:00
1360
2
Aspect
(degrees)
22
22
22
22
22
22
22
22
ELR (degC)/1000m
1400
1
Slope (percent)
29
29
29
29
29
29
29
29
Results
Table 2. Mean temp lapse rates.
1420
Fully Forested Control
Local Aspect
(degrees)
130
26
27
16
24
18
31
34
• Average TaMin and TaMax
was 4.6 °C and 6.8 °C, and
18.3 °C and 28.7 °C for
each respective month.
1440
Temperature transect
Local Slope
(percent)
12
19
29
40
34
38
36
4
ELR ( C)
Study Site – Mica Creek Experimental Watershed
• Elevation range: 975 – 1725 m.
• Size of experimental watershed:
27 km2 (Fig. 1).
• Climate: Continental maritime
• Mean ppt ~ 1450 mm.
• Most ppt occurs as snow (>
70%) from November through
May
• Mean annual air temperature:
4.5ºC (Hubbart et al. in Press).
• Vegetation: ~ 80 year old naturally regenerated mixed conifer.
Elev (m) Treatment
1445 Partial cut
1424
Control
1400
Control
1378
Control
1354
Control
1334
Control
1309
Control
1290
Control
• Increases in temperature with
elevation resulted in an increase in
nighttime D of 0.8 to 1.2 kPa from
the bottom to the top of the slope
(Table 6).
Tree 2
Tree 3
Tree 5
150
Elevation (m)
Table 1. Temperature data logger placement and slope characteristics.
• Measure vertically stratified temperature data in complex mountainous terrain
of the continental/maritime climate region of the U.S.
• Assess the magnitude and persistence of temperature inversions.
• Estimate stratified vapor pressure deficit (D) values at multiple elevation
bands across complex mountainous terrain.
• Model the topographic pattern of leaf water potential (Ψpd), as a function of
distributed D.
• Assess the validity of using predawn ΨL as a surrogate for Ψs.
Discussion/Conclusions
175
Elevation (m)
Study Objectives
Results Continued
esat , ea, D , (kPa)
Methods
• Shielded temperature data loggers (Thermochron iButton®) were installed in
transect at 20 vertical meter distances from peak to base (155 vertical m) on a
fully forested slope (Fig 1, Table 1) during the months of July and Aug 2004.
• Air temperature lapse rates were calculated to quantify lapse rates.
• Stomatal conductance (gs) was measured on foliage clipped from the upper
canopy of 90yr old Douglas-fir, western larch and western redcedar using a
porometer (LiCor 1600) from 20:00 to 08:00.
• Degree of disequilibrium between soil and leaf water potentials for Douglasfir, western larch, and western redcedar was modeled.
gs-noc (mmol m2 s1)
Introduction
When atmospheric conditions are stable, cold air will often accumulate in lowlying areas, resulting in a temperature inversion.
• Several temperate conifer species exhibit stomatal opening at night and thus
when nocturnal vapor pressure deficit (D) exceeds 0.1 kPa transpiration
occurs (Kavanagh et al In Press).
• If transpiration is occurring at night then leaf water potential (ΨL ) is not in
equilibrium with soil water potential (Ψs). The degree of disequilibrium is
directly linked to D which is tightly coupled to the temperature inversion
(Hubbart et al. in Press).
Full Slope
(155m)
33.29
45.29
50.00
45.68
43.61
40.32
38.97
39.68
42.65
44.32
Departure from
AELR (ºC)
39.79
51.79
56.50
52.18
50.11
46.82
45.47
46.18
49.15
50.82
Inversion Layer
(88m)
58.86
75.68
81.82
75.11
73.07
70.80
70.23
72.73
71.25
77.16
ELR=environmental lapse rate (conventionally -6.5ºC/1000m)
AELR=assumed environmental lapse rate
D=vapor pressure deficit (kPa)
Departure from
AELR (ºC)
65.36
82.18
88.32
81.61
79.57
77.30
76.73
79.23
77.75
83.66
Full Slope
(155m)
0.80
1.00
1.20
1.00
0.90
0.90
0.80
0.90
0.90
0.90
Inversion Layer
(88m)
0.80
1.00
1.10
1.00
0.90
0.80
0.80
0.90
0.80
0.90
• Anquentin, S., C. Guilbaud and J.P. Chollet. 1999. The formation and destruction of inversion layers
within a deep valley. J. Applied. Meteor. 37:1547-1560.
• Bond, B.J., and K.L. Kavanagh. 1999. Stomatal behavior of four woody species in relation to leafspecific hydraulic conductance and threshold water potential. Tree Physiol. 19:503-510.
• Hubbart J.A., Link TE, Gravelle JA, Elliot WJ (in Press). Timber Harvest Impacts on Hydrologic Yield in
the Continental/Maritime Hydroclimatic Region of the U. S. In: Special Issue on Headwater Forest
Streams, Forest Science.
• Hubbart, J.A., K.L. Kavanagh, R. Pangle, T.E. Link, and A. Schotzko. In Press. Cold air drainage and
modeled nocturnal leaf water potential in complex forested terrain. Tree Physiology.
• Kavanagh, K.L., R. Pangle and A.D. Schotzko. In Press. Nocturnal transpiration causing disequilibrium
between soil and stem predawn water potential in mixed conifer forests of Idaho. Tree Physiology.
Acknowledgments
• The authors wish to express gratitude to Potlatch Corporation for designing and
implementing the MCEW and for access to the experimental site. This research was
carried out with funding provided by the United States Department of Agriculture,
USFS Research Joint Venture Agreement #03-JV-11222065-068, USDA-CSREES
2003-01264, NSF-Idaho EPSCoR Program and by the National Science Foundation
under award number EPS-0132626, NSF-REU and McIntire-Stennis Funds.