Post on 18-May-2022
An understanding of environmental control on transpiration and its representation in Canadian Land
surface Scheme for two boreal tree species
Shalini Oogathoo
Directeur: Daniel Houle
Co-Directeur: Dan Kneeshaw
26 Avril 2019
1
• Transpiration• important process
• Feeds large amount of water to the atmosphere
2
• Transpiration measured by sapflow techniques and eddy covariance.
Introduction
3
• Adequate representation of transpiration in land surface scheme:
during droughts
• An understanding of the functioning of transpiration (i.e. sapflow) in the natural system:
prior to the growing season during growing season
Introduction
Adds to the understanding of the model pros and cons
4
Temps
ma
i-0
9
juil.
-09
se
pt.-0
9
no
v.-
09
jan
v.-
10
ma
rs-1
0
ma
i-1
0
juil.
-10
se
pt.-1
0
no
v.-
10
jan
v.-
11
ma
rs-1
1
ma
i-1
1
juil.
-11
se
pt.-1
1
no
v.-
11
jan
v.-
12
ma
rs-1
2
ma
i-1
2
juil.
-12
Ra
yon
de
l'a
rbre
(m
m)
0
2
4
6
8
Source: L. Duchesne, 2012
Shrinkage Rehydration
AnnualPlant growthGrowth
• Classification of tree diameter into:
1. winter shrinkage, 2. spring rehydration
(i.e. prior to growing season) and
3. Summer growth.(Tardif et al., 2001)
Three phases:
Introduction
Hypotheses
5
Part I – Hypotheses & Objectives
• VPD and Rad are main drivers of sapflow during the growing season
• Soil water controls sapflow during severe drought events
• Test if VPD and Rad are the main drivers of sapflow
Objectives
• Test the importance of soil water on sapflow
*VPD: vapour pressure deficit; **Rad: solar radiation
• Model may not simulate transpiration (rehydration) prior to the growing season.
• Evaluate the performance of CLASS* for the simulation of transpiration
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Part II – Hypotheses & Objectives
During growing season
During a severe drought event
• Model may underestimate transpiration during drought period.
• Evaluate the performance of CLASS* for the simulation of soil temperature & soil water
Prior to growing season
*Canadian Land Surface Scheme
Hypotheses Objectives
• Study area: two sites (Laflamme and Tirasse) located in Quebec.
7
Part I – Methodology
Laflamme: Balsam fir (3 trees / 2 probes per tree)Tirasse: Black spruce (3 trees / 2 probes per tree
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Sapflow
Climate
Soil θ & T
Dendro
Sonde de
températures
Sonde
d’humidité
du sol
(TDR)
Part I – Methodology
Modelling transpiration
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Part II – Methodology
• Use of Canadian Land Surface Scheme (CLASS)
• Land Surface Scheme for the Canadian Regional Climate Model
• CLASS• Land surface component
• exchange of heat, water and momentum fluxes between land surface and atmosphere
• CLASS inputs:• Climate; Soil; Vegetation
Environmental control during the growing season
10
Part I – Results
VPD and Rad had highest R2
VPD (kPa)
0,0 0,2 0,4 0,6 0,8 1,0
Daily s
ap
flo
w (
cm
)
0
10
20
30
40
50
60
Probe 6
R2: 0.83
VPD (kPa)
0,0 0,5 1,0 1,5 2,0 2,5
Daily s
ap
flo
w (
cm
)
0
10
20
30
40
50
60
70
Probe 3
R2 : 0.81
Relationship of sap flow to vapour pressure deficit (VPD)
11
Balsam fir (2005) Black spruce (2007)
Best-fitted with Gompertz (3 parameters) equation
Part I – Results
Rad (W m-2)
0 200 400 600 800
Daily s
ap
flo
w (
cm
)
0
20
40
60
80
Probe 2
R2 : 0.7
Rad (W m-2)
0 200 400 600 800
Daily s
ap
flo
w (
cm
)
0
20
40
60
80
100
Probe 2
R2: 0.82
Relationship of sap flow to solar radiation (Rad)
12
Balsam fir (2005) Black spruce (2007)
Best-fitted with sigmoidal (3 parameters) equation
Part I – Results
Hourly data: diel hysteresis of sap flow vs VPD and Rad
13
Part I – Results
• Stronger hysteresis with VPD than Rad.
• Hysteresis caused by• Lag between VPD and Rad• Plant and soil water status
Stomatal sensitivity to VPD (a trait of iso/anisohydricity)
14
Part I – Results
• Greater slope = higher stomatal sensitivity to VPD (more isohydry).
• Black spruce is more anisohydry than balsam fir.
• Degree of anisohydricityincrease in 2006 due to extreme heat.
Sap flow during drought
15
Part I – Results
• Rad had highest R2 with sapflowduring the drought period, followed by VPD and precipitation.
• Soil water had a low insignificant R2
for both periods.
Simulated & observed soil temperature
16
• Underestimates in winter slightly for both.• Overestimates in summer for balsam fir
Part II – Results
Simulated and observed soil water
17
• Simulation follows the pattern of observations for both horizons for black spruce, except during winter for horB.
• CLASS converts all liquid water into frozen water
Part II – Results
• For balsam fir: simulation is not good, especially for horizon B
Simulated & observed transpiration prior to and during growing season
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Balsam fir Black sprucePrior to growing season
CLASS simulate well during growing season, but not prior to growing season
Part II – Results
19CLASS does not simulate well from April to May
Part II – Results
R2 : 0.82 R2 : 0.91
Simulated & observed transpiration prior to and during growing season
Simulated transpiration during drought
20
Balsam fir - 2012
Shaded area – drought period
CLASS overestimates transpiration in 2nd
half of drought period
Part II – Results
Conclusion
Hypothesis II was rejected: soil water did not take over control of sapflow
during drought as expected
(suggesting an anisohydric behaviour of balsam fir)
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Our hypothesis I was confirmed: VPD and Rad were found to be the main drivers
of sapflow during the growing season
Conclusion
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• Adequate performance of CLASS for soil temperature
But, important consideration:
soil water varies greatly spatially
• CLASS needs to be improved for the simulation of soil water
• CLASS simulates well transpiration
• CLASS poorly simulates transpiration
During growing season
Prior to growing season &
During drought
Conclusion
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While CLASS overestimates transpiration during drought, other land surface models underestimated transpiration during drought (Ukkola et al., 2016), In spite of the misrepresentation
of anisohydric species in CLASS
Thus, CLASS coupled with Canadian Regional Climate Model will underestimate drought events in future.
Land Surface Scheme being the main driver for climate model, a misrepresentation of transpiration will lead to inaccurate projection of climate model
Merci
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