School
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School ofof
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Ecosystem Scale
Evapotranspiration
Measurement in
Temperate Rainfed
Agriculture
Andrew E. Suyker
University of Nebraska-Lincoln
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of Nebraska-Lincoln
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Water in Bioenergy
Agroecosystems Workshop
June 12-13, 2012
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Objectives
● Quantify evapotranspiration
(ET) of rainfed maize
● Examine some key factors
controlling ET
● Quantify water productivity
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of Nebraska-Lincoln
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● Discuss ET measurement
related issues
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Measurement Sites:
Mead, NE
Rainfed Maize-Soybean
Irrigated Continuous Maize
Irrigated Maize-Soybean
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ET (mm d-1)
8
M
S
M
S
M
S
M
S
M
S
M
6
4
2
0
1/1/01 1/1/02 1/1/03 1/1/04 1/1/05 1/1/06 1/1/07 1/1/08 1/1/09 1/1/10 1/1/11 1/1/12
Rainfed Maize-Soybean Rotation
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ET Summary: Rainfed Maize
Year
2001
2003
2005
2007
2009
2011
Annual
ET
GS
ET
NGS
ET
mm y-1
662
615
611
623
575
607
mm
488
467
476
472
450
481
mm
174
148
136
151
125
126
NGS
GS
portion Precip
%
26
24
22
24
22
21
mm
510
350
340
606
434
502
Yield
Residue
Mg ha-1
7.37
6.52
7.69
8.64
10.14
8.22
Mg ha-1
8.43
6.57
9.07
7.63
11.14
9.12
Growing Season: May - September
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Variability of daily ET:
Growing
Season
Rainfed Maize
1.6
R2 = 0.66
ET/ETo
1.2
0.8
0.4
Adequate Moisture
Dry Periods
0.0
0
2
4
LAI
6
(m2m-2)
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ETo: Reference ET (Allen et al., 1998)
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Non growing season ET
Non-growing season E/Eeq
0.9
R² = 0.71
0.8
2001-02
0.7
2002-03
2003-04
0.6
2004-05
Irrigated and Rainfed Maize
and Soybean
2005-06
0.5
0.0
0.5
1.0
1.5
Surface
Mulch
Biomass
(Mg/ha)
Surface
Residue
(Mg/ha)
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2.0
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ET from Rainfed Maize
(Growing Season)
Nebraska: 450-590 mm
Illinois:
611 mm
Kansas: 411-480 mm
Iowa:
350-500 mm
Irrigated Maize (Mead): 500-580 mm
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ET from other Biofuel Crops
(Growing Season)
Rainfed Miscanthus: 960 mm
(Hickman et al., 2010)
Switchgrass: 760 mm
(Hickman et al., 2010)
Sweet Sorghum: 160-515 mm
(Stricevic et al., 2011)
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Water Productivity
WP = Dry above ground biomass
∑ET
Maize
Soybean
20
Dry Biomass (Mg ha-1)
ET (mm d-1)
8
6
15
4
10
2
5
0
1/1/07
1/1/08
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0
12/31/08
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Normalized Water Productivity
Normalized WP by vapor pressure deficit (D)
WPD = aboveground biomass
∑(ET/D)
D: daytime average VPD
Tanner & Sinclair (1983):
For irrigated and rainfed maize, LAI>2 (Mead)
WPD = 6.9 ± 0.7 Pa
For irrigated and rainfed soybean, LAI>2, (Mead)
WPD = 2.8 ± 0.4 Pa
No significant difference among years
and management practices studied here
(Suyker and Verma, 2010)
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Normalized Water Productivity
Tanner & Sinclair (1983):
Normalized WP by D
Estimated daytime D from daily max/min T
Estimated root biomass (total biomass)
Applying their Procedures:
WPD = 9.9 ± 1.0 Pa for maize
WPD = 4.3 ± 0.2 Pa for soybean
Tanner and Sinclair (1983) values:
WPD = 9.5 ± 1.1 Pa for maize: AZ,CA,CO,NE
WPD = 4.0 Pa
for soybean: KS
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Normalized Water Productivity
Normalize WP by ETo
WPETo = aboveground biomass
∑(ET/ETo)
For irrigated and rainfed maize, LAI>2 (Mead)
WPETo = 27.5 ± 2.3 g DM m-2
For irrigated sorghum (w and w/o fertilization)
WPETo = 25 and 33 g DM m-2
For irrigated and rainfed soybean, LAI>2, (Mead)
WPETo = 14.1 ± 3.1 g DM m-2
For C3 crops (chickpea, wheat, sunflower)
WPETo = 13 g DM m-2
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Steduto and Albrizio (2005):
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ET Measurement
Related Issues
● Energy Budget Closure
Rn - Gs = H + LE
What are the causes for
lack of closure?
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ET Measurement
Related Issues
● Storage term (Gs)
- energy stored in the soil
- energy consumed in photosynthesis
- energy stored in plants/mulch
(Meyers and Hollinger, 2004)
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ET Measurement
Related Issues
● Covariance Averaging Time
(Mauder and Foken, 2006)
- flux averaging time of 30 min not
sufficient to capture low frequency
flux contribution from turbulent
organized structures and thermally
induced mesoscale circulations
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ET Measurement
Related Issues
● Angle of Attack
(Nakai and Shimoyama, 2012)
- flow distortion affects measurement
of u, v, and w which causes fluxes
to be underestimated.
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ET Measurement
Related Issues
● Impact on Closure
(Cava et al., 2008)
Rn- Gs vs H + LE
Raw Flux
AOA
correction
Storage
Term
Averaging
Time
Closure
0.82
0.88
1.02
1.01
R2
0.86
0.87
0.91
0.88
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Foken et al. 2011 Averaging time may not be
as important for short vegetation
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Conclusions
● Rainfed maize growing season
ET ranges 400-600 mm and up
to 960 mm in other biofuel crops
● Leaf area explains a good share
of daily ET variability
● Surface residue explains a
good share of NGS ET
variability
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Conclusions
● Normalized WP seems to
be a conservative value and
needs to be evaluated for
crops at the ecosystem scale
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