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University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Water in BioenergyAgroecosystems Workshop
June 12-13, 2012
Ecosystem Scale Evapotranspiration
Measurement in Temperate Rainfed Agriculture
Andrew E. SuykerUniversity of Nebraska-Lincoln
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Objectives
● Quantify evapotranspiration (ET) of rainfed maize ● Examine some key factors controlling ET
● Quantify water productivity ● Discuss ET measurement related issues
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Measurement Sites:Mead, NE
Rainfed Maize-SoybeanIrrigated Continuous Maize
Irrigated Maize-Soybean
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Rainfed Maize-Soybean Rotation
0
2
4
6
8
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
ET (m
m d
-1) M S M S M S M S M S M
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
ET Summary: Rainfed Maize
Growing Season: May - September
YearAnnual
ETGS ET
NGS ET
NGS portion
GS Precip
Yield Residue
mm y-1 mm mm % mm Mg ha-1 Mg ha-1
2001 662 488 174 26 510 7.37 8.432003 615 467 148 24 350 6.52 6.572005 611 476 136 22 340 7.69 9.072007 623 472 151 24 606 8.64 7.632009 575 450 125 22 434 10.14 11.142011 607 481 126 21 502 8.22 9.12
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Variability of daily ET:
Growing Season
ETo: Reference ET (Allen et al., 1998)
0.0
0.4
0.8
1.2
1.6
0 2 4 6
ET
/ET o
LAI (m2m-2)
Adequate MoistureDry Periods
Rainfed Maize R2 = 0.66
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Non growing season ET
R² = 0.71
0.5
0.6
0.7
0.8
0.9
0.0 0.5 1.0 1.5 2.0
No
n-g
row
ing
sea
son
E/E
eq
Surface Mulch Biomass (Mg/ha)
2001-02
2002-03
2003-04
2004-05
2005-06Irrigated and Rainfed Maize and Soybean
Surface Residue (Mg/ha)
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
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
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
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)
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Water Productivity
0
5
10
15
20
0
2
4
6
8
1/1/07 1/1/08 12/31/08
Dry
Bio
mas
s (M
g ha
-1)
ET (m
m d
-1) Maize Soybean
WP = Dry above ground biomass ∑ET
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Normalized Water ProductivityNormalized WP by vapor pressure deficit (D)
WPD = aboveground biomass D: daytime average VPD
∑(ET/D) 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)
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
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
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
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
Steduto and Albrizio (2005):
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
ET Measurement Related Issues
● Energy Budget Closure Rn - Gs = H + LE
What are the causes for lack of closure?
University of Nebraska-Lincoln
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School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
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● Storage term (Gs) - energy stored in the soil - energy consumed in photosynthesis - energy stored in plants/mulch (Meyers and Hollinger, 2004)
ET Measurement Related Issues
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
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● 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
ET Measurement Related Issues
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
● Angle of Attack (Nakai and Shimoyama, 2012) - flow distortion affects measurement of u, v, and w which causes fluxes to be underestimated.
ET Measurement Related Issues
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
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● Impact on Closure (Cava et al., 2008) Rn- Gs vs H + LE
Foken et al. 2011 Averaging time may not be as important for short vegetation
ET Measurement Related Issues
Raw FluxAOA
correctionStorage
TermAveraging
Time
Closure 0.82 0.88 1.02 1.01
R2 0.86 0.87 0.91 0.88
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
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
University of Nebraska-Lincoln
R
School of Natural ResourcesSchool of Natural Resources
University of Nebraska Lincoln
R
Conclusions
● Normalized WP seems to be a conservative value and needs to be evaluated for crops at the ecosystem scale
Recommended