evapotranspiration (et) from dedicated energy crops ashlynn stillwell ce394k.2: surface water...
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Evapotranspiration (ET) from Dedicated Energy Crops
Ashlynn StillwellCE394K.2: Surface Water Hydrology Spring 2010
5/4/10
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Outline
• Increasing biofuels production• Methods of calculating ET• Results for Mead Irrigated site• CO2 from crops• Future work• Questions
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Increasing Biofuels Production
• Energy Independence and Security Act (2007) mandates 36 Bgal/yr by 2022
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• General consensus that new land will be cultivated
• Additional crops might lead to additional ET
[EISA, Schnoor et al., USDA][earthfirst.com]
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AmeriFlux Sites for Analysis
• ARMS SGP Main– Corn/soybeans rotation– Lamont, OK
• Fermi Agricultural– Corn/soybeans rotation– Winfield Township, IL
• Mead Irrigated– Continuous corn– Mead, NE
• Rosemount G19 Alternative– Corn/soybeans rotation– Rosemount, MN
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[AmeriFlux]
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Methods of Calculating ET
• Reference ET0 (Penman-Monteith equation)– ET0 for well-watered grass
– Kc crop factors adjust to ET
• FAO CROPWAT 8.0 software• Energy balance method• Scaling of latent heat flux (using eddy
covariance instruments)
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Penman-Monteith equation
• Net radiation• Soil heat flux• Air temperature• Wind speed• Vapor pressure
deficit• Slope of vapor
pressure curve• Psychrometric
constant• Pressure
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Reference evapotranspiration ET0
Crop factors Kc
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ET Variation throughout Day
0:00 4:00 8:00 12:00 16:00 20:00 0:00
-5
0
5
10
15
20
25
30
Crop Evapotranspiration (P-M) for Day 204Mead Irrigated | 2005 corn crop | near Mead, NE
Time of Day
ET (m
m/d
) for
hou
rly
time
step
s
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FAO CROPWAT 8.0 software
• Input data– Climate– Precipitation– Crop (some built-in)– Soil (some built-in)
• Output: Crop Water Requirements
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Energy Balance / Scaling of Latent Heat
• Energy balance method
• Scaling of latent heat flux (when measured directly with eddy covariance instruments)
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[Tanner]
[Hyun]
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Comparison of ET Methods
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0 50 100 150 200 250 300 3500
2
4
6
8
10
12
0
100
200
300
400
500
600
700
Mead Irrigated - Evapotranspiration Corn Crop 2005
ET0 P-M TannerLE Cumulative ET (LE)
Day of Year
ET (m
m/d
)
Cum
ulati
ve E
T (m
m)
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Cumulative ET for Methods
ET Calculation Method Cumulative ET(mm) Percent Error
Reference ET0 1,089 66%Penman-Monteith 707 8%FAO CROPWAT software 770 17%Energy Balance (Tanner) 824 26%Scaling Latent Heat Flux 656 baseline
Indirect calculations overestimate ET compared to direct measurements
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CO2 from Crop Cultivation
Crop growing season
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0 50 100 150 200 250 300 3500
100
200
300
400
500
600
700
800
900
Mead Irrigated - CO2 ConcentrationCorn Crop 2005
Day of Year
CO2
(mm
ol/m
ol)
No statistical difference between pre- and post-growth CO2
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Conclusions
• Direct measurement of latent heat flux provides lowest magnitude of ET
• Penman-Monteith equation with Kc provides good approximation
• Crops sequester CO2, but other operations emit CO2
• (Not shown here; in paper)– Highest ET at ARMS SGP Main site (Oklahoma)– Comparable ET from corn vs. soybeans
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Future Work
• Write report• Beyond CE394K.2– Satellite- and GIS-based methods of calculating ET– Change from native vegetation to energy fuel crop
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Questions?
Data AcknowledgmentsOffice of Biological and Environmental Research of the U.S. Department of Energy as part of the Atmospheric
Radiation Measurement ProgramAmeriFlux Network, Oak Ridge National Laboratory
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