1. University of Nebraska Lincoln R School of Natural Resources Water in Bioenergy Agroecosystems Workshop June 12-13, 2012 Ecosystem Scale Evapotranspiration Measurement in Temperate Rainfed Agriculture Andrew E. Suyker University of Nebraska-Lincoln

2. University of Nebraska Lincoln R School of Natural Resources Objectives Quantify evapotranspiration (ET) of rainfed maize Examine some key factors controlling ET Quantify water productivity Discuss ET measurement related issues

3. University of Nebraska Lincoln R School of Natural Resources Measurement Sites: Mead, NE Rainfed Maize-Soybean Irrigated Continuous Maize Irrigated Maize-Soybean

4. University of Nebraska Lincoln R School of Natural Resources Rainfed Maize-Soybean Rotation M S M S M S M S M S M

5. University of Nebraska Lincoln R School of Natural Resources ET Summary: Rainfed Maize Growing Season: May - September

6. University of Nebraska Lincoln R School of Natural Resources Variability of daily ET: Growing Season ET o : Reference ET (Allen et al., 1998) R 2 = 0.66

7. University of Nebraska Lincoln R School of Natural Resources Non growing season ET Irrigated and Rainfed Maize and Soybean Surface Residue (Mg/ha)

8. University of Nebraska Lincoln R School of Natural Resources 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

9. University of Nebraska Lincoln R School of Natural Resources 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)

10. University of Nebraska Lincoln R School of Natural Resources Water Productivity Maize Soybean WP = Dry above ground biomass ET

11. University of Nebraska Lincoln R School of Natural Resources Normalized Water Productivity Normalized WP by vapor pressure deficit (D) WP D = aboveground biomass D: daytime average VPD (ET/D) Tanner & Sinclair (1983): For irrigated and rainfed maize, LAI>2 (Mead) WP D = 6.9 ± 0.7 Pa For irrigated and rainfed soybean, LAI>2, (Mead) WP D = 2.8 ± 0.4 Pa No significant difference among years and management practices studied here (Suyker and Verma, 2010)

12. University of Nebraska Lincoln R School of Natural Resources 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: WP D = 9.9 ± 1.0 Pa for maize WP D = 4.3 ± 0.2 Pa for soybean Tanner and Sinclair (1983) values: WP D = 9.5 ± 1.1 Pa for maize: AZ,CA,CO,NE WP D = 4.0 Pa for soybean: KS

13. University of Nebraska Lincoln R School of Natural Resources Normalized Water Productivity Normalize WP by ET o WP ETo = aboveground biomass (ET/ET o ) For irrigated and rainfed maize, LAI>2 (Mead) WP ETo = 27.5 ± 2.3 g DM m -2 For irrigated sorghum (w and w/o fertilization) WP ETo = 25 and 33 g DM m -2 For irrigated and rainfed soybean, LAI>2, (Mead) WP ETo = 14.1 ± 3.1 g DM m -2 For C 3 crops (chickpea, wheat, sunflower) WP ETo = 13 g DM m -2 Steduto and Albrizio (2005):

14. University of Nebraska Lincoln R School of Natural Resources ET Measurement Related Issues Energy Budget Closure R n - G s = H + LE What are the causes for lack of closure?

15. University of Nebraska Lincoln R School of Natural Resources Storage term (G s ) - energy stored in the soil - energy consumed in photosynthesis - energy stored in plants/mulch (Meyers and Hollinger, 2004) ET Measurement Related Issues

16. University of Nebraska Lincoln R School of Natural Resources 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

17. University of Nebraska Lincoln R School of Natural Resources 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

18. University of Nebraska Lincoln R School of Natural Resources Impact on Closure (Cava et al., 2008) R n - G s vs H + LE Foken et al. 2011 Averaging time may not be as important for short vegetation ET Measurement Related Issues

19. University of Nebraska Lincoln R School of Natural Resources 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

20. University of Nebraska Lincoln R School of Natural Resources Conclusions Normalized WP seems to be a conservative value and needs to be evaluated for crops at the ecosystem scale