METRIC / SEBAL Approaches to Regionalized Remote Sensing of Evapotranspiration

1. METRIC / SEBAL Approaches to Regionalized Remote Sensing of Evapotranspiration Richard G. Allen, Univ. Idaho – Kimberly Development Partners: Wim Bastiaanssen, WaterWatch, the Netherlands Ayse Irmak, Univ. Nebraska-Lincoln Ricardo Trezza, Univ. Idaho Jan Hendrickx, New Mexico Tech Justin Huntington, Desert Research Institute, Nevada Jeppe Kjaersgaard, South Dakota State Univ. NASA/USDA Evapotranspiration Workshop April 5-7, 2011

2. Presentation Coverage • SEBAL/METRIC approach(es) • Operational needs • Model Calibration via Inversion • Interpolation in Time • Applications Evapotranspiration Workshop April 5-7, 2011

3. R n ET = R - G - H n Why Energy balance? • ET is calculated as a “residual” of the energy balance (radiation from sun and sky) ET H (heat to air) Basic Truth: Evaporation consumes Energy G (heat to ground) Evapotranspiration Workshop April 5-7, 2011

4. Energy balance gives us “actual” ET We can ‘see’ impacts on ET caused by: • water shortage • disease • crop variety • planting density • cropping dates • salinity • management Evapotranspiration Workshop April 5-7, 2011

5. METRICtm and SEBAL Mapping EvapoTranspiration with high Resolution and Internalized Calibration METRIC was derived from Surface Energy Balance Algorithm for Land Allen et al., (2002, 2007) Bastiaanssen et al., (1995, 1998, 2005) Evapotranspiration Workshop April 5-7, 2011 METRICtm and SEBAL are, in general, complementary processes

6. North Thousand Springs Wood River Valley Twin Falls recent burn basalt 100 miles Burley Craters of the Moon Lake Walcott Surface Temperature – southcentral Idaho – August 14, 2000 Large Contrast in LST Irrigated Ag. Desert Evapotranspiration Workshop April 5-7, 2011

7. North R n H ET Thousand Springs G Wood River Valley Twin Falls recent burn basalt Burley Craters of the Moon Lake Walcott Heat Flux to Air – southcentral Idaho – August 14, 2000 Evapotranspiration Workshop April 5-7, 2011

8. North R n H ET Thousand Springs G Wood River Valley Twin Falls recent burn basalt Burley Craters of the Moon Lake Walcott Instantaneous ET – southcentral Idaho – August 14, 2000 Evapotranspiration Workshop April 5-7, 2011

9. North Thousand Springs Wood River Valley Twin Falls 100 miles Burley 24-hour ET – southcentral Idaho – August 14, 2000 500,000 irrig. acres Desert 24-hour ET – Magic Valley, Idaho – August 14, 2000 --by METRIC Satellite-based ET Procedure Lake Walcott Evapotranspiration Workshop April 5-7, 2011 ASABE June 22, 2010

10. Satellite Energy Balances are ‘Plagued’ by Uncertainty, Bias, and Error in EB components • Surface temperature • Aerodynamic vs. Radiative Temperature • Bias in Satellite Sensor Calibration • Atmospheric Correction • Air temperature • Albedo calculation • Net radiation calculation (incoming long-wave) • Soil heat flux • Aerodynamic resistance calculation • Wind speed field • Extrapolation of instantaneous ET to 24-hour periods Evapotranspiration Workshop April 5-7, 2011

11. Challenge (BIAS): Up to 2 K different from Trad (satellite) Challenge (BIAS): Uncertain Spatial Distribution of Tair (feedback between EB, Trad, Tair) z2 dT H rah z1 Sensible Heat Flux (H) – “Classical” H = r cp (Taero - Tair) / rah Taero= aerodynamic temperature rah = the aerodynamic resistance u* = friction velocity k = von karmon constant (0.41) Evapotranspiration Workshop April 5-7, 2011

12. Advantage: rah ‘floats’ above the surface and is ‘free’ of zoh and some limitations of a single source approach Advantage: dT is inverse calibrated (simulated) (free of Trad vs. Taero vs. Tair) z2 dT H rah z1 Sensible Heat Flux (H) – CIMEC models H = (r ×cp× dT) / rah dT= “floating” near surface temperature difference (K) rah = the aerodynamic resistance from z1 to z2 u* = friction velocity k = von karmon constant (0.41) Evapotranspiration Workshop April 5-7, 2011

13. H z 2 r dT ah Tveg z 1 Tsoil METRIC and SEBAL assume dT to ‘float’ in the blended EBL above the canopy-soil complex Evapotranspiration Workshop April 5-7, 2011 CGIAR Workshop on “Surface Energy Balance Models of Agricultural Areas from Earth Observation Data” Universidad Nacional Agraria La Molina (Peru), 13 March 2008

14. Solution to dT function: Use Inverse Modeling • Calibrate against known ET at extreme conditions (end points) • Incorporate biases of all inputs into the internal calibration • Biases then fallout during the final estimation process Evapotranspiration Workshop April 5-7, 2011

15. Calibration of SEBAL and METRIC CIMEC models: ASCE Penman-Monteith for 0.5 m (alfalfa) reference is used for the ‘wet’ extreme Inversion of H eqn: Rn – G - 1.05 ETref alfalfa(METRIC) or 0 (SEBAL – classical) Rn - G Evapotranspiration Workshop April 5-7, 2011

16. z z 2 2 H H H r r r dT dT ah ah ah z z 1 1 Near Surface Temperature Difference (dT) • Tair is unknown and unneeded • SEBAL and METRICtm assume a linear relationship between Ts and dT: dT = b + aTs Bastiaanssen ‘breakthrough’ Ts is used only as an index and can have large bias and does not need to represent aerodynamic surface temperature Evapotranspiration Workshop April 5-7, 2011

17. biasRn-G biasH-cal  biasdT  biasH-pixel  LE Calibration of METRIC/SEBAL: The Sensible Heat (H) Function calibrates around Biases in many of the Energy balance components: (Biases exist in: net radiation, soil heat flux, aerodynamic stability, aerodynamic roughness, absolute surface temperature, atmospheric correction) unbiased biases H = Rn – G – LE (for calibration) LE = Rn – G – H (during application) Biases cancel out Evapotranspiration Workshop April 5-7, 2011

18. METRIC analyses suggest that the relationship between Relative ET and Ts can vary with land use (+/- 100%) even within one Landsat Scene and can be nonlinear due to buoyancy effects May 22, 2002 August 26, 2002 Landsat P33, R34-35, New Mexico – Rio Grande Therefore, good to consider albedo, soil heat flux, stability correction, roughness Evapotranspiration Workshop April 5-7, 2011

19. Use ETrF to extrapolate to 24-hours Sugar beets Assume simple scaling with the ET reference Evapotranspiration Workshop April 5-7, 2011

20. Comparing METRIC vs. traditional KcETref methods (relatively good agreement among very independent approaches, with some variation during the ‘shoulder’ periods when ground has partial cover) Evapotranspiration Workshop April 5-7, 2011

21. Uses of Landsat-based ET to Cover Regional Areas Seasonal ET from METRIC and Landsat (100 mile x 200 mile area) Evapotranspiration Workshop April 5-7, 2011 ASABE June 22, 2010

22. Cloud Mitigation Dr. Ayse Irmak, UNL

23. Adjusting for background evaporation at image time to derive “Correct” Monthly ET ET from August 13 1997 not adjusted for background soil evaporation ET from August 13 1997 adjusted for background soil evaporation A daily Gridded Evaporation/Land Process model is run Evapotranspiration Workshop April 5-7, 2011

24. ET – Precipitation ------Indication of Recharge / Depletion Scottsbluff, NE Use in “high-res” hydrologic studies Irrigated areas Rangeland / Dryland Ag. Evapotranspiration Workshop April 5-7, 2011

25. Thanks ET Evapotranspiration Workshop April 5-7, 2011 ASABE June 22, 2010