2. Ground-based energy flux measurements for calibration of the Advanced Thermal and Land Application Sensor (ATLAS) Eric Harmsen, Associate Professor Dept. of Agricultural and Biosystems Engineering Richard Diaz, Undergraduate Research Assistant Department of Civil Engineering

3. Water Use Agriculture is the greatest consumer of water in society. It is estimated that 69% of all water withdrawn on a global basis is used for agriculture.

4. Water Losses Large losses of irrigation water are common. Irrigation efficiencies on the order of 50% are typical.

5. The ability to estimate short-term latent heat fluxes (i.e., crop water use) from remotely sensed data is an essential tool for managing the worlds future water supply. However, validation of these sensors is necessary.

6. Objective The objective of the study was to obtain ground-based estimates of the latent heat flux for calibration of latent heat flux estimates from NASA’s ATLAS sensor. The specific objective of this presentation is to present ground-based estimates of evapotranspiration obtained during the ATLAS fly-over in San Juan, PR.

7. Definition Crop Water Use = Evapotranspiration = Latent Heat Flux

8. The ATLAS Mission On February 11th, 2004, the ATLAS was used to evaluate the Urban Heat Island Effect within the San Juan Metropolitan area. A ground-based study was conducted at the University of Puerto Rico Agricultural Experiment Station in Rio Píedras.

9. Estimating Latent heat flux from ATLAS ρ = density of air C p = specific heat of air VD a = water vapor density of the air VD s = saturated water vapor density of the air at the vegetation canopy, temperature measured from ATLAS channel 4 γ = psychrometric constant, and r s = stomatal resistance

10. ATLAS Spectrum

11. Ground-based methods for estimating the latent heat flux Eddy-Covariance System Accurate Expensive ($20,000) Vapor flux and Energy Balance methods* Easy to use Require estimates of resistance factors Less expensive (less than $5,000) * Methods used in this study

12. Eddy-Covariance System

13. Vapor Flux and Energy Balance Methods

14. Vapor flux method

15. Vapor Flux Equation q = vapor flux ρ a = density of air ρ w = density of water VD 0.2 = absolute vapor density at 0.3 m VD 2 = absolute vapor density at 2 m r s = bulk surface resistance r a = aerodynamic resistance = 400/u 2 u 2 = wind velocity at 2 m

16. Simplified representation of the (bulk) surface resistance and aerodynamic resistances for water vapor flow (from Allen et al., 1989).

17. Energy Balance Method where ET o is the Latent heat flux or Reference Evapotranspiration Δ is the slope of the vapor pressure curve (kPa o C -1 ), R n is net radiation (MJ m -2 d -1 ), G is the soil heat flux density (MJ m -2 d -1 ), g is the psychrometric constant (kPa -1 ), T is mean daily air temperature at 2 m height ( o C), u 2 is wind speed at 2-m height, e s is the saturated vapor pressure (kPa -1 ) and e a is the actual vapor pressure (kPa -1 ).

18. Penman-Monteith Equation The equation applies specifically to a hypothetical reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 sec m -1 and an albedo of 0.23.

20. Results

21. One-second reading of RH Instrument is at 200 cm Height Instrument is at 30 cm Height

22. RH for a single sensor at 30 cm and 200 cm from ground February 11, 2004

23. Air Temperature Differences for a single sensor at 30 cm and 200 cm from the ground February 11, 2004

24. Net Radiation on the Day of the ATLAS Fly-over

25. Soil Heat Flux on the Day of the ATLAS Fly-over February 11, 2004

26. 300 cm 20 cm February 11, 2004 Wind speed at 30 cm and 200 cm above the ground

27. February 11, 2004 Net Radiation on the Day of the ATLAS Fly-over

28. Evapotranspiration Time of ATLAS fly-over Vapor Flux Equation Penman-Monteith

29. Future Work The ATLAS ground surface temperature data are expected to be available in September 2004. Latent and sensible heat flux estimates by several methods will be compared with the ATLAS estimates. An automated devices is currently being developed for obtaining the temperature and humidity at the two heights.

30. Future Work – cont. A study is planned to verify the vapor flux method by comparing it with the eddy covariance system. The vapor flux instrument will be used to verify flux estimates under tropical conditions from other sensors, such as MOTIS.