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. INTRODUCTION The ability to estimate short-term fluxes of water vapor from a growing crop are necessary for validating estimates from high resolution remote sensing techniques, such as NASA’s Advanced Thermal and Land Applications Sensor (ATLAS). On February 11th, 2004, the ATLAS was used to evaluate the Urban Heat Island Effect within the San Juan Metropolitan area. To validate energy flux estimates from ATLAS, a ground study was conducted at the University of Puerto Rico Experiment Station in Rio Píedras (located within the metropolitan area).

4. Objectives To support modeling efforts related to the Urban Heat Island problem. To obtain ground-based measurements and/or estimates of energy fluxes to validate the ATLAS estimates. The specific objective of this presentation is to present estimates of reference evapotranspiration during the ATLAS fly-over.

5. 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

6. Reference Evapotranspiration 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 ).

7. 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.

8. Vapor Flux Equation q = vapor flux ρ a = density of air ρ w = density of water VD 0.2 = absolute vapor density at 0.2 m VD 2 = absolute vapor density at 2 m R s = reference grass stomatal resistance u 2 = wind velocity at 2 m

11. Results

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

13. Relative Humidity Differences

15. Air Temperature Differences

17. 300 cm 20 cm

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

19. Future Work Related to ATLAS Latent and sensible heat fluxes will be estimated by several methods for comparison with the ATLAS estimates. The ATLAS ground surface temperature data are expected to be available in September 2004.