Ground-based energy flux measurements for calibration of the Advanced Thermal and Land Application Sensor (ATLAS) 1 Eric W. Harmsen and Richard Díaz Román, Dept. of Agricultural and Biosystems Engineering University of Puerto Rico, Mayagüez, PR 00681, (787) , Fax: (787) ABSTRACT 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). Short- term measurements (10-second) of micro- meteorological parameters, including soil heat flux, soil temperature, soil moisture and net radiation were measured. Wind speed, relative humidity and air temperature were measured at two vertical positions above the ground. Vertical differences in soil water tension (negative pressure) were also measured continuously. This poster presents results from the ground-based study. Large differences in relative humidity were observed between 20 and 200 cm heights above the turf grass, whereas temperature differences were small. Estimates of evapotranspiration are presented based on the Penman-Monteith method. 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 poster is to present estimates of reference evapotranspiration during the ATLAS fly-over. METHODOLOGY Field Measurements Climatological data were saved on a Campbell Scientific (CS) CR10X datalogger every 10 second. Measurements of air temperature and relative humidity were measured at 30 cm and 200 cm heights, respectively, using a single CS HMP45C sensor. Net radiation was measured using a CS NR Lite Net Radiometer. Wind speed was measured at 30 cm and 300 cm _______ 1 This material is based on research supported by the NOAA-CREST and NASA-EPSCoR (NCC5-595 ). above the ground, respectively. The upper sensor was a MET One 034B wind speed and direction sensor. The lower wind speed was measured using a HOBO wind speed sensor. Soil water content was measured using a CS616 Water Content Reflectometer. Soil temperature was measured using two TCAV Averaging Soil Temperature probes, and the soil heat flux at 8 cm below the surface was measured using a HFT3 Soil Heat Flux Plate. Soil heat flux at the soil surface was estimated using the average soil temperature, soil heat flux at 8 cm and water content data. Reference Evapotranspiration The reference evapotranspiration was estimated using the Penman-Monteith equations (Allen et al., 1998) at 4-minute intervals: where ET o = reference evapotranspiration (mm/hr) Δ = slope of the vapor pressure curve (KPa o C -1 ), R n = net radiation (mm/hr), G = soil heat flux density (mm/hr), γ = psychrometric constant (KPa o C -1 ), T = mean daily air temperature at 2 m height ( o C), u 2 = wind speed at 2 m height (m/s), e s is the saturated vapor pressure and e a is the actual vapor pressure (KPa). The equation applies specifically to a hypothetical reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 s/m and a solar reflectivity of RESULTS Figure 1-7 show the measured climatological data collected on February 11, Figure 8 shows the estimated reference evapotranspiration based on the field measurements. 1 Second Readings of RH (%) Instrument is at 200 cm Height Instrument is at 30 cm Height Figure 1. One-second measurements of relative humidity. Figure 2. Measured relative humidity from 10 AM to 6 PM on February 11, Figure 3. Measured air temperature from 10 AM to 6 PM on February 11, Figure 4. Measured wind speed from 10 AM to 6 PM on February 11, The lower values were from the sensor at 20 cm from the ground. Figure 5. Measured net radiation from 10 AM to 6 PM on February 11, Figure 6. Measured wind speed from 10 AM to 6 PM on February 11, Time of ATLAS fly-over Figure 7. Measured soil temperature from 10 AM to 6 PM on February 11, Figure 8. Calculated reference evapotranspiration from 10 AM to 6 PM on February 11, The total depth of the reference evaporation for the 8-hr period was 3.6 mm.