Development of a Procedure for Estimating Crop Evapotranspiration over Short Periods Dr. Jorge Gonzalez, Professor Dept. of Mechanical Engineering Santa Clara University. Eric Harmsen, Associate Professor Richard Diaz, Research Assist. Dept. of Agr. and Biosystems Eng. Civil Engineering Department University of Puerto Rico

Acknowledgements NASA-EPSCoR (NCC5-595), NOAA-CREST, USDA- TSTAR, NASA-URC, and UPRM-TCESS. Individuals: Javier Chaparro, Antonio Gonzalez, Jose Paulino-Paulino, and Dr. Ricardo Goanaga of the USDA Tropical Agricultural Research Station in Mayaguez, PR. The ATLAS Sensor was provided by NASA Stennis Space Center and the Lear Jet Plane was provided by NASA Glenn Research Center.

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 It is estimated that 69% of all water withdrawn on a global basis is used for agriculture.

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

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.

Objective To describe a relatively inexpensive method for estimating short-term (e.g., hourly) actual evapotranspiration. Present validation results for the method Present application example results from two field studies conducted in Puerto Rico.

Combine Humidity Gradient and Generalized Penman-Monteith Methods Methodology

Generalized Penman-Monteith Method (GPM) ET = evapotranspiration Δ = slope of the vapor pressure curve R n = net radiation G = soil heat flux density ρ a = air density c p = heat capacity of air λ = psychrometric constant T = mean daily air temperature at 2 m height u 2 = wind speed at 2-m height e s = saturated vapor pressure and e a is the actual vapor pressure r s and r a = bulk surface resistance and aerodynamic resistance, respectively.

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

Aerodynamic Resistance (r a ) r a = aerodynamic resistance z m = height of wind measurement z h = height of the humidity measurement d = zero plane displacement h = crop height k = von Karman’s constant u z = wind speed at height z

Humidity Gradient Method (HGM) ET = evapotranspiration ρ a = density of air c p = heat capacity of air ρ w = density of water ρ vL = water vapor density at height L ρ vH = water vapor density at height H r s = bulk surface resistance r a = aerodynamic resistance = ζ / u 2 u 2 = wind velocity at 2 m

Elevator Device

Method Validation

Eddy-Covariance System Eddy Covariance System

ET Station

ET Results - UF Agr. Experiment Station - April 6 th, 2005

RH results over a 15-minute period UF Agr. Experiment Station April 6 th, 2005

Actual Vapor Pressures and Actual Vapor Pressure Differences UF Agr. Experiment Station - April 6 th, 2005

Comparison: eddy covariance system and ET station U of F Agr. Experiment Station April 5 th and 6 th, 2005

Comparison: eddy covariance system and ET station U of F Agr. Experiment Station April 5 th, 2005

Comparison: eddy covariance system and ET station U of F Agr. Experiment Station April 6 th, 2005

Monteith and Unsworth, 1990 Vapor Pressure

Monteith and Unsworth, 1990 Temperature

Application Example No. 1 Estimation of ET and aerodynamic resistance for sugarcane, Lajas, PR

Estimated ET for a sugarcane plot November 9, 2004, Lajas, PR ET = 1.3 mm/day and ζ = 305

Estimated surface and aerodynamic resistances for a sugarcane plot November 9, 2004, Lajas, PR

Measured Net Radiation for a sugarcane plot Oct. 31 st and Nov. 9, 2004 Lajas, PR

Application Example No. 2 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.

The ATLAS Mission

Estimated ET for a grass-covered field Nov. 11, 2004, Rio Piedras, PR ET = 3.7 mm/day and ζ = 208 and r s = 90 sm mm/hr Time of fly-over 2:25 PM

Remote Sensing ET Equation ρ v is the vapor density of the air measured at the ground surface. ρ vs is the actual vapor pressure based on the corrected remotely sensed surface temperature

ATLAS-Estimated Surface Temperature for a grass-covered field Nov. 11, 2004, Rio Piedras, PR ATLAS surface temperature correction: 33.0 o C – 29.8 o C= 4.2 o C

Areal Photo Surface Temperature

31.78 o C o C 4 5

Temperature and ET variation with distance from the ocean The estimated ET varied 0.1 mm/hr over the 20 km transect.

Relative Cost Comparison of Direct ET Methods

Future Work Additional method validation. Utilize the ET station to estimate evapotranspiration rates and factors (r a, r s, K c, K s ). Deploy numerous stations around Puerto Rico to validate/calibrate remote sensing estimates of surface temperature, ET and the surface energy balance.