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Title: REMOTELY SENSED TEMPERATURES AND EVAPOTRANSPIRATION FROM HETEOROGENEOUS GRASS AND CITRUS TREE-CANOPY SURFACES

Author
item Allen Jr, Leon
item HEIMBURG, K - UNIV OF FLORIDA
item BILL, JR., R - UNIV OF FLORIDA
item BARTHOLIC, J - UNIV OF FLORIDA
item BOOTE, K - UNIV OF FLORIDA

Submitted to: Soil and Crop Science Society of Florida Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2004
Publication Date: 12/31/2004
Citation: Allen Jr, L.H., Heimburg, K.F., Bill, Jr., R.G., Bartholic, J.F., Boote, K.J. 2004. Remotely sensed temperatures and evapotranspiration from heteorogeneous grass and citrus tree-canopy surfaces. Soil and Crop Science Society of Florida Proceedings.. v. 63, p.: 1-20

Interpretive Summary: Evapotranspiration (ET) estimates are needed across complex landscapes to predict regional plant water use. Using airborne instruments, ARS and University of Florida scientists at Gainesville, in cooperation with the Kennedy Space Center, obtained color-coded surface temperature images across complex landscapes. Furthermore, methods were adapted for estimating ET from these surface temperatures for each color-coded component of the complex landscape. During a dry spring midday period, surface temperatures differed by as much as 36 degrees F, where bare soil and close-cropped dormant pasture were hottest, and small ponds and tall native trees and shrubs were coolest. They also found that citrus tree foliage was warmer than foliage of native trees and shrubs growing in moist soil. The citrus was warmer because its leaves transpired less water. The area-weighted ET of various methods ranged from 87 to 104% of ET measured by a ground-based system. These remote sensing methods clearly showed differences of ET based on the wide range of surface temperatures. Elsewhere in the USA, this type of technology is already being adapted by ARS scientists for computing ET using satellite data.

Technical Abstract: Evapotranspiration (ET) estimates are needed across complex landscapes to predict regional plant water use. We used surface temperatures from an airborne thermal scanner and methods for estimating surface-to-air resistance of sensible heat flux to determine ET as an energy balance residual. Surface temperatures varied by ~20 degrees C, with bare soil and close-cropped dormant pasture being warmest, and water surfaces and tall native trees and shrubs being coolest. Citrus tree canopy surfaces were warmer than native trees and shrubs growing in moist soil, indicating a lower stomatal conductance for citrus. Rates of ET (latent heat flux) ranged from ~30 watts per square m for warm surfaces with limited water to ~500 watts per square m for cool surfaces where water was not limiting. The area-weighted ET of various methods ranged from 87 to 104% of ET measured by a Bowen ratio apparatus. Three methods for computing surface-to-air heat transport resistance gave similar ET. These remote sensing methods clearly showed ET differences based on the wide range of surface temperatures. The methods can be used to compute ET accurately, provided that reliable surface-to-air heat transport resistances can be obtained.