Monday, 7 November 2005
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Investigating Evapotranspiration in Jordan Using Modis Surface Temperature and Alarm.

Ayman Suleiman, University of Georgia, 1109 Experiment St., Bio & Ag. Eng. Dept., Griffin, GA 30223 and Jawad Al-Bakri, University of Jordan, Dept. of Land, Water and Environment, Faculty of Agriculture, Amman, 11118, Jordan.

 

Estimation of evapotranspiration (ET) is needed for many applications in diverse disciplines such as agriculture, hydrology and meteorology. MODIS-based leaf area index (LAI) and surface temperature (Ts) were used to examine ET at six sites in Jordan using the dimensionless temperature DT, defined as (Ti –Ta)/(Tmax – Ta), where Ti is the aerodynamics surface temperature, Ta is the air temperature and Tmax is the surface temperature that would occur if all the net radiation (Rn-G) was converted to sensible heat flux and no evaporation occurred. An analytical-land-atmosphere-radiometer model (ALARM) has been used to convert Ts taken at any zenith view angle to Ti, at a defined scalar roughness length. The Ti is the temperature that gives the correct value of H at a specified value of the scalar roughness length. For all locations of the study but Irbed, the minimum DT (DTmin) ranged from 0.36 to 0.52 while the maximum DT (DTmax) ranged from 0.74 to 0.80. The minimum ET (ETmin), for all locations but Irbed, ranged from 0.7 to 1.5 mm d-1 while the maximum ET (ETmax) ranged from 2.4 to 3.6 mm d-1. For Irbed, DT ranged from –0.60 to 0.61 while ET ranged from 1.2 to 6.5 mm d-1. The lower DT and the higher ETmax in Irbed was due to wet conditions prior to the period of estimation and the sensible heat advection. Results of the study coincided with the meteorological records that indicated the occurrence of advection during that period, a well-known phenomenon that usually results in crop failure in the area. Irbed's ET for the wet days was in close agreement with the potential ET estimated using Priestley-Taylor method, indicating the potential role of the contemporary technology of remote sensing in providing real time and spatial data on ET in the different ecological zones.


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