The difference of Modified SEBAL from SEBAL on ET estimation from satellite data
Our Modified SEBAL differs from SEBAL on ET estimation from satellite data. Table 1.
shows the basic difference regarding data source, the calculation of reflectance and
albedo, G/Rn, Rah, and u * . Where G is the soil heat flux, Rn is net radiation, NDVI is
normalized difference vegetation index, Rah is the aerodynamic resistance for heat
transport, and u * is the friction velocity (Campbell and Norman, 1998; Morse, et. al.,
2000).
Table 1. The difference between Modified SEBAL and SEABLE on Et estimation.
Data
Temperature, G/Rn
Stability effects on
Stability
source Reflectance
=f(x)
Rah
effects on
and Albedo
x=NDVI
u*
SEBAL Landsat Calculated
ln( z 2 / z1)   h ( z 2) Consider
0.3  (1  0.98 x 4 )
Rah 
from
stable,
u * k
radiance data
unstable,
and
neutral
effects
4
3
Modified Aster
Ordered
 2.696 x  3.98 x R  ln( z 2 / z1)   h ( z 2)   h ( z1) Consider
ah
SEBAL
from Aster
unstable,
u * k
 1.64 x 2  0.11x
and
 0.40
neutral
effects
Data source
SEBAL uses Lansat data for special area such as Idaho. Authough Landsat temperature
data has better resolution 60 by 60 m compared with 90 by 90 m of Aster data, the
Landsat data is rarely available for Las Cruces. So, we use available Aster data. The data
frequency is about two weeks.
Temperature, Reflectance and Albedo
SEBAL calculates temperature, reflectance and albedo from the Lansat radiance data. But
we order Temperature and Reflectance from the Aster web site. From the reflectance data,
we calculate the albedo according to Liang, et. al., (2002).
G/Rn
SEBALE uses regression equation from Bastiaanssen, et. al., (1998) to calculate G/Rn.
We obtained a better-fitted regression equation from data in Bastiaanssen, et. al. (1998).
Please see Figure 1 for our regression equation.
Stability effects on Rah
SEBAL calculates Rah only consider the atmospheric correction at 2 m height and does
not use the correction at 0.1 m because they think the correction at 0.1 m is negligible.
But the correction is not negligible. Otherwise, the Rah will be unreasonable. So we
consider atmospheric corrections at both heights.
G/Rn
Stability effects on u *
SEBAL considers unstable, stable, and neutral effects on u * calculation. But our data is at
noon time, the atmosphere should be unstable. So when the calculated condition is stable,
it will be set to neutral. So in the Modified SEBAL, unstable and neutral conditions
effects on u * are considered.
y = -2.6955x 4 + 3.9817x 3 1.6401x2 - 0.1102x + 0.4079
R2 = 0.6633
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.5
1
NDVI
Figure 1. The regression equation for G/Rn calculation.
References:
Bastiaanssen, W.G.M., M. Menenti, R.A. Feddes, and A.A.M. Holtslag. 1998. A remote
sensing surface energy balance algorithm for land (SEBAL): 1. Formulation. J.
Hydrology 212-213, p. 198-212.
Campbell, G. S., and Norman, J. M. (1998). An introduction to environmental biophysics
(Second edition). Springer.
Liang, Shunlin, Chad J. Shuey, Andrew L. Russ, Hongliang Fang, Mingzhen Chen,
Charles L. Walthall, Craig S.T. Daughtry, Raymond Hunt Jr. 2002. Narrowband to
broadband conversions of land surface albedo: II. Validation. Remote Sensing of
Environment 84, 25–41.
http://weather.nmsu.edu/teaching_Material/soil470/sebal/downloadedfiles/references/Lia
ng2002AlbedoCalculation.pdf
Morse, A., Tasumi, M., Allen, R. G., and Kramber, W. 2000. Final Report. Application
of the SEBAL Methodology for Estimating Consumptive Use of Water and Streamflow
Depletion in the Bear River Basin of Idaho through Remote Sensing. Idaho Department
of Water Resources, University of Idaho, Department of Biological and Agricultural
Engineering.
The file is stored as ”FinalAllenReport.pdf” and can be downloaded from our website:
http://weather.nmsu.edu/teaching_Material/soil470/sebal/downloadvisualizationinstructio
n/FinalAllenReport.pdf