Discussion
There
are often wide discrepancies between ET values calculated using MODIS data via
the Penman-Monteith formula and those calculated using weather station data via
the Kimberly-Penman equation. The reasons for this discrepancy are not
immediately apparent. A previous study suggested that performance of the
Penman-Monteith formula in arid conditions is erratic and tends to
underestimate evapotranspiration (Allen, et al., 1998).
MODIS
data has the advantage of incorporating a wide variety of parameters specific
to the landscape in the analysis of ra and rs
to compute ET using the Penman-Monteith formula most effectively. These
parameters include land cover classification, vegetation cover fraction,
albedo, aerodynamic resistance, leaf stomatal conductance, among others. Eddy
covariance data is also included in the analysis to produce more accurate
values of ET. In contrast, using the Kimberly-Penman equation is limited in
that the weather stations used for this study may not be in environments in
which this equation would be useful, as they are not alfalfa fields or crop
fields. More measurements of ET at these locations would be needed to calibrate
the Penman-Monteith formula for the specific conditions observed.
