difference vegetation index(Kustas and Daughtry, 1990).
The empirical equation estimating soil heat tlux can be
expressed as below:
M 4-—TM3
Gz(0325 — 0208 S TMS )- Rn ------- 13)
TM4*- TM3
3-4. Compute of instantaneous evapotranspira-
tion, ET,
From the above energy balance equation, the
instantaneous evapotranspiration(mm / hour). £7, can
be computed as follows:
ET, = 3600 - En-G-H 14)
L
where L is the latent heat of vaporization
(=[2501 - 2.37 T,] - 10°: J/ kg).
4. COMPUTE OF DAILY EVAPOTRANSPIRATION
As above described, the instantaneous evapotranspira-
tion, ET, can be calculated from LANDSAT TM data at
one time of day and in conjunction with other
meteorological data. The daily change of radiation and
evapotranspiration in the field assume a sine
form(Jackson and Hatfield, 1983).
ETER CSRILN) ener d$)
where ET,,. is the maximum evapotranspiration of a
day, N is the day length between sunrise and sunset, f
is the time of LANDSAT overpass after sunrise.
Therefore, the daily evapotranspiration(mm/day) was
computed as follows:
e N RN
EL =], ET = || Ela SPAN) m --16)
=2N-ET a / 7=2N-ET, / x-sin(xt/N)
PES
max
*
=
=
S
Lal
5e
$
=
>
©
sunrise t sunset
hour
Figure 4. Graphic representation of the estimation of
daily evapotranspiration from instantaneous
evapotranspiration.
5. RESULTS AND DISCUSSION
Table 2 summarized daily averaged surface energy
balance at each land cover and composition ratio of land
cover at study area from LANDSAT TM.
Table 2 Daily averaged surface energy balance
at each land cover
le ratio(%)
Rn(Wm?)|EcWm?) |LE(Wm2)
tidal flat 20 513 155 333
vegetated 39 306 290 191
nonvegetated 41 — 470 145 83
Figure 4 shows the estimated evapotranspiration
according to method of chapter 3 and 4. The latent
heat flux which equal to evapotranspiration in water
balance is greater on the tidal flat where contains lots of
water in the soil. nonvegetated surfaces such as urban
and reclamated area shows the half amount of latent
heat rather than vegetated surfaces(Table 2). In
nonvegetated areas, there is a little amount of
evapotranspiration, but vegetated area especially in
forest. shows the high evapotranspiration rate
(> 2mm/day).
According to the national development program. in
the study area, west coasts of Korea. the land cover/land
use change is very severe. This situation can be seen in
Figure 3. The tidal flat(or intertidal zone) was
reclamated to use of industrial complex, construction of
new residence section. :
As a result. those land use/land cover changes will
induce the potential change of thermal environment. For
example, when existing tidal flat will reclamated, and
changes to non vegetated land surface, the effect on
surface energy balance become:
ARn - - 43: reclamated surface area
AH z-10 reclamated surface area
À LE = -250- reclamated surface area
As known this results, the change of latent heat flux
due to change of land use/land cover is greater than that
of net radiation and sensible heat flux. Decreasing net
radiation(increase of urban area, decrease of tidal flat or
vegetated surfaces) due to increase of surface albedo
works on lowering surface temperature. In the other
hand, decrease of latent heat which is greater at least
six times than decreasing net radiation and sensible heat
flux works on highering surface temperature.
In this study, the author estimates of the
evapotranspiration using LANDSAT TM data with
ancillarv ground-based meteorological data and topolo-
34
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
Siamo