246
From the plot of near infrared to red (Thematic
Mapper band 4 to 3; wavelength respectively 776 to
905 nm and 624 to 693 nm) it can be concluded that
most units with the exception of a small group show
spectral curves representative for soils with a low
to absent vegetation cover. Comparing feature space
plots of the first four bands (band 1 452-518 nm
and band 2 529 to 610 nm) in the visible and near
infrared part, three main types can be
discriminated:
1. a small group with vegetation (palm trees in
the oasis) with a low reflectance for blue, green
and red and intermediate reflectance in the near
infrared and a high 4/3 ratio.
2. a group of (almost) bare soils changing to a
range of intermediate to very low reflectance
values in band 4.
3. a group of bare soils, having an intermediate
to very high reflectance in all 4 bands.
Introducing also band 7 (2097 to 2347 nm) the
above mentioned groups can be characterised as
follows:
1. the reflectance of the oasis is very low in
band 7, as can be expected for vegetation.
2. this group has like in band 4 an intermediate
to very low reflectance.
3. two extremes develop within this group:
1. one extreme has both in band 7 and in band
4 the intermediate to very high reflectance.
2. the other extreme has in band 7 low values
in stead of intermediate values and intermediate
values in stead of very high values.
Band 5 (1568 to 1784 nm) can be considered as in
termediate between the first four bands and band 7.
The following characteristics can be given of the
3 groups. The oases, covered with palm trees in the
upper layer, are group 1. Group 2 consists out of
units of different parts of the playa and playa
border zone. The relatively dry footslope and dune
areas are the third group. The large decrease of
reflectance in band 7 in 3.2 is due to the presence
of gypsiferous sand or gypsum crust at the surface.
Differences in reflectance within the footslope and
playa group are due to different amounts of stones,
crust, gypsiferous or non-gypsiferous sand,
vegetation, surface roughness, exposition,
moisture, salt content and mineralogy of the top
surface.
3. CAUSES OF DIFFERENCE BETWEEN JANUARY AND MAY
DATA
The differences between the two days can be divided
into differences of the surface itself and those
due to the effect of sun elevation at Landsat
overpasstime and turbidity of the atmosphere.
3.1 Difference of the surface
The most probable differences of the surface may be
caused by the effect of rain, wind and vegetation.
Due to the effect of rain the surface layer may
be moist. An increase in moisture content in the
surface layer leads to a decrease in reflectance.
The cause and relation between reflectance and
moisture content are described a.o. by Angstrom
(1925), Bowers and Hanks (1965) and Planet (1970).
The decrease in reflectance will last for a longer
time if the groundwater table is close to the
surface. Therefore, if the moisture effect of rain
is visible; this will be in the playa and playa
border zone. Field measurements showed that this
moisture effect in the top surface layer has
disappeared in the footslope area within a day.
A second effect of rainfall is the redistribution
of £>alt. Salt efflorescence occurs in the playa and
playa border zone. It was observed in May 1985 that
even in the relatively high parts of the playa
border zone (up to 1 meter) this phenomenon occured
and lasted at least for a week after rainfall.
A third effect to be mentioned is the occurence
of slaking after rainfall. This effect is also
important in the relatively dry areas even in the
dunes and may last for long periods.
Erosion and sedimentation is a fourth
possibility, which may affect reflectance if
rainfall occurs.
Due to the wind in sand areas the roughness of
the surface may change as a function of the
windvelocity before and at the time of Landsat
overpass. Moreover dunes may migrate. Since sand
grains or sheets are often an important part of the
surface, the effect on reflectance may be present.
The vegetation in May in this area will in
general be more green and healthy than in January.
Moreover annuals may be present on places, where in
January no plants are present. In order to estimate
the effect of vegetation the spectral reflectance
curves in relation with the soil surface have to be
taken into account.
All effects mentioned above may differ in place
not only due to the characteristics of the surface
but also due to irregular distribution of rain,
runoff and wind within the area.
3.2 Other differences
The solar zenith angle at the acquisition dates
differs significantly (29 degrees in May and 60
degrees in January). Moreover differences in
turbidity of the atmosphere at the two days may
exist. The turbidity and solar zenith angle
influence the relation between reflectance at the
top of the atmosphere and the surface reflectance.
Different approaches and formulas relating these
two types of reflectance can be found (Otterman and
Fraser 1976, Nack and Curran 1978, Chen and Ohring
1984, Koepke et al 1985). The relationship between
surface albedo (a ) and planetary albedo (ax;
albedo at the top of the atmosphere) can^ be
approximated according to Koepke et al (1985) by:
a=a+ba (1)
where a^= path raâiance
b = two way transmittance
A nomogram is presented by Koepke et al (1985) to
find values for a and b at different solar zenith
angles and turbidity factors. In table 1 a and b
values are presented for 29 and 60 degrees solar
zenith angles with albedo expressed in reflectance
percentage. As can be expected path radiance will
increase with increasing solar zenith angle and
tubidity of the atmosphere, while the two-way
transmittance will decrease with these two factors.
It may be clear that transmittance is wavelength
dependent. Hence the correction for albedo bands
cannot be applied directly on the different bands.
Table 1. Path radiance (a) and two-way
transmittance (b) values for different turbidity
factors (T at 550 nm) and solar zenith angles of 29
and 60 degrees (after nomogram presented by Koepke
et al. 1985).
zenith angle 29° 60°
a
b
a
b
3.5
.79
5.7
.73
3.6
.77
5.9
.71
5.7
.62
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