s of absorption
icients of
volume of each
minerais, the
d to be :
eq. 4
on of the mi-
on of minerai i
minerai i
he results of
amples of pure
.h the aid of
.ion of three
ive been calcu-
icident radia-
a function
quartz, gypsum
don ( - 1 p 2 )
licite
. . 188
. .062
). 969
) . 905
) .855
). 8 2 1
). 793
). 77 1
). 7 4 1
3.724
3.688
erals in a
reflectance
aid of equa-
the minera-
icle size of
urfaces (van
ve been air-
alcite and
n small cups
>s and mean
) i1 s ample s.
¡ample B
'O
!5
). 2
.25-250
ilculated and
>fotometer;
and 2b.
L,
100 _
"-'80 ..
o
<1>
£40 ‘
0)
u
20 ■■
# ! measured
X * : calculated
_i i i 1 . i . i . i i L u i 1 I
' 1.0 1.4 1.8 2.2
wavelength
tyumf
Figure 2a. Calculated and measured reflectance
of sample A.
1.0 1.4 1.8 2.2
wavelength (um)
Figure 2b. Calculated and measured reflectance
of sample B.
In general, the same trend is observed for
the measured reflectance and the calculated
reflectance. The presence of gypsum in sample
B is clearly recognised in the characteristic
dips around 1.4 and 1.9 um in the measured
reflectance curve. These dips, although less
pronounced, are also observed in the calcula
ted reflectance curve. Differences in reflec
tance value are due to differences in cali
bration of the two spectrometers; to differen
ces in sample preparation (the depth of the
Hunt and Salisbury samples was only 3.5 mm);
and to differences in shape and surface
structure between the minerals used by Hunt
and Salisbury and those in the Tunesian soil
samples.
The influence of soil water
The influence of soil water on reflection
from soil surfaces is complex and involves
many variables. Adding water to a sample of
dry soil introduces an extra component of ab
sorption which optical characteristics depend
on its solute content and on the tension at
which it is bound to the soil mass (pF). An
attempt has been made here to include soil
water as a single parameter in modelling at
the intrinsic soil surface level. Again, re
ference is made to Bowers and Hanks (B&H,
1965): "The effect of moisture content on re
flection from a Newtonian Silt Loam". Plots
of Ln(r) versus moisture content at different
wavelengths showed a linear relation between
Ln(r) and moisture content. It is assumed
that the influence of soil water on mean pe
netrated layer thickness can be neglected
when compared to its effect on the coeffi
cient of absorption. On this assumption, the
following formula for the coefficient of ab
sorption of the total soil mass is proposed :
2
k = k + 9 k (m ) eq.5
s w w
in which : k = coefficient of absorption of
the total soil mass
k = coefficient of absorption of
the dry soil mass
k = coefficient of absorption of
the soil water
9 = moisture content in percentage
of volume
Applying this equation to the measurements
of Bowers and Hanks raised the problem of
finding some input for the mean penetrated
layer thickness d. Since no specifications
concerning aggregate size of the Newtonian
Silt Loam could be found, arbritary values
for the mean aggregate diameter 0 have been
chosen. In figure 3,calculated values for
k , using equation 5 and 3 with 0 = 100, 500
and 1000 |im,are plotted against wavelength
of incident radiation.
Figure 3. Calculated values of k
tion of wavelength, and k derived
et al. , 1965 . W
as a fune-
from Allen
The shape of these curves compares well with
that of the water absorption curve of Allen
et al. , 196-5. Considering the method used for
obtaining some value for mean penetrated la
yer thickness, no comparison between absolute
values for k can be made. Also, the influen-
■i w
ce of soil water composition and soil water
tension on k has not been taken into ac
count.
CONCLUDING REMARKS
The combination of equation 3, equation 4
and equation5 leads to the following formu
la, describing reflectance from a moist mix
ture of mineral matter in a specific parti
cle size class :
r = exp ( - ( Y*c . k . + 9 k ) (vf^Ln ( 0/X) ) ) eq.6
11 w w
Modelling of intrinsic soil surface reflec
tion has to include more variables such as
shape and surface structure of the particles
and aggregates, nature and content of organic