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