Full text: Remote sensing for resources development and environmental management (Volume 1)

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Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
Theoretic reflection modelling of soil surface properties 
B.P.J.van den Bergh & B.A.M.Bouman 
Agricultural University, Wageningen, Netherlands 
ABSTRACT: For a theoretical approach to surface reflection modelling,soil surface properties are divided on two 
levels of distinct influence on reflection. On the first level, an attempt is made to formulate the influence 
of particle size, mineralogical composition and moisture content and reflection. The developed equations are ba 
sed on the Lambert attenuation law of radiation in diffusing media. 
A natural soil surface is a heterogeneous combination 
of various elements of different composition, size, 
shape and spatial distribution. The combination of the 
se elements can be described in terms of soil proper 
ties such as textural class, mineralogical composition, 
organic matter and moisture content. Further, a soil 
surface has a certain roughness. There may be the 
presence of aggregates, rills, crusts and/or the ef 
fects of human influence like ploughing or harrowing. 
Soil surfaces may also contain stones, boulders, plant 
debris and other materials. Plant life in various 
forms between algae and trees may be present, while 
animal activity can have an important impact on the 
structure of the surface. Finally, this whole com 
plex of surface elements has a certain spatial orien 
tation viz: slope and exposition. 
For a theoretical approach of reflection from a natu 
ral land surface, a division in two levels of surface 
properties is suggested. The first will be called 
"the intrinsic soil surface" and the second the "bare 
land surface" (the presence of vegetation will not be 
considered in this study). 
Modelling reflectance from bare land surfaces 
The intrinsic soil surface is defined in terms of 
optical parameters:"a heterogeneous combination of 
reflective and absorptive elements in a solid, liquid 
or gaseous state, arranged in a specific spatial dis 
tribution and orientation. The dimension of these ele 
ments is such that the reflective process at this 
intrinsic level is governed by multiple reflections 
and re-reflections from these elements. The reflec 
tion from this intrinsic surface is thus the result 
of multiple internal reflections (mutually influen 
cing each other) and hence the theory of diffuse re 
flection should serve as a basis for further theore 
tical model building. On this level grain size and 
grain size distribution, shape and frosting of the 
grains, mineralogy, content and nature of organic mat 
ter and moisture content should be included as para 
meters in a model. 
The bare land surface is defined as a combination of 
intrinsic soil and non-soil surface segments, arran 
ged in acertain spatial distribution and orientation. 
Here, following Cooper and Smith, 1985, only varia 
tions in reflection arising from macroscopic features 
of a soil (large enough that diffraction by the irre 
gularities of the surface may be neglected) are consi 
dered. A bare soil surface has a certain roughness of 
many geometrically different intrinsic soil surface 
segments. Also, non-soil elements like stones and boul 
ders are included here, as well as slope and exposition 
of the surface as a whole. 
Above division in two surface levels is based on the 
dimensions of the surface elements relative to the re 
flective processes invoved. By this division, a hier 
archical ordening in the reflection influencing surfa 
ce properties is achieved. Modelling of the bare land 
surface reflection is super imposed on modelling of the 
intrinsic soil surface reflection. The approach is gra 
dual in which, started from the skeleton of the soil, 
different parameters are introduced in the model. In 
this paper, attention will be paid to modelling at the 
intrinsic soil surface level only; more specifically 
to the parameters particle size, mineralogy and moistu 
re content. 
At this level, the equation of Lambert-Beer serves as 
a basis for further elaboration. This equation descri 
bes reflectance r from diffusing media in the optical 
r = i/l = exp(-kd) eq.l 
and hence: 
Ln(r) = -kd eq.2 
in which: r = reflectance 
I = intensity of incident radiation (W/sr/u) 
I = intensity of reflected radiation (W/sr/ju) 
k = coefficient of absorption 
d = mean penetrated layer thickness 
The coefficient of absorption k is a characteristic of 
the absorbing components of the medium, while mean pe 
netrated layer thickness d depends on the geometrical 
structure and texture of the medium. Both properties 
are a function of the wavelength of incident radiation. 
By calculating Ln(r), whereby r is measured for soil 
samples of different variables, attempts can be made to 
relate the coefficients k and d to these variables. 
Since only limited laboratory measurements of reflec 
tance could be performed, use has been made of measu 
rements presented in literature. Own laboratory mea 
surements have been carried out with a NIWARS-spectro- 
fotometer, measuring bidirectional reflectance in the 
range of 0.3 to 2.4 ^om. (Bunnik, 1978). 
The influence of particle size 
The mean penetrated layer thickness was studied in re 
lation to particle diameter of sorted soil samples. In 
1965, Bowers and Hanks (B&H) published the results of 
a study to the effect of particle size on reflectance 
at different wavelengths in the optical range. These 
reflectance measurements of samples of kaolinite and 
bentonite clays are used here for further investiga 
tion, thereby introducing the following two hypotheses 
1) d is dependent on wavelength and on particle size 
2) k is dependent on wavelength but not on particle 

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