SPECTRAL SIGNATURES OF WATER BODIES WITH DIFFERENT TURBIDITIES
by
K.R.Rao, B.L.Deekshatulu, K.Krishnam Raju
National Remote Sensing Agency (NRSA), Secunderabad, India
Abstract: This paper presents analysis of the data pertaining to
different water bodies having relative turbidities varying from
3 to 25 and the corresponding spectral reflectance values. Secchi
depth measurements and bottom reflectance effect measurements were
taken. The depth of extinction was determined for waters of different
turbidity values. The data was collected on five water bodies
around Mandya town which is about 95 kms. south of Bangalore on
Bangalore-Mysore highway.
Correlation of ground data with that of the M?S data collected at
the same time has been attempted. From the data (Fig.1, 2) it is
established that the spectral reflectance increases with increasing
turbidity. This phenomenon is more prominent in bands 4 and 5
(i.e. 0.5-0.6um; 0.6-0.7um). The spectral reflectance increases
much faster in the lower turbidity values. It is also established
that the absorption of radiation in water is of exponential
nature (Fig.16-19).
1. Introduction:
1.1 There are a number of papers published in recent years re-
lating to water quality and corresponding spectral signatures
measured from the ground as well as from aircraft and spacecraft.
Ritchie Etal (4) studied the reflected and incident solar radiation
on six northern Mississippi reservoirs and showed that 0.7 to
0.8um is the best band for suspended solid concentration studies.
They have also established the fact that the sun angle has a
definite effect on the reflectance relationships. Mc Cluney etal
(3) have discussed the approaches for development of techniques
for the analysis of remote multispectral imagery of natural
resources of water. A set of maximum surface truth measurement
parameters and techniques are suggested in this paper. Bartolucci,
L.A. etal (5) measuring spectral reflectances of a highly turbid
water body and a low turbidity clear water body, established the
increases of spectral reflectance with turbidity. Bottom reflec-
tance effect has also been measured.
1.2. Turbidity: Turbidity is caused by the presence of colloidal
particles in water. Colloidal particles are larger than true ions
and molecules dissolved in liquids and smaller than settleable
solids. Colloidal suspensions have some unique properties. They
scatter light and the amount of light scattered is related to
the degree of turbidity and particle surface area. Sources of
turbidity in water are clay, silt soil, sewage bacteria, indus-
trial wastes, etc. The concentration of turbidity is measured
by the degree of light scattered by the sample. The turbidity
values given here are not absolute values but are relative.