AERIAL SURVEY QF WATER QUALITY: AN INDIAN CASE STUDY
S.Thiruvengadachari, N,G.Inamdar, S.V.Kotbagi,
H,Tambralli and B,L,Deekshatulu
1. INTRODUCTION
The encouraging results from recent investigations (Scherz
1977, Moore 1978, Piech et al 1978, Mckeon, Rogers and Smith 1977,
Blackwell 1979) on remote sensing of water quality coupled with
the availability of expertise and equipment led National Remote
Sensing Agency (NRSA) to initiate a project to evaluate the use of
remote sensing techniques for monitoring water quality. A major
objective of the project was the assessment of the capabilities
and limitations of this new technology in regard to Indian water
quality surveys. The investigation centered around the weed infes-
ted Husainsagar Lake in Hyderabad city (Fig.1) and the stretch of
Godavari river upstream of Rajahmundry town (Fig.2) where the eff-
luent from the nearby paper mill enters the river system. Labora-
tory and boat level spectral studies coupled with appropriate
water sampling were conducted first to obtain a clear understand-
ing of energy - water interaction under controlled and field envi-
ronments. The culmination of the project was in the aerial sca-
nner-cum-camera surveys condueted over Husainsagar lake and Goda-
vari river during April 1981 at the time of maximum pollution con-
ditions. Colour coded water quality maps were generated for these
two areas. This paper presents some of the important results ob-
tained in this project, details of which were reported in the
final report submitted in July 1981 to the Indian Space Research
Organisation which sponsored this investigation.
2. BOAT LEVEL INVESTIGATION
Boat level investigations were extensively conducted to obtain
a clear understanding of the energy-water interaction in the field
environment, to precede the crucial aerial surveys. Spectral mea-
surements were obtained using a variety of instruments such as the
Exotech radiometer, quantaspectrometer, Barnes Spectralmaster etc.
Simultaneous water sampling enabled correlation of the spectral
response to the quality of water. Though the use of secchi disc
depth as a measure of turbidity has been on the decline, it can
still be a useful fielà measure of the water clarity. The second
depth was found to be related to turbidity which is related to
suspended solids. The secchi depth measurement is however affec-
ted by the type of day, cloud cover, acuity of observer's vision
etc. Quantaspectrometer and secchi depth measurements were com-
bined in a theoretical model for bottom reflectance to yield a
criterion for selection of sample points where the bottom reflec-
ted radistion may be safely ignored (Fig.3). Sample points were
to be chosen at locations with total depth exceeding twice the
secchi disc depth. Spectral measurements in Husainsagar clear ard
Codavari river clear waters indicated that the difference in the
attenuation characteristics is essentially due to the colour of
water and hence in the absorption characteristics. The propor-
tional relationship between secchi depth and backscatter contrary
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