Mapping chlorophyll-a in upper lake, Bhopal using IRS-1C data
Dr. Nitin Kumar Tripathi & Pradeep Singh
Field Sampling
Field sampling in the lake has been conducted on Nov 7, 1997. The date
was fixed keeping in mind the pass of the satellite over the study area during
field sampling so that accurate relationship of chlorophyll -a with sensor
records may be established. IRS-IC passes over the same area after 24 days.
Water samples were collected from various points inside the lake and brought
for laboratory sampling. To accurately mark the location of sampling station
inside the lake, a Global Positioning System (GPS) (SOKKIA-Spectrum) was used.
Water samples from 45 different locations were collected. These samples were
then tested in laboratory for chl-a concentration. The chl-a concentration has
been determined by the spectrophotometric method (APHA-AWWA-WPCF, 1985).
Methodology
In this study ILWIS 2.1 GIS package has been
used. Different bands of IRS-IC data were initially geo-referenced with the help
of Survey of India (SOI) toposheet (1:50,000) of Bhopal. The lake from toposheet
is digitized using ALTEK A0 size digitizer. This map is overlain on digital
image data and the portion of the lake is extracted for each band. Knowing the
co-ordinates (lattitude, longitude) of the sampling stations from the GPS
observations, reflectance values were extracted for a 2*2 grid of pixels and the
mean value was taken for all the mean reflectance values of the four bands and
the correspondence chl-a concentrations at the corresponding location. Of the
three bands it was found that chl-a gives best correlation with band 2
(0.52-0.58um). The image of the lake in band 2 (green band) is shown in the
Figure 1. Green band has also been found to give best correlation for
determination of chl-a in productive lakes by George (1987). The model obtained
is of the form
Chl-a=3.73565 R2 – 230.6679
Where, chl-a is
chlorophyll -a concentration in mg/cu.m. and R2 is reflectance in band2. This
model has offered coefficient of correlation of 0.895. The model was found to
yield satisfactory result at 5 percent confidence level in Chi-square test.
The Chlorophyll model was applied on the band2 image of the lake. This
produced a map of the lake (Figure 2) with chl-a values at every point. This map
was then density sliced to give four zones of chlorophyll -a distribution.
Conclusion
As can be seen in the Figure 2, the highly
polluted zones are appearing in red colour. The areas showing high concentration
of chl-a are the areas which are receiving untreated sewage from a hospital
(Hamidia hospital) and some densely populated slum dwellings (in Karbala,
Koh-e-fiza, and Khanugaon area) of adjoining areas of lake. It is also observed
that near the outfalls severe algal bloom occurs. The prolific growth of
macrophytes are observed near upper left banks of lake due to agricultural
runoff. It is observed that a large area in middle right portion of the lake is
showing very low chlorophyll – a concentration as it is away from point and non
– point sources of incoming nutrients.
The conventional GIS approach of
mapping chl-a distribution may be the point interpolation method. But, it is
realised that this method gives poor results for points situated at a larger
distance from the sampling station. Hence to get an accurate estimate fairly
large number of samples are required . Whereas the methodolgy adopted in this
study is based on satellite sensor records and needs fewer sampling points and
offers very accurate regional picture of the whole lake for chl-a distribution.
Similarly other water quality parameters such as SSC, total dissolved solids
(TDS) and several other chemicals may be modelled and a regional map may be
generated using the reflectance data from spaceborne sensors.
Acknowledgement
Authors are grateful to Sri S. K. Katiyar,
Lecturer, MACT Bhopal for rendering useful help during fieldwork. Authors also
thank for the kind help offered by Mrs Leela Ayyagar , Senior Scientific
Officer, IIT Kanpur in carrying out chemical Laboratory analysis.
References
APHA-AWWA-WPCF, 1985, Standard Methods for the
Examination of Water and Wastewater, 16th edition.
George, D. G., 1997,
The airborne remote sensing of phytoplankton chlorophyll in the lakes and tarns
of the English Lake District. International Journal of Remote Sensing, vol. 18,
no. 9, pp 1961-75.
Harper, D., 1992, Eutrophication of freshwaters:
Principles, Problems and Restoration(London: Chapman & Hall) Peavy, H.S.,
Rowe, D.R. and Tchobanoglous, G., 1985, Environmental Engineering, (New York:
McGraw Hill Company).
Tamot, P. and Srivastava, P., 1987, Study of upper
lake catchment area with special reference to factors influencing its
environmental status, specially water quality and ageing. Mapcost project:
E.N.V. – 72/87
Vollenweider, R.A., 1968, Scientific fundamentals of the
eutrophication of lakes and flowing waters with particular references to
nitrogen and phosphates as factors in eutrophication Technical Report OECD,
DAS/CSI/68.27, Paris.
Authors
Pradeep Singh is doing his M.
Tech. in Geoinformatics from Indian Institutes of Technology-Kanpur and has done
B.E. (Civil Engineering) from Poona University. He is currently in the process
of giving last touch to his M. Tech. Thesis. The topic of his thesis is
“Determination of chl-a, SSC and SDD using integrated GIS, GPS and Remote
Sensing”. This study has been carried out on Upper Lake-Bhopal. The area of his
interest is “GIS application for Environmental Studies and other Applications”.
Dr. Nitin Kumar Tripathi is B. Tech. In Civil Engineering from R.E.C.
Warangal. He has done his M. Tech. And Ph.D. in Remote Sensing from Indian
Institutes of Technology Kanpur. He has been involved in development and
restructuring of Post Graduate programme of Geoinformatics and developing the
Laboratory. He has around sixteen research papers to his credit. His research
interest includes: digital processing of remotely sensed data for various
applications, GIS, GPS and digital photogrammetry. He has guided eight M. Tech.
Thesis on these topics. He has received 1994 DAE Young Scientist Award from
Dept. of Atomic Energy and also Career Award for Young Teachers in 1997 from
AICTE.
