Mapping chlorophyll-a in upper lake Bhopal using IRS-1C data: a study

This paper on the GIS Development site describes the findings of a study that aimed at mapping chlorophyll-a in upper lake, Bhopal using Irs-1C data. Field sampling in the lake was 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 was determined by the spectrophotometric method (APHA-AWWA-WPCF, 1985).

The study found that areas showing high concentration of chl-a were the areas, which were 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 lakes. It was also observed that severe algal bloom occured near the outfalls. The prolific growth of macrophytes were observed near upper left banks of lake due to agricultural runoff. It was observed that a large area in middle right portion of the lake was showing very low chlorophyll - a concentration as it was away from point and non - point sources of incoming nutrients.

The study concluded that conventional GIS approach of mapping chl-a distribution could be the point interpolation method. However, it was realised that this method gave poor results for points situated at a larger distance from the sampling station. Hence to get an accurate estimate, fairly large number of samples were required . The methodolgy adopted in this study was based on satellite sensor records and needed fewer sampling points and offered 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 could be modelled and a regional map could be generated using the reflectance data from spaceborne sensors.