Maharashtra’s mangrove cover: Reports by state and Centre differ

Two different reports on mangrove cover in Maharashtra – one by the Union environment ministry and the other commissioned by the state forest department – show a discrepancy of 33sq km (equal to 6,111 football fields) in the extent of mangroves.

Though experts said the discrepancy falls within the acceptable realm of error for satellite imagery analysis, environmentalists have expressed concern over this difference from an administration and conservation standpoint.

A survey conducted by the Indian Institute of Space Science and Technology (IIST), at the behest of the Maharashtra forest department, showed a mangrove cover of 353.1sq km. The survey used satellite data up to the year 2019. HT had reported the data on March 4.

In contrast, the Forest Survey of India’s (FSI) biennial report (last released in 2019), pegs the total extent of mangrove cover in the state at 320sq. This report, however, used satellite data only between October 2017 and January 2018. A comparison between the sensors, scale, resolution and minimum mappable units used in each of the above surveys is not publicly available.

“While an increase in the mangrove cover between 2018 and 2019 would have certainly happened, it can’t be 33sq km in such a short period of time. And even if the discrepancy is plausible due to differences in the scale and spatial resolution of images used, it only shows that there is no real substitute for an on ground assessment of the true figure,” said Stalin D, director, NGO Vanashakti, who has recently filed a fresh plea over non-compliance of the Bombay high court’s 2018 judgment on mangrove protection in the state.

Virendra Tiwari, APCCF, mangrove cell and independent mappers attributed the discrepancy to differences in methodology.

“The difference is most likely due to different spatial resolutions used for analysis,” Tiwari said.

Raj Bhagat, a remote sensing expert currently working with the World Resources Institute (India), explained, “There will always be differences in the result when different organisations conduct the same assessment. It is not just the resolution of images. Seasonality of the images and the definitions of coastal boundaries, for example, will also make a difference. I have always maintained that satellite imagery is only an alternative to better quality data, which can be gathered from a local assessment of the grid boundaries. In the present instance, the 33sq km difference is within the acceptable 5-10% margin of error for remote sensing, but from an administrative standpoint, it is not a fair number.”

There is also a more significant discrepancy in earlier data. For example, the IIST report pegged the total extent of mangrove cover in Maharashtra for the year 2005 at 304sq km, while FSI’s 2005 report pegged the state’s mangrove cover at nearly half that number, at 158sq km. This indicates an on ground discrepancy of more than 27,000 football fields. Even accounting for differences in data collection periods and methodology, experts said this difference is too large to be rationally explained.

Nevertheless, experts were in agreement that the total extent of mangroves in Maharashtra is seeing a rapid increase. Going only by FSI reports, between 2005 and 2013, the mangrove area in Maharashtra grew by 17%. In the past six years alone, the increase has been a whopping 72%, as stated by the mangrove cell in an email to Hindustan Times on July 24.

However, if one were to consider the IIST report from March, the growth of mangroves between 2005 and 2019 was only 16%. HT had reported this increase on March 4. This large inconsistency, and contradictory statements made by authorities, has drawn the ire of environmentalists.

“Ground truthing is a bit harder in terms of mangroves because they are not accessible on foot, but there are still more reliable ways to go about this than just relying on remote sensing data: carry a handheld GPS (global positioning system), mark the grid boundaries, and then calculate the distance between coordinates using a geographic information system. It’s inexpensive, and moreover it is the duty of authorities to provide rigorous, trustworthy data to the public,” said Stalin D.