Mumbai: On December 5, the Ministry of Environment, Forest and Climate Change (MoEFCC) announced a 30% drop in the number of open fire incidents in Punjab and a 48% drop in Haryana during this year's paddy stubble burning season from September 15 to November 30. But experts say that satellite data alone are not enough to establish an actual decrease in stubble burning. This is because data accuracy depends on meteorological conditions, the resolution of satellite images and the period for which the satellite is recording observations from the area.

The government said that efforts by the central and state governments and other stakeholders meant that in Punjab, Haryana, Delhi and the districts of the National Capital Region in Uttar Pradesh and Rajasthan, stubble burning incidents reduced from 78,550 in 2021 to 53,792 in 2022. Experts say cloudy skies in 2022 and farmers only burning part of the stubble could have led to an underestimation of this year's stubble fires.

What is stubble burning and why is it necessary to track it?

Stubble burning is the process of burning straw stubble, the residue left in the soil after the harvesting of paddy and wheat, before sowing of the next crop begins.

Stubble burning releases carbon dioxide and unburnt smoke particles into the air, adding particulate matter pollutants thinner than human hair (PM 2.5 and PM 10). The contribution of farm fires to the national capital's air pollution, however, varies widely depending on wind speed and direction, besides local weather conditions, we reported in December 2021. Based on an analysis during October and November 2022, the maximum contribution of these farm fires to the daily PM 2.5 levels in Delhi was 34% on November 3. Besides leading to poor air quality, farm fires also impact the soil health and fertility, kill microflora and other organisms in the soil.

To reduce stubble burning, the government has taken several steps to encourage use of methods during the sowing process (in-situ methods), such as using super seeders which help sow the wheat crop while mixing the paddy straw into the soil, and other measures, such as mapping the demand of stubble from industries and biomass-based power plants to whom farmers can supply the stubble. Despite this, some farmers continue to opt for the faster and more economical method to dispose of this stubble by burning it since there is a short window available to prepare for sowing of the next crop.

How is stubble burning measured?

Based on the Commission for Air Quality Management's protocol, the MoEFCC has used NASA's Fire Information for Resource Management System (FIRMS) to track open fires during the paddy stubble burning season between September 15 and November 30 since 2021.

The data used for detecting fires are received from NASA's Visible Infrared Imaging Radiometer Suite (VIIRS) and Moderate Resolution Imaging Spectroradiometer (MODIS). The VIIRS sensor aboard the Suomi National Polar-Orbiting Partnership (SNPP) satellite has a resolution of 375 metre square, which means it can locate a fire accurately within an area of 375x375 sq. m--approximately the size of 17 cricket grounds. A single fire within this area will be attributed to the entire area, and similarly, multiple fires in this area will be counted as one fire. In comparison, the average size of a farm in Punjab is 3.6 hectares (36,000 sq m), which means that the resolution of the VIIRS SNPP satellite is about four times the size of an average farm in Punjab.

MODIS Aqua and Terra have an even higher resolution of 1 km. This makes it difficult to estimate the area of farm land which is being burnt and also the accuracy of the number of fires across these farms.

2022 fire numbers impacted by clouds, partial burning

Satellite data alone would not be enough to give a clear picture of the change on the ground as compared to last year, L.S. Kurinji, a researcher with the Council on Energy, Environment and Water (CEEW), told IndiaSpend. "Although there was a decline in farm fires this year, it is worth noting that the peak burning period witnessed three cloudy days which may have prevented the satellites from spotting the fires. Therefore, the decline may not be as steep as reported." Further, she added, prevalence of partial burning was high this year, where farmers burn the top portion of the stubble and then use super seeders for sowing the next crop. "This was followed mainly to reduce the operational cost and smoothen the operation of in-situ machines. We don't really understand the extent of the area where partial burning is being practised, hence it's difficult to estimate and compare the area, number of farm fires and the impact to last year's. Partial burning must not be treated as an interim win and the push should be towards the adoption of zero-burn alternatives."

Since the SNPP satellite gives global coverage every 12 hours, there are periods when an area is not being monitored. "Unlike forest fires, which continue for a longer period of time over a larger area, agricultural fires are small incidents," explained Vinay K. Sehgal from the Consortium for Research on Agroecosystem Monitoring and Modelling from Space (CREAMS). "It occurs in one field after another and persists for hardly 15-20 minutes to half an hour. So, if the satellite is not there at this time, those fires are not picked up." He adds that "some of the farmers have over time learned when the satellites are coming," and they time stubble burning for when a satellite will not be overhead. "As per ground reports, this has become an issue in this particular year."

In order to work around these limitations, the CREAMS team uses a different optical data set along with the thermal data sets provided by the satellite feeds to detect fire events. "We use optical data set resolution of 10 metres to map burnt fields, both harvested and not harvested, on multiple dates to estimate the burnt area," said Sehgal. The analysis of this large data set is time-consuming and will be available shortly for this paddy season.

Agriculture fires during summer

While the paddy residue burning season is between mid-September to November, farmers burn wheat stubble in March and April. NASA FIRMS open fire data show that the highest number of open fires, including agricultural fires as well as forest fires, is during the summer months of the year. The data suggests that wheat residue burning is on par with that of paddy.

In normal conditions, the warm air, along with the pollutants, rises to higher altitude and gets dispersed, but during winter, thermal inversion causes the pollutants to get trapped closer to the earth's surface. "But the fires during this period [summer] should be given as much attention, if not more, because these fires are easier to tackle."

He explains that one of the reasons farmers choose to burn the stubble after harvesting paddy is because of the short window before the next crop is planted. But in Madhya Pradesh, Chhattisgarh and other states that grow wheat, farmers have more than two months to plant the next crop. "Moreover, unlike paddy residue, wheat residue can be utilised as fodder for animals, especially during the drier months of May and June."

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