Shifting crop-burn timing reduces air pollution

Moves in India to change farming practice are paying big dividends. Biplab Das reports.

In India, crop residue burning significantly contributes to air pollution.


Groundwater conservation policies designed to delay planting and harvesting of rice may reduce air pollution caused by crop residue burning in India, researchers have revealed.

In a paper published in the journal Nature Sustainability, scientists led by Andrew McDonald from Cornell University, US, show that delayed planting and harvesting also shift residue burning from the last week of October to the first fortnight of November, a period during which prevailing winds are weaker and the dispersion of pollutants slows.

Agricultural burning is a global problem that affects air quality and human health. Recent studies have estimated that around 18 to 30% of crop residue is burned while the material is still in fields. One study found that in India more than 100 million tonnes of crop residue was put to flame.

Such burning produces fine particles, greenhouse gases, organic carbon, volatile organic compounds and polycyclic aromatic hydrocarbons. Despite laws that ban the practice, approximately 23 million tonnes of rice residues are burned annually in the Punjab and Haryana regions alone.

The fine particles, with diameters of 2.5 micrometres and smaller, then travel to neighbouring regions such as New Delhi, contributing significantly to air pollution, and consequent cases of respiratory diseases.

Against this bleak backdrop, McDonald and colleagues demonstrated a solution. They showed that groundwater conservation policies, including specific legislation, improved air quality in northwest India through changing patterns of rice production and agricultural burning.

Groundwater laws – in particular the Punjab Preservation of Subsoil Water Act and the Haryana Preservation of Subsoil Water Act, both promulgated in March 2009 – prohibit transplanting rice before 20 June in any year.

The influence of the acts is clear, the researchers say, with less than 40% of the total rice area planted on or before 28 June since 2009. Rice harvest has shifted accordingly. Before 2009, approximately 40% of the rice crop in Punjab was harvested by 26 October. Today it has declined to 14%.

Using time-series satellite data, McDonald and colleagues showed that residue burning declined within the last fortnight of October but significantly increased in the first three weeks of November.

Since 2009, the period of maximum burning has shifted to the first fortnight of November, when temperatures in New Delhi are three degrees Celsius lower and winds are weaker. These conditions favour atmospheric stability and discourage the dispersion of pollutants.

The researchers say that resorting to various agronomic technologies may also help further reduce crop residue burning.

One such technology, they note, is a seeder that permits crop planting into residues without prior burning.

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