The rice-wheat cropping system is the most widely practiced cropping system in Northwest India – particularly in Punjab, Haryana and western Uttar Pradesh. It generates huge amounts of stubble or crop residue.
In the absence of a robust stubble value chain and limited utilization of this residue, farmers in the region engage in large scale stubble burning at the end of the summer (kharif) and winter (rabi) harvests. It is estimated that 620 million tons of crop residue is generated annually in India, of which 16% is burnt in the fields. The major contributors to stubble burning are paddy straw (43%) and wheat straw (21%), followed by sugarcane straw (19%).
Annually, Punjab accounts for 21.32 million tons and Haryana for 9.18 million tons of stubble burning. Unfortunately, due to lack of proper waste collection and disposal systems, most of the stubble is disposed of through open field burning.
Over the years, stubble burning has become a troublesome phenomenon with numerous economic, social, health, and environmental issues brought on with the introduction of mechanized harvesting.
Paddy is a water-intensive crop that requires plenty of groundwater. Punjab and Haryana, which primarily grow paddy as a summer crop, have witnessed a drastic depletion in groundwater in their states. Both states enacted the law ‘The Preservation of Subsoil Water Act, 2009’ (TPSW) to help arrest the falling groundwater table.
The law mandated delaying the transplanting of paddy to beyond June 10 every year, when the most severe phase of evapotranspiration is over. Farmers were forbidden from sowing paddy before May 10 and transplanting it before June 10.
The rate of groundwater storage (GWS) had steadily depleted between 2002 and 2017, except for the period from 2010 to 2013. After the enforcement of the Act in 2009, there was improvement in the groundwater storage rate during the 2010-2013 period (compared to 2002-2009).
However, after 2014, there was a drastic deterioration in the GWS rate in Punjab and Haryana (see Fig. 1). In 2017, the GWS depletion rate observed in Punjab was (-)16.02 ± 2.1 millimeter per year(mmyr-1) whereas in Haryana it was (-) 21.5 ± 2.8 mmyr-1.
However, since 2014 there is again a drastic deterioration in the depletion of GWS change rate at (-)3.57 and (-)4.45 mm yr-1 in Punjab and Haryana, respectively, in comparison with 2010-2013.
The shift in transplantation of paddy by over a month led to the shrinking of the window between harvesting and planting the next rabi crop. This led to further intensification of stubble burning. Paddy stubble burning begins in the second week of October and lasts till the fourth week of November (about 48 days). It reaches its peak during the first week of November.
This leaves farmers with little time to clear the paddy fields and start sowing the next rabi crop. In order to clear the fields, the farmers resort to burning stubble in the open fields; this releases aerosol particulate matter into the atmosphere.
An analysis of 19 years (2002-2020) of MODIS satellite data showed that after the implementation of the TPSW Act, that is, in the post-2010 era (2010-2020), paddy stubble fires had increased by 21% compared to the pre-2010 era (Fig.2). This revealed the ineffectiveness of the TPSW law. The peak burning for paddy stubble shows a substantial shift of 32% from October to November (Fig. 3).
The shift in the stubble burning pattern, which intensifies by the first week of November, coincides with the festive season and low wind speed in north India. The release of highly anthropogenic carbonaceous dust, compounded with stubble burning smoke and aerosol particulate matter (PM2.5, PM10), adversely affects air quality.
The smoke advection to the National Capital Region (NCR) and parts of the Indo-Gangetic Plain, increases the high level of pollutants, such as PM2.5, PM10, in the atmosphere. This situation is aggravated by the steady northwesterly winds that transport pollutants to NCR and get trapped by the low boundary layer (<500 m), low wind speed (1-2 m/s), and relative humidity; local meteorology prevents the vertical mixing of pollutants.
Lidar satellite data Calipso and air mass trajectory models have observed an increase in aerosol pollution loading by 9.6% and 17.5% in PM2.5, which is advected (70% of PM2.5 air mass clusters from these regions) with a time lag of 4-6 days to NCR and downwind regions, leading to deterioration of air quality in North India by as much as 4-5 times the permissible Indian standard.
However, with the implementation of the National Policy for Management of Crop Residue and increase in policy and awareness programs among farmers by the government, the stubble fire incidents and related pollution over NCR over the last four years has declined marginally by 5%. This has resulted in the reduction of average PM2.5 concentration over Delhi-NCR.
Besides, in our study, we recommend the use of shorter duration rice varieties, direct seeding of rice (water use is reduced by 20-25%), diversification of kharif crops (summer crops) and providing economic solutions to farmers for stubble management. These methods and options could help reduce stubble burning and minimize its contribution to air pollution.
