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For at least three years now, western disturbances have displayed unusual behaviour in India. The cyclonic storms, which originate in the Mediterranean region, travel more than 9,000 km to bring winter rains to northwest India.
But since 2019, winter has been largely dry across the country. Western disturbances arrived late in 2023 and began to intensify around March. They reached a climax in May, which received 175 per cent more rainfall than normal and saw maximum and minimum temperature 10 per cent and 20 per cent below normal, respectively. The effect was largely limited to northern India. The change in weather has affected rabi crops, particularly the harvest and marketing of wheat grains, causing losses to farmers.
There is no doubt that the behaviour of western disturbance will change in a warming world. But how will it behave and how will it affect agriculture?
Researchers at Dr Y S Parmar University of Horticulture & Forestry in Nauni, Solan district, Himachal Pradesh, offer some clues. The researchers analysed the trend of western disturbances in Solan for the past five decades to see how their behaviour was changing.
They say that between 1971 and 2021, the annual duration of a western disturbance in Solan, which on an average lasts for 139 days from October to May, has reduced at a rate of 0.29 days or about eight hours per year. While the onset of the events, has advanced by 0.19 days per year, their withdrawal has delayed by 0.11 days per year.
Change in onset of western disturbances leads to variation in sowing of rabi crops, particularly in regions that lack irrigation facilities and where farmers largely depend on rainfall for agriculture. Similarly, delayed withdrawal could lead to a late harvest, exposing crops to excess rains and leading to losses.
Farmers in Solan have also experienced for the last three decades that there is no rain when needed and more rain than normal when rain is actually not necessary, say the researchers.
For instance, the highest number (398) of western disturbances was recorded in summer (March to May) followed by winter (December to February). The lowest number (90) of the event occurred in October and November, the post-monsoon months, just when wheat crops would be sown.
In terms of rainfall, shorter-duration western disturbances showed an increasing trend across months, while longer-duration events were on a decline. Western disturbances can last from one to five days, or sometimes go for longer duration.
Highest amount of rainfall was received through two-day western disturbances, followed by one-day events. Events lasting five days and longer had the lowest amount of rainfall. Average annual rainfall from western disturbances during the study period was 69.65 mm, with February receiving the highest 70.13 mm, followed by March which received 69.17 mm.
Such distribution can be either favourable or unfavourable for overall yield, depending on the intensity during the respective crop growth stage. For instance, a storm event in February, just before wheat is harvested, is favourable for a higher grain yield.
Similarly, rainfall due to western disturbances had a positive impact on production when it occurred at growth and development stage for barley, tomato, capsicum, peach and plum. In contrast, impact was negative when rainfall occurred during the maturity and harvesting stage of the crops.
In 2021, a research led by scientists from Punjab Agricultural University noted similar trends of the systems in the state; the lowest number of western disturbances in Punjab had been observed in November and highest in May, says the study. This indicates a pattern for change in western disturbances to be studied.
MS Jangra is principal scientist and Prakriti a research scholar at the Environmental Science Department of Dr Y S Parmar University of Horticulture & Forestry, Nauni, Himachal Pradesh. The analysis in the article is based on their research
This was first published in the 16-30 June, 2023 print edition of Down To Earth