Tracking monsoon variations across India

3 months ago 44
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Half the monsoon is over. The India Meteorological Department (IMD) has forecast a normal monsoon for the second half of the season, as it did for the first half. Aggregate statistics, however, hide the wide variation over time and geographies. While states on the east coast have received excess rainfall for much of the first half, rainfall fizzled out on the west coast before recovering in August. In flood-hit Assam and Bihar, too, the rainfall has remained in the excess category so far.

Wide variations in monsoon precipitation over time and regions, between highs and lows and rain deficits and surpluses aren’t beneficial for agriculture in a country where two-thirds of the population depend on farm incomes for a living. The output of summer crops such as rice, sugar, lentils and edible oil seeds depends on monsoon rains.

For India as a whole, a lot of rain seems to have fallen early in the season. For most of June, for example, the cumulative monsoon rainfall was in the “excess” category. This means that India received much more rainfall in June than past statistics would have expected it to. A departure from the Long Period Average (LPA) of between 20% and 60% is considered excess rainfall. But, as June moved to July and then August, this cumulative rainfall has slowly trended towards normal. A rainfall departure of up to 20% is considered normal. The LPA is calculated as the mean of the rainfall from 1961 to 2010.

This India-wide trend is also true for states such as Rajasthan, Madhya Pradesh, Gujarat (barring the Saurashtra and Kutch regions), Haryana and Delhi, Jharkhand, Odisha, and Gangetic West Bengal.

This hasn’t been the case in some regions of east India. In Assam, Bihar, and eastern Uttar Pradesh, which have seen floods this season, cumulative rainfall in the first half of monsoon has been either excess or large excess. Rainfall departure of above 60% of LPA is considered large excess. Deviation of cumulative rainfall from normal in eastern Uttar Pradesh started decreasing by the end of June, but reached the normal range only by the end of July. That is not the case for Bihar, where cumulative rainfall is still in the excess range. While the deviation from the LPA has been lower in Assam compared to Bihar and eastern Uttar Pradesh for most of this season, it was steady between the 20% and 40% range before August. Western Uttar Pradesh, on the other hand, has been rain-deficient throughout this monsoon.

The two coasts of India have seen slightly different trends. Barring the Konkan and Goa region, the west coast has received relatively normal monsoon rainfall this season compared to the states on the eastern coast. The Konkan region had large excess and excess rainfall in June, but stabilized to normal values from July. However, the coastal Karnataka region and Kerala have seen a sudden rush of rainfall in August. Coastal Karnataka was 11% rain-deficient (though within the normal range) on August 1. It had a 3.6% excess by August 10. Similarly Kerala was 28% rain deficient on July 28. This deficit became a 0.8% surplus by August 10. This suggests that these two states received a lot of rain in August. This quick revival to normal levels of monsoon rainfall explains the floods that have hit these states this month.

While the west coast has seen a decrease in departure from normal in July, the east coast saw an increase. Tamil Nadu and Andhra Pradesh have both seen cumulative rainfall rise to above normal values in July, which has continued into August. Odisha, however, has seen a decline from above normal values in early June to normal values in July.

Apart from causing floods, such uneven distribution of rain – despite aggregate rainfall being normal – is not good for agriculture, according to experts. “Sometimes it rains 70 mm in a day and after that for 20 days there is no rain. That destroys all the crops because most farmers don’t have rain water harvesting structure,” said Professor Jeet Singh Sandhu, vice chancellor of SKN Agriculture University, Jobner, Jaipur. Regular rainfall of less intensity is also needed for maintaining moisture in the soil for application of fertilizers, Sandhu added.

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