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India welcomes monsoon rains
It is vital to the country's agricultural output. But what causes the monsoon and how dependable is it?
Last Modified: 28 Jun 2011 16:48
Monsoon rains in central and northern parts of India have lagged somewhat this year [EPA]

The arrival of rain in Delhi on Sunday brought relief from the early summer heat across the city. More importantly, these monsoon rains are vital to the lifeblood of the country. But what causes the monsoon and how dependable is it?

Whilst the arrival of the rains resulted in a drop in daily temperatures from 38 C to more comfortable 32 C, it was the rain itself which would have been met with the greatest appreciation.

Northern India is particularly important to the country's agricultural output. Not for nothing is the Punjab known as the "breadbasket of India".

This region produces 20 per cent of India's wheat, nine per cent of its rice, as well as other cereal crops, fruit, vegetables and cotton.

The fertility of this region has contributed greatly to its wealth but the dependency on regular rainfall puts it at great risk when the rains are late or do not fall in sufficient quantities.

Many people think of the Indian monsoon as being a moist southwesterly airstream produced by hot air inland over Asia "sucking" the air in from the Arabian Sea.

As a generalisation this may be true. Think of the monsoon as a giant sea breeze if you wish. This, though, is very much a simplification of the true pattern.

In fact, the rains which broke the drought in Delhi did not arrive from the southwest. They came instead from the east, having formed from an area of low pressure, known a monsoon depression, which began life in the Bay of Bengal.

This depression moved very slowly northwestwards but by the time it reached Delhi, rainfall had decreased drastically. In the states of West Bengal, Bihar, Jharkhand and eastern Uttar Pradesh, daily rainfall totals exceeded 100mm causing widespread flooding.

By the time the rain reached Delhi, barely 9mm fell.

Western side

More consistent monsoon rain falls along the western side of the country, extending from Gujarat in the north, through Goa, to the Western Ghats in the south.

Even here, rainfall varies greatly, and between May and September there are "active" and "break" periods when daily rainfall can vary between 60mm and no more than 2mm or 3mm. These pulses are the result of meandering troughs of low pressure high in the atmosphere.

Climatologists are able to produce maps showing the mean position of the northward progress of the summer monsoon, to the day, over the entire country.

On many occasions the monsoon's arrival will coincide with these averaged dates. This year, the monsoon has behaved largely as expected, although in central and northern parts of India it has lagged somewhat.

For Delhi, the India Meteorological Department say that the monsoon arrived three days early this year, but in the last 30 years it has arrived as early as June 15, 2008,  and as late as July 26, 1987.

These averaged positions do disguise the fact that, in some years, the monsoon can fail to appear or be very weak.  Droughts occur on a regular basis throughout the subcontinent.

Between 1890 and 1975 there were nine years of extreme drought.

Concerns regarding any shortfall in monsoon rains in the Punjab are heightened by the dramatic fall in groundwater levels in recent years.

NASA satellites revealed that as a result of the pumping of groundwater for irrigation and other human activities, water levels are dropping at the rate of more than 25cm every year.

Leaving aside the potential impact of any future manmade climate change, analysis of the monsoon over the last 30 years has shown an increase in both the variability and the number of heavy or extreme rainfall events.

With India's population set to overtake that of China by 2025, the pressures on the agricultural sector to feed this growing population cannot be underestimated.

It is likely that the monsoon rains, which give life to the country’s fields, will be studied ever more intensely in the years ahead.

Source:
Al Jazeera
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