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The drenching monsoon downpours are essential to life in India, bringing with them an end to the dry heat and much-needed rainfall for farmers.

This year's rainy season onset has brought a mixture of chaos and joy, with central India in particular rejoicing after a brutal heatwave and crippling drought — so bad that one city resorted to daily deliveries of drinking water, by train.

Forecasting the monsoon's arrival is a serious business, and as Correspondent's Report found out, India's mechanism for predicting the annual downpour is well overdue for an upgrade.

The monsoon is extremely serious business in India, and for the last few months, in the country's centre things have been pretty grim.

When I visited Latur — a city of half a million in rural Maharashtra — in April, I was told of how people there would be forced to leave without the daily trains that were hauling in millions of litres of water inland.

"If we don't get our drinking water also, how can we survive? That's the most important part of our life," one resident from Latur said.

The reason: overuse of groundwater, compounded by successive poor monsoons.

So you can imagine the excitement, amid the height of the despair, on hearing the forecast of coming relief.

"After two years of drought and as 10 states currently reel under a severe water crisis, there is finally some good news. The Met department (India Meteorological Department) is saying India will have an above normal monsoon this year," one news report said.

The rains over the three monsoon season months bring 75 per cent of the annual rainfall to India, where half of its more than 1.2 billion people depend directly on agriculture for their livelihoods.

And the monsoon's ripple effect is huge — good crops keep food prices and inflation down, put money in farmers' pockets, which they go and spend, and so on.

That first forecast was two months back now.

Then one month ago, came some bad news.

"Our top story, the heat in many parts of the country is unbearable, and the bad news is that the monsoon will be late by a week," another news report said.

"The Met department made the announcement today, saying that the rain will now hit Kerala on the 7th of June. It'll then move onto Kolkata on the 10th, Mumbai on the 12th, and by the time it reaches Delhi on the first, it'll have caught up."

So this week, I called Arun Samudre, a Latur journalist who had helped us arrange interviews while we were there, to see how things were going.

"We've had good pre-monsoon showers," he said. "And we're now looking forward to the monsoon. We are hoping it will arrive by next week."

"People are really happy here. Since the pre-monsoon was good, they are hoping that there will be heavy rainfall during the monsoon season."

British colonial prediction models still in use

Latur in Maharashtra is almost smack bang in the middle of India, which means the monsoon is still late, even going on the updated forecast.

The predictions are made with models that factor in things like prevailing winds, temperatures, El Nino and La Nina patterns, even how the Ganges flows into Bay of Bengal and mixes with the salty seawater.

But at the heart of all that, India's scientists say, is a statistical model that dates back to the 1920s, when Britain was still under colonial rule.

Mr Rajeevan, a scientist at India's Institute of Earth Sciences, said their target was to prepare block level forecasts for agriculture for the next three to four days.

"That is one. Next target is that we are issuing seasonal forecasts. That means June-September, what will happen for the state level. That is the target," he said.

Mr Rajeevan wants to radically improve the accuracy of the Met department's forecasts.

So they are ditching the old British model in favour of a US one, reworked for India.

"We will be able to generate forecasts at every 10-12 kilometres, that's what we have planned," he said.

But he admits they will need a serious boost in computer power to run it.

"Our present computer will not be enough, so we need to multiply by about 10 times. So, we are going to have another machine, a large-scale computer which is 10 times faster than the present computer," he said.

Good news for farmers, but for the hundreds of millions of Indians who call its cities home, poor drainage means several months of havoc and delays as roads turn to rivers.