With India in a "low epoch" since the 1990s, a lower average rainfall forecast was likely. (File)

The weather department said today that it was set to cut estimates for average monsoon rainfall after decades of below-normal downpours, with climate change causing greater variations.

India is grappling with a severe water crisis, with emergency supplies sent to Chennai after the drought-hit city saw only a fraction of the rain it usually receives during June and July.

The India Meteorological Department climate research chief Sivananda Pai said the country was in the middle of a multi-decadal epoch of low rainfall.

"If you take an average of 30 to 40 years, compared to say a 100 years of normal rainfall, we are passing through a below-normal rainfall," he told news agency AFP.

The current average of 89 centimetres (35 inches), he said, was based on the agency's observation from 1951-2000. The government agency revises the "normal" rain baseline every decade.

With India in a "low epoch" since the 1990s, meaning average rainfall has been below normal, a lower average rainfall forecast was likely, Mr Pai said.

"It was around 88 centimetres during the period 1961 to 2010. When the new normal is extended to 2020, a further decrease is possible," he added.

Rainfall for June in India was 112.1 millimetres compared to the average of 166.9 millimetres, a deficit of 33 percent according to the weather agency.

Mr Pai said while average rainfall levels can change over the decades due to natural variability, "we can't ignore the linkages to climate change".

"Heavy rainfall and long dry periods can be linked to climate change. This has been the case across the world," he said.

In contrast to the crisis in Chennai, other parts of the country's north and east have been grappling with heavy flooding which has killed hundreds of people.