An Indian think-tank, which aims to forecast the annual rainfall based on an analysis of historical data said on Wednesday that the overall monsoon this year would be 15 per cent below the 120-year long-term average.

The forecast by the Bangalore-based Centre for Mathematical Modelling and Computer Simulation was worse than that given by the Indian Meteorological Department in April.

The IMD said this year's monsoon rains in the June to September season would be 96 per cent of the long-term average.

Anything between 90 and 102 per cent of the average is considered good for the economy, which was hit last year by India's worst drought in 15 years.

"Basically, there is a 15 per cent deficit for the whole country for the entire year," Prashant Goswami, scientist at the Bangalore institute, told Reuters.

The Centre uses sophisticated software to predict rainfall based on an analysis of patterns of past data.

Goswami said that last year the method had failed as it forecast rainfall at 99 per cent of the long-term average compared to actual rainfall of only 82 per cent.

But he said its forecasts had been right in seven of the eight years for which it has made predictions so far, with an average error of less than four per cent until 2002.