NEW DELHI (Reuters) - Rainfall in India’s annual monsoon season was below average and less than forecast, with key crop-growing northern states among areas that received less rain than needed, weather office said on Sunday.

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Rains were 91 percent of the long-term average at the end of the July-September monsoon season, compared with a forecast of 97 percent, marking the fifth straight year in which the weather office has overestimated the likely rainfall.

The monsoon, which delivers about 70 percent of India’s annual rainfall, is critical for the farm sector, which accounts for about a sixth of India’s $2 trillion-plus economy and employs about half of the country’s 1.3 billion people.

The Indian Meteorological Department (IMD) defines average, or normal, rainfall as between 96 percent and 104 percent of a 50-year average of 89 cm for the entire four-month season.

The IMD adopted a so-called dynamic model, based on a U.S. model tweaked for India, for the first time last year to improve the accuracy of its forecasts.

Despite low rainfall overall, the distribution was erratic, with some parts of the country experiencing extreme rainfall and flash floods that killed hundreds of people and damaged crops and property.

India, in its annual economic survey for 2017/18, said that the proportion of extremely dry or wet weather conditions has increased steadily because of climate change, with rain-dependent areas suffering a 14.3 percent fall in agricultural revenue owing to “extreme rainfall shocks”.

India’s rainfall was below average mostly because of a lack of precipitation in the rice and maize-growing states of Bihar and Jharkhand and in the cotton-growing state of Gujarat.

Major farm-dependent states, such as the oilseed and pulses-growing central state of Madhya Pradesh and the northern, rice-growing state of Uttar Pradesh also received rainfall that was lower than the long-term average.

The drop in rainfall could lift food prices and stoke inflation, which is expected to harden in coming months because of rising fuel prices.