Monsoon this year, which ended on Monday, was the driest in the past 10 years.

A spate in the Ganga threatened to cause a flood in the city this year but at the end of the monsoon, it emerged that the rainfall had been less than normal. The rainfall deficiency in Patna was around 40 per cent this year. Last year, it was only five per cent.

Not only Patna, 35 more districts got a dry monsoon. Only Araria and Kisanganj got sufficient rainfall this year.

The rains are expected to retreat by October 5. The extremely dry weather is expected to lead to an early onset of winter.

Ashish Sen, director, India Meteorological Department (IMD), Patna, told The Telegraph on Monday: “The state has received 30 per cent deficient rainfall this monsoon. According to IMD standards, it is moderately deficient. The main reason for it is the prolonged southward shift of the monsoon trough from its normal position.”

The monsoon trough line normally passes through Pakistan, Punjab, Haryana, Uttar Pradesh, Bihar and Bengal.

“However, this year, the trough line was normal till central Uttar Pradesh but titled from east Uttar Pradesh and passed through Chhattisgarh, Odisha and the Bay of Bengal right through the monsoon. As precipitation occurs on the southern side of the trough line, states like Odisha, Madhya Pradesh and Chhattisgarh received good rainfall. Bihar and Jharkhand, on the other hand, remained mostly dry.”

Another reason for the scanty rainfall was the absence of monsoon depression in the region.

“The monsoon depression is associated with the trough line, which forms in the Bay of Bengal and moves along the trough line. Around six to eight depressions were formed this year but only one reached Bihar. The rest moved southward because of the shift in the trough line,” said Sen.

The rain-bearing winds had entered the state on June 15. The entry of monsoon was quite good. The state received nine per cent surplus rainfall in June. However, it seemed to fall flat in July as the state received 47 per cent deficient rainfall. September also witnessed deficiency of 27 per cent.

It was because of the huge deficiency in August and September, which are considered crucial for the transplantation of Kharif paddy seedlings, that the government declared drought in 33 of the 38 districts. “While agricultural drought has more to do with weekly distribution of rainfall during the peak farming season and the consequent impact on agricultural activities, meteorological drought is declared when the rainfall deficiency at a place is more than 50 per cent,” said Sen.

Nine districts in Bihar qualified for meteorological drought this year with the highest deficiency (63 per cent) at Vaishali and Lakhisarai.

With regard to onset of winter, Sen said: “The onset of winter would be early this year, around the first week of November. This is because of the low moisture content in the atmosphere and early flow of dry westerly winds.”

Residents are worried with the dry monsoon.

Ashiana Nagar resident Arpita said: “There was hardly any monsoon this year. It seems the number of rainy days in Patna is going down every year.”

Experts are curious about the erratic distribution of rainfall.

“It rained for some days at length, while it remained completely dry on others,” said P. Partha Sarathi, faculty member and academic co-ordinator, Centre for Environmental Sciences, Central University of Bihar. “A study needs to be conducted about why localised and erratic rainfall is occurring.”