* Monsoon heaviest so far in third week
* Rains to recede next week in flood areas
* Showers erase drought spots in south, western India (Adds quotes from experts with next week's forecast)
By Ratnajyoti Dutta
NEW DELHI, June 20 (Reuters) - India's monsoon rains could ease soon after hitting 89 percent over averages in the week to June 19, according to weather office sources, in a third straight week of downpours that have caused major flooding in north India.
This year's monsoon has drenched the country in record time, almost a month ahead of schedule. Heavy rains in north India have swollen the Ganges, India's longest river, and floods have swept away houses, killing more than 80 people and leaving tens of thousands stranded.
For India's farmland, above average early rains help soften the soil, leading to plantings of summer crops such as rice, soybean, cane and cotton, and the June to September monsoon is crucial for the 55 percent of arable land without irrigation.
India's monsoon rains are expected to ease in the next week and forecasters see normal rainfall in July and August.
"Rainfall activities are expected to weaken in north, northwest and southern India," said D.S. Pai, the lead forecaster of the India Meteorological Department.
If flooding persists, stagnant water could delay sowing or damage early rice shoots.
The farm sector accounts for about 15 percent of a nearly $2-trillion economy, Asia's third-biggest.
"The monsoon has been beneficial so far over drought areas of south and western India," said Sudhir Panwar, president of Kishan Jagriti Manch, a farmers' group.
Weather officials also said the monsoon rains will be more on the west coast next week, while the rain-starved northeast region will also have increased rainfall. (Editing by Jo Winterbottom and James Jukwey)