The monsoon, the lifeblood of India, has come to this country's parched northern plains, bringing optimism that a good new crop will alleviate a year of drought and famine.

This year's season of drenching rains started strong and early on the Indian subcontinent in late May, and they now have reached the northern tier of the country.

Last year the monsoon was late and weak, and the ill effects still are being felt 12 months later.

More than 200 million Indians aresuffering from the worst drought of the last 100 years. Itcut foodgrain production by nearly 10 percent and caused the deaths of an estimated 1,755 persons. Most of them died because of severe shortages of drinking water.

Only its grain stocks from good harvests of past years -- brought aboutby strong monsoons -- saved India from the severe famine that accompanied previous droughts.

Indeed, for the 80 percent of the Indian population living in villages and dependent on agriculture, the monsoon is a blessing, although often a mixed one.

The prospect of a bountiful harvest in 1980,along with the break in the constant heat of more than 110 degrees that New Delhi has sweltered in since early April, is enough to make residents here put up with the temporary inconvenience of flooded streets, mildewed closets and what seems to be an epidemic of ants and bugs.

Even the extreme humidity, which leaves clothes soaked with perspiration minutes after they are put on, seems bearable.

Each day now the clouds open up and rain pours down in sheets.

The monsoon is a time of happy release. Children show it most vividly as they strip to their shorts and frolic in the deep puddles that quickly form on streets and sidewalks. Five children sat on the steps of a house the other day, their faces turned to the sky and their arms outstretched as if they were begging for more rain.

Even adults do not seem to mind getting wet. Instead of dashing for shelter as Americans might, Indians walk calmly through the downpours, apparently unconcerned that their clothes are getting drenched. Women stroll with their normally loose, flowing saris plastered to their bodies, while men ride bicycles or motor scooters through the rain as if the day were clear and balmy.

The rains come with violent intensity. The humidity and pressure seemto build and build until, finally, they can no longer be contained. Then the skies open up, and the rains pour down.

The temperatures have dropped to 98 degrees. Even though that may seem extreme, at least it is cool enough to give air conditioners a fighting chance.

Parched brown lawns have turned green overnight, and the dust that has choked the city for the past three months has magically disappeared.

In its place, though, are pools of mud and badly flooded streets. Although New Delhi claims to have a drainage system, it clearly does not work.

As a result, an eight-foot-deeplake suddenly appeared on a main street in the center of New Delhi during a rain storm last week, submerging a city buswith 36 passengers on it. The New Delhi fire brigade rushed two boats, four life preservers and two rubber rafts to the scene to rescue the passengers.

Even the flood-control headquarters was caught unprepared by this year's monsoon. The first rains poured into the chief secretary's office through a badly leaking roof.

While the floods in New Delhi cause only temporary discomfort, they are a serious matter in other parts of the country, which seem to have either too little rain or too much.

The United News of India reported today that floods in western and southern India had taken 24 lives so far and forced hundreds of thousands of Indians to evacuate their homes. About 5,000 people have been relocated in relief camps as part of a $375 million government flood-control program, officials said.

Two dozen dams in the west central Indian province of Gujarat are threatened by heavy downpours that officials fear could cause a flash flood like the one that drowned 1,000 persons in that same area last August.

At least 2,000 homes have been damaged therealready, several villages and large areas of farmland are under water, and thousands of persons have been evacuated to higher ground.

That is the price of the monsoon. Yet without it, India would be an arid wasteland. Its growing population of 650 million would face starvation.