Heavy rain deluged some drought-hit areas as Congress began working on a package of relief for America's farmers, and the first shipment of hay airlifted from Oregon was to arrive in Kentucky Wednesday.

"Everything looks so good with a bit of rain over the past 24 hours," farmer Herman Krone said Tuesday at his 2,800-acre farm in southern Illinois, which President Reagan plans to visit Thursday during a tour of the drought-stricken area.However, Krone said, almost one-third of his 1,200 acres of corn already have been lost.

Very heavy rain fell Tuesday in sections of Texas, Louisiana, Oklahoma and Arkansas.

In Texas, more than 61/2 inches of rain fell in just 75 minutes in Mesquite, washing a pickup truck and several cars into a creek and leaving some homes 8 inches deep in water. Water was up to the tops of cars in Garland and street flooding was reported in Heath. Nearly 6 inches fell in DeQueen, Ark.

Showers and thunderstorms were widespread over the Middle Atlantic States, the Tennessee Valley and the Gulf Coast states.

Thunderstorms boomed Wednesday over the Dakotas and Minnesota.

In Washington, the House and Senate Agriculture committees began working on wide-ranging legislation which would provide payments of up to $100,000 each to farmers whose crops have been wiped out by the dry spell.

House Agriculture Committee Chairman E. "Kika" de la Garza, D-Texas, called for final action on the measure by mid-August.

Meanwhile, the Agriculture Department released a report predicting severe damage to the harvest from the drought. It estimated, for example, that corn output this year would shrivel to 5.2 billion bushels, down from 7.06 billion last year.

The dry weather is "the largest drought that our nation has ever experienced," Agriculture Secretary Richard E. Lyng told legislators.

"I think history will show that it's truly more mammoth than any of us has ever experienced," Lyng said.

The drought has dried up pastures, forcing livestock farmers to go far afield for hay.

The first of 15 20-ton air shipments of hay to Kentucky from Oregon was to arrive Wednesday, but figuring out a way to distribute it fairly will take at least a week, said state Department of Agriculture spokesman Roger Nesbitt.

The department had short notice of the shipments, which were announced Friday, and must coordinate its efforts with extension agents in 120 counties, Nesbitt said Tuesday.