WASHINGTON (AP) _ American farmers are pumping more money than ever into irrigation - with costs growing by nearly 500 percent in the past decade - and the expansion is likely to continue, although at a slower rate, the Agriculture Department says.

A continuing study by the department's Economic Research Service shows on- farm pump irrigation grew by 27 percent from 1974 to 1983, the latest year for which figures were available.

That meant another 9.5 million acres were added to irrigated cropland, bringing the total to 44.6 million acres.

But while the acreage was growing gradually, higher energy costs sent the farmer's spending for pump power skyrocketing.

By 1983, pump energy accounted for 23 percent of all energy used on the farm for crop production. Energy bills rose from $551 million in 1974 to $2.5 billion in 1983.

Electricity remained the leading pump energy source, followed by natural gas, diesel fuel, liquefied petroleum gas and gasoline. Price increases during the period ranged from more than 100 percent for electricity to 700 percent for natural gas.

Sales from irrigated farms accounted for 30 percent of overall farm sales in 1983, up 6 percent from 1974, the department said in a report by Gordon Sloggett.

Higher costs are making farmers take a closer look at conservation, including ways of recovering irrigation runoff and use of low-pressure center- pivot irrigation systems, the report said. Center-pivot sprinklers saved about $72 million in energy costs in 1983, it noted.

The outlook is for irrigation to expand by 3 million to 4 million acres in the Great Plains by the year 2020, the department said, with potential for additional gains in the eastern half of the United States.

''Soil and water resources are adequate to support an additional 18-20 million acres of irrigated land,'' the department said, while adding that further expansion depends on profitability of expanding production.

Strong prices in the early part of the study decade accounted for much of the expansion of irrigation, with the pace of increase slowing in later years as crop prices weakened.

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WASHINGTON (AP) - Thieves who have always had to contend with automobile serial numbers or cattle brands now have a new pitfall to avoid: wheat fingerprints.

An Agriculture Department researcher has developed a computerized bank of printlike patterns characteristic of the 88 most commonly grown varieties of domestically grown wheat.

In a laboratory, a chemist now can put wheat protein from a suspected stolen lot through an analysis, then compare it to the unique ''signatures'' of the cataloged varieties.

George Lookhart, the chemist who developed the comparison process, says it's as good as fingerprints for identifying where a load of grain came from.

In one case, that of a 1983 heist of five semitrailer loads of wheat near Atchison, Kan., the procedure was accepted as evidence in court.

Lookhart's computer positively identified the grain as the same variety recently missing from the Atchison River Terminal Farmers Co-Op Elevator - about $100,000 worth of wheat.

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WASHINGTON (AP) - Rice and cotton farmers eligible for income subsidies on their 1985 crops will receive a total of about $1.2 billion in the so-called deficiency payments, the Agriculture Department announced.

The USDA said Monday cotton farmers will get paid at the rate of 23.7 cents per pound, an amount equal to the difference between the 81-cent-per-pound target price and the national average loan rate of 57.3 cents. Total payments are expected to be about $800 million.

Rice farmers will receive $3.90 per hundred pounds, the difference between the $11.90 target price and the $8 loan rate. That will total abot $385 million, of which $185 million has already been paid out in advance benefits.