Angry farmers in eastern Washington say the U.S. Bureau of Reclamation badly bungled water forecasts this spring, causing them to spend millions of dollars on unneeded drought-relief measures.

The bureau, an arm of the Interior Department, admitted in late May that it had been "overly conservative" in estimating the amount of water that would be available to valley farmers this year. By that, the bureau meant that a drought of historic proportions had suddenly turned into a year of close-to-normal irrgated water supplies.

But the news came too late for many farmers here who had plowed their fields under, invested in expensive deep water wells, sold their water rights or moved entire crops (like mint) to other counties with more secure sources of water.

Local bureau officials said in May they had erred in computing water seeping back into the Yakima River from irrigated farms because their computer in regional headquarters in Boise, Idaho, was programmed only to handle surpluses such as floods - not shortages.

It was an excuse many farmers found hard to swallow because the bureau had been forecasting water supplies as low as 5 per cent of normal in some irrigation districts, only to turn around later and up these figures to 71 to 80 per cent.

Being that far off the mark was inexcusable, farmers told Yakima regional manager Bill Gray in late June. Gray apologized and said the bureau would try to do better next time.

The Yakima Reclamation District, one of the West's older federal water projects, covers about 6,000 square miles in Yakima, Benton and Kittitas counties in southeastern Washington.

The region produces two-thirds of the country's hops, an important ingredient in making beer, and a third of the nation's mint. About 500,000 acres in the district are irrigated family farms.

Perennial crops like hops, mint, asparagus, seeds and fruit trees make up a third of the total farm acreage. The orchards are especially vulnerable to drought because, unlikely dryland wheat, for example, the fields cannot be left fallow during a year of dry weather and replanted the next. It often takes eight to 10 years to bring orchards to minimal production if they are replanted.

So, many farmers were more than a little alarmed when the magnitude of the western drought became apparent in late 1976.

Three exceptionally dry months had left the Cascade Mountains, which feed the Yakima River, practically denuded of snow.

In the Yakima Valley concern turned to panic in January, when the Bureau of Reclamation began issuing water forecasts for the coming growing season showing some irrigation districts could receive as little as 5 per cent of their normal water supplies - hardly enough to keep even the irrigation canals in operation, much less bring in a crop.

As the drought outlook worsened, many farmers began looking to the state and federal government to bail them out with water schemes that eventually turned out to be too expensive or too impractical to be much use this year.

In Olympia, the region's legislators managed to win approval for a $125,000 program to seed clouds over the Caseades, despite some objections from Idaho about "stealing" moisture that might otherwise drift over that state. The cloud-seeding program was carried out with some apparent success, since the snowpack in some parts of the mountains reportedly doubled after the clouds were seeded.

By March most farmers had begun to realize that it would take individual initiative - some spring rains - to get them through what appeared to be the worst drought in Washington history.

Rather than wait for the Carter administration's drought-relief program, many began to take matters into their own hands. Some moved entire crops out of the Yakima Valley and planted in the Columbia Basin, less affected by the drought this year. Others invested in expensive wells to tap water 500 to 1,500 feet below the surface, making some parts of the valley look like Texas oil fields.

Some farmers with ro crops like corn or sugar beets let their land lie fallow and sold their water rights at $300 an acre to orchardists who couldn't afford to lose their trees this year.

It was only after these commitments had been made and aftter it was too late to plant anything but corn silage and Sudan grass that the bureau unexpectedly revised its forecast upward drastically.

Irrigation districts that had expected only 5 per cent of their water-supplies in Marcch suddenly found in May they would get 50 per cent (later revised again to 70 to 80 per cent) of their normal water supplies.

"That's the difference between merely saving perennial crops and having a chance to harvest a crop," complained Henry Vancik, manager of the Roza Irrigation District.

A farmer, Paul Allison, said he had spent more than $25,000 to drill a well on his farm and would have done things differently had he known earlier that the drought was easing.

Bureau officials said in May they made a mistake in not calculating return flows, but appeared to put the blame on an unusual, unpredictable year. "We've never had a year like this before. There weren't any analysis procedures and formulas set up to determine base flows," Gray told a Prosser Chamber of Commerce meeting after the revised forecast.

Yet, the bureau's report to the commissioner, released in late June, conceded that officials had failed to follow established regulations that require they take into account as separate items the natural runoff, water accumulated in storage and the amount of water returning from irrigated fields in water forecasts.

The report and the bureau's promise to do better in the future, such as cooperating better with the National Weather Service, hardly mollified bitter farmers who are pressing Sen. Henry M. Jackson (D-Wash.) to begin an investigation of the bureau and its water practices.