The great drought of 1988 is the costliest natural disaster in America's history, with economic losses exceeding those of any previously documented drought, earthquake, hurricane or flood - and consumers are likely to pick up the tab.

Farm production alone will be reduced by an estimated $11 billion to $15 billion, the government's Interagency Drought Policy Committee reported Friday.Consumers, rather than farmers, will pay for most of the losses, and the impact will linger well into 1989, the committee and other economists agreed.

The lost farm production will affect one of the country's most important economic indicators, the gross national product, reducing the index's growth rate slightly this year, the report said.

"I think in current dollar terms there is not much doubt this is the biggest physical disaster that (agriculture) has encountered," said Neil Harl, an agricultural economist. "And except for war . . . it ranks well up with all kinds of disasters."

Prior to this year's drought, the nation's costliest national disaster in terms of economic losses - not in terms of death or injury - was Hurricane Agnes, according to disaster researcher James Cornell.

That storm caused losses in Pennsylvania and southern New York State totaling $10.24 billion, when translated into 1988 dollars.

In the 1988 drought, "Consumers will be net losers," said Mark Drabenstott, assistant vice president of the Kansas City Federal Reserve Bank.

And although many individual farmers will suffer significant losses, net cash farm income - the key measure of farm economy health - is likely to approach the levels of 1987, when the weather was normal. The reason: The drought has inflated prices farmers are receiving for commodities they do have to sell.

Most of the losses consumers pay will come in the form of higher food prices. The drought could hike the average annual food bill by $23 per person this year and by between $40 and $50 next year, based on one set of government indicators.

"When you have a disaster of this nature (with a demand for food that remains constant despite price increases) it boosts price and profitability disproportionately so it tends to cover up the loss, and it extracts it from the consumer," said Harl.

The government's drought relief legislation, which President Reagan signed into law Thursday, will also help to shield the farm sector from the drought's effects.

Although it will take months to fully assess the drought's damage to the national and rural economies, its wide-ranging impact is beginning to emerge in government data. For example:

_The value of corn lost because of drought is an estimated $5.24 billion while lost soybean production is valued at an estimated $2.41 billion. (Those figures are based on the average price farmers recieved for those crops over the last four years. Using that formula was suggested by several economists consulted by the Los Angeles Times who said using this year's drought-inflated prices to compute losses would be unrealistic.) The corn and soybean loses are likely to increase if the bad weather persists _ as government forecasters predict _ and the autumn harvest falls short of current government expectations.

_The barge industry, which moves billions of tons of farm commodities, coal, petroleum and chemicals on the nation's drought-lowered inland waterways could lose up to $1 billion this year, according to Friday's drought assessment.

_Although there is no dollar estimate yet, the drought will affect the timber industry for the next 10 years to 20 years. So far this year, 62,000 wildfires have damaged 2.7 million acres _ more than double the damaged in 44,000 fires reported by this time last year.

_As hydroelectric power facilities are affected by reduced river flows, utilities are being forced to buy energy at higher costs to meet local and regional demands, raising electricty costs in some regions. So far, hydroelectric generation nationwide is down by 13 percent from last year. Losses to power companies are likely to run into the hundreds of millions of dollars.