Washington — The United States is as wet as a waterlogged sponge. Record storms have soaked large areas of the country, ruining weekends, turning cities into lakes, and loosing walls of mud that rumble downhill like runaway semitrailers.
So far this spring, rainfall for the whole country is about 150 percent above normal, according to the National Weather Service.
In the West, the problem is that winter lasted too long. Unusual spring storms have mantled mountains with record-breaking blankets of snow; in the Sierras, snowpack is 200 percent of normal.
Typically, Western peaks are almost free of snow by now. But this year the snow has barely begun to melt.
''This is the wettest weather I've seen in 40 years,'' says Robert Clark, chief hydrologist for the National Weather Service.
This run-off was a major factor in the flooding that hit Nevada and Utah last week. Over the next month, as weather warms and more melted snow comes gushing down from the heights, floods could spread to California, Colorado, and New Mexico, Dr. Clark says.
''There's going to be a lot of water running off for a long time,'' he says.
In the East and the southern Mississippi valley, the problem is ground that's sopping wet. Much of the Eastern US is so saturated that two inches of rain in three hours would cause flash flooding. In the lowest-lying states - Louisiana, Mississippi, and Arkansas - almost any rain at all might loose a flood, Clark says.
But widespread flooding isn't inevitable, he adds. If temperatures climb into the 80s in the East and helps dry up soggy soil, and if the Western snowpack melts relatively slowly, serious problems can be avoided.
And in any case this spring's weather is far from the most destructive to hit the US in the recent past.
''It's no comfort if your feet aren't dry,'' says Bob Blair, an official with the Federal Emergency Management Agency (FEMA), ''but we've had much more severe weather'' within the past 12 years.
In the spring of 1973, Mr. Blair points out, Mississippi River crested above flood stage six times, reclaiming 16 million acres of land and causing $1.2 billion worth of damage. In 1972, tropical storm Agnes went ripping up the East Coast, causing $2.5 billion in damage, according to FEMA figures.
This year's harsh weather has affected larger areas of the country than the storms of previous years, but in terms of damage caused and people injured, ''things are better this year than in some previous years,'' says Blair.
So far in 1983 President Reagan has issued nine federal disaster declarations , all but one because of flooding. Parts of Louisiana, Washington, California, Mississippi, Utah, and the Virgin Islands have been labeled disaster areas, making them eligible for federal cleanup aid.
It will cost about $460 million in federal obligations to clean up these areas, according to preliminary FEMA estimates. The money will provide a wide variety of services that states and localities often can't afford on their own.
Uncle Sam pays for temporary housing costs for those fleeing a disaster zone, for instance. Three-fourths of the cost of fixing roads, sewers, and other public works is picked up by the federal government.
''A plethora of federal agencies are involved in disaster relief,'' says Jim Jennings, a disaster program specialist with the Small Business Administration.
The SBA provides disaster loans to businesses and individuals to cover uninsured losses. Farmers whose crops are damaged can have their losses covered by low-interest loans from the Farmers Home Administration.
Even the IRS has something to offer. If you live in a federally designated disaster area and your home or business gets swept away, you can write off the loss against last year's taxes - enabling you to collect an immediate tax refund.