WASHINGTON — The federal government started calling agencies and departments Thursday to tell officials which of them would stay open and which could close in the event of a partial government shutdown Saturday.

It also started telling federal employees who'd keep working — albeit with IOUs in lieu of paychecks — and who'd be furloughed. Roughly 800,000 would be laid off, their pay subject to an act of Congress later.

Among the federal offices and services that would close: parts of the Internal Revenue Service, national parks and monuments, most of the Education Department.

Many agencies would keep operating. Airports would remain open and air traffic controllers would work. The National Weather Service would keep issuing alerts for floods or tornadoes. Social Security checks would go out. The mail would be delivered.

All of it was part of an intricate contingency plan put together by the White House Office of Management and Budget, which manages the executive branch of the government. Congress made similar plans for the legislative branch; the Administrative Office of the U.S. Courts handled the planning for the judicial branch.

All would run out of money at 12:01 a.m. Saturday unless Congress and the president reach agreement on a budget for the rest of the fiscal year, which runs through Sept. 30.

"We still hope that we do not have to execute the plans, but we are prepared to implement if necessary," said Jeff Zients, the deputy director of the OMB. "If there is a shutdown, it would have very real effects on the services the American people rely on, as well as on the economy as a whole."

Generally, the government would keep operating any services that are deemed essential to protecting life and property, as well as any that are financed through user fees or multi-year appropriations already approved by the Congress and the president, among others.

Those who work would be paid retroactively. Those who were furloughed were paid retroactively the last time the government shut down, in 1995 and early 1996. President Barack Obama would support that again, aides said, but it's unclear whether Congress would agree.

Here's how a partial shutdown would affect some parts of the federal government:

OPEN:

• Air traffic control. Air traffic controllers would work, and airports would remain open.
• National Weather Service. It would stay open to monitor floods, storms and tornadoes.
• Federal courts would be open, completely staffed for a time thanks to fees and other funding not provided by Congress. "We are going to be open for at least 10 days," said Karen Redmond, a spokeswoman for the Administrative Office of the U.S. Courts. "Beyond that, each court is making contingency plans."
• Customs inspectors and Border Patrol agents would work.
• Federal prisons would remain open, guards on duty.
• Food stamps would be distributed.
• NASA's satellite missions would remain operating.
• Military. Troops would remain on duty. They'd be paid retroactively once Congress and Obama sign a budget deal.
• Mail. The Postal Service would still deliver the mail.
• Social Security. Checks would still be sent out to current beneficiaries.
• Medicare. Payments would still be made.
• FBI and other federal law enforcement. Would keep working.
• Courts. Federal courts would be open.

• Parks. National parks and monuments would close. The Smithsonian Institution's museums and the National Zoo would be closed.
• Small Business Administration. The SBA would stop approving applications for loans from small businesses.
• Federal Housing Administration. The FHA would stop guaranteeing mortgage loans, which could have a significant impact heading into the spring home-buying season, the year's busiest.

• IRS. E-filed tax returns would be processed. Payments would be collected and refunds sent out automatically. But paper-filed returns — about 30 percent of the total — wouldn't be processed, and refunds would be held until furloughed employees could return to work. Audits would be postponed.
• Civilians at the Department of Defense. Those whose work helps protect life or property would keep working. Others would be sent home, apparently without pay.
• Environmental Protection Agency. The EPA would continue to monitor for radiation reaching the United States from the damaged nuclear power plant in Japan. It would stop doing environmental impact statements and issuing permits.
• National Institutes of Health. Would continue to treat patients in clinical trials, but would stop accepting new patients or starting new clinical trials.
• Congress. Each member of Congress would decide who among their staffs would keep working and who'd be furloughed. Rep. Darrell Issa, R-Calif., for example, said that all his staff members were essential. Sen. Joe Manchin, D- W.Va., said he'd return his paycheck to the Treasury for work during a shutdown.

• White House. The president and Vice President Joe Biden would keep working. The Secret Service would remain on guard. But many political aides would be sent home.

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