As most of the country's farmers face federal farm program cuts and the gloomiest economic picture since the Depression, Congress is about to extend big new subsidies to irrigation-water users in the West.

A coalition of western agribusiness groups, irrigators and legislators is pushing through a generous rewrite of the 1902 Reclamation Act, which provides below-cost irrigation water to farmers in 17 states.

A House-passed bill and a more bounteous version pending in the Senate would broaden, in a variety of ways, the tax-subsidized benefits available to federal water users in the arid reclamation states.

With support from the Reagan administration, despite its professed insistence that farmers pay their way without federal subsidy, the western coalition is pressing for quick final action to avoid the strict enforcement of the 1902 law ordered by federal courts.

The fight, bubbling for years in Congress, appears to be virtually over.

Opponents--family farmer, environmental and religious groups--concede that their hopes for reform are down the drain. Traditional liberal critics of the water subsidy, typified by Rep. George Miller (D-Calif.), have thrown in the towel, saying they have done all they can.

Senate floor action on a committee-passed bill has been blocked temporarily by Sen. Richard G. Lugar (R-Ind.), who questions increased water subsidies while farmers elsewhere are being urged to get along with less federal aid. Lugar, aides say, may offer a substitute measure.

Although details remain to be worked out, the new legislation would:

* Increase the limit on subsidized water from the present 160 acres (640 per family) to at least 960 acres, even though about 96 percent of reclamation farmers already are under the 640-acre limit. The House bill sets a 960-acre limit; the Senate bill allows up to 2,080 acres.

* Require that "full cost" be paid for irrigation water. True full cost, according to the Congressional Budget Office, would produce $16 billion in new payments to the government. New revenue from the House "full-cost" version is estimated at $3 million. The federal subsidy in some areas ranges as high as $800 per acre.

* Provide permanent subsidies to big companies and wealthy growers; authorize absentee and new corporate ownership; allow owners of land in excess of acreage limits to determine who will get their water rights; permit leasing of additional land at higher, but still subsidized, water rates.

* Exempt U.S. Army Corps of Engineers projects in California, where major corporations receive irrigation water, from the law's limitations. Corporate operators in those projects include the J.G. Boswell Co., the world's largest cotton grower, Superior Oil, Tenneco and Chevron USA.

The House bill, passed earlier this month on a 228-to-117 vote, was virtually unchanged from the version adopted by the Interior Committee, dominated by westerners and lobbied heavily by the irrigation interests.

Their power drowned out protests by the National Farmers Union (NFU) and the National Grange, among others, who charged that the measure would guarantee an end to the small-farm development envisioned by the 1902 law.

NFU president George Stone said both the House and Senate bills "propose to legalize vast subsidies to a relatively few, but large, corporate farms and investment syndicates."

Rep. James Weaver (D-Ore.) agreed, calling the House version "a bald-faced anti-family farm package of direct subsidies to the richest of America's agribusiness interests . . . . How can Congress justify a fat subsidy to the richest agribusiness concerns when the rest of the nation's farmers are facing ruin?"

Reps. David F. Emery (R-Maine) and Dale E. Kildee (D-Mich.) made similar criticisms. Kildee, for example, said the bill "would really legalize that which the court has declared to be illegal."

Their arguments made little impact.

Rep. Abraham Kazen Jr. (D-Tex.), chairman of the subcommittee that drafted the bill, said, "There have been some abuses, as there are in any program of this nature. But there have been substantial investments made in labor, as well as in money, by persons who in good faith have relied upon the representations of their government. They must be treated fairly, as must the government itself. This is a fair bill."

Miller, long a critic of some of the features included in the new bill, called it "the absolute limit" of what the House could accept. He said the Senate version was "so horrendous and injurious to the public interest" that the House should not consider a conference to work out differences unless it is changed "radically."

The reclamation dispute boiled up in the mid-1970s, when National Land for People, a small-farmer organization in California, sued over what it termed illegal enforcement of the 1902 law.

A federal court upheld Land for People, ordering the Department of the Interior to prepare new regulations that would assure that acreage limits, residency requirements and other strict features of the law were enforced.

The alternative, the court held, was for Congress to rewrite the law. This set off a massive lobbying effort by the western water interests to avoid the tough enforcement that looms if the new regulations take effect.