A federal judge here, chiding the Reagan administration for "benign neglect" of federally designated wilderness areas, has ruled that wilderness lands have the same legal right as farms and cities to water in the arid West.

The decision by U.S. District Court Judge John L. Kane Jr. -- the first ruling that wilderness areas have a "reserved water right" -- sent shock waves through the water industry in the West because it establishes a new competitor for the region's sparse water supplies.

In western water law, a "water right" is the authority to use a certain amount of water each year from a specific lake, reservoir or river.

The water rights -- as precious as gold or oil -- are divided carefully by state governments.

Kane's decision says that Congress intended to reserve specific "water rights" for wilderness lands when it passed the Wilderness Act in 1964.

Wilderness areas are defined in that law as lands in a natural state "where man is a visitor who does not remain."

Under Kane's ruling, the federal government may be forced to go to state water courts in the West and demand that specific amounts of water be reserved for the plants and animals on wilderness land.

That would leave less water for agriculture and municipal use.

Kane's decision is a clear rebuke to the administration, which has argued -- in Kane's courtroom and wherever else the question has arisen -- that wilderness lands have no "reserved water right."

During Jimmy Carter's presidency, the Interior Department issued an opinion holding that there is a "reserved water right" in wilderness areas.

That regulatory decision was cheered by environmentalists but denounced by farm and city water users throughout the West.

When Reagan named James G. Watt to be interior secretary in 1981, Watt quickly reversed the department's opinion.

In his decision this week in Sierra Club v. Block, Kane in effect ordered the Reagan administration to abide by the Carter administration's regulatory decision.

"I am dismayed by the federal government's benign neglect of this issue of federal reserved water rights in the wilderness areas," Kane said in his opinion.

" . . . If the Reagan administration had carefully analyzed the legislative history of the Wilderness Act, Congress' intent to reserve water for the wilderness areas would have been apparent," the opinion said.

Kane ordered the National Forest Service and the National Park Service to come up with a plan by April "to protect wilderness water resources."

Attorneys on both sides of the case said that, to comply with Kane's order, the federal government will likely have to go to water court and assert a right to the water.

That court action would put the federal agencies in direct opposition to farm and city water users who want rights to the same water.

Kane's decision is binding only on the 24 wilderness areas in Colorado.

But since his is the first federal court ruling on the question of a wilderness "water right," it is likely to influence similar cases in other western states, including Nevada, Montana and Utah, where the question is in dispute.

Attorneys for the Denver Water Board, the Colorado Farm Bureau and the National Cattlemen's Association -- which joined the Reagan administration in the suit opposing the wilderness "water right" -- said they would appeal Kane's decision to the 10th U.S. Circuit Court of Appeals here.