WASHINGTON (AP) _ A House subcomittee heard calls Tuesday for an end to an ''insanity'' that finds taxpayers subsidizing irrigation water used by western farmers to produce crops that other growers are paid not to plant.

The assault on the system of conflicting subsidies came from eastern and midwestern lawmakers, environmental and watchdog lobbying groups and the chairman of the subcommittee, Rep. George Miller, D-Calif.

''This is not rational,'' Miller said as he opened a hearing by his Interior water and power resources subcommittee. ''This is a waste of taxpayer dollars.''

Miller said surplus crops such as barley, rice and cotton are grown on as much as 45 percent of the 9.7 million acres in 17 western states getting low- cost, subsidized water from Bureau of Reclamation projects.

''I have calculated that the total annual subsidy to provide irrigation water for the production of surplus crops ranges between $260 million to $376 million,'' said Miller, a persistent critic of the bureau.

The hearing centered on legislation sponsored by Sam Gejdenson, D-Conn., and 21 other House members to require farmers using bureau water for surplus crops to pay the full cost of delivering the water - a move that could force them to switch to other crops.

The proposal would apply only to new and renegotiated contracts between the bureau and farmers, who on the average pay only about 15 percent of what it costs the government to provide the water, according to Gejdenson.

He said that while the bureau's projects are encouraging surplus crop production in the West, taxpayers are spending about $24 billion a year in subsidy and other payments to discourage farmers from growing the same commodities.

''This kind of insanity destroys popular support'' for agricultural subsidies, Gejdenson said. ''Reducing the production of surplus crops will increase commodity prices and farmer income.''

''There is a clear contradiction between our agriculture and our irrigation policies,'' said Rep. Thomas Petri, R-Wis.

Opponents of the plan, including California cotton and wheat growers, said it would have little impact on surpluses because less than 2 percent of the nation's excess crops are produced on land served by the bureau.

The opponents predicted that bureau farmers would switch to other crops, perhaps creating surpluses in these commodities. They said raising water prices to bureau farmers would be discriminatory. Miller and Gejdenson said the present system discriminates against farmers who don't get subsidized water.

Savings to the government would be insignificant, according to Wayne Marchant, deputy assistant Interior secretary for water and science, and Richard Goldberg, deputy Agriculture undersecretary for international affairs and commodity programs.

Marchant said the proposal would have little impact over the next 20 years because water contracts coming up for renegotiation during the period cover less than 1 percent of the nation's land producing surplus crops.

He said the Reagan administration is working on a plan that would change the way the cost-benefit ratio is calculated for future irrigation projects in a way that will discourage using subsidized water for surplus crops.