The chairman of a House panel charged Thursday the Interior Department is purposely understating the cost of federal irrigation subsidies for Western farmers in a "politically motivated" effort to defuse controversy over the program.

Rep. Sam Gejdenson, D-Conn., said a recent Congressional Budget Office study showed the department greatly underestimated the total cost of irrigation subsidies to the government in responding to requests from Gejdenson for such information.Gejdenson, chairman of the House Interior and Insular Affairs Oversight Subcommittee, said the department told him the cumulative cost of federal irrigation subsidies over the past 80 years was $9.8 billion.

However, Gejdenson said the Congressional Budget Office, in a report he requested, put the total cost at between $33.7 billion and $70.3 billion.

Furthermore, Gejdenson said his subcommittee staff found internal documents in the department's files that showed officials in the Bureau of Reclamation, the agency in the Interior Department directly responsible for irrigation programs, put irrigation costs at $22.4 billion.

"Needless to say, this figure is a far cry from the department's `official' estimate of $9.8 billion," Gejdenson told Interior Secretary Donald Hodel in a letter.

Gejdenson said one economist in the department, Benjamin Simon, wrote a May 27, 1988, memo to his superiors expressing concern about the bureau's estimate.

"We continue to have reservations about the bureau's analysis and about the way they addressed Representative Gejdenson's request," Simon wrote. "In addition, the response does not accurately portray the analysis we performed."

Gejdenson said the internal department documents clearly showed "the department purposefully distorted the truth and refused to use standard accounting practices.

"These documents, written by the department's own economists, confirm the findings of the CBO report that the department's `official' estimates of the irrigation subsidy have no basis in reality and are politically motivated," Gejdenson said.

Gejdenson said the irrigation subsidies go to Western farmers that also receive agricultural commodity payments, amounting to an unwarranted "double subsidy."

He said the department underestimated the irrigation costs "to provide Congress with the lowest possible estimate of irrigation subsidies in order to thwart congressional action to prevent (Bureau of Reclamation) farmers from receiving double subsidies.

"I hope the department will take a close look at the CBO's report and will reconsider its opposition to ending double-subsidies," Gejdenson wrote. "Farmers should have to choose their subsidy. They should either receive cut-rate water or commodity benefits, but they should not receive both."