The Western Area Power Administration should pay close attention to the economics of rural areas when it decides on a new marketing plan for its hydropower, some of the federal agency's customers said at a hearing Tuesday night.

The new plan "will have a direct and significant effect on the cost" of power sold to rural communities, many of whom get as much as half their power from WAPA, said Carolyn McNeil, general manager of the Sandy-based Intermountain Consumer Power Association.But WAPA should understand that national park system facilities are also powerful contributors to the economy of Western states, environmentalists and recreational users countered. They urged the agency to put endangered downstream ecosystems in the parks high on the list of concerns to be included in a new power marketing plan.

The hearing, held in Salt Lake City, was the first of five WAPA has scheduled to allow the public to comment on the scope of a new environmental impact statement that will address the sale of power from the federal dams that make up the Colorado River Storage Project.

The scoping hearings are the first step in a process scheduled to end in 1993. At that time, issues including how much power WAPA will sell from its nine hydro plants on the Colorado River, what time of day the power will be sold and who will be allowed to purchase the power will be decided.

Hydroelectricity is generated when dam operators release water to turn turbines. Because the releases are timed to produce maximum power at peak need times, river flows can fluctuate radically in the course of a single day.

The power is generated without air pollution. At the same time, releases cause downstream damage. For example, flows released from the Glen Canyon Dam have ripped out beaches and damaged fish, wildlife and recreation in the Grand Canyon. And because the dam stops natural downstream silting, the beaches and ecosystems are not easily replenished.

The Colorado River Basin dams affect the Grand Canyon and Canyonlands national parks; the Curecanti and Glen Canyon national recreation areas; and the Black Canyon of the Gunnison and Dinosaur national monuments.

While environmentalists and recreational users at Tuesday's hearing spoke of the need to halt downstream degradation of these and other recreation areas caused by heavy releases, representatives of rural electric companies emphasized the economics of their communities and said that recreational users should help pay for the benefits the dams have brought.

WAPA had completed a new marketing plan in 1986, but lawsuits filed by Utah Power & Light Co. and a coalition of environmental and recreational organizations caused the federal agency last year to capitulate to demands to prepare an environmental study before setting new contract criteria.

Though all WAPA contracts expired in 1989, the agency is operating with interim contracts while the environmental study process is ongoing. The EIS process is necessary before new long-term sale contracts can go into effect.

The agency is soliciting comment on the scope of the environmental study from the public at hearings through this month, and will accept written comments until Nov. 16.