The Interior Department's plan to crack down on big agricultural users of water from federally financed projects has raised questions about the future of the massive Central Arizona Project (CAP).

The $1.79 billion project is a complex of man-made lakes and canals designed to bring water from the Colorado River west of the Rockies 300 miles to the desert land to east. It is regarded as key factor in the continued development of the booming Southwestern Sun Belt.

Over the next 50 years, the water is expected to be used for large-tract agriculture mainly absentee-owned - until the land is taken over for housing for the region's sprawling urban areas. CAP would then supply water for drinking and industry.

The Interior Department's plan may change all that. Under a 1976, order from a federal court in California, the department announced in Auguest a broad set of proposed regulations to enforce a 1902 law that limits ownership of federally watered land to 160 acre plots.

The regulation have drawn bitter critism, particularly from owners of large and middle-sized farms, and President Carter is expected to discuss the regulations in his meetings on Western water problems in Denver and California this weekend.

Most of the attention on the effect of the regulations has been focused on big farmers in California's Central Valley, but the impact here in Arizona and western New Mexico could be even more far-reaching.

Most of the more than 1,000 farmers scheduled to be getting the bulk of the CAP water for the next 50 years are large landowers.

Under the proposed regulations these farmers would have to divest themselves of all but 160 acres of land for each member of the landowning family living within 50 miles. Here in Arizona many farmers and their families do not live on or even near the land they farm.

Some farm experts here believe that the regulations would force farmers to drop requests for CAP water. The project could thus be left lacking consumers, despite earlier requests by farmers for three times the amount of water the CAP plans to deliver.

"You need about 700 acres minimum to make it farming here," said Ivan J. Shields, head of the Maricopa County Extension Service, which covers Phoenix and the surrounding area. "A lot of farmers won't be able to use the CAP water if they have to reduce their acreage. It will just have to flow on by somewhere else, or maybe, if there's no one to pay for it, not flow at all."

Because of the way the project is financed - a federal loan to the state with repayment to be made out of user fees - Arizona taxpayers could end up bearing a staggering burden, paying for water no one will need for several decades, according to Wesley Steiner, head of the Arizona Water Commission, which is in charge of allotting CAP water.

An average of 1.2 million acre feet of CAP water is scheduled to begin flowing annually by 1985 from the Colorado River 300 miles across the state to the Phoenix area. Tucson will tap into the system two years later, according to the CAP schedule.

At first less than 20 per cent of CAP water is scheduled to go the Phoenix and Tucson with most of the rest allotted to agriculture around the cities.

Both cities now use water pumped from underground supplies, but ground water levels are dropping by an alarming average of 10 feet per year across the state. By 2020 when the CAP is finished officials have scheduled half the project's water for the cities' use and half for agriculture.

If the agricultural demand drops because of the 160 acre regulations, however, some state officials here predict that municipal or state taxpayers could end up buying unneeded water from the CAP from the start in order to provide the user fees neccessary to pay off the project. Under the CAP financing structure they would have to pay $32.50 per acre foot for the water compared to $2 per acre foot which farmers would be charged.

State and federal officials said this week that while the possibility of such a problem exists they do not expect to have trouble finidng agricultural buyers for CAP water even with the 160 acre land restriction. But they acknowledged that they don't know how many of their potential CAP customers are large land owners who could be affected by the ruling.

Earlier this month, R. Keith Higginson, head of the Interior Department's Bureau of Reclamation, which is building the CAP, warned officials here not to challenge the adoption of the land regulations.