Phoenix, Ariz. — Those cracks slicing the earth here are still the most blatant clues to the burden southern Arizona cities place on the American Southwest. In places the scrubby, caramel-colored earth has cracked like a moist cookie, creating crevasses one can barely leap across, sometimes tens of feet deep.
Why? Since the early 1920s, Arizona has been pumping water out from under itself faster than it can be replenished, so the ground is sinking. It has dropped five feet beneath the random urban sprawl of northeast Phoenix and is still dropping there six inches a year.
This is how badly southern Arizona needs the water that, beginning in 1986, it will siphon from the Colorado River through the massive Central Arizona Project - known casually here as ''the ditch.''
Arizona Gov. Bruce Babbitt puts it in more expansive terms: ''Without that project, Tucson would eventually go the way of the cities of Mesopotamia.''
Tucson could not live out the next century, he says, on its groundwater supplies alone. Phoenix, though somewhat better fixed for water, would also face limits to growth eventually without the Central Arizona Project (CAP). The project will pump 1.3 million acre-feet of water in an average year from Lake Havasu on the Colorado to Phoenix and Tucson.
It won't fix Arizona's water problems. When the 330-mile, $2.5 billion set of aqueducts is complete, around 1992, urban Arizona will still be overdrawing its water supplies, and the ground will still be sinking. But CAP will cut 60 to 70 percent of present water overdraft, and actual balance in the water table - a legislated goal in the state by 2025 - will become reasonable.
Diverting water from the Colorado across the Arizona desert has been discussed for a century. Engineers began proposing plans in 1918. The current project was first put before Congress in 1947. It was 1968 before it authorized the project.
Meanwhile, in more than 30 years of legal suits with California and Indian tribes, Arizona has established its rights to the river - 2.8 million acre-feet of the Colorado's 15 million acre-foot flow in a normal year.
(An acre-foot is enough water to cover an acre to a depth of one foot.)
To the extent to which the water comes out of anyone else's supply, it comes out of California's. Well over a fifth of the Colorado River's normal annual flow goes to irrigate the farms in California's dry southeastern corner. This farmers' portion won't change.
But the Metropolitan Water District (MWD), which supplies 70 percent of San Diego's water and 2 percent of Los Angeles's, will have its share of the river cut by over half when the CAP starts pumping. The MWD plans to make up the difference with water from northern California. It will need expensive new aqueduct systems to get it, but the state actually has twice as much water as it uses. San Diegans are not likely to be caught short.
Arizona is on a much more stringent water diet.
More than half of the water used in the region CAP will service - a broad swath of south-central Arizona that takes in Phoenix and Tucson - is for agriculture.
Farm acreage here has virtually been capped. Ninety-nine percent of all Arizona farmland requires irrigation, and growers here are prohibited from irrigating any land that was not already under plow in the late 1970s.
Further, for every acre-foot of CAP water that a farmer buys, he must cut his use of groundwater by an acre-foot. Heavy water users will be gradually eased toward a standard usage of about five feet of water per year.
Municipal water users, who are steadily taking over farmland for housing tracts, shopping centers, and industrial parks, will face a different kind of discipline - a 10 percent across-the-board cut in water use.
How this cut is made - whether by raising prices or allocating quotas - will be left to each municipal water company. Everyone has priorities. The affluent Phoenix suburb of Scottsdale, a state water official notes, sprinkles 25 to 30 percent of its water on golf courses. In rural Buckskin to the southwest, some farmers actually have to pump groundwater out of their fields to keep them from saturating.
Arizonans are currently waiting for Interior Secretary William Clark to approve ''plan six,'' a billion-dollar set of southern Arizona dams and dam improvements that would add another 150,000 acre-feet of water a year to the CAP canals. (These are federal projects, but Arizona users eventually pay for them.)
All this considered, urban Arizona may still be drawing 200,000 to 225,000 acre-feet each year beyond what returns to the water table, according to Dennis Sundie, a state water planner.
If so, water engineers may try water-saving strategies like weather control and coating hillsides with salt solutions to improve the watershed. The last resort, Mr. Sundie says, is for the state to begin buying farms to take them out of production.