We weren't the first. The enterprising Mesopotamians, on the banks of the Euphrates, also built a civilization on irrigated agriculture. So did the ancient Zunis of New Mexico and others in dry regions around world. But one by one, salt did them in.
``There never has been a society that depended upon irrigated agriculture that has survived,'' notes Robert Sweazy, director of the Water Resources Center at Texas Tech University in Lubbock.
The US has some technological answers to the salinity problem -- which is caused by irrigation. But in some important farming areas, the problems have become severe and the answers may be too expensive.
Today, the West uses more than 90 percent of the nation's irrigation water. The Southwest, more than all other regions, was developed on irrigated agriculture.
Salt build-up is probably cutting crop production in a third of the nation's irrigated land, according to Jim Rhoades, research leader at the US Department of Agriculture's Salinity Laboratory in Riverside, Calif.
Salts of all kinds are found in both soils and water. Particularly in hot, dry climates, some irrigation water evaporates, leaving a residue of salt in the soil. Often, when irrigation water passes through the soil, it gathers salt with it as it runs off the field or seeps down to the water table.
The problem came to a head last spring at Kesterson Wildlife Refuge in central California.
Waterfowl were being killed by heavy doses of selenium-a natural salt in the region's soil, that had been concentrated by water draining into the refuge from local farms.
The Interior Department threatened to close down farms in the area, but decided instead that farmers who irrigate must divert all drainage from Kesterson by next June.
But Kesterson is merely the most dramatic instance of a problem that affects as many as 1.5 million acres of irrigated land in California -- including the nation's most productive fruit and vegetable regions.
Salinity problems can be found throughout the Southwest, but most are minor compared to the acreage affected in California's breadbasket, the San Joaquin Valley.
To keep salts from building up, farmers must put more water on the fields than the crops require, in order to wash the salt out of the soil. This works, as long as there is somewhere to put the excess salty water.
All along the Colorado River, farmers drain used irrigation water back into the river, sometimes making it 16 times more salty when it reaches the Mexican border than it was near its source. In the Imperial Valley of southeastern California, fields are drained into the Salton Sea, which is now becoming so salty it risks becoming a dead sea within a decade or so.
But on the west side of the San Joaquin Valley, the drainage water has nowhere to go. The federal project that irrigates much of the region was supposed to include a drain into the Sacramento-San Joaquin Delta, but the drain was never completed. The delta's ecology is fragile and there is great opposition to draining brackish water there.
The valley has a shallow water table, sealed underneath with a clay layer. The water table has grown salty and risen to the root level of the crops, because of years of irrigation. Some farmers use tile drains to drain the water from under their fields. Many more need to, but there is no place to send the excess water.
Ideas for solving the salinity problem, or for buying time, range from the obvious to the exotic. But as new sources of fresh water become more expensive, all the ideas begin to look more practical.
The most logical way to help solve the salinity problem, is to allow less excess water to end up on the ground instead of on the crops. An example is the Wellton-Mohawk Irrigation District in Arizona. The district was emptying salty water into the Colorado River, as do many others, and Mexico complained about the salt increase in the river.
So the US Bureau of Reclamation put Wellton-Mohawk farmers on a crash program of water-dieting. Six-thousand acres of citrus crops were removed from sandy soils that didn't hold water well. Sandy patches in fields have been replaced with better soil. Ditches were lined with concrete, and dikes were built around fields. Crews with special moisture-sensing probes now help farmers schedule irrigation precisely when crops need it. The district's flow has been cut by half.
The remaining flow will be filtered through what will be the world's largest reverse-osmosis desalting plant. After careful cleansing, the water will be forced through a plastic membrane to strain out salts. The desalter will be completed in 1989 or 1990 at a cost of $255 million. Some water engineers and soil experts, frequently point out that it may have been cheaper to cut Wellton-Mohawk's brackish drainage by buying out all the farms in the district. California's Department of Water Resources is building a small reverse-osmosis desalter of its own in Los Banos, in the central valley. The goal is to produce clean water from salty water, at a cost of $350 to $450 per acre-foot (one acre of water one foot deep). This cost would be comparable to that of new or more traditional water sources.
Mr. Rhoades of the US Salinity Lab wants to recycle drainage water. By rotating between salt-tolerant and salt-sensitive crops, and between irrigation with fresh water and drainage water, he claims crop production can be maintained using less fresh water.
In the central valley, this could reduce drainage to one fourth of its present flow, says Rhoades. He has been testing the idea on lab farms for seven years with good results. But farmers are by habit reluctant to put salty water on their fields.
Blaine Hanson, a specialist in irrigation and drainage for the California agricultural extension service, proposes that farmers manage their shallow water tables at a depth of about three feet below the ground. Salt-tolerant crops with deep roots could then draw on that water, cutting the need for fresh irrigation.
Farmers near Kesterson are also looking into deep-well injection -- pumping the brackish water into the ground well below any groundwater deposits.
Another idea has also been suggested. Ralph Abascal, general counsel for California Rural Legal Assistance, proposes allowing San Joaquin Valley farmers to retire their land and sell their federal water to Los Angeles at a profit until they have recovered the cost of their land. Ariculture take money and invest elsewhere. Of the federally-irrigated land in California, he notes, fully 59 percent is planted in surplus crops.
Mr. Abascal's notion may be radical, but the economics of farming the central valley are already changing in some degree. All over the West, water purity is becoming a greater concern as finding new sources of fresh water grows more expensive. The old bromide that ``the solution to pollution is dilution'' becomes less practical.