TWO hours east of San Diego along the Mexican border, the sun-bleached desert monochrome abruptly gives way to the damp, sweet smell of alfalfa and a green patchwork of fields embroidered in a gridwork of irrigation canals.Most who have ever even heard of the Imperial-Mexicali Valley best identify it with bug-splattered windshields and 100-degree-plus temperatures - just a rest stop on the freeway, a crossing at the international border. But like a farmer's muddy boots and jeans which modestly disguise his millions, the rural flavor and remoteness of this valley can be deceptive. This is where the United States meets rancho grande Mexico, industrialized world meets third world. Immigration, pollution, poverty, trade, agribusiness, bilingualism, water rights - issues important to nations - take a human face here, alternately bonding or disrupting the rural border culture. Only palm trees and water towers, not high rises, break the horizon to the north and south. And yet Mexicali, capital of the Mexican state of Baja California, is home to nearly 1 million people. Almost a million square acres are under cultivation here. They look common enough for a rural area. But because of the climate and virtual carte blanche use of Colorado River water, the green is a year-round phenomenon - making crop values in the portion of the valley in the United States the eighth-richest agricultural center in the nation. The fact that anything grows here at all is the legacy of turn-of-the-century pioneer dreamers who, through a mammoth engineering and financing feat, siphoned Colorado River water across 80 miles to feed the cross-border canal system. The Imperial Irrigation District's canal system even preceded the building of roads. The valley echoes the traditional border symbiosis of northern power and wealth and southern labor and poverty. But what is different at this border point, observers say, is the small-town atmosphere and the desert climate. Even today, when air-conditioning is in common use on both sides of the border, only the hardiest of people can last a summer here: There is rarely a day, June through August, when temperatures don't reach triple digits. This heat has kept huge migrations of settlers away from the valley. It remains the kind of slow-paced, friendly place where people of all classes sink in their roots and do well.