Ninety percent of our country's population growth is due to immigration and the high birth rates of immigrants. At the current growth rate, our population will triple from today's 300 million to almost 1 billion before the end of the century. The Senate immigration bill, with its massive amnesty and its impossible enforcement provisions, would probably boost that number even higher. The issue is not the color or race of these 600 million additional people, it's their sheer numbers.

How many people can this country sustain, and at what quality of life? The first limiting factor is water. Nowhere is the clash between population growth and water more dramatic than here, in Southern California. This has been the driest year in recorded history, and the snowpack in the Sierras, from which we import most of our water, is at historic lows.

Our other water source, the Colorado River Basin is, itself, in the grip of an eight-year drought. This, climate scientists tell us, is no anomaly, it's the future. It's also the past. Tree-ring studies from living and fossil trees show that prolonged droughts, some lasting for decades, are historically normal weather for this region. Add in global warming, which is predicted to reduce average precipitation in the Rockies and the Sierras by at least a third and the flow of the Colorado River, lifeblood of seven fast-growing states, by up to one-half.

Occasional floods and hurricanes notwithstanding, water shortage is the story over most of the country. As we drain our underground water sources to support a growing population, natural vegetation that depends on groundwater is left high and dry. Across the country, vast forest fires, on a scale we have never before seen, have incinerated millions of acres of drought-stricken trees. Even tropical Florida has recently been ravaged by drought-invoked fires.

We have already exploited all our major rivers to the limit and, nationwide, we now extract about 25 percent more from underground sources than is replaced by rain and snowfall each year.

The deficit is far higher in the Ogallala aquifer, the great, natural underground reservoir that underlies our Great Plains. This water, a remnant of the last ice age, is what has enabled us to turn an arid prairie into the "amber waves of grain" that feed our country and much of the rest of the world. We extract 4 to 6 feet of water from the Ogallala each year, while nature puts back less than an inch. Already, over vast areas, wells that once watered fields of wheat and corn now come up dry no matter how deep they're driven.

Meanwhile, our nation adds 1.2 million new water consumers every year. Unless we do something real to curb immigration both legal and illegal (the current bill does neither), we face a future with one-third less surface water, with much of our groundwater exhausted, and with three times the number of people to share what's left.

Lynnette M. Perkes lives in Poway.