many parts of the SRP have declined significantly, and it is gradually becoming clearer that the ground water supply, which is renewed very slowly, cannot be mined indefinitely just because it is the cheapest alternative at hand. Dealing with the ground water over-draft is a little like dealing with our federal budget deficit: it is relatively cheap and painless in the short run just to let the deficit ride or even grow, but this course passes the burden to future generations. Slowly but surely, even in the conservative Phoenix basin, where government intervention is despised as much as any place else in the country, the necessity of government control of ground water mining is slowly becoming accepted. As taxes are levied to decrease consumption, ground water will become more and more expensive to pump.

Yet, the situation is not at a crisis point in the Phoenix area—at least not yet. There is ample room for conservation to expand supplies. Conservation by both agricultural and urban users is really just beginning. Reuse of sewage effluent will become much more important and will provide the valley with additional water for either future growth or for reducing the ground water overdraft. There is still a great deal of water use by agriculture, and this water will gradually, whether in an orderly or a disorderly fashion, be shifted to urban uses. So water will be argued over, traded, swapped and recirculated, but there will not be shortages severe enough to curtail urban growth for some time to come—several decades at least. Supply variability will be met by varying ground water pumping, by increasing conservation and effluent reuse, and by some water transfers.

The time will come in the next century when these various sources of water will no longer be sufficient to sustain Sun Belt growth rates. Planners might want this to occur relatively quickly. They might want to shift growth to other parts of the state or country where air quality is better, water is more plentiful, and the environment is less fragile. Given the history of the West and of the United States in general, however, it is highly unlikely that political restrictions on urban growth will ever be successful. Somewhere in the future, market-like mechanisms will begin to take hold. Either water will become very expensive or the quality of life will become so degraded (or both) that growth in areas like Phoenix and Tucson will slowly grind to a halt. By that time, we will have much larger urban populations, much less agriculture, and many more people expecting a steady supply of water. This, in turn, will make it more difficult to deal with increased supply variability that the greenhouse effect may bring about.