SMALL BUT IRRITATING shortages of gasoline have begun to hit the United States and that, if you will reflect on it, is a good thing. Small shortages now may make Americans cautious, reducing the chance of any great disastrous shortage further down the road. So the present inconveniences are a useful warning.

They originated in the shutdown of the Iranian oil fields at the end of last year. Because oil tankers travel slowly, the actual impact took about two months to reach the United States. As imports fell below the levels that they had expected, oil companies began drawing down their stocks. That process has now gone as far as the governmernt considers prudent and, in the case of home heating oil, a bit further. Last month the government and the industry imposed a rough but effective kind of rationing at the wholesale level.

The basic principle is that the brunt of any shortage has to fall on the automobile. It's unpleasant for the motorist never to know when the next filling station will be open. But that's infinitely less dangerous than a shortage of industrial fuel on which people's jobs depend, or of heating oil on which people's lives sometimes depend. The stragedy now is to rebuild stocks of heating oil for next winter, which takes more oil away from the gasoline market. Then there are all the priorities-the farm tractors, the police cruisers, the trash trucks, the school buses. That's why an oil shortage of only several percent works out to a drop in retail gasoline sales of nearly 20 percent. Meanwhile, demand keeps rising with more people and more cars on the roads.

As frequently happens, California has become the extreme case. Some of the reasons for the panic there are technical; the shortages are in fact a little worse. But they are far from extreme and part of the explanation for those long lines of cars at the filling stations is related to the character of California life-a life that requires cars and a people who, even more than most of their countrymen are unaccustomed to the idea of shortages of any commodity.

How long will the present stringency last? Iran is now exporting oil again, but at little more than half of last year's volume. Whether other countries will increase exports in the months ahead is uncertain. It is at least as possible that Saudi Arabia will reduce exports, and this country is now crucially dependent upon Saudi oil.

Perhaps the present anxiety and exasperation at the filling stations will prove an effective reminder of the real and rising dangers of depending too heavily on foreign oil. Perhaps, if Americans are both wise and lucky, they will have to back off that dependence without any abrupt or painful disruption of their lives. If that happens, this year's gasoline shortages will have served a valuable purpose that neither presidential exhortation nor congressional legislation has been able to achieve.