''The only substantial constraint on El Paso growth,'' says Timothy Roth, a University of Texas economist, ''is water.''
Seated in his El Paso campus office overlooking new construction in all directions, Professor Roth quickly adds that while supplies of water available for residential and industrial use may seem limited, ample supplies could be guaranteed by cutting back on irrigated argiculture. ''A 10 percent reduction of utilization of water in agriculture,'' he said, ''could sustain a much more rapid rate of industrialization.''
John Hickerson, general manager of the El Paso Water Utilities Public Service Board, has a long list of other well-documented options drawn up to ''prove to outside people that we do have an adequate long-term water supply.''
El Paso is suffering from having legal rights to only 10 percent of the water resources within a 100-mile radius of the city. Mr. Hickerson says this low figure is despite the foresight of city planners who began actively buying up water rights 30 years ago.
Part of the problem is geographical. El Paso marks the extreme western tip of Texas, tightly wedged between Mexico to the south and New Mexico to the north and west. The city is waging a legal battle to win the right to drill wells in New Mexico.
Another problem is that the federal government is refusing to allow El Paso to tap the underground water under vast federal land holdings.
But the city is prepared to keep the water flowing even if it never squeezes another drop from New Mexico or federal lands. If these lower-priced routes are blocked, El Paso will rely more on treating sewage water for reuse. To make certain this is a viable alternative, the city is constructing a $30 million recycling plant able to contribute 10 million gallons of purified water per day to offset the city's average daily use of 90 million gallons.
If the underground bolson supplying El Paso continues to be depleted at current rates, city wells could turn salty in another 50 years. So the city is studying the relatively expensive option of treating brackish water.
Mr. Hickerson considers listing such expensive options a city responsibility. But his objective is to win the water battle primarily with a zero-cost option: conservation. In past skirmishes, the city finally switched from offering summer discounts for lawn watering to increasing per-gallon water charges for heavy users.
One victory in the conservation battle may come from experiments with native desert plants at Texas A & M University's agricultural research center in El Paso. Here, horticulturist Jimmy Tipton is working with the creosote bush, the desert willow, dyssodia, and other desert plants to develop varieties that can be propagated by commercial nurseries. His goal is to provide homeowners with trees and shrubs that will be colorful, yet survive without any water except El Paso's annual 7.7 inches of rainfall.
Professor Tipton says that ''El Paso, like a lot of the Southwest where you have a declining water supply and an influx of people with a traditional concept of lawns and trees, is using over 30 percent of its water on landscape watering.''
Tipton, who is working with John Hickerson to develop a pleasing, water-conserving alternative, said that ''our approach is to encourage a different type of landscape, one that is more attuned to what occurs in the desert itself, one that essentially can survive on natural rainfall.''
Herbert Black, sales manager for Aldridge Nursery Inc., near San Antonio, looks forward to the domestication of more desert plants and feels their use will spread as more cities begin rationing water for yard use. He cautions, though, that any reward is at least 10 years away, because ''first the basic research has to be done, and then the idea has to be sold to landscape architects.''
But limited water supply has already proved to have one benefit for El Paso: it keeps out heavy ''smokestack'' industries while more desirable, nonpolluting industries such as electronics manufacturers move into town.