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Phreatophytes

Phreatophytes are plants that depend for their water supply upon ground water that lies within reach of their roots. Although not confined to the arid regions of the Western United States, their occurrence there is more common, more spectacular, and, because of their effect on water supply, more important than it is in humid and subhumid regions. Most phreatophytes have low economic value, and consequently, the water they use and return to the atmosphere without substantial benefit to man is defined as consumptive waste.

Some phreatophytes are widespread throughout the entire West, and others, such as saltcedar, are confined to the river valleys of the Southwest. In all, they waste tremendous quantities of ground water each year. It is estimated that phreatophytes (excluding beneficial species such as alfalfa) cover about 16 million acres in the 17 Western States and discharge as much as 25 million acre-feet of water into the atmosphere annually. Although little has been done so far to prevent this waste, much of the water undoubtedly can be salvaged by converting consumptive waste to consumptive use. There are two basic methods: reducing of consumptive waste by diverting water from the plants to other uses, and increasing the efficiency of water use by substituting beneficial for nonbeneficial plant species. These methods, to be successful, require an understanding of the factors that affect the occurrence and amount of water used by phreatophytes: climate, depth to, and quality of ground water and soil.

More than seventy plant species have been classified as phreatophytes; this report lists information concerning them according to their scientific names. The available information about the phreatophytic characteristics of most of the species is meager, but for eight, pickleweed, rabbitbrush, saltgrass, alfalfa, cottonwood, willow, greasewood, and saltcedar, there are sufficient data to warrant separate discussions. The annual use of water by phreatophytes ranges from a few tenths of an acre-foot per acre to more than 7 acre-feet per acre.

In the Southwest, saltcedar, an exotic plant that develops a junglelike growth, has invaded and choked the normal overflow channels of streams, so as to produce a flood hazard that must be reckoned with. In addition, the ponding effect of the dense growth results in above-normal sediment deposition in the area of growth, and reduced deposition downstream, as was observed at the McMillan Reservoir on the Pecos River in New Mexico.

In the interest of conserving water to meet an ever-growing demand and to reduce flood hazards in the Southwest, more and more attention must be given to the phreatophyte problem