DISCHARGE METHODS 333 



In arid regions plants of certain species habitually utilize water from 

 the zone of saturation. For such plants the name phreatophyte (meaning 

 a well-plant) has been proposed. Salt grass (Distchlis spicata), rabbit 

 brush (Crysothamnus graveolens) , big greasewood (Sarcohatus vermicu- 



latus), a certain type of mesquite {Prosopis ), samphire {Spiros- 



fachys occidentalis) , rye grass (EJymus condensatus) , yerba mansa 

 (Anemopsis Calif oi-nica) , and birch, sycamore, and willow trees are 

 among the most common phreatophytes in the arid West. 



According to observations by C. W. Eiddell in Steptoe Valley, Nevada, 

 the water-table is generally within 20 feet of the surface where grease- 

 wood predominates, within 15 feet where rabbit brush predominates, and 

 within 7 feet where salt grass predominates. Other observations seem to 

 indicate that a certain type of mesquite will send its roots to a depth of 

 as much as 50 feet to find water. 



Except for tracts containing plants of uncertain significance, the areas 

 of ground-water discharge can be shown on a map with considerable ac- 

 curacy. Thus, it was found that in Big Smoky Valley ground-water is 

 discharged from a total area of 130,000 acres, exclusive of the grease- 

 wood belt, and in a number of other Nevada valleys there are discharge 

 areas of comparable size, large parts of which are bearing luxuriant vege- 

 tation. Such information, of itself, gives valuable^ clues as to the rate of 

 discharge from an aquifer and as to the quantity of ground-water an- 

 nually available. This was strikingly impressed on me recently, when I 

 went from some of the relatively well watered valleys of east-central Ne- 

 vada, with their large areas of luxuriant phreatophyte vegetation, directly 

 to the more arid Mohave and Colorado deserts of southeastern California, 

 where, as a rule, the areas of discharge are much smaller and have less 

 luxuriant vegetation. In a valley having 130,000 acres of ground-water 

 discharge there may be a question as to whether the annual water supply 

 is sufficient to irrigate 50,000 acres or only 5,000 acres, but it is certain 

 that there is a substantial irrigation supply. On the other hand, in a 

 desert valley, with only a few hundred acres of stunted vegetation that is 

 discharging ground-water, irrigation developments should be made with 

 caution, even though wells of satisfactory yield are obtained. 



A very valuable series of experiments on the rate of discharge of 

 ground-water from soil and vegetation in Owens Valley, California, was 

 made several years ago by Charles H. Lee.* In experiments made in 

 tanks filled with soil supporting a growth of salt grass, he found that the 



* Charles H. Lee : An intensive study of tlie water resources of a part of Owens Val- 

 ley. California. F. S. Oeol. Survey Water-supply Paper 294, 1912. 



