17 



used are determined by quantitative measurements. The principal 

 tank investigations of natural vegetation have been made in 

 California, Colorado, Idaho, New Mexico, Oregon, and Utah. 



(2) Soil-moisture studies are generally conducted in 

 areas where the water table is some distance below the root zone. 

 The amount of precipitation retained in the soil is measured by 

 means of soil samples taken from definite depths, before and 

 after each rainstorm, and the moisture content of the sample is 

 determined. In clay, loam, or sandy soils the soil tube is used 

 in collecting the samples, but in rocky or gravelly places sam- 

 ples are taken from open pits. 



This method is suitable for areas of deep-rooted natural 

 vegetation. It may be used for weeds, native brush and grass, 

 trees, and agricultural crops. The principal soil-moisture in- 

 vestigations of native growth have been in California. 



(3) Consumptive use of v/ater by alders, cottonwoods, 

 sycamores, and other riparian gro\irth common to small streams has 

 been found by measurement of the stream flov; at two or more con- 

 trol points where the underflow is forced to the surface, the 

 decrease in flow between controls representing use of water by 

 the vegetation affected. The danger of nonmeasurable inflow to 

 the stream from the canyon sides is a factor not to be overlooked, 

 and this difficulty makes many canyons undesirable for such in- 

 vestigation. However, where conditions appeared stable, this 

 method has been used by the Division of Irrigation in southern 

 California. Literature, with a few exceptions, (2, 4) appears 



to have given the subject little attention, yet there is a field 

 for this type of study in many localities. 



(U) Approximate measurements of ground-water discharge 

 by plant growth may be made by translating the daily rise and 

 fall of the water table into inches of depth of water consumed 

 by the overlying vegetation. This requires a knowledge of the 

 specific yield of the soil from which the water is withdrawn, 

 specific yield being the amount of water which will drain from 



