35 



this device was designed for irrigation canals carrying water at 

 a relatively low velocity and that it should be used in mountain 

 areas with some caution. 



WATER-TABLE FLUCTUATIONS 



The three methods of measuring consumptive use of water by 

 vegetation previously discussed - tank, soil moisture, and stream- 

 flow investigations - are applicable to somewhat limited soil- 

 moisture conditions or types of growth. The tank method is suit- 

 able for the smaller vegetation, but it is evident that it cannot 

 economically be used for studies of consumptive use of water by 

 large trees. Extensive soil-moisture investigations generally 

 are not undertaken in areas of high ground water, and it is evi- 

 dent also that canyon-bottom investigations are limited. Observa- 

 tions .of daily ground-water fluctuation in relation to vegetative 

 discharge has been little used. Beyond its application in Arizona 

 by Smith and in California (33) and Utah (38), it apparently has 

 not received much attention, largely because of the difficulty of 

 determining the specific yield of large areas of soil in place. 

 Water-table fluctuations provide a basis of estimating the con- 

 sumptive use of water by overlying vegetation but present diffi- 

 culties in arriving at precise measurements of quantity. 



During the early spring, as vegetation is beginning its 

 growth, the daily fluctuations of the water table do not reach 

 the same degree of amplitude that occurs during periods of maxi- 

 mum growth; and conversely, in the fall, as vegetation is matur- 

 ing and transpiration decreases, the daily fluctuations become 

 progressively smaller. Fluctuations do not occur where there are 

 bare lands or plowed fields under which the ground water is below 

 the reach of plant roots, nor during winter months when plants are 

 either dead or dormant. Lower temperatures, cloudiness, or rain- 

 fall decrease the size of the fluctuations; warm sunshine, low 

 humidity, or hot winds increase them. In short, any cause affect- 

 ing transpiration influences also the diurnal changes of the water 



