36 



table. Fluctuations begin approximately at the same time each 

 day, the surface lowering as the transpiration increases, although 

 the result of the increased transpiration is not immediately ap- 

 parent and there is a noticeable lag between cause and effect. 

 During the growing season, when demand is greatest, the dajrtime 

 draw-down generally exceeds the nighttime recovery. The result 

 is a steadily falling water table which continues \intil transpi- 

 ration ceases. When plants become dormant the normal recharge of 

 the basin increases ground storage until a resumption of the vege- 

 tal demand occurs in the following spring. 



Because the water usage of some plants is greater than 

 that of others, the draft of the different species on the ground- 

 water supply varies with the type as well as with density of the 

 natural cover, so that the amplitude of the fluctuations varies 

 with the vegetation. The fluctuations are widest where water- 

 loving vegetation constitutes the dominant growth and are least 

 where drought-resistant or salt-resistant plants occupy the great- 

 est area. Not only does the type and density of plant growth 

 affect the quantity of water withdravm, but the depth to water 

 affects the total consumed, as is evident from the stunted growth 

 found where the water is at considerable depth. 



Interpretation of the ground-water fluctuations in inches 

 of depth of v/ater consumed by vegetation is accomplished by the 

 following method: First, ground-water wells are equipped with 

 water-stage recorders which provide continuous records of changes 

 in water levels. Second, the specific yield of the soil near the 

 wells is determined by driving metal cylinders over columns of 

 undisturbed soil. Third, the ground -water discharge in inches of 

 depth is computed by means of the formula Q, = ^(24r + s_) where ^ 

 is the consumptive use of water, ^ the specific yield of the soil 

 for the area investigated, and r the hourly rate of recharge of 

 the water table during the hours of least transpiration demand. 

 This period is between midnight and 4 a.m. The factor s_ is the 

 net difference in the height of the water table in 24 hours. 



