23 



FACTORS AFFECTING THE USE OF WATER BY 

 VEGETATION GROM IN TANKS 



It has already been stated (page 16) that the use of water 

 by tank crops varies in some degree from natural field use, and 

 this difference must be compensated for by applying a reduction 

 factor to tank records. A knowledge of the influencing factors 

 and an effort to carry on an investigation under the most natural 

 conditions will go far to equalize the use of water between tank 

 grov/th and natural fields. The factors affecting tank investiga- 

 tions are many and are related to soil, water, plants, and envi- 

 ronment . 



Methods of placing the soil in the tank, density of vege- 

 tation, unnatural environment of growth, injury to root systems, 

 limitation of the amount of soil as affecting root growth and 

 soil fertility, aerial spread of foliage, and entrance of rain 

 water, act upon the growth of tank vegetation or the amount of 

 water it consumes. Each of these factors is important, as esti- 

 mates of field consumptive use by natural cover will be in pro- 

 portion to the accuracy of the tank determinations. 



Experience with the Santa Ana investigation (2) has 

 demonstrated a satisfactory method of filling soil tanks without 

 soil disturbance. The recommended practice is to force the open- 

 bottom inner shell of the double cylinder tank down over a core 

 of undisturbed soil, cutting off the soil column by jacking the 

 bottom plate into place when the shell is filled. (Plate I-A. ) 

 This requires an excavation around the tank as the work proceeds. 

 Once the plate is bolted in place the filled tank may be hoisted 

 above ground with tripod and chain-block and lowered into the 

 outer tank previously set in the location selected. This pro- 

 cedure leaves the soil in the tank in its original condition and 

 has the advantage of sometimes capturing a growing crop without 

 serious disturbance to its root system. 



The relation of density of tank growth to natural field 

 growth is a contributing factor in determining the reduction 



