27 



tank surface has advantages. Investigations involving use of 

 water by plants under natural conditions where the ground water 

 fluctuates from day to day and from season to season undoubtedly 

 require that rain be allowed to enter the tank soil. On the 

 other hand, in those investigations which are carried on with a 

 fixed water table, the entrance of rain water into the soil dis- 

 turbs the normal distribution of capillary moisture. In its 

 natural distribution the largest percentage is immediately above 

 the water table and the least at the ground surface. If rainfall 

 is allowed on the tank, the entire soil mass becomes filled to 

 field capacity, and conditions relative to a fixed water table no 

 longer exist. 



In a series of soil tanks having different depths to water 

 table, each with overflow pipes to drain off excess soil water, 

 the soil moisture varies with the depth to the water table. For 

 instance, if the water table is near the surface and rainfall is 

 heavy there will be much overflow from the waste pipe, as the 

 shallow soil is unable to hold all the excess; but if the water 

 table is deep in the tank there will be little overflow as the 

 greater volume of soil holds more rain. Thus all the rain might 

 be retained in a deep tank while but a small portion would be 

 held where the water table was near the surface. Under these 

 conditions the changed moisture distribution resulting from rain- 

 fall penetration is different for each tank or for each depth to 

 water table. Hence, while the treatment of a series of tanks may 

 be uniform as regards soil moisture during the dry season, it is 

 far from uniform during the wet season. It is evident, therefore, 

 that the procedure to be followed for tank protection will depend 

 largely on the object in view. 



