20 



J^ 



Supply 

 tank 



■ Glass tube 



-Meter stick 



Trench 



4 Galvanized iron pipe 



To be at same level 



12-0 





'^ 



W^sV-'VW^c7jl-V<'^x-7/'/c.'-"W, 



Soil 



tank 



V 



Water surface 



mn 





FIGURE l.--Mariotte tanlc connected to soil tank to maintain 

 a constant water level in the soil and supply water evapo- 

 rated or transpired. Santa Ana station, 1929-32. 



Figure 1 the water table in the soil tank is at such a depth that 

 it is necessary to extend the vent tube downward into a well be- 

 low the level of the connecting pipe. 



Experience has determined that variations in temperature 

 cause changes in vapor pressure in the Mariotte tank, resulting 

 in fluctuations of the water level. Thorough insulation is there- 

 fore necessary. Tanks at Santa Ana were completely buried in the 

 ground except for a small entrance provided with a narrow doorway 

 which opened upon the graduated scale. Protection against changes 

 in soil moisture around the tanks due to rainfall was provided by 

 a section of roof which excluded precipitation while permitting a 



