46 



DIVISION OF WATER RESOURCES 



soon as possible and removed when the rain ceased. The amount of addi- 

 tional moisture the tanks received from rainfall Avas negligible and was 

 soon evaporated. 



There is objection to using covers as the shade they give reduces 

 both evaporation and transpiration. At the time they w'ere used, 

 however, both evaporation and transpiration were at a minimum on 

 account of overcast sky and increased humidity. Also, because covers 

 were raised above the soil tanks by the legs on which they stood, normal 

 air movements were not restricted. 



Beginning with the 1931-32 rainy season, it was decided to have 

 all soil tanks exposed to rainfall and covers were not used as during 



PLATE IV 



CIRCULAR METAL COVERS TO PROTECT SOIL TANKS FROM RAINFALL 

 WHILE ALLOWING FREE CIRCULATION OF AIR OVER THE 



TANK SURFACE. 



the two previous seasons. Under these conditions each rain changed 

 the water content of the soil. The additional soil moisture was dis- 

 posed of by evaporation and transpiration or the excess drained off 

 through the overflow outlet to be caught and mea,sured in a container. 

 On account of changes in soil moisture, withdrawals of water from 

 the Mariotte tanks did not give a correct index of consumptive use by 

 the soil tanks. Recourse to taking soil moisture samples from each tank 

 at the beginning of each month was therefore necessary. Consumptive 

 use was computed as the algebraic sum of the change in soil moisture, 

 rainfall during the period, water drawn from the supply tank, and 

 waste water measured in the overflow containers. Certain inaccuracies 

 inherent to this method could not be avoided. Probably the most 

 important of these was caused by inability to obtain soil moisture 

 samples immediately above the water table in the soil tank. This soil 

 was saturated to such an extent that it could not be held in the soil 

 tube, and the samples obtained indicated less moisture than actually 

 existed. 



