98 Irrigation and Drainage 



cover simply what has been found necessary to main- 

 tain that amount against surface evaporation from the 

 soil under the best of conditions and through the crop 

 itself. In the San Joaquin valley there is a long inter- 

 val, from the end of July until the fall rains begin 

 in November, when some evaporation is taking place 

 from the surface soil, and enough rain must have 

 fallen to bring the soil up to a good standard condi- 

 tion of soil moisture before the crop is started in it, 

 and the amounts in the table would need to be in- 

 creased by so much, at least, as would be required 

 to establish this condition. 



How much water would need to be added to the 

 soil in the San Joaquin valley by the fall rains, in 

 order to restore the proper amount of soil water, or 

 how great the evaporation may be between harvest and 

 seeding time, we do not know. We do know, however, 

 that the rate of evaporation from the surface of a dry 

 soil is not very rapid. In illustration of this, it may 

 be stated that after removing a crop of oats from four 

 of our cylinders in the field, a record was kept of the 

 loss of moisture from them between Aug. 2 and Aug. 

 25, and it was found that the total evaporation from 

 7.068 square feet was 5.3 pounds. In another case, 

 six cylinders in the field lost by surface evaporation 

 between Jan. 10, 1894, and March 12, 41.8 pounds. 

 The loss per 100 days expressed in inches in the first 

 case was .6268, and in the second 1.243. 



Taking the first of these two figures, which is likely 

 to be more nearly true for the district in question, the 

 total loss would be .79 inches, and at the second rate 



