56 



IRRIGATION PRACTICE 



water vapor into the atmosphere, and the less effectively 

 can the sunshine heat the evaporating surface. The 

 best investigations on this subject are those recently 

 conducted by Fortier, and Fortier and Beckett, under 

 true arid conditions. These experiments were made at 

 five different points in the arid region in California, 

 Montana, Nevada, Washington and New Mexico so that 

 the validity of the results could be checked under the 

 varying climatic conditions of the irrigated region. 

 Immediately after each irrigation, "fine, dry, granu- 

 lated soil mulches," of different depths, were placed upon 

 the soil, and the water losses were determined during a 

 period of four weeks. Some of the average results are 

 as follows: 



The thicker the mulch placed upon the soil the smaller 

 was the evaporation, varying from 1.75 inches, when no 

 mulch was applied, to .22 inch or 12.5 per cent, when a 

 9-inch mulch was spread over the soil surface. 



In another series of experiments, a 10-inch mulch 

 practically stopped evaporation. When the mulch is 

 made by cultivation, similar results are obtained, the 

 difference being the loss immediately after irrigation 

 and just before cultivation, discussed above. 



It may be said safely that the deepest cultivation is 

 the most effective for the checking of evaporation from 



