RUN-OFF DURING TORRENTIAL RAINS. 



15 



lost through subsequent evaporation even when the surface of the soil 

 is cultivated after the rain, since the rainfall does not penetrate below 

 the depth of the mulch. These small rains are, however, usually 

 sufficient to pack the mulch and form a surface crust, which must be 

 broken by cultivation as soon as possible; otherwise, evaporation will 

 go on so rapidly from the surface of the soil that far more moisture 

 will be lost than was gained through the rain. At North Platte, 

 Nebr., during the month of August, 1908, the total rainfall was 1.9 

 inches, which came in the form of nine showers. None of these rains 

 was sufficient to wet thoroughly the mulch on the summer fallow 

 and all were useless so far as storing water in the soil was concerned, 

 and yet they necessitated a good deal of work in order to keep the 

 mulch in good condition. Such rains are, of course, useful to grow- 

 ing crops that cover the ground, but for storing water in the soil 

 showers of one-half inch or more are necessary. 



RUN-OFF DURING TORRENTIAL RAINS. 



If the rainfall is of a torrential character, the rain falling at the 

 rate of an inch or more per hour, the loss through run-off becomes 

 very serious. These torrential rains occur occasionally in all parts 

 of the West. The records of the Weather Bureau show that they 

 occur more frequently in the southern part of the region. It is impor- 

 tant for the dry farmer to study the best methods of handling his 

 land so as to absorb as much of these torrential rains as possible. 

 Our recent fleldwork has shown that even on summer-fallowed land 

 provided with a good mulch the loss through run-off during a torrential 

 rain may be very great. 



Measurements made by Mr. F. D. Farrell at the Nephi substation, 

 Utah, in 1908, showed that during a heavy rain of 2.5 inches in four 

 hours summer-tilled land absorbed only 0.5 inch of water, while 

 near-by wheat stubble land, which was very dry and contained many 

 surface cracks, absorbed 1.5 inches. In other words, the run-off on 

 the summer-tilled land was 80 per cent of the total water falling, 

 while the loss on the stubble was 40 per cent. The high loss on the 

 summer- tilled land was probably due to the driving rain, which packed 

 the fine surface soil so that it would not absorb the water freely. 

 Similar measurements by one of the writers (Belz) at the experiment 

 farm of the Office of Dry-Land Agriculture Investigations, at Dalhart, 

 Tex., in 1909, showed that during a torrential rain of 2 inches the run- 

 off from cultivated plats was from 20 to 60 per cent of the total rain 

 falling. In another instance, with a rainfall of 1.1 inches, the average 

 loss through run-off from seven cultivated plats amounted to 35 per 

 cent of the total rainfall. All of these measurements were made on 

 practically level land. 



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