230 



SOILS. 



served in the arid region, when on the advent of the summer 

 drought, young plantlets whose tap roots have reached a cer- 

 tain depth continue to flourish and develop, while others prac- 

 tically of the same age, but slightly behind, quickly succumb, 

 though the feeding roots of both may draw upon the same soil 

 layer. It is especially in sandy soils that moisture is naturally 

 thus conserved in the upper layers, because of the failure of the 

 water to rise by capillary ascent so as to evaporate from the 

 surface layer. It is often surprising to find a good amount of 

 moisture in the sandy soils of desert regions at the depth of 

 eight of eight or ten inches, when the surface is so hot as to 

 scorch the fingers; and this moisture continues very uniformly 

 to great depths, probably to bottom water lying twenty or 

 more feet below the surface, which in such materials may 

 readily by reached by tap-rooted plants such as the " sage- 

 brush " (Artemisia tridentata), the saltbushes (Atriplex) and 

 others. 



Injurious Rise of Bottom Water resulting from Irrigation. 

 In the deep, pervious sandy lands of the arid region, especi- 

 ally where the rainfall is very low and can wet the soil annually 

 only to two or three feet depth, the substrata are sometimes 

 found to be barely moist to depths of thirty and forty feet, 

 and the short-lived spring vegetation carries off during its 

 growth all the moisture supplied by the winter rains. When 

 such lands are subjected to irrigation and the ditches carrying 

 the water are simply dug into the natural sandy land, the thirsty 

 soil absorbs the water greedily, so that even a considerable 

 volume of water makes but slow progress toward the farther 

 end of the canals. Gradually, as the rapidity of absorption 

 decreases, the diminution of flow becomes less sensible, but 

 still the loss thus experienced may be a very considerable per- 

 centage of the whole supply. Thus in the Great Valley of 

 California, as well as in portions of Wyoming (Bull, 61, p. 

 32), the permanent loss from seepage is in the case of some 

 extensive irrigation systems estimated at fully 50 per cent. 

 When such lands have a considerable slope, the injury com- 

 monly ends with the loss of the water, which in many cases is 

 again gathered and utilized at a lower level. But when the 

 lands have but a slight slope, the drainage may become so slow 

 as to permit of the gradual rise of the seepage water in the 



