2l6 SOILS. 



bursts " occasionally occurring within these limits are usually 

 confined to mountainous regions, and the water they pour 

 down on the dry soil is rarely of any direct benefit to agricul- 

 ture; hence they cannot be properly counted in the general 

 estimate of the effective rainfall. A region of high rainfall 

 (up to 100 inches and over), however, extends along the Pa- 

 cific coast from northern California through western Oregon 

 and Washington across British Columbia to Alaska, to sea- 

 ward of the Sierra Nevada, Cascade, and Alaskan coast ranges. 



In the country east of the Mississippi river, the average an- 

 nual rainfall ranges from 30 inches in the region of the Great 

 Lakes, and 45 to 50 inches on the north Atlantic coast, to 60 

 inches in Louisiana and up to eighty in southern Florida. The 

 average of the Mississippi Valley and Atlantic coast States is 

 usually stated at about 45 inches, which is distributed more or 

 less evenly throughout the year, excepting usually from six 

 to eight weeks of more scanty precipitation in the latter part 

 of August and in September the " Indian summer" season; 

 so that the winter is the season of greatest total rainfall. 



Natural disposition of the Rain Water. The rainfall is 

 naturally first disposed of in two ways, viz., a portion which 

 is absorbed by the soil, and another which is at once shed from 

 the surface and constitutes the " surface runoff." The portion 

 absorbed into the soil is subsequently disposed of either by 

 soakage downward into the subdrainage and through springs 

 and seepage x into the streams and rivers ; or by evaporation. 

 The latter again occurs in two different ways, viz., from the 

 soil-surface itself, or through the roots and leaves of plants. 

 The importance of each of these modes is sufficiently great to 

 entitle each to detailed consideration. 



The Surface Runoff. This portion of the disposal of rain 

 may range all the way from nothing to almost totality, accord- 

 ing to the nature of the soil and the condition of its surface. 2 



1 The quiet seepage from the banks and beds of streams plays a much more im- 

 portant part in the increase of volume of flow than is commonly supposed, because 

 unperceived save by measurement of the tributaries and comparison with the 

 main streams. This is especially true of the drainage in the arid region, where 

 the deep and pervious soils favor diffuse seepage as against definite spring flow. 



2 Tourney (Yearbook U. S. Dep't Agr. 1903) states that in the San Bernardino 

 mountains in southern California, the first rainfall (in December) was absorbed to 

 the extent of 95 in forested areas, against only 60 in the non-forested ; but 



