604 



Agriculture YearbooJ^ 1949 



ing, devastation logging, overgrazing 

 of livestock and game, and fire. 



Changes took place at the ground 

 surface that altered the manner in 

 which precipitation entered the soil. 

 The storage capacity of the soil was 

 also altered. Those changes threw the 

 original control of water and of soil 

 stability out of balance. The result has 

 been widespread accelerated erosion, 

 sediment in the streams, erratic stream 

 flow, and damaging floods. Nature's 

 original controls were maintained by 

 vegetation. Today, better land-man- 

 agement practices must be inaugurated 

 to restore a more favorable plant cover 

 and soil structure if we wish to main- 

 tain land and stream conditions to 

 serve our present and future needs for 

 usable water. 



THE SOIL and the underlying rock 

 mantle is the key to understanding the 

 control of water on the land. Soil is ca- 

 pable of storing water. Some of this 

 water is retained by the soil just as 

 water is held behind a dam. But the soil 

 also releases water when the mantle is 

 filled to capacity. 



Soils on forest and range lands can 

 absorb and retain against the force of 

 gravity from 1 to 3 inches of water 

 per foot of mantle depth. Fine- textured 

 soils with a high content of organic 

 matter have a greater retention-stor- 

 age capacity than coarse soils a dry 

 soil mantle 4 feet deep can absorb and 

 hold from 4 to 12 inches of rain or 

 water from melted snow without yield- 

 ing a drop of runoff. This retention- 

 storage function is the same as that 

 performed by a dam. Removal of the 

 soil by erosion, or otherwise, reduces 

 the capacity of a site to retain water 

 and so increases the chances for greater 

 runoff and flood discharges in the same 

 way as would the lowering of a dam. 



Retention storage is only one of the 

 storage functions of the watershed 

 mantle. After a soil mantle is wet to 

 its capacity to hold water against the 

 force of gravity, it is not yet saturated. 

 Air space still remains between the wet 

 soil and rock particles. This additional 



storage space may be equivalent to as 

 much as 2 inches a foot of mantle 

 depth. Water that enters these spaces 

 is not retained by the mantle but moves 

 downward to the subsurface aquifers, 

 where it may replenish the ground- 

 water levels, or may emerge in channels 

 or at springs to sustain stream flow. 



The percolation of the free water 

 through the soil and rock mantle of a 

 watershed takes time much longer 

 than the escape of water over the 

 spillway of a dam. The slowness of the 

 percolation process is attested by the 

 fact that streams continue to flow for 

 periods as long as a year after free 

 water disappears from the soil mantle. 



The delayed yield of water is one of 

 the most important and valuable func- 

 tions of watershed lands. Communities 

 and industries pay millions for a sus- 

 tained yield of water and one of the 

 major purposes of billions of dollars 

 worth of dams is to catch spring floods 

 and make them useful in the autumn 

 droughts. The same functions are per- 

 formed by the soil on many millions of 

 acres of forest and range watershed 

 lands. These natural and beneficial 

 functions of the soil must be main- 

 tained through good management. 



PLANTS herbs and shrubs, as well 

 as trees are important in maintaining 

 an efficient watershed mantle. 



All who have sought shelter under a 

 tree during a rainstorm do appreciate 

 that vegetation intercepts precipita- 

 tion in its descent to the earth. In a 

 40-inch rainfall belt, an old-growth 

 hardwood forest will prevent 6 or 7 

 inches of rain from reaching the 

 ground during the course of a year. 

 This means that insofar as the soil un- 

 der the forest is concerned there is 

 really only about 34 inches of rain in- 

 stead of 40. During individual storms 

 the plant canopy may intercept up to 50 

 percent of the precipitation. The plant 

 canopy, in other words, is an integral 

 part of the watershed reservoir with the 

 special function of intercepting and dis- 

 sipating a part of the precipitation be- 

 fore it reaches the soil mantle. 



