93 



cording to the relntive anioniits of sand and clay. Watei* is also found in 

 this in more or less tubular channels a few inches in diameter as well as 

 in the interstices between the particles. Sands and gravels are very por- 

 ous, the water sinking into beds of such material and the whole mass being 

 saturated with water below the water level. Clay is very impervious to 

 water. 



In Rock. — Water found in the pores of roclis is given up readily only 

 vn the coarser rocks such as sandstones. The waters found in finer grained 

 recks are generally from joint, fault, or foliation planes. In limestone the 

 water occurs mainly in channels and caverns whicli have been dissolved 

 out or eroded by water. The amount of absorbtion also depends upon tlie 

 inclination of the pcjrous beds, the gently inclined ones absorbing more 

 than the steeper ones. 



Water-tahle. — As the water passes down through the ground it soon 

 leaches a level at which the soil is completely saturated. The surface of 

 this saturated zone is kncnvn as the water level or water-table. Above 

 this plane the soil contains a large percentage of moisture which is a most 

 important factor of plant and animal life, but only the water beneath this 

 is generally included in the term underground water. The water-table in 

 general follows the contours of the overlying soil, but the angles and slopes 

 are much less abrupt than the surface of the land. The deittli of the 

 water-table below the surface of the ground varies greatly in different 

 localities. In regions of abundant rainfall it is generally within a few 

 feet of the surface, while in arid countries it may be hundreds of feet 

 below. Moreover, the water level of any locality is subject to changes 

 because of seasonal variations of rains and drouth. Underground water. 

 l>esides being drawn up as soil moisture by caijillarity, also creeps later- 

 ally, its direction and I'apldity of flowing depending upon the porosity of 

 the soil and rock through which it i^asses. 



Forests and Water Level. 

 Regarding the effects of forests upon water level, it is evident from the 

 above considerations that any factors which tend to increase the condi- 

 tions that make it possible for a larger per cent, of the precipitated water 

 to enter the soil, will aid in raising the water level of the region on which 

 the rainfall occurs, and any agent which tends to increase evaporation, 

 surface run-off, etc., will help to lower it. Let us now consider the im 

 portance of the forest as a factor in both of these conditions. 



