170 



FORESTRY AND IRRIGATION. 



" He calleth for the walers of the sea, and poureth them out upon the 

 face of the earth, the Lord is His name. " Amos, 800 years B. C. 



CHAPTER XXIV. 



SOME RELATIONS BETWEEN FORESTS AND WATER 



SUPPLY. 



By H. HAWGOOD, M. Inst. C. E. 



We must start out with the fundamental principle that the waters 

 of the earth derive their existence from the heavens above. There is no 

 spontaneous process of production, no water manufactory in the re- 

 cesses of the earth. The water which falls upon the earth in the form 

 or rain or snow passes away in flowage and evaporation. By flowage 

 is meant the underground movements, as well as the visible surface 

 streams of the springs, brooks and rivers. Evaporation includes the 

 direct evaporation from the surface of the ground, from objects animate 

 and inanimate, and the water which is taken up by plants and tran- 

 spired as vapor through their leaves. And while not strictly correct, 

 the much smaller portion of water that is permanently absorbed into 

 plant structures may, for simplicity's sake, be charged under the com- 

 mon head of evaporation. 



We now have a simple expression of supply and demand. Total 

 rainfall equaling the sum of flow and evaporation. It is from the flow 

 that practically all waters for the use of man are drawn. For the primi- 

 tive direct collection of the rain from house tops and other exposed 

 surfaces, is on too minute a scale to be given a place. 



Evaporation decreases the quantity of flow. Floods decrease the 

 flow available for economic purposes. The total quantity of water 

 being fixed, whatever rushes past in a torrent to the sea is lost for 

 useful ends. 



