The Forester. 



Vol. V. 



NOVEMBER, 1899. 



No. 11, 



Effect of Forests on Water Supply. 



Being a Paper Read at the Summer Meeting, Los Angeles, Cal., 1899. 



-INVESTIGATIONS REGARDING RAINFALL AND PERCOLATION AS RELATING TO 

 WATER SUPPLY THROUGH FOREST INFLUENCES. 



The waters of the earth derive their 

 existence from the heavens above and have 

 no other source. This is the fundamental 

 principle underlying all discussions on 

 water-supply. There is no spontaneous 

 process of production, no water manufac- 

 tory in the recesses of the earth, no source 

 but the clouds. 



Forests interpose between earth and sky 

 and become factors in the engineering 

 problems of water-supply. Their action 

 is in the direction of increasing precipita- 

 tion, decreasing evaporation, and modify- 

 ing floods. It is proposed in this paper to 

 review some of the more salient relations 

 between forests and water. 



Many meteorological stations in connec- 

 tion with forestry have been established in 

 France, in Germany, in India and else- 

 where. In this country the subject has 

 also received attention, but on a more 

 limited scale. It is an unfortunate fact 

 in regard to all subjects connected with 

 rainfall that systematic observations must 

 have been conducted over a long series of 

 years at least thirty-five in order to 

 obtain data upon which to predicate posi- 

 tive results. 



The records of a few years may be, and 

 frequently are, very misleading. The 

 secular meteorological changes tend to 

 move in cycles of wet periods and drouth. 

 No conclusions can safely be accepted that 

 are not based on records extending suffi- 



ciently back into the past to embrace and 

 give full weight to these cycles. 



The records of precipitation at Phila- 

 delphia extend back to 1S25. Charting 

 the precipitations gives a wave-like curve 

 descending very low in 1S25 but with its 

 sinuosities all well above that point until 

 1SS1, when the minimum precipitation of 

 1S25 was again closely approached. The 

 period of extreme low precipitation was 

 fifty-six years. The years of maximum 

 precipitation were 1841 and 1S67, the 

 period being twenty-six years. 



Taking the total of sixty-four years and 

 averaging the annual rainfall by periods 

 of four, eight, sixteen and thirty-two years 

 we get the following results : The averages 

 by four years run from 22 per cent, low, 

 to 19 per cent, high as compared with the 

 average for the entire period of sixty-four 

 years. The eight-year groupings gave re- 

 sults from 11 per cent, low, to 1 1 per 

 cent, high and the thirty-two-year 2 per 

 cent, low, to 2 per cent. high. 



Analvzing the recorded rainfall at Los 

 Angeles for the past twenty-seven years 

 and averaging by periods of five years 

 gives results ranging from 35 per cent. 

 below to 16 per cent, above the average 

 seasonal rainfall for the entire period of 

 twenty-seven years. The extreme low 

 points of the Los Angeles precipitation 

 curve are twenty-two years apart, being 

 from the season of 1876-77 to 189S-99. 



