182 



THE FORESTER. 



August, 



reservoirs in our mountains that hold 

 large bodies of water. In the technical 

 sense a reservoir is "a basin, either 

 natural or artificial, for collecting and 

 retaining water or other liquids." 



There are two essentials to make a 

 reservoir a success : First, there must 

 be means for collecting the water ; and 

 second, means for retaining it until it is 

 needed. When we speak of the moun- 

 tains as reservoirs, the word is not used 

 in its ordinary sense, for I believe that 

 the great volumes of water that contin- 

 uously flow from our mountains are 

 held in the interstices of the soil and 

 rocks. My own investigations show that 

 our different soils hold from about 17 to 

 26 per cent of water, although some 

 authorities make a much larger percent- 

 age. 



Different kinds of soils vary in regard 

 to their porosity, and the same soils 

 vary to a very great degree in regard to 

 their absorptive power of water, de- 

 pending upon the amount of moisture 

 already contained in them. For ex- 

 ample, here is a sample of red maca 

 soil it is hard and dry, containing but 

 a trifle over one per cent of moisture. 

 Water was turned on it and it absorbed 

 only one-twentieth of the amount of 

 water absorbed but a short distance from 

 it by soil of the same kind, which con- 

 tained, at the beginning of the experi- 

 ment, about 8 per cent of moisture. 

 This experiment was carried on on com- 

 paratively level land ; but if on a hill- 

 side you see that 95 per cent of the 

 water would have run off. 



This may be an extreme case, but it 

 is remarkable how much water will run 

 off from the soil when it is dry. We 

 see the same effect if we dip a dry 

 feather in water : when we pull it out it 

 comes out dry. But if we moisten it 

 and then dip it in water, it comes out 

 saturated. 



It seems necessary then in order to 

 have our land absorb the maximum 

 amount of water that it should retain a 

 goodly percentage of moisture. Or in 

 other words, if we wish to fill our moun- 

 tains with water and preserve the great- 



est amount of rainfall, they should be 

 kept moist. 



Having shown that it is necessary to 

 have some moisture in the soil in order 

 to have it absorb the rainfall readily, and 

 thus make our mountains a reservoir, 

 let us look at the other side of the case 

 that of retaining the moisture ; and I 

 regret to say that the experiments are 

 not so complete and numerous as they 

 should be, as they have only been fairly 

 begun. 



In the first place, let me call your 

 attention to the fact that capillary action 

 in soil is in every direction from a given 

 point. W T ater spreads out sidewise, as 

 well as upwards and downwards by this 

 action. Soil that had been thoroughly 

 irrigated was taken and the amount of 

 water determined at 26.12 per cent. 

 Some of this soil was put in beakers, 

 filling them about half full, and placed 

 in the laboratory. On the following day 

 66 per cent of the moisture had dried 

 out. Tin cans, without either bottoms 

 or tops, were pressed down into the soil, 

 and the soil taken from the side of the 

 can, and a slide passed under the can, 

 thus cutting off connection from the 

 earth beneath. It was found that about 

 the same amount of water had disap- 

 peared from these cans as had disap- 

 peared from the beakers. Where these 

 cans had been pressed some inches be- 

 low the surface of the ground, and the 

 soil above raked, or cultivated, there 

 was practically little loss of moisture 

 where they had been covered with a 

 mulch. 



Conclusions from these facts are very 

 obvious that in order to make reser- 

 voirs of our mountains, it is necessary 

 to keep them in such condition that they 

 will readily absorb water and retain it. 

 And that this result can be brought 

 about only by keeping them covered 

 with a product of growth, or in other 

 words, with the forests, as these forests 

 make a covering, or mulch, for retaining 

 the moisture. 



S. M. WOODBRIDGE, Ph. D. 



Lenapuente Experimental Ranch and 

 Laboratory, South Pasadena, Cal. 



