SOIL MOISTURE AND TO EVAPORATION. 13 



face very rapidly, but always stood in the. pockets and hollows for 

 several hours after the cessation of the rain. The surface soil became 

 saturated anew with each shower, but usually dried out quite thoroughly 

 before the next. The downward penetration of the water into the 

 lower soil layers continued, however, between the showers, and by 

 August 1 the soil had become quite moist to the depth of 20 or 30 cm. 

 This downward movement of water is hindered, as has been stated, by 

 the puddling of the soil, but is hastened by the presence everywhere of 

 oblique rock surfaces down which water movement is much more rapid 

 than it is through the soil itself. It is thus seen that during the rainy 

 season the deeper layers of soil receive considerable quantities of water 

 by direct downward movement from the surface. It is also probable 

 that greater or smaller amounts of water find their way through the 

 soil of the upper part of the mountain to the underlying rock and thence 

 flow down the slope beneath the soil surface and penetrate into all the 

 crevices, whether or not these are closed by caliche, the latter being 

 always quite readily permeable. Altogether, it is highly probable that, 

 for periods of many days during the latter part of the summer rainy 

 season, the entire soil of the mountain, with the exception of the first few 

 centimeters, is very moist and offers abundant opportunity for growth of 

 roots. The most superficial layers themselves are often very moist for 

 periods of several days at a time, when showers follow one another at 

 frequent intervals. Thus seeds might germinate at or near the surface, 

 when the soil contains sufficient moisture, and the seedlings might, by 

 the rapid downward development of roots, easily attain, before the end 

 of the rainy season, to depths where the water content is permanently 

 as great as 10 or 12 per cent. 



Measurements of the rate of downward movement of water in the 

 soil when air-dry were made by several experiments. Six cylindrical 

 tumblers 5 cm. in diameter and 11 cm. high were filled to a depth of 

 of about 9 cm. with air-dry soil moderately tamped, water was poured 

 upon the surface of each so as to stand about 1 cm. above the soil, and 

 measurements of its rate of downward penetration were made from 

 minute to minute for a period of 15 minutes. The water above the soil 

 was kept at a nearly constant level by adding more as it disappeared. 

 The average rate per minute for the several intervals was determined 

 for the six soil columns. During the first minute the water penetrated 

 3.1 cm., during the second 0.8 cm., during the third 0.5cm., during the 

 fourth, 0.5 cm., and the rate of advance gradually diminished until at 

 the end of 15 minutes it had fallen to 0.2 cm. per minute, the decrease 

 being now exceedingly slow. During the entire period of 15 minutes 

 the water had penetrated, according to these averages, to a depth of 

 7.5 cm. 



