no RELATIONS OF SOIL TO WATER 



nine years after the establishment of the gauges. The years 

 are arranged in the order of their rainfall. 



The rainfall in the seasons mentioned shows a large range of 

 variation, 2^-9-42-7 inches; but the amount of the evaporation, 

 though varying somewhat from year to year, seems quite 

 independent of the amount of rain. The large evaporation 

 credited to the deeper soil in 1870-1, is doubtless above the 

 truth. The gauges were constructed in the dry summer of 

 1870, and the blocks of soil were during the construction 

 suffering evaporation from the side as well as from the surface. 



The comparative constancy of the rate of evaporation from 

 the bare soil is doubtless due to the fact that the two factors 

 which tend to produce a large evaporation do not occur to- 

 gether. In a wet season there is an ample supply of water 

 to be evaporated, but evaporation is hindered because the 

 sky is cloudy and the temperature low ; while in a fine hot 

 season evaporation is checked as soon as the surface of the 

 soil has dried, and its amount is controlled by the scantiness 

 of the rainfall 1 . 



All descriptions of soil will evaporate a similar amount of 

 water when they are in a perfectly saturated condition. In 

 the case of very permeable soils, consisting of coarse particles, 

 this condition continues only a short time after rain has 

 ceased ; when this point is passed, the capacity of a soil to 

 evaporate water will largely depend on the quantity of water 

 it is capable of retaining near the surface. 



1 The amounts of evaporation shown inTableXIX have not been maintained 

 in subsequent years (compare Table XXII). This diminution of evaporation, 

 with its consequent increase in percolation, is apparently due to the fact that 

 the soils have been left undisturbed though kept free from weeds, and their 

 surface is now more occupied by stones than at first ; a slight growth of moss 

 has also taken place. 



