EVAPORATION FROM BARE SOIL m 



An extreme instance of the behaviour of a very permeable 

 soil is furnished by the drain-gauge filled by Greaves with 

 fine gravel, prepared by sifting through a screen with eleven 

 wires in two inches (Table XXII). In this very coarse soil, 

 with an extremely free percolation, the winter evaporation is 

 only i -2 inches, and that for the whole year 4-2 inches. The 

 amount of evaporation is thus largely diminished when only 

 a small quantity of water is retained near the surface. 



The speed with which a soil dries depends greatly on its 

 mechanical condition. When a soil is in an open, loose 

 condition, and is thus readily permeable to air, evaporation 

 may go on within its mass as well as on its external surface. 

 A soil thus dries quicker at the surface when in the crumbly 

 pulverulent state known as ' good tilth,' and the general effect 

 of tillage is in the same direction. The practice of ploughing 

 clays in autumn, and leaving the land in ridges through the 

 winter, not only yields a better tilth, but also a drier seed-bed 

 when the land is harrowed in spring. Soils composed of 

 coarse particles not only retain but little water, they also dry 

 quickly at the surface. 



Evaporation from the soil is greatly diminished when it is 

 shaded from the sun's rays, and protected from wind. The 

 effect of protection from sun and wind in diminishing evapor- 

 ation, is seen most strikingly in the case of a forest soil. 

 Ebermayer (Lehre der Waldstreu, 182) describes experiments 

 made during five years, at six forest stations, on the com- 

 parative amount of evaporation from artificially saturated 

 soils within the forest, and freely exposed without. During 

 the six months, April to September, the evaporation within 

 the forest was on an average only 47 per cent, of that 

 observed from similar soil in the open. Land covered by a 



