WATER CIRCULATION AND ITS CONTROL 



271 



most vital thing in making a crop, by 

 seeing to it that rain goes into the soil 

 and not over it or away with it. 



EVAPORATION 



When dry air passes over a moist 

 surface it absorbs moisture from the 

 latter. This is the simple process of 

 evaporation, which is, however, subject 

 to intricate relations of temperature, 

 pressure, and humidity. Considering 

 it as a way in which water that we want 

 in the ground escapes into the air, we 

 are interested to limit evaporation. This 

 is possible by keeping the surface cool 

 and the air above it moist, as under 

 trees ; and in sufficiently arid regions it 

 is accomplished by covering the surface 

 with a dry mulch, which cuts ofif the 

 upward rise of moisture. The action 

 of the woods in keeping the air and 

 ground cool and moist is not open to 

 question ; even on the semiarid plains 

 a wind break notably reduces evapora- 

 tion from the fields in its lee ; but it is 

 held by some writers that the trees 

 draw in through their roots and trans- 

 pire through their leaves more mois- 

 ture than would evaporate were the 

 soil beneath them bare. This is true 

 with regard to the soil reached by their 

 roots, particularly on plains. On hill 

 slopes it is also true, but trees so in- 

 crease the amount of water stored in 

 sloping ground, over that which can 

 be stored in the same slope if bare, that 

 they more than make up for what they 

 transpire. 



In regard to the manner of evapora- 

 tion from soils under arid and humid 

 conditions we may best quote as fol- 

 lows from Buckingham ■}' 



When a moist soil dries by contact with 

 the air above it the loss of water takes place 

 by evaporation close to the surface, the 

 amount lost by direct evaporation from points 

 several inches below the surface being in 

 general negligible. As the surface soil dries 

 out a moisture gradient is established, and 

 the dry surface soil draws up water from 

 the moister region below by capillary ac- 

 tion. If this capillary flow of water be pre- 



vented or lessened, as by the use of a 

 mulch, the escape of water is decreased, 

 because the evaporation has, on the 

 whole, to take place from farther below the 

 surface, so that it encounters greater re- 

 sistance and is slower. 



The flow of water in a soil which is not 

 very wet has to take place through the thin 

 films in which a part of the water is dis- 

 tributed over the soil grains. If the soil 

 becomes very dry it is to l)e expected that 

 these films will become thinner or break, 

 the resistance to capillary flow increasing- 

 very much in either case. Hence it is to be 

 expected that in very dry soils capillary flow 

 will be slow. 



Suppose a soil could be made so dry that 

 no capillary flow at all would take place in 

 it even when it was in contact with a moist 

 soil. Such a layer of soil would act toward 

 the moist soil below it as a protecting mulch. 

 A soil can probably not be made so dry as 

 to lose its power of capillary conduction of 

 water entirely, but we may get an approxi- 

 mation to this limiting case. 



Suppose that after a rain the soil surface 

 be exposed to very arid conditions with a 

 high surface temperature and a hot, dry 

 wind. The surface of the soil will lose 

 water much faster than it can be brought 

 up from below by capillary action, and if 

 the arid conditions be kept up a layer of 

 dry soil will be formed on the surface, which 

 may be so dry as to act in the manner sug- 

 gested, i. e., as a protecting mulch. We 

 shall thus have an initial period of very 

 rapid evaporation, followed by a period of 

 slow evaporation, taking place largely from 

 below the surface. 



If the same soil had been subject to less 

 arid conditions the initial loss would have 

 been less. The capillary flow from below 

 would have been sufficient to prevent the 

 surface soil from becoming dry enough to 

 act as a mulch ; hence, though the initial rate 

 of loss would be smaller, that rate would 

 not fall off so rapidly. It might even happen 

 that in the long run the soil under the arid 

 conditions would actually lose less water 

 than the same soil under humid conditions. 



From these statements and the care- 

 ful experiments on which they are 

 based it appears that in humid regions 

 exaporation from a bare surface is not 

 readily controlled, whereas in arid re- 

 gions the ])reci()us moisture may be 

 retained by a mulch of sufficiently dry 

 soil. 



.SURFACE FLOW 



Surface tluw is the water in swamps, 

 lakes and streams. It is all derived 



'Buckingham, Edgar: Studies in the Movement of Soil INIoisture. U. S. Department of 

 Agriculture, Bureau of Soils, Bull. No. 38, pp. 18-19, 1907. 



