THE: SOIL AND WATER 145 



losses of water by percolation and evaporation. The composition 

 of the drainage water may also be studied. 



A drain gauge is a water-tight vessel rilled with soil and ex- 

 posed to the rain under natural conditions. The water which 

 passes through a given depth of soil is collected, measured, and 

 otherwise examined, as desired. The soil may be kept bare, or 

 cultivated or planted to various crops. The drain gauges at 

 Rothamsted are filled with undisturbed soil of that place. Excava- 

 tions were made along the side of the block of earth desired; it 

 was bricked up and isolated. Each drain gauge consists of a 

 rectangular mass of heavy loams with flints, of an area of i/iooo 

 acre, 20, 40, and 60 inches deep, respectively. All the rainfall 

 either passes through this mass of earth or evaporates. Drain 

 gauges filled with loose earth represent unnatural conditions, and 

 time should be allowed for the earth to consolidate before 

 measurements are begun. 



Another method of studying drainage waters is to measure the 

 water going off through tile drains, but this is not a good method, 

 since only the excess of ground water passes off through the 

 drain. A large portion of the percolating water passes into the 

 country drainage. 



Evaporation. The rate at which water is lost by evaporation 

 depends upon the nature and moistness of the soil, its capillary 

 condition, the temperature, velocity of wind, humidity of air, etc. 



The wetter the soil, the less the humidity of the air, the greater 

 the velocity of the wind, and the higher the temperature, the 

 greater is the loss by evaporation. 



If the upper layer of the soil is loose and porous, it will dry out 

 quickly, and the rate of loss will then be influenced by the rate at 

 which water is brought to the surface by capillary action. 

 Evaporation is greatly checked if the connection between the top 

 soil and the under layer is broken by cultivation. Evaporation is. 

 also influenced by the rapidity with which the water penetrates 

 the soil. Much larger quantities of water will be lost by 

 evaporation if the water is retained near the surface, than if it 

 sinks into the soil. 



