THE CONTROL OF SOIL-MOISTURE 



217 



the amount of rainfall. Even in humid regions, where the 

 annual rainfall is ample for maximum crop production, the 

 yields are frequently reduced below the profit point by pro- 

 longed periods of dry weather in the growing season during 

 which the loss of water from the plants, coupled with the loss 

 from the soil and through weeds, exhausts the moisture sup- 

 ply very rapidly. 



While run-off and percolation are directly proportional to 

 the rainfall, loss by evaporation does not vary to such a de- 

 gree. The loss by percolation depends almost directly on the 

 amount of rainfall above the retentive power of the soil. In 

 years of heavy precipitation losses by percolation increase. 

 Evaporation from the soil depends largely on the length of 

 time that the soil surface is moist, and this will not vary 

 markedly from year to year. The following figures from the 

 Rothamsted x sixty-inch drain gauge may be quoted in this 

 regard : 



Table XLIII 



RAINFALL, DRAINAGE AND EVAPORATION AT THE ROTHAMSTED 

 EXPERIMENT STATION, 1871 TO 1912. 



A rough calculation may be made which will show the ap- 

 portionment of the yearly rainfall in a humid region of the 

 temperate zone between the four forms of losses — run-off and 

 percolation, evaporation, and transpiration. The percolation 

 under a rainfall, say, of 28 inches, as shown by the Rotham- 



1 Hall, A. D., The Booh of the Bothamsted Experiments; p. 22, New 

 York, 1917. 



