270 SOILS: PROPERTIES AND MANAGEMENT 



being where deep, large cracks occur, which allow thermal 

 loss directly from the subsoil. This loss of water by 

 direct evaporation from the soil may be excessive and 

 may result in direct reduction of the crop yield — a type 

 of loss so familiar that examples hardly need be cited. 

 In the results with the Rothamsted rain gauges about 

 50 per cent of the annual rainfall was regained in the 

 drainage water. Since the gauges bore no crop, the 

 remaining 50 per cent must have been lost by evapora- 

 tion. It should be noted that in the summer months 

 under warm temperature this loss was greatest, amount- 

 ing to 75 per cent of the rainfall. Correspondingly, in 

 the semiarid and arid sections of the country, where 

 there is little or no drainage, the rainfall is all lost by 

 evaporation. Investigations indicate that about 70 per 

 cent of the precipitation on the land surface is derived 

 from evaporation from land surface. Even in humid 

 regions, where the annual rainfall is ample for maximum 

 crop production, the crops are frequently reduced below 

 the profit point by prolonged periods of dry weather in 

 the growing season, during which the loss of water from 

 the plants, coupled with the loss from the soil, exhausts 

 the moisture supply. 



While run-off and percolation are directly proportional 

 to the rainfall, loss by evaporation does not vary to such 

 a degree. The loss by percolation depends almost 

 directly upon the amount of rainfall above the retentive 

 power of the soil. In years of heavy precipitation, 

 losses by percolation must increase. Evaporation from 

 the soil depends largely upon the time that the soil 

 surface is moist, and this will not vary markedly from 

 year to year. The following figures from the Rothamsted 

 drain gauges may be quoted in this regard : — 



