216 NATURE AND PROPERTIES OF SOILS 



to pay. If rapid efficient drainage can not be assured at a 

 reasonable cost and under such conditions that the increased 

 crops will return a good profit on the investment, tile drains 

 should not be installed. 



112. Evaporation losses. — Evaporation of soil-water takes 

 place almost entirely at the surface, exceptions being 

 where 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 reduc- 



FiG. 40. — Cement block at the outlet of a tile drain. 



tion of crop yield, a type of loss so familiar that examples 

 hardly need be cited. In the results with the Rothamsted rain 

 gauges (see page 207), about 50 per cent, of the annual rain- 

 fall was regained in the drainage water. Since the gauges bore 

 no crop, the remaining 50 per cent, must have been lost by 

 evaporation. It should be noted that in the summer months 

 under warm temperature, this loss was greatest, amounting 

 to 75 per cent, of the rainfall. Correspondingly, in the semi- 

 arid and arid sections of the country where there is little or 

 no drainage, the rainfall is almost all lost by evaporation. 

 Evaporation from land surface has an appreciable effect on 



