INFLUENCE OF A CROP ON EVAPORATION 125 



Woburn Experimental Fruit Farm the ground surrounding 

 certain young apple trees has been differently treated, in 

 some cases grass has been sown round the tree, in other cases 

 natural weeds have been allowed to accumulate, while in 

 other cases the land has been kept clean throughout the 

 summer by hoeing. The difference in the growth of these 

 trees during the dry summer of 1896 was most marked, the 

 trees standing in the bare soil worked with a hoe making an 

 abundance of new wood, while hardly any growth took place 

 in thejcase of the trees surrounded by vegetation. Weeds 

 are far more injurious in a dry climate than in a wet. 



The amount of evaporation from a soil largely affects the 

 amount of percolation through it. In the case of ordinary 

 permeable soils, the amount of rainfall and of evaporation 

 determine the quantity of water which will pass through and 

 appear as drainage. In Table XIV we see the influence of 

 evaporation on percolation. In the winter, 70-80 per cent, of 

 the rain comes through the drain-gauges at Rothamsted, while 

 in summer only 20-27 per cent, passes through the soil. A 

 great part of this summer drainage is derived from heavy 

 storms, a portion of the water passing through the open 

 channels in the soil before the body of the soil is saturated. 

 The same quantity of rain in the form of light showers would 

 produce no percolation. 



In Table XIX we have seen striking illustrations of the 

 fact that the annual evaporation from a bare soil is a fairly 

 constant quantity, so that all rainfall over this amount appeal's 

 as drainage. Doubling the rainfall has in fact increased the 

 drainage five or six fold. If, however, land is covered with 

 vegetation, the amount of evaporation ceases to be a constant 

 quantity, every increase in rainfall up to an excessive amount 



