REPORT ON FORESTS. 141 



evaporation on the Lee and Thames rivers, and Wendover 

 springs, which data we give later in the table of stream meas- 

 urements, it is found that these three streams give an average 

 evaporation of 20.7 inches annually from catchments covered 

 with the prevailing vegetation of Hertford and Middlesex, 

 against 16.68 from ground without vegetation. We seem to 

 have here quite reliably indicated an excess of 4 inches in 

 annual evaporation caused by vegetation. Lawes and Gilbert 

 themselves are of the opinion that from 3 to 4 inches repre- 

 sents a fair allowance for the excess of evaporation caused by 

 the ordinary vegetation around London over evaporation from 

 fallow soil kept hoed. 



The experiments of Charles Greaves at Lee Bridge, and of 

 Evans and Dickinson at Nash Mills, appear to show an evapora- 

 tion from fallow soil almost exactly equal to that shown by the 

 catchments of the above rivers, and on the basis of these experi- 

 ments there would be no increase of evaporation due to vegeta- 

 tion, but we believe that the above estimate of 3 to 4 inches, 

 based on the experiments of Lawes and Gilbert, is fairly accurate. 

 In our Report on Water-Supply, of 1894, we estimated from 

 the data furnished by the gaugings of the Passaic that evapora- 

 tion on that water-shed was not increased by the existing 

 vegetation to a greater amount than 6 inches per annum.* 

 This vegetation is made up of 58 per cent, forests ; about 6 per 

 cent, wet meadow, which is in grass ; 6 per cent, fallow land ; 

 15 per cent, short grass, or pasturage, and 15 per cent, under 

 cultivation. After a recent revision of these estimates our 

 opinion is that the effect of vegetation, such as the above, is to 

 increase the evaporation over what would take place from the 

 same area with no vegetation whatever to the extent of not 

 more than 5 inches per annum. Our own figures are therefore 

 in fair accord with the results of Lawes and Gilbert, and both 

 are based on data derived from conditions closely approximating 

 to those which we find in nature, and confirmed by other esti- 

 mates which it is impossible to set forth in this paper. We feel 

 justified, therefore, in concluding that the effect of vegetation of 

 any and all kinds upon evaporation has in many cases been 

 much over-rated, and that such vegetation as occurs in the 



* Report on Water-Supply, page 332. 



