REPORT ON FORESTS. 151 



heavily against the theory that forests, or any other cause 

 except rain-fall and temperature, affects evaporation to any 

 important extent. 



The large excess of evaporation on the Genessee is unaccount- 

 able. No such excess appears on the Skaneateles, which has a 

 similar climate, and is even more deforested, but the excess on 

 the Desplaines and Kansas is fully accounted for by the large 

 excess of spring and summer over fall and winter rain-fall. 



The excess on the Wandle, in England, is possibly attribut- 

 able to escape of the water downward into the chalk, but the 

 figures for the Lea, Thames and Wendover Springs agree quite 

 closely with our formula. It is remarkable that the Sudbury, 

 with 14 per cent., the Merrimac, with 40 ; the Connecticut, with 

 53 ; the Skaneateles, with 12 ; the Raritan, with 13 ; the Hacken- 

 sack, with 60 ; the Passaic, with 58 ; the Ramapo, with 75, and 

 the Egg Harbor and Batsto, with 88 per cent, of forest area, all 

 conform closely to the formula, showing that a range from 12 to 

 88 per cent, in forest area is not sufficient to materially affect the 

 evaporation. Allowing for differences due to seasonal distribu- 

 tion of rain-fall, it seems to be sufficiently clear that a formula 

 based only upon rain-fall and temperature can be made to 

 harmonize the results of stream gaugings as closely as the 

 unavoidable errors of gauging and rain-fall measurement will 

 admit of. Indeed, if it seemed warrantable, for the purposes of 

 this paper, to introduce extended computations by months 

 instead of by years, it could be shown that all influences, except 

 temperature and rain-fall, are practically negligible in determin- 

 ing evaporation and stream yield, so far as is shown by the best 

 records of measurement now available. Consequently, we con- 

 clude that stream gaugings show no decrease of evaporation 

 owing to the presence of forests upon the catchments. 



Another class of experiments often quoted in this connection 

 has no bearing at all upon the relative evaporation from a 

 wooded or an open country, so far as it affects the streams. I 

 refer to those, such as Dr. Ebermayer's, which show that the 

 evaporation from a water-surface, or from the soil beneath the 

 forest trees, or beneath the litter of the forest, is less than in the 

 open. This we readily concede to be true, but we must deter- 

 mine whether or not this is offset by the large volume of water 

 taken up by the roots of the tree and transpired from its leaves 



