REPORT ON FORESTS. 149 



Wind increases evaporation, but its influence to cause variation 

 between catchments is limited because the variation in the total 

 amount of wind through the year is limited. If we had a catch- 

 ment on which the wind blows all the time to compare with one 

 on which there is little or no wind, no doubt the diiference in 

 evaporation would be appreciable. 



In case the summer rain-fall is large, the evaporation will be 

 greater than that given by the formula, which is based upon 

 such rain-fall as that of the Eastern and Middle States, which is 

 quite evenly distributed through the year. This difference may 

 be taken into account by monthly or seasonal formula, but this 

 is a matter of too much detail for the purposes of this paper. 



Such a computation as we have indicated is made in the 

 following table. The streams are grouped into deforested catch- 

 ments, having less than 40 per cent, of the area in forests and 

 forested catchments having more than this percentage. 



Observed Evaporation Compared with Evaporation Computed 

 on the basis of Rain-fall and Temperature. 



DBFORESTED CATCHMENTS. 



Per cent. Observed Computed Differ- Differ- 

 STRBAM. of Annual Annual ence, ence. 



Forest. Evaporation. Evaporation. Inches. Percent 



Sudbury 14 23.6 24.4 — 0.8 — 3 



Cochituate, Small. 24.3 23.8 -I-0.5 +2 



Mystic, Small. 23.6 22.6 o o 



Croton 30 24.9 25.0 — 0.1 o 



Genessee, 27 27.2 20.6 -|-6.6 -f24 



Skaneateles, 12 18.9 19.6 — 0.7 — 4 



Musconetcong, 39 21.0 23.7 — 2.7 — 13 



Pequest 18 20.5 24.0 — 3.5 — 17 



Paulinskill, 27 22.2 25.2 — 3.0 — 14 



Raritan 13 23.7 25.9 — 2.2 — 8 



Tohickon, . . 24 21.3 28.2 — 6.9 — 32 



Neshaminy 6 24.4 28.3 — 3.9 — 16 



Perkiomen 25 24.2 28.2 — 4.1 — 17 



Muskingum, Small. 27.3 25.7 +1.6 +6 



Desplaines, Small. , 25.0 19.8 +5.2 +21 



Kansas, Small. 24.1 20.5 -I-3.6 +15 



Sweetwater, Small. 21.9 22.2 — 0.3 — i 



I,ea, Small. 21.8 21.3 +0.5 +2 



Wandle, Small. 29.3 24.5 +4.8 +17 



Thames Small. 20.3 20.9 — 0.6 — 3 



Wendover Springs, Small. 20.0 21.4 — 1.4 — 7 



