84 
A NEW METHOD OF ESTIMATING STREAM-FLOW 
to be substituted therein. This applies with special reference to the wind velocity, 
w, which is ordinarily observed at an elevation of approximately 100 feet above 
the ground surface, and is not the wind velocity at or very near the ground or 
water surface. 1 
COMPARISON OF COMPUTED EVAPORATION FROM LAKE MICHIGAN-HURON WITH DIRECTLY 
OBSERVED FALL OF LAKE SURFACE. CORRECTED FOR THE VARIOUS INFLUENCES 
By substitution of the derived values of E x and E 2 from Solution V h shown 
in Table 25, into the observation equations for that solution shown in Table 24, 
the evaporation from the lake surface was computed for each day, or group of days 
combined into one equation, and also the residuals, v, were evaluated (shown in 
Table 28). The evaporation from the lake surface so computed is shown in the 
eleventh column of Table 23 marked "computed evaporation." The algebraic 
sum of this computed evaporation for any day and the "net rise," (/), is the 
residual, v, for that day. An examination of the computed evaporation in Table 
23 shows that: 
(a) The maximum evaporation computed on any day was 0.029 foot on 
July 25, 1910; 
(6) The minimum evaporation computed on any day was 0.000 foot on 
August 25, 1910, and on July 24, 1913. 
(c) The total and mean evaporation, computed, is as follows, expressed in 
feet of depth on Lake Michigan-Huron: 
May 
June 
July 
August. . . . 
September. 
October. . . 
Total 
Mean monthly. 
Mean daily. . . . 
1909 
(0.198) 
.151 
( .123) 
( -162) 
.634 
.158 
.0052 
1910 
(0.177) 
.165 
.249 
.202 
.100 
.168 
1.061 
.177 
.0058 
1911 
(0.150) 
.137 
.245 
.160 
.108 
( -132) 
.932 
.155 
.0051 
1912 
(0.177) 
.142 
.139 
.099 
.104 
( .228) 
.889 
.148 
.0048 
1913 
(0.172) 
.215 
.268 
.180 
( -137) 
( .133) 
.105 
.184 
.0060 
Mean 
monthly 
0.169 
.165 
.220 
.158 
.114 
.165 
Mean 
daily 
0.0055 
.0055 
.0071 
.0051 
.0038 
.0053 
The values enclosed in parentheses are the values from the table corrected 
for the rejected days. 
(e) The average evaporation for the whole period of observation used in 
this investigation, July to October 1909, and May to October of 1910, 1911, 1912 
and 1913, was 0.0054 foot of depth per day on Lake Michigan-Huron. 
(/) The maximum variation in evaporation from one day to the next was 
0.022 foot from July 25 to 26, 1910. This represents a variation of over four 
times the average daily evaporation for the summer months, 0.0054 foot. There 
are numerous instances of variation in evaporation from one day to another of 
2 and 3 times the average daily evaporation. 
1 For a discussion of its accuracy and a comparison of this evaporation formula with others, see Appendix 
to Part I, page 121. 
