126 
A NEW METHOD OF ESTIMATING STREAM-FLOW 
mate theory used in the investigation of the effects of winds and of barometric 
pressures. 
(/) The average total evaporation on Lake Michigan-Huron for the months 
May to October inclusive, as computed by Freeman, using his formula, is 15.33 
inches of depth on the lake area, or 0.00694 foot of depth per day (page 145, 
Regulation of the Great Lakes). This is 0.00144 foot of depth per day, or 28 per 
cent, greater than that computed by the formula developed in this investigation 
( == 1.281, and is a difference of the same sign, and approximately of the 
same amount, as that noted in paragraph (e), above. The difference between the 
overall evaporation computed by the two formulas is probably too great to be 
accounted for by errors in Assumption No. 5. 
(g) The average daily rate of evaporation for the summer months May to 
October inclusive, as finally adopted by Freeman (column 7 of table on page 145, 
Regulation of the Great Lakes) is 0.0056 foot of depth per day, an amount in almost 
exact agreement with the value 0.0054 derived in this investigation. But the 
comparison month by month is not so close. The average monthly values, and 
their accumulated sums, expressed in units of one foot of depth on the lake area 
are as follows : 
Month 
Average evaporation 
(ft.) 
Accumulated sums of 
evaporation 
(ft.) 
This 
investigation 
Freeman 
This 
investigation 
Freeman 
May 
July 
Aug 
Bept 
Oct 
0.169 
.165 
.220 
.158 
.114 
.165 
0.0250 
.0333 
.133 
.250 
.308 
.300 
0.169 
.334 
.554 
.712 
.826 
.991 
0.025 
.058 
.191 
.441 
.749 
1.049 
From this tabulation it is apparent that the rate of evaporation as computed in 
this investigation is larger in May, June and July, and smaller in August, September 
and October than the rate as finally adjusted by Freeman. 
COMPARISON WITH HORTON'S RESULTS 
A comparison of Horton's formula 1 directly with the one developed in this 
investigation is not feasible on account of its difference in form. 
The average monthly evaporation on Lake Michigan-Huron for the six months 
May to October of 1909 to 1913 inclusive — the years of observation used in this 
investigation — as computed by Horton is 22.74 inches or 1.895 foot (Table 71, 
page 184). This is at the rate of 0.0103 foot of depth per day, which is 91 per cent 
greater than the value 0.0054 foot of depth per day derived in this investigation. 
COMPARISON WITH FORMULAS OF ABASSIA, FITZGERALD. CARPENTER AND STELLING 
In the monthly Weather Review for July 1907 (page 313), Professor Bigelow 
presents a comparison between four evaporation formulas, all based on the Dalton 
1 Report of the Engineering Board of Review of the Sanitary District of Chicago on the Lake Lowering Controversy 
and a Program of Remedial Measures. Part III — Appendix II. Hydrology of the Great Lakes, by Robert E. 
Horton in collaboration with C. E. Grunsky, page 174 (1927). 
