A NEW EVAPORATION FORMULA 
17 
It is now proposed, first, to use equation (8) in evaluating the ratio — , second 
to apply this ratio in equation (7) to obtain I e . Equation (8) will be applied in 
three cases — 
Case 1: Involving Lakes Superior, Michigan-Huron, Erie and Ontario; and the average 
year. 
Case 2: Involving Lakes Superior, Michigan-Huron and Erie; and the average year. 
Case 3: Involving Lakes Superior and Michigan-Huron; and the average year. 
The estimates of rainfall, evaporation, inflow and outflow for use in equation 
(8) for each case were made from sources partly external to this investigation and 
need not be given in detail. The necessary land and water areas are shown in 
Table 7. 
Table 7 — Areas of land and water surfaces on the Great Lakes drainage basin. (The unit is 1 square mile.) 
Lake 
Superior 
Michigan . . . 
Huron 
St. Clair 
Erie 
Ontario 
Total 
Area of 
lake 
surface 
32,060 
22,336 
22,978 
603 
9,968 
7,243 
95,088 
Drainage area 
including lake 
surface 
76,134 
65,799 
72,008 
6,194 
34,573 
32,980 
287,688 
Land area 
excluding 
lake area 
44,074 
43,463 
49,030 
5,691 
24,605 
25,737 
192,600 
Ratio of 
lake to drain- 
age area 
1:2.37 
1:2.95 
1:3.13 
1:12.31 
1:3.47 
1:4.55 
1:3.02 
From "Report of International Waterways Commission on Regulation of Lake Erie" (1910), page 9. The 
values given in the above table were the best available at the time. In subsequent literature more accurate 
values, no doubt, are given. Where feasible and advisable, the latest values were used in this investigation; that is, 
the computations based upon the early values were corrected, but in analyses where the results would bo affected 
very little by the later values, the original computations were allowed to stand. 
Consider the above three cases in turn, and let the unit be one foot of depth 
over the whole land area considered in each case. 
Case 1: 
The total land area = 192,600 square miles. 
The total area including water surface = 287,688 square miles. 
The total water area = 95,088 square miles. 
The total rainfall for a year =34.7 inches over the whole area. 
'287,688> 
the total rainfall for a year is (^l\ f 287 ' 688 ^ - 
V 12 / \192,600/ 
In the adopted unit 
4.31. 
,192,600, 
The evaporation from the water surface = 31.1 inches per year over the whole water 
area. In the adopted unit it is f— ^ / 95,088^ 
1.27. 
,192,600/ 
The run-off down the St. Lawrence River in one year is 39.7 feet on the surface of 
Lake Ontario (an area of 7,243 square miles). In the adopted unit, it is 39.7 X 
/ 7243 \ ■ 
\192,600/ 
1.49. 
Hence, from equation (8), Lake Superior being the highest lake in the area 
considered and there being no decrease in storage in the average year, in the adopted 
unit, evaporation from land = i?j = 4.31 — 1.27 — 1.49 = 1.55. This value, 1.55, is 
the evaporation from each unit of area of land expressed in feet of depth of water 
evaporated per year. 
