18 A NEW METHOD OF ESTIMATING STREAM-FLOW 
31 1 
The corresponding rate of evaporation from the water surface = — -=2.59 
feet, as that is the depth evaporated from each unit of water area per year. Hence 
for Case 1,^ = ^=0.60. 
E w 2.59 
Case 2: 
The total land area = 166,863 square miles. 
The total area including water surface = 254,708 square miles. 
The total water area = 87,845 square miles. 
The total rainfall for the average year = 34.7 inches over the whole area considered in 
Case 2. In the adopted unit the total rainfall for the average year is ( — — ] 
/25V08\ = U2 7 
\166,863/ 
The total evaporation for a year for this area is estimated at 30.5 inches. In the 
adopted unit the total evaporation for a year is ( ^ \[ ° 7, ° .) = 1.34. 
\ 12 /\166,863/ 
The run-off through the Niagara River for the average year = 24.1 feet on the surface 
of Lake Erie, an area of 9,968 square miles. In the adopted unit, therefore, the 
run-off for Case 2 is 24.1 ( 9 ' 968 ^ = 1.44. 
i / 9,968 V 
\ 166,863/ 
Hence from equation (8), Lake Superior being the highest lake in the chain 
of Great Lakes and there being no decrease in storage in the average year, in the 
adopted unit evaporation from land = i?* = 4.42 — 1.34— 1.44 = 1.64. Hence, for 
Case 3: 
The total land area =136,567 square miles. 
The total area including water surface = 213,941 square miles. 
The total water area = 77,374 square miles. 
The total rainfall for the average year = 34.7 inches, or in the adopted unit the total 
rainfall is ^V21MilV4.51. 
\ 12 Al36,567/ 
The total annual evaporation from the water surfaces of Lakes Michigan-Huron and 
Superior is estimated at 25.0 inches, or, in the adopted unit, it is ( 5 " )( ''^74 ) =1.18. 
\ 12 ) V136.567/ 
The run-off through the Detroit River for the average year = 5.13 feet on the surface 
of Lake Michigan-Huron. In the adopted unit it is 5.13 [ 45 > 314 ) = 1.70. 
\136,567/ 
Hence, from equation (8), Lake Superior being the highest lake in the series 
and there being no decrease in storage in the average year, in the adopted unit 
evaporation from land = E x = 4.51 — 1.18 — 1.70 = 1.63. Hence, for Case 3, 
^ = L63 = 
E„ 2.59 
-p 
The adopted value for the ratio — ' for the Great Lakes region is taken as the 
E w 
Ei 
mean of the three values obtained in the above three cases or — - = 0.62. 
E a 
