8 A NEW METHOD OF ESTIMATING STREAM-FLOW 
(6) The problem of estimating the evaporation from other large open surfaces 
of water, such as the surfaces of reservoirs, lakes and rivers, in the design of con- 
trolling works for power, irrigation, navigation and sanitation. 
(c) The problem of estimating the evaporation from large land surfaces as a 
basis for estimating the run-off from such surfaces for use in various engineering and 
scientific problems. The uses to which run-off estimates are put in engineering 
problems need not be enlarged upon. As one example of the use of such an estimate 
in a scientific problem is the use made of it in the problem of formulating the laws of 
stream-flow, in Part II of this publication. 
ORDER OF PRESENTATION 
The order of presentation of the material in Part I of this publication is 
as follows: 
(1) The final form of the observation equation used which served to determine 
the evaporation from Lakes Michigan-Huron and Superior is presented. The 
complete list of 755 daily observation equations on Lake Michigan-Huron is given, 
together with the residuals from them. These equations and their residuals served 
to fix the constants and their probable errors in the evaporation formula as derived 
from the observations on Lake Michigan-Huron. The principal facts from the two 
final least-square solutions for determining the evaporation are given, and the 
adopted weighted-mean values of the constants in the final evaporation formula as 
developed in this investigation are presented. 
(2) Some of the evidence, which led to the final adopted form of the formula for 
expressing evaporation, is presented. 
(3) The method of evaluating the constant part of the run-off into the lake 
is stated, and a brief discussion is given of the effect upon the final results of neglect- 
ing the variable part of the run-off into the lake. 
(4) A brief discussion of the accuracy of the computed evaporation is given, 
together with a comparison of the formula developed in this investigation with 
others (Appendix to Part I). 
FINAL FORM OF OBSERVATION EQUATION USED FOR DETERMINING EVAPORATION 
The final form of observation equation used for determining the rate of 
evaporation from the surfaces of the Great Lakes is the following: 
+eE l +e[(£ i -x)\Ei+I = v (1) 
[(luu-*)] 
The current day is the day to which the observation equation is credited in 
listing the equations; the next earlier day will be called the preceding day. The 
meanings of the quantities in equation (1) are as follows: 
e = mean difference for the lake between (1) the saturation vapor-pressure corresponding to 
the mean air temperature for the lake for the two days ending at midnight at the end 
of the day to which the observation equation refers; and (2) the mean actual vapor- 
pressure for the 24-hours ending at 12:30 p. m. on the date to which the observation 
equation refers. The unit is 0.01 inch of mercury. The difference (1) — (2) may, for 
convenience, be called the vapor-pressure potential. 
^i=that part of the evaporation which is proportional to the vapor-pressure potential. 
