A NEW EVAPORATION FORMULA 
INTRODUCTION 
The investigation of the laws of evaporation and stream-flow, which is the 
subject of this publication, under the general title "A New Method of Estimating 
Stream-flow based upon a New Evaporation Formula," was brought to a stop in 
the latter part of December 1924 at which time Dr. John F. Hayford, who was con- 
ducting the investigation, was stricken with apoplexy, which resulted in his death 
in March 1925. In June 1926 the investigation was resumed under the direction of 
the writer, with a view principally to bringing it to a stage of publication as soon as 
possible with due regard to the work already accomplished by Dr. Hayford, and 
without undertaking any new studies beyond those deemed necessary for rounding 
out in good form the work already done. The results presented in this publication 
are therefore those achieved mainly by Dr. Hayford, with the writer as his principal 
assistant. At the time of his death in March 1925, the evaporation formula as 
hereinafter presented had been derived, and its application to the investigation of 
the laws of flow of Stream A, Wagon Wheel Gap, Colorado, had been practically 
completed. Since June 1926 the studies on Stream A have been completed and 
similar studies on Stream B have been made by the writer. In addition, during this 
time interval, the constants C, F and M in Part II of this publication were increased 
in accuracy and reliability. 
The ultimate object of this investigation was to obtain a much better formula- 
tion than the engineering profession now has of the laws governing the amount of 
stream-flow. The engineer is confronted at the outset in his design of works for 
controlling the waters of a river for power, flood-control, sanitation and navigation, 
with the necessity for knowing as accurately as possible the future flow of the river 
and its variations. The more accurately this can be determined at the outset, the 
more accurately it is possible to design the works with economy and safety. 
It appeared at the beginning of this investigation that progress toward the 
formulation of better laws governing the amount of stream-flow probably hinged 
largely upon obtaining a better knowledge of the laws of evaporation from large 
water and land surfaces. In order to secure this better knowledge of the laws of 
evaporation, it seemed advisable to supplement the numerous intensive studies of 
evaporation which had been made from evaporation pans a few square feet in area 
by a study made on the full scale of nature and under natural conditions. For this 
purpose it was proposed to consider each of the Great Lakes in turn as an evapora- 
tion pan and to evaluate from day to day (1) the change of content, (2) the income, 
and (3) the outgo, including evaporation. These evaluations have been made with 
sufficient accuracy to segregate that part of the outgo which is evaporation, and the 
laws which control the rate of evaporation have been determined with a fair degree 
of accuracy. The laws so determined have been applied in the formulation of better 
laws governing the amount of stream-flow with a confidence based upon the fact 
that the experimental evidence in establishing the evaporation laws, which form the 
basis of the stream-flow study, has been on the full scale of nature and under 
natural conditions. 
The principal obstacle at first toward formulating the laws of evaporation from 
observations on the Great Lakes was the difficulty of evaluating with sufficient 
accuracy the change of content of the lake from day to day. The change of content 
