A NEW METHOD OF ESTIMATING STREAM-FLOW 137 
wind, in miles per day, over the watershed, as measured by the nearest Weather 
Bureau anemometer or anemometers located about 100 feet above the ground sur- 
face. The expression 1.49e ( r^r — 2.6] enters the equation only when it is 
positive, or only when w exceeds 2G0 miles per day or 10.8 miles per hour. 
-p 
— ' is a constant for each watershed. It is derived from observations on the 
E w 
watershed in terms of the observed precipitation as rain, computed net melting of 
the snow and ice, observed run-off, and computed evaporation from water. Appar- 
ently the best basis for the evaluation of the ratio — is the idea that in general for 
Iil U 
any long period, especially for a long period which begins and ends at the same 
season of the year, the net rate of addition to storage must be nearly zero. That is 
(Rainfall) + (Net melting) — (Evaporation from Land) — (Run-off) . . 
Length of period ^ ' 
must be numerically small, and must tend to get smaller as the length of the period 
is increased. 
In equation (35) the rainfall is observed, the net melting is computed in a 
manner presently to be described, the run-off is observed, the evaporation from a 
water surface is computed, hence setting the equation equal to zero and solving for 
the only unknown, 
Ei Rainfall + net melting — Run-off /n _ . 
ir = 5 (35a) 
In equation (35a) since E w and the net melting are in units of 0.01 inch per 
day the observed run-off (stream-flow) must be converted into the same unit before 
substitution therein. 
The use of equation (34) to compute the evaporation from land in the r's in 
equation (33) involves three assumptions, which may be stated at this point, 
namely: 
Assumption No. (6) — It is assumed that the evaporation from a land surface 
(including the surfaces of trees, shrubs, etc.) in the summer months, when the 
precipitation is all in the form of rain, follows the same laws, to the degree of accur- 
acy which can be detected from the observations themselves, as the evaporation 
from a free, open, water surface as stated in equation (23), modified only by the 
constant — - . 
E w 
Assumption No. (7)— It is assumed that the loss by evaporation from a land 
surface in the warmer months, to which assumption No. 6 applies, follows the same 
laws, to the degree of accuracy which can be detected from the observations them- 
selves, when the trees and shrubs are covered with leaves in the spring, or when the 
crops are growing, as in the fall when the branches are bare and the ground covered 
with leaves, or after the crops have been harvested. 
Assumption No. (8) — It is assumed that the evaporation from the surface of 
ice and snow in the winter months, when precipitation is all in the frozen form, 
follows the same laws, to the degree of accuracy which can be detected from the 
