A NEW METHOD OF ESTIMATING STREAM-FLOW 227 
period of decreased rate of melting, the net melting is to be computed by equation 
(67), page 156, in which are substituted the final values of C, F, M and T" from 
(91), page 193. 
(5) Using the residuals of equation (59), viz, —D, as the absolute term in (60), 
derive the corrections to the estimated S e of step (1) and the estimated R'\, R'\, 
R" z , ... of step (2). These corrections will be R h R ly R,, . . . of equation (60). 
Adding these to the assumed values according to (61) will give the normal-flow 
constants of the stream, S c , R\, R\, .... 
(6) Estimate the flood-flow constants of the stream, R" fl , R" /2 , R" ft , ... by 
comparison with other streams studied in connection with the general ideas set 
forth on pages 222 to 225. Obtain the G — D' curve of the stream by studying the 
residuals of equation (60) and the observed discharge, D'. 
(7) Compute r /h r /2 , r /t , . . . from the values of r, computed in step (4) and 
the value of G established in step (6). Use these in equation (79), page 172, 
together with the R"/s estimated in step (6) and the residuals of equation (60) as 
absolute term, taken from days of heavy rain and rapid rates of melting. 
(8) Take the residuals of equation (79) as the absolute term in equation (78) 
and compute the corrections R n , R /2 , R /3 , ... to the assumed values, R"/i, R" '/», 
R" n, ... to get the flood-flow constants of the stream R' fl , R' /2 , R' /8 , . . . 
according to (80). 
(9) The normal-flow formula obtained in steps (1) to (5), and the flood-flow 
formula obtained in steps (6) to (8) can now be used to estimate the stream-flow for 
years for which no direct observations of flow are available, in terms of the observed 
meteorological elements on the watershed, by the process stated in general terms 
on pages 138 and 139. 
The eight steps outlined above for deriving the discharge formulas of the 
stream represent the probable least amount of labor which will be required for that 
purpose. To carry out those eight steps would require about 100 to 200 days of 
work of one computer. It is probable that additional refinement will be necessary, 
in addition to those eight steps, as in the case of Solution P, Stream A (see pages 
166 to 168), which would probably bring the labor required to nearer 200 than 100 
days of work for one computer. It is estimated that with the maximum time 
stated, the constants for any stream in the eastern two-thirds of the United States, 
in a region of 20 inches or more of annual rainfall, could be developed with greater 
accuracy than those now available for Stream B, after the meteorological data and 
stream-flow data have been gathered for 3 years. 
The rate at which the stream-flow can be estimated by use of the discharge 
formulas after they are derived has not been determined. It appears, from general 
considerations, however, that less than 10 man-hours of time should be required to 
compute the daily stream-flow for one month from the meteorological elements 
after the formulas are derived. 
In connection with the estimate of accuracy with which this method may be 
applied on a new stream, made in the second preceding paragraph, it is noteworthy 
that the most important part of the duration curve for power estimates on run-of- 
river power plants is the lower part. This happens to be the most accurately 
determinable part by this method. In this type of plant very low flows can not be 
utilized because of insufficient volume of water to turn the turbines. Very high 
flows can not be utilized because of the decrease in effective head on the turbines 
due to rise of tailwater. If a run-of-river plant be considered as installed on 
