month. The difference in intercepts on the run-off axis at the b^#-
ning and end of month will give the total run-off in billions of cubic
feet in a month if the rate were continued for that period. For con-
venience in determining rates of flow the diagram in the upper left hand
corner of Plate VIII has been prepared with lines corresponding to various
rates of flow. The slope of any part of the curve can be compared
readily with this diagram and the rate of flow read directly or inter-
I)olated.
4. The average rate of flow for any period of time can be obtained by
determining the slope of the line connecting the ends of the curve for
that period as explained under 3.
5. The ordinate intercepted between the curve and a line connecting
any two points on the summation hydrograph shows whether the total
flow of the stream from the beginning of the period to the date indicated
by that ordinate is greater or less than the total flow that would be
produced during the same period by the rate of flow indicated by the
slope of the straight line. If the ordinate is positive, i. e., measured
above the straight line, it shows the amount by which the total flow of
the stream is greater than that produced by the flow of the draft line;
if negative, it shows the amount by which it is less than the quantity
produced by the flow corresponding to the slope of the draft line.
