Mass inflow is a curve which represents the cumulative flow in a reservoir at any particular instance.
It is a plot between cumulative inflow in the reservoir with time.
It can be prepared with the help of a hydrograph of the river for the dam site for a large number of years.
We know hydrograph is a plot or curve between discharge versus time.
Figure 9.4 (a) shows a hydrograph of a river for the dam site for a number of years and Fig. 9.4 (b) a corresponding mass curve prepared from the hydrograph of Fig. 9.4 (a). Let starting year of the hydrograph is 1998.
The total quantity of water that has flown through the river from 1998 to a time t1 (say 2001) has been represented by the hatched area.
In mass curve, the corresponding ordinate at time t1 (ordinate AB) will represent the total volume of water indicated by the hatched area of hydrograph curve.
Similarly the ordinates of the mass curve corresponding to other years can be determined from the hydrograph curve and plotted.
When all such points are joined free hand we get the mass curve corresponding to the hydrograph.
It is evident that a mass curve will continuously rise as it is the plot of accumulated inflow versus time.
Periods of no inflow would be indicated by horizontal lines on the mass curve.
The slope of the mass curve at any time gives the rate of inflow at that time.
Mass inflow curve is sometimes known as Ripple diagram also.