Contents preface (VII) introduction 1—37

2.6.2.1. Flow-Mass Curves
Flow-mass curve or runoff-mass curve or inflow mass curve or simply mass curve is cumulative flow volume V versus time curve. The mass curve ordinate V (m³ or ha.m or cumec-day) at any time t (in days or weeks or months) is given as
T

V = _zt0 Q . dt (2.19)
where, t₀ is the time at the beginning of the curve. Obviously, mass curve, Fig. 2.16, is an integral (i.e., summation) curve of a given hydrograph, Art. 2.7. Also, slope of the mass curve at any point on the plot i.e., dV/dt equals the rate of stream flow (i.e., stream discharge) at that time. Mass curve is always a rising curve or horizontal (when there is no inflow or runoff added into the stream) and is a useful means by which one can calculate storage capacity of a reservoir to meet specified demand as well as safe yield of a reservoir of given capacity.

Massinflow(Hectare–metre)

1970

Storage

K

L

Spilling

refilling

I

J

G

H

Depleting

E

F

C

D

Max. spill

Mass-flow curve

X Hectare

B

Demand

meter

A

line

1 year

Demand rate = slope

1971

1972

1973

Year