To prepare a rainfall mass residual curve
Obtain a long period
of rainfall data and derive long-term average values.
For each period that
you wish to examine you then compare the recorded rainfall value with the
long term average and subtract away the average.
This gives you a residual.
To create
a cumulative plot
Add together the residual values over time
and graph them against time.
Where the slope of the curve is rising, during
that
period rainfall exceeds long term average meaning wetter periods.
Where
the slope of the curve is falling, during that period rainfall is less than
long
term average meaning generally drier periods.