LESSON 3.7

Question 1

The consistency of a rainfall record is tested with

Answer 1

double-mass analysis

Question 2

This method compares cumulative annual (or alternative, seasonal) values of station “X” with
those of a reference station. The reference station is usually the mean of several neighboring
stations. The cumulative pairs (double-mass values) are plotted in an xy arithmetic coordinate system
as shown in Figure 1, and the plot is examined for trend changes. If the plot is essentially linear, the
record at station X is consistent. If the plot shows a break in slope, the record at station X is
inconsistent and should be corrected. The correction is performed by adjusting the records prior to the
break to reflect the new state (after the break). To accomplish this, the annual rainfall data prior to the
break are multiplied by the ratio of slopes after and before the break. In the example shown, each of
the station X values up to and including the breakpoint year would be multiplied by a ratio m2/m1.

Answer 2

Double Mass Analysis