Double mass curve, station X and 15 stations, 2013-2022

The data are first arranged in chronological order (2013 to 2022) and the cumulative values are found.

Consistency check

Plot cumulative X (y) against cumulative average of the 15 stations (x). The ratio X/avg is about 1.1 to 1.45 up to 2017, and below 1 from 2018. So the points follow one straight line up to the 2017 point and a flatter line after it.

• Slope up to 2017: $M_a = \dfrac{1895}{1450} = 1.307$
• Slope after 2017: $M_c = \dfrac{3089 - 1895}{2788 - 1450} = \dfrac{1194}{1338} = 0.892$

The slopes differ, so the record of station X is not consistent. The change in regime occurred after 2017 (from 2018).

Adjustment

The earlier data (2013-2017) are adjusted to the present regime (slope $M_c$):

$$P_{cx} = P_x\times\frac{M_c}{M_a} = P_x\times\frac{0.892}{1.307} = 0.683\,P_x$$

Data for 2018-2022 are unchanged.

Answer: Station X is inconsistent; change in regime from 2018 (break after 2017); the 2013-2017 values are multiplied by 0.683 to give 298.4, 232.2, 208.3, 218.5 and 336.6 mm.