Doublemass analysis—The trend of the rainfall records at a station may slightlychange after some years due to a change in the environment (or exposure) of a station either due to coming of a new building, fence, planting of trees or cutting of forest nearby, which affect the catch of the gauge due to change in the wind pattern or exposure. The consistency of records at the station in question (say, X) is tested by a double mass curve by plottting the
cumulative annual (or seasonal) rainfall at station X against the concurrent cumulative values of mean annual (or seasonal) rainfall for a group of surrounding stations, for the number of years of record (Fig. 2.9). From the plot, the year in which a change in regime (or environment) has occurred is indicated by the change in slope of the straight line plot. The rainfall records
of the station x are adjusted by multiplying the recorded values of rainfall by the ratio of slopesof the straight lines before and after change in environment.
Example 2.2 The annual rainfall at station X and the average annual rainfall at 18 surrounding stations are given below. Check the consistency of the record at station X and determine the year in which a change in regime has occurred. State how you are going to adjust the records for the change in regime. Determine the a.a.r. for the period 19521970 for the changed regime.