Hydrologic analysis requires data collected over a long period.
The longer the period, the greater is the chance of that the consistency of the data is affected by changes in gage location, instrumentation, environmental conditions or observational procedures. The consistency of the data can be tested by Double Mass Curve Technique, and it is possible to identify when the change in regime occured and also to adjust the data. Plot the cumulative values of the annual or seasonal precipitation of that station against the concurrent cumulative values of the mean of like observations of a group of nearby base stations. If the data is consistent, the two sets of data will be proportional and the slope of the resulting line will give the constant of proportionality. If there is a break in the slope of the line, a break in consistency or a change in regime is indicated.