-------------------------------------------------------------------------------help doubmass-------------------------------------------------------------------------------

Double mass plot

doubmassyvarxvarlist[ifexp] [inrange] [,ratioscatter_options]

Description

doubmassplots the cumulative sum ofyvaragainst the average of the cumulated sums of each variable inxvarlist. Ifxvarlistcontains just one variablexvar,doubmassreduces to a plot of the cumulative sum ofyvaragainst the cumulative sum ofxvar.It is assumed that data are tsset and that no more than one panel is considered in any call. Otherwise put,

doubmassexpects a data structure in which contemporaneous panels constitute parallel time series.

RemarksIn hydrology double mass plots are a standard method of checking the consistency of time series. For precipitation or stream discharge in particular, nearby sites often have averages differing by a multiplier, so that their ratio is constant, to a good approximation. Any breaks of slope on the graph may indicate discontinuities such as station shift, instrument breakdown, or changes in site conditions.

This technique appears not to be covered in literature on cusum charts for quality control (e.g. Woodward and Goldsmith 1964).

Options

ratioplots the slopecumulative sum of

yvar-------------------------------------- average of cumulative sums ofxvarlistagainst average of cumulative sums of

xvarlist.

graph_optionsare any of the options allowed with scatter.

Example

. doubmass site1 site2

AuthorNicholas J. Cox, Durham University, U.K. n.j.cox@durham.ac.uk

ReferencesClarke, R.T. 1994.

Statistical Modelling in Hydrology.Chichester: John Wiley,Dingman, S.L. 1994.

Physical Hydrology.Englewood Cliffs, NJ: Prentice-Hall.Dunne, T. and L.B. Leopold. 1978.

Water in Environmental Planning.San Francisco: W.H. Freeman.McCuen, R.H. 1998.

Hydrologic Analysis and Design.Upper Saddle River, NJ: Prentice-Hall. pp.193-197.Searcy, J.K. and C.H. Hardison. 1960. Double-mass curves.

United StatesGeological Survey Water-Supply Paper1541B.Woodward, R.H. and P.L. Goldsmith. 1964.

Cumulative Sum Techniques.Edinburgh: Oliver and Boyd for Imperial Chemical Industries.

Also seehelp for scatter