[R-sig-eco] Time Series Analyses of Environmental Water Chemistry

[Rich Shepard]

   If this is not the appropriate mail list for my questions, please point me
to the proper list.

   Two projects require time series analyses of water chemistry. As is common
with permit compliance monitoring data the frequencies are not regular; for
example, sometimes the stream channel is dry, other times it's covered by a
few meters of snow and ice. Also, sampling locations can be visited on
different (usually adjacent) days, and quite often (particularly with
metals) concentrations are left-censored below the reporting levels.

   I have data for each chemical in a separate data frame. Applying zoo() to
this structure gives me a warning about multiple date instances; for
example,

  as.d.z <- zoo(x=as.d$quant, order.by=as.d$sampdate, frequency=NULL)
Warning message:
In zoo(x = as.d$quant, order.by = as.d$sampdate, frequency = NULL) :
   some methods for “zoo” objects do not work if the index entries in
‘order.by’ are not unique

   For now I want to analyze the data regardless of site (in the above
example that would be for all dissolved arsenic). There may be a need in the
future to look at the time series for each site (I hope not!!) or a few
specific sites.

   Since all this is a new area of R for me I seek advice on how to apply R
for time series analyses of these data.

Rich