character string identifying the time period to summarize by.
Defaults is hydrologic year ("hyear"), other choices are "year" and "month".
The "month" option will return the number of missing data points for each
month in the time series.
hyrstart
optional argument, define start month of hydrologic year
Value
Returns a numeric vector of the number of missing observations per
summary period. The "times" attribute of the returned vector provides the
corresponding year, hyear, or month.