#' Calculation of QMNAs (annual minimum of mean monthly flows)
#'
#' @param x either [POSIXt] dates, or a [data.frame] with a first column of [POSIXt] dates and flows in the other columns, or a \code{airGR::OutputsModel} which is an output of [airGR::RunModel()]
#' @param threshold [numeric] minimum ratio of available data in a year or a month to take it into account
\item{x}{either \link{POSIXt} dates, or a \link{data.frame} with a first column of \link{POSIXt} dates and flows in the other columns, or a \code{airGR::OutputsModel} which is an output of \code{\link[airGR:RunModel]{airGR::RunModel()}}}
\item{threshold}{\link{numeric} minimum ratio of available data in a year or a month to take it into account}