Computes the monthly mean values of discharge, concentration, flux, flow-normalized concentration and flow-normalized flux (Q, Conc, Flux, FNConc, and FNFlux) in SI units
Note that the Flux and FNFlux values are average flux values (not totals). For discharge they are in m3/s, concentration is mg/L, and flux is kg/day.
It returns a data frame containing month, year, decimal year, and mean values of DecYear, Q, Conc, Flux, FNConc, and FNFlux.