R script to calculate daily PAR from solar radiation data

Generating estimates of daily reference
photosynthetically Active Radiation (PAR). We show the procedure to
generate estimates of daily reference PAR using solar radiation data.
The input for the R script (CalculateDailyPAR.R) is a raw time series of
hourly solar radiation (stored in variable ‘ws’) that for our case was
obtained from the CIMIS website (station id: 105) [California Department
of Water Resources, 2015]. The script processes the data set to format
the date and time columns, and to identify missing data points reporting
their position within the time series (variable ‘na.id’). The user
fills the gaps using adequate strategies and creates a new input file
(stored in variable ‘fill.points’) containing the values to fill in
within the time series. A reference PAR estimate is obtained as a
constant fraction of solar radiation using the conversion factor
proposed by [Meek et al., 1984]. The script then calculates an average
daily value of solar radiation and integrates the reference PAR over the
daytime period to obtain a daily value. The script ends by generating a
final table (‘ws.results’) reporting daily values of solar radiation
(maximum and mean in W m-2), and maximum, mean, and minimum reference
PAR values in units of (μmol m-2 d-1) and (mol m-2 d-1).