In cropgrowdays: Crop Growing Degree Days and Agrometeorological Calculations

When modelling crops, agronomists typically specify dates as the day of year. Several functions are available for day of year calculations and converting these back to dates. In R, dates, times and timezone data are easily manipulated using the lubridate package.

The day_of_year function is used to convert a date to the day of year, which could be based on the calendar year starting on 1 January, the Australian financial year starting on 1 July or an arbitrary starting date.

##  Day of Calendar Year
day_of_year(ymd(c("2020-12-31", "2020-07-01", "2020-01-01")))
day_of_year(ymd(c("2020-12-31", "2020-07-01", "2020-01-01")), return_year = TRUE)

## Day of Financial Year
day_of_year(ymd(c("2020-12-31", "2020-07-01", "2020-01-01")), type = "financial")
day_of_year(ymd(c("2020-12-31", "2020-07-01", "2020-01-01")), type = "fin",
            return_year = TRUE)

To convert a day of year to a date, use date_from_day_year noting that while the calendar year is the default, we can specify the Australian financial year or an arbitrary starting date.

## Convert day of year to a date
date_from_day_year(21,2021)
date_from_day_year(21,2021, type = "fina")

Finally, while we can use day_of_year to obtain the day of the current year, if a crop is planted near the end of the year then we way wish to know the day of harvest which will fall in the next year. The day_of_harvest function provides the day of year in the year of sowing which can be used to calculate other quantities like day of flowering etc. Thus, quantities like the number of days between harvest and sowing are easily calculated taking into account that the crop may grow past the end of the year. Alternatively, these quantities are also easily computed directly on the dates by using the lubridate package. For instance the convenience function cropgrowdays::number_of_days is essentially a call to as.numeric(finish_date - start_date) + 1.

## Day of harvest using the first day of the year of sowing as the base day
day_of_year(ymd("2021-01-05"))
day_of_harvest(x = ymd("2021-01-05"), sowing = ymd("2020-12-20"))  # > 366

Note that the first calculation simply assumes the first day of the year is 1 January 2021 whereas the second calculation yields a result assuming the first day of the year is 1 January 2020. Hence, since 2020 is a leap year containing 366 days, then the day of harvest is $366 + 5 = 371$.