chill_units function

Calculation of chill units from hourly temperature data (Utah model)

The function calculates chill units using the Utah model (Richardson et al, 1974). This model is characterized by differential weighting of temperature ranges, including negative weights for temperatures above 15.9°C. This model recognizes that different temperatures vary in effectiveness in accumulating chill as well as a negative influence of high temperatures on previously accumulated chill. Chill Units (Utah or Anderson model) perform better than chill hours for a wider range of climates, and it could be considered as the 'reference' method nowadays, but it is ill-suited for warm or Mediterranean conditions. To date, Chill portions is the best existing model for most growing regions, so chill fulfilment should preferably be calculated using that method, especially when transferring varieties from one region to another, or in studies on climate change.

chill_units(climdata, Start)

Arguments

• climdata: a dataframe with hourly temperature data. It must contain the columns Year, Month, Day, DOY, Temp.
• Start: parameter indicating the day of the year when chill accumulation is supposed to start.

Returns

dataframe with the chill accumulated for all the seasons in the dataset. Seasons begin at the start date and end the day before the start date of the following year. It contains the columns Year, Month, Day, DOY, Chill

Examples

# Generate hourly temperatures
library(magrittr)
library(dplyr)
library(lubridate)
Weather <- Tudela_DW %>%
   filter (Tudela_DW$Year<=2002)
Tudela_HT <- hourly_temps(Weather,42.13132)
# Calculate chill as chill units, starting on DOY 305
Chill_u <- chill_units(Tudela_HT,305)

References

Richardson EA, Seeley SD and Walker DR, 1974. A model for estimating the completion of rest for Redhaven and Elberta peach trees. HortScience 9, 331-332.

Author(s)

Carlos Miranda, carlos.miranda@unavarra.es