Predicting chill accumulation requirements using different models

Chill accumulation models are used to estimate the days from the end of a season until bud break in the following season. Trees have specific chill requirements that are related to the number of accumulated hours within a known temperature range. A new chill accumulation model was developed to improve the estimation of bud break date. In this model, we use chill days (CD), which are defined as the cumulative number of hours below a pre-selected threshold temperature divided by 24 hours per day (to express as chill days). We also use "anti-Chill days" (CA), which are defined as the cumulative number of hours above the same pre-selected threshold temperature divided by 24 hours per day. The CD values are given a negative sign and the CA values are given a positive sign. In the model, the negative CD values are accumulated until they reach a pre-selected value that is identified as the chill requirement (CR), which is also a negative number. The chill requirement is met when the SCD is less or equal to CR. On the following day, the model no longer adds CD values but begins to add daily anti-chill days to CR. On the date that CR + SCA is greater or equal to 0, that date is selected for bud break. The optimal value for the chill temperature threshold and for CR are determined using trial and error until the root mean square error between predicted and observed days between harvest or leaf drop and bud break is minimized.