Counting Chill Better – Using the Chill Portions Model

Part 2 of 3 in the series - What can we learn from the low chill winter of 2013-2014In my last post, Is Last Year’s Warm Winter the New Normal?, I discussed low chill winters like last year’s coming more often in the near future. The first step to preparing for those warmer winters is counting chill better – moving from counting in chill hours to counting in chill portions.What makes the chill portions model (also called the Dynamic Model) better? There are three basic difference between the chill hours model and the chill portions model.

• Chill hours counts any hour between 32°-45° F as the same. Chill portions gives different chill values to different temperatures. No more wondering about the value of ‘warm’ chill hours. Temperatures between 43°-47° F have the most chill value. The chill value on either side of that range are lower, dropping to no value at 32° F and 54° F.
• Chill hours only counts up to 45° F. Chill portions count up to 54° F. This makes chill portions better able to approximate how the trees we grow, most of which evolved in fairly mild climates, count chill.
• Chill hours does not subtract for warm hours. Chill portions can. The math is tricky, but the concept is simple: Chill portion accumulation is a two-step process. First, a ‘chill intermediate’ is accumulated, but can be subtracted from if cold hours are followed by warm hours. Second, once the chill intermediate accumulates to the certain threshold, it is converted into a ‘chill portion’ and the chill intermediate count starts over from zero. The chill portion cannot be undone by later warm temperatures.

It’s hokey, but I like to think of it as filling ‘chill buckets’ that then fill a ‘chill tank.’ Cold hours add to what’s in the bucket until it’s full. Warm hours along the way can spill out some of what’s in the bucket. But once the bucket is full and dumped into the tank, that chill can’t be spilled or lost. Last January shows the need for this component. Warm January day temperatures subtracted from the cool temperatures of the night before, but did not subtract from cool temperatures in November and December.There’s two ways you can track chill portion accumulation – let UC do it for you with CIMIS station data, or do it yourself with your own weather system.To use the UC resource, go to the Fruit & Nut Research & Information Center’s Cumulative Chill Portions calculator. Click your nearest CIMIS station, put in the start date of November 1^st and click “View Data.” The site will show a graph of the chill accumulation this winter and the previous five, and a long table of numbers. The column next to the dates is the daily chill portions for this year. The next columns shows the running total for this winter and the last five.*To use your own weather system, you need a file with hourly temperature data. Then visit Kitren Glozer’s chill model website and download the “Dynamic Model” Excel file and the step-by-step “Dynamic Model & Chill Accumulation Guide” pdf. Follow the pdf instructions to copy-paste your hourly temperature data into the Excel file and drag down rows with embedded formulas.Now that you can figure out how much chill you got, how much do you need? I’ll deal with that in my next post. *The winters are labelled by the year they started. For example, this winter of 2013-2014 will be labelled 2013. You might see letters or numbers next to the chill counts. This indicates that for that day CIMIS is not sure the data is correct.