Are The Calculations Understood?

by lan Nichols, Ontario Weather Network

The crop heat unit (CHU) index continues to be an important yardstick for determining the relationship between crop development and growing season temperatures. The index value comes from two different equations. One equation calculates the nighttime contribution that temperature makes to crop development and another equation calculates the daytime contribution. These values are averaged to arrive at the daily CHU total. The daily calculation itself is straightforward and can be automated in a simple computer program. Tables are also published so any device that can accurately record the daily high and low temperature can easily be used in the determination of CHU. (Dr. Murray Brown of the University of Guelph developed this CHU index after conducting experiments in the late 1950s and early 1960s with several corn hybrids and soybean varieties that covered the range of maturities for Ontario at that time. He described the index in an OMAF Factsheet shortly after it was introduced in 1964. He and Andy Bootsma, Agriculture Canada, Ottawa, revised the CHU map and the procedure for starting and ending the crop growing season each year in the early 1990s and described these procedures and the new map in an OMAF Factsheet published in 1993.)

While the procedure for daily calculations are easy, determining the official season start date requires an examination of each weather station's temperature record to determine when two different conditions have been met. The rules for starting the season can create a wide range of season start dates when they are applied correctly. However, some computerized routines in recent years have not considered both conditions and when producers go looking for information, they may not realize the significance of the errors.

The starting threshold date is the date at each station when the average temperature reaches 10° C (Chart 1 -green box). This threshold is the condition that must be met before the second rule can be applied. Because this threshold date changes for different weather stations this is the rule that tends to be ignored. The second rule, which states that CHU accumulation starts on the third day of three consecutive days when daily average temperature reaches 12.8°C or higher, is an easy check for a computer. A very unusual warm spell occurred in 2002 when much of the southern part of the province reached the third day of 12.8 or higher on April 17. Some computerized CHU calculators kicked into gear at this time and started the seasonal tally (Chart 1 - tan boxes). A check for 12.8° C temperatures should not have even occurred before the average 10° C date (Chart 1 - green boxes). In 2002, some corn was planted early but then it turned cool and wet in many locations and the spring went into hibernation. The official 2002 CHU start, when both rules were applied, occurred 41- 42 days (Chart 1 - purple boxes) later than the 12.8 rule trigger alone. Note though that Toronto, a bit of a hot spot, was officially 3 weeks earlier than Hamilton. Much confusion resulted.

As more and more analyses are done between growing seasons and between adjacent farming areas, comparing current year values to the past year and to the elusive normal year, erroneous conclusions can result. It is difficult for producers to relate relative hybrid performance to past growing seasons when they have to be concerned about the consistency of CHU calculations.

One possible option for the CHU index is to standardize the start date, perhaps on May 1st (Chart 1 - yellow boxes). The argument for doing so is not just rooted in convenience and standardization. Producers once waited for soils to be warm before planting. With larger acreage, better seed and more effective seed treatments, producers tend to plant earlier. When the soil is fit, or nearly ready, the planters roll and many corn crops, and sometimes beans, are in the ground and making some progress before the CHU calculation for the season officially begins. Seed dealers suggest that farmers routinely ask for longer season corn than the official area ratings for their early plantings. If CHU is to remain a relevant measure we better be counting it when crops are in the ground.

The Ontario Weather Network at Ridgetown College studied the ramifications of adjusting to a May 1 start date by computing the CHU adjustment at numerous weather stations over the past 30 years (Chart 2). In years when the official rules, properly applied, declared an early start to CHU accumulation, the adjustment is minimal (Chart 2 - green bar). For London Ontario, this is about 50 CHU. When official CHU rules declare a late spring, a May 1st, CHU start adds considerably more to the season accumulation. How much more? The extreme case in 30 years for London is about 125 CHU (brown boxes). In an average year the CHU adjustment would run between 100 and 200 CHU across the province. However, the adjustment may actually be less if the farmer's perspective is considered. In a cool late planting year, the crop goes in at any possible opportunity and often before the CHU triggers have been met.

The question now has to be asked -can producers gain a better understanding of crop growing conditions by calculating CHU from a standard May 1st. start date each year? Consider these points.
1. The rules would be consistently applied - far less confusion.
2. Comparisons between years would be based on the same number of calendar days.
3. The basis for the "normal year" calculation would be easily understood.
4. It accommodates earlier planting into cooler soils.
5. Early May CHU accumulations are typically low most years, but if crops are planted, CHU should be counted.
6. Late start seasons CHU's are often understated compared to actual planting dates.
7. Comparisons between nearby weather stations would be more consistent in the case where one station would have otherwise narrowly missed the 3-day criteria. (Toronto-Hamilton 2002)

A couple of final notes:
1. Some weather programs generate a CHU value for a single warm day -even in January. Be careful how you request your statistics!
2. CHU's are guidelines and do not take into account microclimate affects such as soil type and topography. The published CHU map is classed by 200 CHU intervals and considerable variability exists within these zones.
3. A Western Canada fall 2000 article, Temperature and Plant Development from AAFC-Research Highlights, states "Frequently the start date is set at May 15th." We don't appear to be cutting new ground with a fixed start date proposal - the guys out west are doing it!
4. Ending dates are also variable from year to year and from station to station. A killing frost occurring in low-lying pockets while not in other areas can really complicate the determination of the end of the CHU season.

The Ontario Weather Network (OWN) operates from the Ridgetown Campus of the University of Guelph. OWN specializes in climate monitoring and in producing advisories and crop management models for agricultural clients. OWN has developed the analytical tools and historical database to test new CHU index applications.