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Agronomy 541 : Lesson 4a
Derived Temperature Indices
Introduction
Developed by D. Todey
It is suggested that you watch Video 4A and complete the exercise in the video before continuing with the
lesson.
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Regular measurements of climatic temperature data include maximum and minimum temperatures at numerous
climatological stations. Interpreting how these temperatures affect crop growth is not always straightforward.
But other indices derived from these data can illustrate how crops should be responding to the temperatures.
While not completely indicative of crop condition, indices such as the Growing Degree Day can be descriptive
when summed over the growing season. There are several other indices which can be derived from maximum
and minimum temperature measurements; these include Stress Degree Days, Aridity Index, Cooling Degree
Days, Heating Degree Days, and Chill Units. These indices have agricultural and personal comfort application.
We will define and discuss applications of these indices in this part of lesson 4.
What You Will Learn in This Lesson:
How temperature and moisture stress in crops can be compared using various indexes.
About temperature indexes.
About combined temperature and moisture indexes.
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Stress Degree Days
Average temperatures and precipitation are the starting point for comparison of weather and climate between
years and regions. Average maximum temperatures and precipitation for Ames, IA (Fig. 4.1) are included for
reference values. Examining historical climate and yield records has produced a statistical relationship for
monthly temperature with yield (Thompson, 1986) (Fig. 4.2). If the July temperature is near normal in the U.
S. Corn Belt, a near normal crop yield is likely. If the July temperature is 4° F above average, the yield might be
diminished by 600 kg/ha, and if the temperature is 4° F cooler than average in July, the yields may be
enhanced by 300 kg/ha. There have been cold temperatures and low yield, and cold temperatures another
year with very high yield, but there is a relationship that gives some predictability. If the temperature is a few
degrees cooler than usual, it helps the yield. This result implies that normal July temperatures are slightly
stressful to crops in the central United States.
Fig. 4.1 Average monthly maximum and minimum temperatures (°F)
and precipitation (in.) for 1951-1997.
Three degrees colder than usual seems to be the optimum. If it is even colder than that, then it starts to drop
off again. Of course, average temperature means average yields. This relationship allows the July contribution
of temperature to yield potential to be evaluated.
The largest single-month statistical effects on the crop are from July and August. This is because of the
stressful conditions that may occur during the critical reproductive periods of those months. The idea with
temperature indices is to try to quantify some of the stress crops may experience.
Fig. 4.2 The response of corn to weather variables in
the five central Corn Belt states.
Although averages are useful for assessing expected conditions, the range and variability are also of great
concern. Most stressful years will occur on the extremes of the range of climatic possibilities, not near the
means. The range of average high and low temperatures during the month of August over the past 100 years
for Ames gives an example (Figure 4.3).
Fig. 4.3 August monthly averaged maximum and minimum temperatures
for Ames, IA.
Study Question 4.1
How many degrees above average were August 1995 maximum temperatures?
°F Check Answer
Study Question 4.2
How many degrees above average were August 1995 minimum temperatures?
° F Check Answer
Figure 4.3 indicates that August 1995 was not particularly one of the hotter Augusts of the past century. It is in
the warmest 50 percent, but not in the warmest 10 percent. High temperatures were not stressfully hot.
Minimum temperatures in August 1995 were some of the very warmest nighttime temperatures in the history of
records kept in Iowa.
This is a significant situation. When the overnight temperatures are high, there will be a marked influence on
growing degree days, more of an influence than if daytime temperatures were high. Remember, growing
degree days cut off at 86° F. What will happen to corn development? The growing degree days will be greatly
accelerated by the high overnight temperatures. Compare the expected August growing degree day for 1995
with 1988 and with an "average" August (Fig. 4.4).
Fig. 4.4 Growing Degree Days for 1988 in Ames, IA (Blue line is actual
GDD accumulation; red line is normal GDD accumulation-left axis.
Green line is the difference in accumulated from the average-right
axis.).
Study Question 4.3
How many GDDs are normally accumulated by July 1 at Ames, IA?
GDDs Check Answer
Study Question 4.4
What was the difference in accumulated actual growing degree days from average on July 1, 1988?
GDDs Check Answer
Fig. 4.5 Growing Degree Days for 1995 in Ames, IA (lines
are the same as Fig. 4.4).
In the 1995 situation we have enhanced corn growth and GDD accumulation in August without increasing
stress. In 1988 there was a great increase in stress caused by increased maximum temperatures. There are
ways of calculating this stress using temperature values. Note that the GDD accumulation in August 1995
exceeded that if the very stressful 1988 (Fig. 4.4 and 4.5).
FYI : July Temperatures
Monthly averages and other summary information for climatological stations around the state can be found at the ISU
Climatological Data page:
http://www.agron.iastate.edu/climodat
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IN DETAIL : Comparison of Phenomena
When working with comparing amounts or evaluating phenomena for a particular year, always compare it to the
average. It is necessary to make an "eyeball" guess. To do this, place and adjust a ruler on the chart until it is in the
approximate center of the values or estimate what the average temperature may be. Notice the average temperature
for the highs. Note what it was in 1995. Note what it was the previous hot year, 1988 in this case for the high
temperatures. Compare what happened this year with the previous ones and make an estimate, remembering that we
have been looking at growing degree days when we study the factors that influence crop growth.
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Agronomy 541 : Lesson 4a
Derived Temperature Indices
Heat vs. Water Stress
Some measure of assessing the stress on crops is necessary without going into the field to determine the plant
status, since field measurements are not always available. One of these methods uses daily maximum
temperatures, called the Stress Degree Day (SDD). Similar in calculation to the growing degree day, the
concept of the stress degree day is to use temperature only to measure the stress on crops. We often hear
about heat stress on a crop. True heat stress does not occur very often. Experimental work indicates that the
crop is under severe heat stress when the temperature of the air is in excess of 112°> F (44.4°C). Actually,
leaves begin to cook (experience protein breakdown) when the leaf temperature reaches 117°> F (47.5°> C).
Conditions such as these are rare in Iowa. Table 4.1 lists the number of days and the year when 112°F
(44.4°C) or higher temperatures have been recorded. Note that there have not been any since 1940. Even the
warmest summers of recent history have not produced such extreme temperatures.
Table 4.1 Occurrences of greater than 112°F (44.4°C)
temperatures in Iowa
Heat Stress Occurrences
Year Number of Days
1894 1
1901 2
1911 2
1918 2
1930 1
1934 9
1936 12
1939 1
1940 1
At any temperature below that, they are not cooking. They may be using an extraordinarily high quantity of
water because of being at a high temperature. Often that which is referred to as heat stress is really water
stress induced by elevated temperature.
At what temperature does this water stress that is induced by high temperatures (or other factors) become a
significant factor? First, look at the optimum temperature for a crop. For an individual plant, probably the
optimum temperature for photosynthesis and for crop development is 92°> F (33.3°C). If the temperature
raises above 92°> F (33.3°C), changes begin to take place because of temperature induced increased
respiration rate. It is possible that various activities are not working correctly in the plant chemistry. Between
92°> F (33.3°C) and 117°> F (47.5°C) plant development decreases rapidly. Remember, whenever the leaf
temperature hits 117°> F (47.5°C), the leaf dies suddenly. As the temperature drops below 92°> F (33.3°C),
the photosynthesis and efficiency of the plant drop off and often stop near the temperature of freezing (around
32°> F, 0°C) (Fig. 4.6).
Fig. 4.6 Crop growth, stress, and death
conditions as a function of air temperatures.
At any temperature there could be water stress. It begins at about 86°> F often enough that air temperatures of
86°> F or higher are considered to induce stress, or be stressfully high temperatures. The base temperature at
86°> F is used as the base temperature calculation point for stress degree days (SDD).
Stress Degree Days are calculated similarly to growing degree days. If the low temperature is 70°> F (21°C)
with a high temperature for the day of 90°> F (32.2°C), the low temperature is set to 86, which has been
determined to be the average base temperature for stress (if the minimum temperature is above 86°> F, use
the minimum temperature. Add that to 90 and divide the sum by 2, giving the average of 88°> F. Subtract the
base, which is 86, to obtain 2 stress degree days for the 24-hour period. Stress degree days are determined in
much the same way as we keep track of the growing degree days by summing daily throughout the season.
Equation 4.1
Study Question 4.5
Yesterday's high was 98°F and the low was 79°F. Compute the 24-hour stress degree day contribution.
SDD Check Answer
Study Question 4.6
Under conditions of a high of 102°F and a low of 88°F, compute the 24-hour stress degree day contribution.
SDD Check Answer
Stress degree days are a valuable indicator of anticipated crop yield for the Midwest. Figure 4.7 is a chart of
the accumulated stress degree days for the state of Iowa from 1948 through 1988.