Calculating Thermal Time from Weather Data

2.8 Calculating Thermal Time from Weather Data

Reports of thermal time for predicting crop or pest development are gen- erally based on calculations from daily maximum

and minimum temperatures using

At.

If the average of the maximum and minimum temperatures is less than the base temperature or greater than some maximum temperature, zero is

added to the sum for that day. Several assumptions are implicit in using Eq. (2.9):

1. the growing region of the plant is at air temperature

2. the hourly air temperature does not go below the base temperature or above the maximum temperature during a day

3. the process being predicted is linear with temperature between the base and maximum temperatures.

The time increment, At, is taken as one day. The progress toward comple- tion of a developmental stage is reported in day-degrees above a specified base temperature. Day-degrees required for completion of a developmen- tal stage are used to determine completion or progress toward completion of development. The role of extreme temperatures in calculation of degrees is discussed in the next section. Errors from the growing point temperature not being at air temperature can be significant. For example, the growing point in corn is below the soil surface in early developmental stages, and failure to use soil temperature during this time can result in errors of five days or more in predictions of tasselleling date.

The base temperature and thermal time requirements of organisms depend, of course, on species and developmental stage. There is some evidence, however, that base temperatures may be relatively constant for developmental processes within a species and genotype. Angus et al. (1981) report the base temperatures of 30 species, including both tem- perate and tropical crops. Selected values are shown in Table 2.1. Note

that the base temperatures fall into two groups, one centered around C, and the other around 1 C. The former are representative of temperate species such as wheat, barley, pea, etc., and the latter of tropical crops such as maize, millet, and sorghum. Base temperatures and thermal time requirements can be estimated using the values from Table 2.1,

but it should be recognized that considerable genotypic variability

Calculating Thermal Time

Weather Data

2.1. Base temperature and thermal time requirement for emergence of selected temperate and tropical crops (from Angus et al. 1980).

T A BLE

Species

(C) day-deg

Species

(C) day-deg

10.6 48 field pea

2.6 79 pigeon pea 12.8 58

ists. This variability is very useful in fitting specific genotypes to specific environments.

The thermal time concept has been useful in applications where one wishes to predict harvest dates or emergence dates from planting dates, for finding varieties (with known thermal time requirements) which are best suited to a given climate, and for predicting disease or insect development.

Example 2.6. Using the weather data in the first three columns of the following table, and assuming the seed temperature is the same as the air temperature, how long would it take to germinate seed of

which was planted on day

Solution. From Table 2.1, has a base temperature of

C, and

4 in the table is the quantity in brackets in Eq.

a thermal time requirement of 43 day degrees for emergence.

and column 5 is the summation according to Eq. (2.9). Emergence takes place on day 199 since the thermal time requirement is completed in that day.

Temperature