calculation of heat units

CALCULATION OF CALCULATION OF HEAT UNITSHEAT UNITS

ByBy

Linda De WetLinda De Wet

INTRODUCTION-HEAT UNITSINTRODUCTION-HEAT UNITS There is a wide selection of hybrids and varieties There is a wide selection of hybrids and varieties

for most crops. Most of the warm-season crops for most crops. Most of the warm-season crops have a wide range of maturities. Growing have a wide range of maturities. Growing Degree Days (GDD), Degree Days (DD), Crop Degree Days (GDD), Degree Days (DD), Crop Heat Units (CHU) or Thermal time (TT) provide Heat Units (CHU) or Thermal time (TT) provide an indexing system to assist farmers to an indexing system to assist farmers to selectselect the most suitable hybrids and varieties for their the most suitable hybrids and varieties for their area. This area. This indexing systemindexing system was originally was originally developed for field corn and has been in used in developed for field corn and has been in used in Ontario for 30 years. The crop heat unit ratings Ontario for 30 years. The crop heat unit ratings are based on the total accumulated GDD for the are based on the total accumulated GDD for the frost-free growing season in each area (Brown frost-free growing season in each area (Brown and Bootsma, 1997).and Bootsma, 1997).

ManagementManagement Prediction of stage of crop developmentPrediction of stage of crop development

Timing of pesticide applicationTiming of pesticide application Scheduling of orderly harvestScheduling of orderly harvest Synchronising the flowering of cross-Synchronising the flowering of cross-

pollination cropspollination crops Hybrid seed productionHybrid seed production

Thermal time is the principal approach used to Thermal time is the principal approach used to estimate the estimate the duration duration of crop growth and not of crop growth and not temperature.temperature.

INTRODUCTIONINTRODUCTION

Growth Growth Division + expansion of cellsDivision + expansion of cells Dry matter accumulationDry matter accumulation

DevelopmentDevelopment Orderly process – birth to deathOrderly process – birth to death Differs between organismsDiffers between organisms Development stages and processes - different TaDevelopment stages and processes - different Ta E.g. - E.g. - timetime for egg stage of the Fruit fly, for egg stage of the Fruit fly, Dacus Dacus

cucurbitaecucurbitae Constant at a temperature of 10 Constant at a temperature of 10 ooC - 35 C - 35 ooCC Short 20 Short 20 ooC - 30 C - 30 ooCC Longer at extremes - T > 37 Longer at extremes - T > 37 ooC and < 15 C and < 15 ooCC

INTRODUCTIONINTRODUCTION

If we calculate rate of development as 1/time, typical If we calculate rate of development as 1/time, typical curves can be drawn with Z development rate versus curves can be drawn with Z development rate versus Ta. These same curves hold for many biological Ta. These same curves hold for many biological processes.processes.

Figure 1Figure 1 Typical curve for biological processes showing Ta vs Typical curve for biological processes showing Ta vs Z development rateZ development rate

Typical growth curve

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50

T (oC)

Z D

evel

op

men

t ra

te

(1/t

)

1/t

Table 1Table 1 Calculation of development Calculation of development (1/t = development rate)(1/t = development rate)

Time (h/d)Time (h/d) Ta (Ta (ooC)C) 1/t1/t 1/t1/t

00 00 0.000.00 00

33 55 0.330.33 0.330.33

66 1010 0.170.17 0.50.5

99 1515 0.110.11 0.610.61

1212 2020 0.080.08 0.690.69

1515 2525 0.070.07 0.760.76

1818 3030 0.060.06 0.820.82

2121 3535 0.050.05 0.860.86

2424 4040 0.040.04 0.910.91

Table 1 Table 1 A time period (day or hour) is chosen A time period (day or hour) is chosen Ta is loggedTa is logged Rate at that Ta for change in time (Δt) is Rate at that Ta for change in time (Δt) is

calculatedcalculated (1/t) is calculated(1/t) is calculated The rate x t = development that takes placeThe rate x t = development that takes place The total time to completion is time till sum at oneThe total time to completion is time till sum at one So, the assumption that is generally made, is that So, the assumption that is generally made, is that

the calendar is correct basis for development, the calendar is correct basis for development, but but development actually depends on Tadevelopment actually depends on Ta, and not , and not date, as illustrated abovedate, as illustrated above

THERMAL TIMETHERMAL TIME When using TT some assumptions need to be When using TT some assumptions need to be

made:made: Development rate is constantDevelopment rate is constant Straight line between Ta and development rateStraight line between Ta and development rate Ta is always between base (Tb) and maximum Ta is always between base (Tb) and maximum

temperature (Tmx)temperature (Tmx) Base temperature Tb is Ta where growth stopsBase temperature Tb is Ta where growth stops Tmx is Ta at maximum growth rateTmx is Ta at maximum growth rate

TT is the sum of (ΔT from Tb) x Δt (length time TT is the sum of (ΔT from Tb) x Δt (length time at current Ta)at current Ta)

So the equation for thermal time isSo the equation for thermal time is

TT or DD = TT or DD = (Tave-Tb) Δt(Tave-Tb) Δt

The conditions for above is that The conditions for above is that (Tave-Tb) > 0 where (Tave-Tb) > 0 where

Tave = Tmx+Tmn/2Tave = Tmx+Tmn/2 If Tave = Tb, then TT = 0If Tave = Tb, then TT = 0

THERMAL TIMETHERMAL TIME

Time step is chosen so that Ta in that time is Time step is chosen so that Ta in that time is constantconstant

Unit for TT is degree day (Unit for TT is degree day (ooCd) or degree hour Cd) or degree hour ((ooCh)Ch)

So thermal time is the calculation of the time for So thermal time is the calculation of the time for development at a maximum rate in degree daysdevelopment at a maximum rate in degree days

Total thermal time value can be used to Total thermal time value can be used to calculate time taken at another Ta alsocalculate time taken at another Ta also

Thermal time can be calculated from weather Thermal time can be calculated from weather data, i.e. Tmx and Tmn for each day, to get data, i.e. Tmx and Tmn for each day, to get Tave.Tave.

TT = TT = [(Tave) - Tb)] Δt[(Tave) - Tb)] Δt


If Tave < Tb then add only 0 (not If Tave < Tb then add only 0 (not negative nos)negative nos)

Time period Δt is one day and is in Time period Δt is one day and is in degree-days above Tbdegree-days above Tb

TT depends on specific organism, TT depends on specific organism, age and development stage but Tb is age and development stage but Tb is constant for a species (Table 2)constant for a species (Table 2)


Table 2Table 2 Base temperature (Tb) for different speciesBase temperature (Tb) for different species Species Tb (oC) TT for emergence

Wheat 2.6 78

Barley 2.6 79

Oats 2.2 91

Peas 1.4 110

Lentils 1.4 90

Rape 2.6 79

Maize 9.8 61

Pearl millet 11.8 40

Sorghum 10.6 48

Peanut 13.3 76

Cowpea 11 43

Pigeonpea 12.8 58

Thermal time is used to predict harvest dates Thermal time is used to predict harvest dates and emergence from planting date with the and emergence from planting date with the current year’s weather data. Usually the daily current year’s weather data. Usually the daily values are used values are used

Calculation of TT on an hourly basis is more Calculation of TT on an hourly basis is more accurateaccurate

Environmental factors can also be included with Environmental factors can also be included with TTTT

For plant disease - moisture on leaves is NBFor plant disease - moisture on leaves is NB Here the current vapour pressure (ea) or dew Here the current vapour pressure (ea) or dew

point temperature (Td) or RH is used, when the point temperature (Td) or RH is used, when the leaf is wetleaf is wet


Table 3Table 3 Example for DAILY DD calculated in the Example for DAILY DD calculated in the summer. Use Tb for Maizesummer. Use Tb for Maize

Day Date Tmx Tmn Tave TT TT

(oC) (oC) (oC) (oCd) (oCd)

274 1/10/1997 24.3 12.7 18.5 8.7 8.7

275 18.9 11.2 15.1 5.3 14

276 20.3 9.4 14.9 5.1 19

277 20.1 9.3 14.7 4.9 23.9

278 25.8 10.1 18 8.2 32.1

279 15.3 11.5 13.4 3.6 35.7

280 24.1 9.5 16.8 7 42.7

281 31.6 13.4 22.5 12.7 55.4

282 30.9 14.4 22.7 12.9 68.2

283 22.7 10.6 16.7 6.9 75.1

284 22.7 10.6 16.7 6.9 81.9

TIME (h) Rad(W/m2) Ta (oC) Tw (oC) WIND(m/s) RAIN (mm) RH (%) TT(oCh) (oCh)

1 0 5.9 5.6 0.31 0 95

2 0 6.1 5.7 0.42 0 95

3 0 6 5.7 0.46 0 96

4 0 5.4 5.2 0.14 0 97

5 0 4.4 4.3 0.01 0 98

6 0 4.5 4.4 0.01 0 99

7 0 4.5 4.4 0.01 0 99

8 0 4 4 0.43 0 99

9 115 6.5 6.2 0.36 0 96

10 149 8.9 8.3 2.48 0 93

11 126 8.8 8.4 2.23 0 95

12 182 9.7 9 1.64 0 92

13 384 13.1 11.2 1.37 0 80

14 491 16.3 12.7 1.75 0 66

15 306 16.9 12.6 1.92 0 61

16 262 17.4 12.5 1.77 0 57

17 82 16.9 12 1.56 0 55

18 0 15.4 11.1 0.38 0 59

19 0 12.1 9.3 0.01 0 70

20 0 10 8.2 0.01 0 79

21 0 9.8 8.3 0.01 0 82

22 0 10.2 8.6 0.01 0 81

23 0 9.4 7.9 0.01 0 82

24 0 8 7 0.01 0 88

PRACTICAL PRACTICAL HEAT UNITS FOR CROP-ECOTOPE MATCHINGHEAT UNITS FOR CROP-ECOTOPE MATCHING

Degree Days to Predict Harvest DatesDegree Days to Predict Harvest Dates Use the weather data provided (Blackboard) to calculate the Use the weather data provided (Blackboard) to calculate the

thermal time for a ………..crop. Use the base temperatures thermal time for a ………..crop. Use the base temperatures provided in the table and predict the germination, emergence and provided in the table and predict the germination, emergence and harvest dates (harvest being after ………… degree days have harvest dates (harvest being after ………… degree days have accumulated). Tabulate results.accumulated). Tabulate results.

The planting can only occur after …………….. and after 25 mm The planting can only occur after …………….. and after 25 mm rain has been received. rain has been received.

Use the Heat Units map to decide which areas in S.A. your crop Use the Heat Units map to decide which areas in S.A. your crop would be best suited to.would be best suited to.

Check Assessment form for details concerning handing in Check Assessment form for details concerning handing in requriements.requriements.

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calculation of heat units

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