Thermal Time

Daily thermal time (also known as growing degree days or heat units) is the primary driver for phenological development in the Lentil model. It accounts for the cumulative effect of temperature on plant development rates.

Overview

Thermal time is calculated daily using a three cardinal temperature response function with different parameters for different developmental phases. The function uses minimum, optimal, and maximum temperatures to define the temperature response.

  • Pre-emergence thermal time is accumulated from sowing until emergence
  • Post-emergence thermal time is accumulated from emergence onward
  • Different phases may have different thermal time requirements
  • The thermal time accumulation is the primary mechanism driving phenological transitions

Calculation Method

Thermal time (in °Cd per day) is calculated as:

\[TT = \max(0, \min(T_{opt} - T_{min}, T - T_{min}, T_{max} - T))\]

Where: - \(T\) is the daily mean temperature (°C) - \(T_{min}\) is the minimum temperature (base temperature) (°C) - \(T_{opt}\) is the optimal temperature (°C) - \(T_{max}\) is the maximum temperature (°C)

Temperature Thresholds

Different growth phases have different temperature thresholds:

Pre-emergence Phase

  • Base temperature (\(T_{min}\)): Varies by cultivar (typically 0-5°C)
  • Optimal temperature (\(T_{opt}\)): ~25°C
  • Maximum temperature (\(T_{max}\)): ~45°C

Post-emergence Phases

  • Base temperature (\(T_{min}\)): Typically 2-5°C
  • Optimal temperature (\(T_{opt}\)): ~24°C
  • Maximum temperature (\(T_{max}\)): ~40°C

Cultivar-Specific Thermal Time Requirements

Each phenological phase has specific thermal time requirements that are cultivar-specific:

Phase Thermal Time Duration (°Cd) Notes
Germinating Variable Pre-emergence, soil-dependent
Emerging ~200-300 Variable by cultivar
Juvenile 400 Base value, may vary
Inductive Variable Cultivar-specific
FirstFlowersDeveloping 200 Time from budding to flowering
FirstPodsDeveloping Variable Pod development phase
FirstGrainsDeveloping Variable Grain initiation phase
FirstGrainsFilling Variable Critical grain-filling phase
FinalPodsDeveloping Variable Late pod development
FinalGrainsFilling Variable Final grain maturation
Ripening Variable Ripening to harvest maturity

Practical Applications

Thermal time is used for:

  1. Predicting phenological stages: Accumulating thermal time predicts when major growth stages will occur
  2. Comparing cultivars: Different cultivars have different thermal time requirements for similar developmental stages
  3. Assessing seasonal effects: Warm seasons accumulate thermal time faster than cool seasons
  4. Yield forecasting: Early prediction of flowering and grain-filling timing helps forecast grain yield
  5. Management decisions: Knowing thermal time requirements guides irrigation, nutrient, and pest management timing

Examples

  • Warm season: A season with higher average temperatures will accumulate thermal time faster and reach flowering earlier
  • Cool season: A cool growing season will accumulate thermal time more slowly, potentially extending the growing season
  • Cultivar comparison: A cultivar requiring 400°Cd to reach a stage will reach it earlier in warm regions than a cultivar requiring 600°Cd

The thermal time model in APSIM accurately represents the temperature-dependent development of lentil under field conditions.