Microclimate & Canopy Data

📚 Biophysical Modeling of Protected Crop Transpiration

Plant transpiration accounts for over 90% of total greenhouse water consumption and serves as the primary evaporative cooling mechanism for the crop canopy. Sizing drip irrigation pumps, substrate moisture replenishment, dehumidification heat pumps, and ventilation capacity requires precision modeling of hourly latent heat flux.

1. The Penman-Monteith Canopy Equation

The Penman-Monteith equation (FAO-56 standard adapted for CEA greenhouse canopies by Stanghellini) couples the radiative energy balance with convective boundary layer vapor transfer:

• λ : Latent heat of vaporization of water (2.45 × 10⁶ J/kg).
• E : Evapotranspiration mass flux (kg/(m²·s), equivalent to mm/s or L/(m²·s)).
• Δ : Slope of the saturation vapor pressure curve at air temperature (kPa/°C).
• R_n : Net radiation intercepted by the vegetative canopy: R_n = R_g × (1 - e^(-k × LAI)) where k ≈ 0.65 is the canopy extinction coefficient.
• ρ_air · c_p : Volumetric heat capacity of moist greenhouse air (≈ 1200 J/(m³·K)).
• VPD : Vapor Pressure Deficit of greenhouse bulk air (kPa).
• r_a : Aerodynamic boundary layer resistance of leaves (s/m), governed by forced convection and air velocity u.
• r_s : Bulk canopy stomatal resistance (s/m), regulated by guard cell turgor pressure and light-activated ion pumps.
• γ : Psychrometric constant (≈ 0.066 kPa/°C).

2. Crop Stomatal Resistance & Morphological Benchmarks

3. Translating Transpiration into Irrigation Dosing & Leaching

To prevent root-zone osmotic stress and electrical conductivity (EC) spikes, total water injected through drippers must compensate for plant transpiration plus a target drainage fraction:

For high-wire Dutch tomatoes cultivated at 2.5 stems/m² with a cumulative daily ET of 3.2 L/m²·day and a target 30% leaching fraction (Drain = 0.30), each plant requires: (3.2 / 2.5) / (1 - 0.30) ≈ 1.83 Liters/plant/day delivered across 8 to 14 discrete pulses synchronized with solar radiation accumulation (e.g. 1 pulse per 150 J/cm²).