What is a Net Radiometer? The Basis of Energy Balance and Irrigation Decisions in Agriculture
A net radiometer is a measurement sensor that directly answers the question, “How much of the energy from the sun actually heats and evaporates the plant and soil?” It reveals the energy balance of a surface (soil, vegetation, water surface, etc.) by measuring the total radiation energy it receives and loses.
What Does a Net Radiometer Measure?
A net radiometer evaluates four basic radiation components together:
- Incoming shortwave radiation (Solar radiation)
- Reflected shortwave radiation (albedo effect)
- Incoming longwave radiation from the atmosphere
- Outgoing longwave radiation from the surface
The difference between these components is expressed as follows:
Net Radiation = Incoming − Outgoing (shortwave + longwave)
Therefore, a net radiometer measures the energy actually gained or lost by the surface.
What Does a Net Radiometer Do?
A net radiometer is of critical importance in the following areas:
Agricultural Irrigation Planning
- It is the primary input for crop evapotranspirasyon (ET) calculations
- It is used to determine when and how much irrigation should be performed
Microclimate and Climate Analysis
- It explains the soil–atmosphere energy exchange
- It plays a critical role in frost risk, heat stress, and heat island analyses
Water and Energy Balance
- It directly affects soil warming, evaporation, and plant transpiration
Plant Stress Analysis
- Rₙ: Net radiation (MJ m⁻² day⁻¹)
- G: Soil heat flux
- Δ: Slope of the saturation vapor pressure curve
During daylight hours, 60–80% of ET₀ is directly related to net radiation. That is, the net energy entering the plant-soil system determines water consumption.
Net radiation:
- Heats the soil and the plant
- Provides the energy required for water evaporation
Behavior of Net Radiation
- Rₙ = 0 → Even if there are wind and humidity differences, ET drops significantly
- As Rₙ increases → The plant's water consumption increases
Clear and sunny summer days:
→ Rₙ high → ET₀ high → irrigation requirement increases
On cloudy / short days:
→ Rₙ decreases → ET₀ decreases
Night:
→ Rₙ is usually negative (the surface loses energy)
How is Rₙ Calculated? (According to FAO-56)
FAO-56 divides net radiation into two parts:
- Shortwave (solar) net radiation
- α (albedo) ≈ 0.23 (reference grass)
- Rₛ → Incoming solar radiation
2. Longwave net radiation
Rnl
- Heat difference between incoming from the atmosphere + outgoing from the surface
- It is related to temperature, humidity, and cloudiness
3. Total net radiation
Rn = Rns – Rnl
What is a Pyranometer?
A pyranometer measures the shortwave solar radiation reaching a surface.
Radiation it measures
- Solar-derived shortwave
- Wavelength range: ≈ 0.3 – 3 µm (300–3000 nm)
What does it measure?
- Global solar radiation
- Direct solar radiation + scattered radiation in the atmosphere
What is a Pyrgeometer?
A pyrgeometer measures longwave (thermal) radiation emitted from the surface and the atmosphere.
Radiation it measures
- Thermal infrared (longwave)
- Wavelength range: ≈ 4.5 – 50 µm
What does it measure?
- Incoming longwave radiation from the atmosphere
- Outgoing longwave radiation emitted by the surface
How does a pyrgeometer provide information about frost risk?
Why does frost occur?
Radiation frost in agriculture mostly occurs under these conditions:
- Clear and cloudless night
- Weak wind
- The surface (soil/plant) losing net heat to the atmosphere
The main pathway for this heat loss is:
emission of energy into space via longwave radiation (thermal IR).
What does a pyrgeometer measure?
A pyrgeometer measures longwave radiation:
- L↓ (incoming longwave from the atmosphere)
- L↑ (outgoing longwave from the surface)
Critical difference for the night:
Lnet = L↓ – L↑
- Negative value → The surface is losing heat to the atmosphere
- Growing negative value → Accelerating cooling → frost risk is increasing
Difference between clear and cloudy nights (critical)
Clear sky (high frost risk)
- L↓ from the atmosphere is low
- L↑ emitted from the surface is high
- Net longwave loss is large
- Plant temperature drops rapidly
A pyrgeometer measures this directly
Cloudy night (low frost risk)
- Clouds re-emit IR
- L↓ increases
- Net energy loss decreases
A pyrgeometer indicates that the risk of frost has decreased.
Agricultural decision mechanism
Thanks to the pyrgeometer:
- Warnings are given before frost occurs
- Wind machines, fogging, and irrigation are started on time
- Unnecessary intervention is prevented
The pyrgeometer does not measure frost itself; by measuring the nighttime radiative heat loss that causes frost, it reveals the risk of frost hours in advance.
Don't Guess Irrigation Without Measuring Net Radiation
What determines how much water a plant needs is not just temperature or humidity, but the net energy that actually enters the plant-soil system.
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